The endometrium during the window of implantation: endocrine, immunological and morphological changes
Decidualization is the combination of changes on endometrium structure and hormone profiles, along with gene expression, cell differentiation and tissue modification, that allow for implantation of the embryo.
Authors: Inés Abad, Roberto de la Fuente and Sara Sanz
The menstrual cycle in humans lasts 28 days in regular terms, being the first day of menses considered the first day of the cycle. But in actuality, cycle refers to a series of intermingled events mainly related to three aspects: the ovary, the uterus (or specifically the endometrium) and the hormones participating in the process. As a consequence, one may refer to either the follicular or the luteal phase regarding to the ovary, the proliferative or the secretory phase of the endometrium or even to the hormone with highest blood levels. Any of them refers to a specific moment of the cycle (Fig. 2).
The window of implantation (WOI) is the term used for that phase in which the uterus is ready to receive the conceptus, around day 9 after ovulation (depending on when fertilization actually occurs). In an ideal 28-day cycle, the window of implantation is considered to start at day 20 and to last for about 5 days (3). During this interval, there is a series of physiological changes in the woman (see below) aimed to facilitate the implantation of the embryo and its survival and development in the endometrial microenvironment. Because many cases of infertility have been associated with a reduced receptivity of the endometrium, a proper WOI is currently considered a fertility-determining factor (4, 5).
Hormone fluctuation within the cycle
The entire menstrual cycle is controlled by levels of different hormones, some of the most important being the gonadotropins. These hormones are glycoproteins secreted by the anterior pituitary through GnRH (gonadotropin-releasing hormone) stimulation, which is strictly controlled by the hypothalamus (reviewed in (6)). The main gonadotropins involved in regulation of the menstrual cycle are FSH (follicle-stimulating hormone) and LH (luteinizing hormone).
The beginning of the cycle is regarded as the beginning of the follicular phase of the ovary and the proliferative phase of the endometrium. During this phase, cohorts of follicles at different stages progress and develop due to an increase in FSH levels. The largest follicles, called Graafian follicles (~18mm), present granulosa cells that are able to produce not only oestrogen, like early smaller follicles, but also progesterone (7). The granulosa cells are the cells that conform the follicle structure, nourishing and providing support to the oocyte. Whereas progesterone levels are relatively low and constant during the first half of the cycle, there is an increment in the blood levels of oestrogen (mainly oestradiol 17β) due to its production from the follicles. However, during the last days before ovulation, largest follicles also acquire the ability to produce progesterone, and so its levels increase significantly at that time. Oestradiol secretion creates a positive feedback that stimulates LH production; but increased levels of LH are also the result of progesterone-mediated regulation (8).
Follicle growth and development means that granulosa cells not only respond to FSH-stimulation, but also to LH, as well. As a consequence, levels of oestradiol that had been increasing from ~day 5 become so high that at the preovulatory phase they trigger a rapid and highly significant release of LH from the pituitary (LH surge) (9). The LH surge causes a negative feedback on the steroidogenic pathway from the ovary, and so there is a switch from the production of oestrogen to the production of progesterone (10) (Fig. 3).
The LH surge coincides with ovulation (~day 14) and promotes the final stages of egg maturation to occur. At this moment, the huge amount of cells left at the follicle after ovulation form the so-called corpus luteum (CL), highly active in the production of progesterone, mainly from the granulosa cells. Oestrogen production, in turn, is back to low levels although maintained relatively constant.
The rapid increase in progesterone produced by the CL has another consequence: a negative feedback that causes release of LH to cease due to inhibition of translation of the mRNA into protein, and also suppression of the growth of antral follicles due to the depression of FSH production. By the end of this phase, in the absence of conception, steroid levels are reduced at luteolysis. This results in the relaxation of the negative feedback originated by progesterone, and the restoration of positive feedback that leads to the production of oestrogens, and to FSH and LH levels reaching a plateau. As a consequence, preantral follicle development is resumed along with the next cycle (7).
Fig 3. Profile of the main hormones involved in the menstrual cycle, with details on the different phases. Pulsatile stimulation of GnRH is seen, triggering release of LH and FSH (green profile lines). The overall levels of steroids, E2 and P, are depicted as purple and yellow lines, respectively. This image shows the clear predominance of E2 (oestrogen) in the proliferative/follicular phase vs. the main action of P (progesterone) in the secretory/luteal phase. Modified from (7).
The endocrine basis of implantation
It has been mentioned earlier that oestrogen (E2) is a key player during the proliferative phase of the endometrium. Besides its role in promoting stromal cell proliferation, oestrogen also induces the expression of the progesterone receptor (PR) (11), which will be of utmost importance during the secretory phase.
The invasion of the endometrium by the blastocyst entails a rupture of the luminal epithelium in order to achieve implantation. For this to occur, it is necessary a series of changes in the uterine structure, as well as in the profile of several transcription factors and hormones. This way the mother adapts to the progression of the conceptus and, at the same time, controls its invasion. The development of all these changes is a process known as decidualization, which in humans starts by the mid-luteal phase, ahead of potential implantation (12).
The significantly increased production of progesterone by the CL makes it the main character of the second half of the menstrual cycle. The action of progesterone has different simultaneous effects: it reduces proliferation of epithelial cells (regulated by oestrogen) while inducing their differentiation (13), and it promotes stromal cells proliferation, stimulating the glandular secretory activity. There is also a potential increase in the action of specific immune cell types (like uNKs, see below) to respond to local production of chemokines (14). Additionally, it is known that progesterone regulates the contractile activity of the junctional zone (JZ), so that it allows the conceptus to develop in a safe and controlled fashion underneath the myometrium (15).
Progesterone acts partially by binding to its type A receptor (PR A) (16, 17), which in turn downregulates type B receptor (PR B), both being isoforms of the same transcriptional factor (18). The combined action of the two receptors in both stromal and glandular epithelial cells results in a modulated decidualization-like response of the endometrium in preparation for implantation. However, progesterone is not the only inducer of decidualization; in vitro progesterone only-induced differentiation of stromal cells would take up to 8-10 days under normal circumstances (19, 20). Rather, it is actually the cAMP pathway activation the main process that initiates decidualization (21, 22). In fact, many of the factors produced and secreted after ovulation like prostaglandins or gonadotropins, among others, stimulate cAMP in the stromal cells of the endometrium (22). Interestingly, cAMP levels detected from biopsies during the secretory phase are higher than during the proliferative phase (23, 24). After stimulation of cAMP, during the first stages of pregnancy, progesterone stimulates the expression of a group of interrelated genes in the endometrium, preparing the uterus for the implantation (25).
Hormone profile at the beginning of gestation
It has been exposed that during the luteal phase, the key acting hormone is progesterone, produced by the CL, the remnants of the Graafian follicle after ovulation. So, by the time the oocyte reaches the uterus, the endometrium is already prepared for implantation. However, if fertilization has not occurred, decidualization results useless and the physiological and endocrine profiles must be reversed.
Thus, the transition from luteal to follicular phase starts around 2-3 days before the end of the menstrual cycle, with the so-called intercycle rise of FSH. This characteristic increase in FSH or FSH window is responsible for the development and eventual selection of one of the follicles in the first half of the cycle. However, after the rise of FSH, oestradiol (and inhibin B) concentration increases again, inhibiting FSH secretion, and so FSH levels are maintained (26).
In humans, fertilization usually takes place in the distal third of the Fallopian tube. From there, the fertilized oocyte (now zygote) travels along the oviduct while undergoing a series of rapid divisions (cleavage), until it reaches the blastocyst stage. The blastocyst must now hatch from the hardened zona pellucida to be able to implant in the endometrium, a process that normally takes place between day 8 and 10 after ovulation (27).
Not only maternal hormones regulate the process of decidualization, but the embryo itself also contributes to preparing the right microenvironment for implantation. As early as the 8-cell stage, the embryo produces hCG (human chorionic gonadotropin) (28, 29), its first active regulator of implantation. From here on, hCG secretion by the syncytiotrophoblast increases progressively after implantation, reaching detectable levels in maternal blood by day 10 of gestation (which can be normally used for pregnancy tests) and peaking around the 10th week (30).
hCG has been known for decades and its current use is quite extended in reproductive medicine. In fact, it is employed to stimulate final follicle maturation before oocyte collection for IVF cycles (31), which along the external administration of progesterone for luteal phase support, helps to avoid potential problems of ovarian hyperstimulation syndrome (OHSS) derived from the use of LH. After fertilization, embryonic/placental production of hCG seems to be related with maintaining the role of the CL in secreting progesterone to sustain pregnancy (32). The need to rescue the CL from luteolysis is related to the luteoplacental shift, which occurs around the 6th week of gestation (33). This transition coincides with the decline in both progesterone and hCG serum levels detected following the 4th week and recovery thereafter (34). A crucial balance of hCG and progesterone must be reached between the CL and the syncytiotrophoblast to avoid luteal insufficiency, one of the main reasons for implantation failure during the first trimester (35).
IMMUNOLOGICAL CHANGES DURING THE WOI
The proliferative phase of the human endometrial cycle is characterized by the active growth of different cell types, including stromal, epithelial and vascular cells (36). On the other hand, the secretory phase supposes an increase of the glandular secretory activity and a significant increase in the number of immune cells. The major cell types in this phase are T cells, uterine NK (uNK) cells and macrophages (37).
In the secretory phase, the endometrium is prepared for nidation of a conceptus, so it undergoes a series of changes during which the immune system plays an important role. Mainly, the aim of this role is to create an environment of maternal tolerance towards the conceptus, and at the same time protection against infections within the uterus (37).
Immunological changes from proliferative to secretory phase
T cells, essential components of the immune system and originally formed from stem cells in the bone marrow, are known to help protect the body from infections. They comprise around 45% of leukocytes in the proliferative endometrium and their number stays constant throughout the secretory phase (38). However, their relative number decreases because of a large increase in uNK cells during this phase and early pregnancy (37).
During implantation, uNK cells comprise 70-80% of the total leukocyte population (39). The NK cells are a type of leukocyte or white blood cell that acts in the immune system as a first line of defense against foreign invaders. They are cytotoxic cells (40), whose function in the endometrium is to act as a source of important cytokines, including leukemia inhibitory factor (LIF), tumor necrosis factor (TNF)-α, interferon (IFN)-γ, granulocyte macrophage-colony stimulating factor (GM-CSF), and IL-10 (41). Cytokines are proteins that regulate the function of the cells that originate them on other cell types. They are primarily produced by activated lymphocytes and macrophages, and they are the agents responsible for intercellular communication, inducing the activation of specific membrane receptors, cell proliferation and differentiation, chemotaxis, growth and modulation of immunoglobulin secretion (40).
Gene expression in the WOI
Several genes related to the immune response are specifically regulated during the WOI. Worth to mention, among others: decay accelerating factor, indoleamine 2, 3 dioxygenase (IDO), interleukin (IL) 15, IL-15Rα, interferon regulatory factor (IRF)-1, lymphotactin, natural killer-associated transcript-2 (NKAT2) and granulysin NKG5. Some of these genes' functions are: to promote uNK cell proliferation, chemotaxis, to inhibit NK cell cytolytic activity, to inhibit cell growth and to inhibit the classical complement pathway. The functional diversity these genes represent likely serves the goal of preparing the endometrium for implantation, thereby adapting the maternal system to accommodate the immunologically ‘foreign’ fetus (42).
Lobo et al. demonstrated by semi-quantitative RT-PCR (Fig. 4) and Northern analysis the upregulation of these genes in secretory versus proliferative phase (37). The mRNA of all these genes, while expressed in proliferative endometrium, demonstrated increased expression in the secretory phase. However, and taking into account the fact that uNK cells increase in number during the secretory phase, it is likely that the apparent upregulation of the above-mentioned genes is actually reflective of an increase in cell number rather than a true increase in gene expression (37).
Fig.4. Graph bar showing semi-quantitative analysis of the expression of several genes, comparing proliferative (light bars) and secretory (dark bars) human endometrium. Gene expression analyzed by RT-PCR is normalized to GAPDH. There is a significant upregulation of all genes shown in the secretory phase (*P < 0.05, **P < 0.01). Modified from Lobo et al. (37).
Genes that regulate an increase in uNK cell number
IL-15 is a cytokine localized in the endometrium during the secretory phase of the human cycle. Its main function is to induce proliferation of uNK cells (43, 46). Some studies have demonstrated that IL-15 expression is upregulated in secretory versus proliferative phase (37, 44, 45).
Additionally, Lobo et al. demonstrated the upregulation of IL-15Rα subunit from proliferative to secretory endometrium. This subunit belongs to the receptor of IL-15, and it confers high-affinity binding of cytokine to its receptor (37).
IRF-1 is a transcription factor that regulates expression of IL-15 (45). IRF-1 expression has also been shown to increase during the secretory phase of the endometrium (37, 47).
Genes that inhibit NK cell cytolytic activity
Natural killer cells are classically considered to be cytolytic to non-self cells. Therefore, one would expect a decrease in their numbers from implantation through pregnancy to allow the fetus to be safely carried to term. Instead, there is a dramatic increase in NK cells during implantation. The answer to this paradox lies in the fact that uNK cells have effects that extend beyond their killer function, and that cytokines secreted by uNK cells are essential for implantation. In fact, NK cell-deﬁcient mice exhibit decidual cell degeneration and endothelial cell distortion or displacement from supporting cells (48).
It is believed that, at the maternal-fetal interface, human leukocyte antigen (HLA) class I molecules expressed on trophoblast cells interact with inhibitory receptors on uNK cells to protect the trophoblast from being cytolysed by uNK cells. An example of one of these inhibitory receptors is NKAT2, which shows an upregulated expression in the secretory phase (37).
Genes that inhibit cell growth
The uterine epithelium provides a physical barrier to infection. One mechanism for inhibiting such infections is through the secretion of endometrial peptides with bactericidal properties, which becomes especially evident at the beginning of implantation (49).
NKG5 is an isoform of granulysin that exhibits antimicrobial activity against a huge spectrum of pathogens (50). The expression of this gene is especially high in NK cells surrounding the glandular epithelium in secretory endometrium (37).
IDO is another example of genes that inhibit cell growth in the endometrium. It is an enzyme from the tryptophan catabolic pathway that depletes tryptophan in local tissue environments, thus suppressing proliferation of cells in the vicinity. IDO may not only inhibit T-cell proliferation, but it could also be bactericidal by this mechanism (51). An upregulation of this gene in secretory versus proliferative endometrium has been demonstrated (38).
MORPHOLOGICAL CHANGES DURING IMPLANTATION WINDOW
In the evaluation of women fertility, morphological studies are one of the most common procedures. These studies began with Noyes’ work, which describes the particular features of the endometrial histology, correlating it with the specific days of the menstrual cycle (52). In fact, these studies are still important and also improved and complemented thanks to the recent advances in technology.
Endometrial thickness and pattern
At present, there exists a data conflict between the various studies focused on endometrial thickness measurements by ultrasonography. Several papers agree in that, during an IVF cycle, the endometrium should be ≥ 7mm on the day of human chorionic gonadotropin (hCG) administration and ≥ 8mm thick on the embryo transfer day. Likewise, thickness < 6 mm has been reported to adversely affect implantation rate (53). Nevertheless, controversy arose when some studies documented no association between implantation rates and endometrial thickness. These articles claim that the important factor is the endometrial pattern, which changes throughout the entire menstrual cycle (54).
Regarding endometrial pattern, the structure of a triple line is correlated with a receptive endometrium. Ultrasonography as a tool to study the endometrial pattern has a high sensitivity (79-100%), but also an elevated percentage of false positives (57-91%) (53). However, it is possible to achieve pregnancy with a “non triple-line” pattern, although at a low frequency (55).
In the receptive endometrium, a complex secretory environment is established in order to active the blastocyst and to create both proper uterine receptivity and stromal decidualization. The structures in charge of creating this environment are the uterine glands.
Once ovulation takes place, P4 levels increase and the endometrium switches into a secretory phase. The endometrium is divided into two layers, the upper one formed by loose stromal and glandular tissue in its majority, with dynamic structure and function during the cycle. On the contrary, the lower layer is structurally stable and it is formed by spiral glands and a dense stroma. During the time in which the endometrium is receptive, glycogen vesicles are disposed below the nucleus of the glandular epithelial cells, and their content is transported by microfilaments to the apical region, where glycogen is actively secreted (56).
Uterine blood flow
During endometrial receptivity the arterioles acquire spiral form, and there is also a similar growth of the subepithelial capillary plexus, both at basal and functional endometrium. Regarding spiral arteries, it has been reported that when glomerular capillary length reaches a certain limit, a new capillary loop is generated in order not to increase the resistance to blood flow. Additionally, during the early and mid secretory phase the vessel length, which has achieved its maximum density during the proliferative phase, experiments an increase in the number of vessel junctions (57).
Recent studies have found that the existence of correct blood flow in the border area between the endometrium and myometrium plays an important role in endometrial receptivity. Uterine artery blood flow can be studied through the impedance, a term that includes the pulsatility index (PI) and the resistance index (RI) (58). Several research lines suggest that high blood flow resistance is associated with pregnant failures, whereas lower PI is a good indicator for pregnancy (Fig. 5).
Applebaum et al (59). define the vascularity zones as follows:
It has been shown that pregnancy rates increase if the vessels reach the zone of the endometrium and the subendometrial halo. This fact demonstrates the relation between endometrial perfusion and endometrial growth, since the endometrium becomes thicker because of the presence of vessels. On the contrary, the absence of blood in the endometrial and subendometrial areas correlates with low probability of pregnancy, in which case uterine resistance is higher (60).
Ultrasonography is also used in the study of uterine blood flow due to its non-invasiveness, real-time monitoring and repeatability. hCG administration day is normally the day for ultrasonography to be performed, because it is at this moment when highest sensitivity and specificity are achieved, although it can also be performed the day of embryo transfer (60).
Uterine junctional zone
The JZ is not only a distinct structure from the myometrium and endometrium, but it also presents functional differences (Fig. 6). There is supporting evidence for an important role of the JZ in the implantation process. On one hand, the movements of this layer allow the sperm to move rapidly to the dominant ovarian follicle. On the other hand, these contractions must decrease once fertilization has occurred. Otherwise, implantation might result impaired, reason why it is better to perform the embryo transfer during another cycle or, at least, at day 5-6 (58,61).
Human endometrial transcriptomics
Although morphological evaluation is very useful, more advantages are needed in order to study the window of implantation. Actually, transcriptomics can be used to analyze the expression of certain genes involved in the implantation process (62). Nowadays, two existing products that can be found in the market for studying receptivity using transcriptomics are “ER Map” and “Test ERA”. These products can be used in those cases in which the uterus is apparently normal, the endometrium presents the proper thickness and yet recurrent implantation failure occurs (3 or more in case of young women and 2 or more in elderly women), especially if the embryos transferred were of good quality (63,64).
The high versatility of the endometrial structure responds to the function of housing the conceptus for implantation at the right time of the cycle. It is essential that all the endometrial features adapt to the needs of both the embryo and the mother. In order to cover its nutritional requirements and, at the same time, to regulate this extraordinary changes of the endometrium, blood supply must be increased, and so there is a significant process of angiogenesis that involves restructure of the endometrium itself.
The combination of all changes needed to support early embryo implantation and further development is known as decidualization. This is triggered and controlled mainly by the action of a series of hormones, mainly progesterone from the mother and hCG from the conceptus. Production and dynamics of both hormones are in turn controlled and regulated by a network of molecular interactions, involving hormones, enzymes and other factors acting in delicate balance. This allows for adaptation to the actual and potential situations, with the goal of being always prepared during the WOI, should conception happen.
The menstrual cycle exhibits extreme complexity, given the high amount of factors involved in its regulation. Even though the aspects described in the present text (hormone profiles, gene expression and physiological transformation) have been studied for decades, further research is needed to fully understand the roles of other molecular factors. Unravelling complete interaction networks of these players may uncover potential roles in implantation, thus helping find new solutions to diverse causes for infertility.
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57. Gambino LS, Wreford NG, Bertram JF, Dockery P, Lederman F, Rogers PAW. Angiogenesis occurs by vessel elongation in proliferative phase human endometrium. Hum Reproduction. 2002; 17(5): 1199-206.
58. Abdallah Y, Naji O, Saso S, Pexsters A, Stalder C, Sur S, et al. Ultrasound assessment of the peri-implantation uterus: a review. Ultrasound Obstet Gynecol. 2012;39(6):612–9.
59. Sardana D, Upadhyay AJ, Deepika K, Pranesh GT, Rao KA. Correlation of subendometrial-endometrial blood flow assessment by two-dimensional power Doppler with pregnancy outcome in frozen-thawed embryo transfer cycles. J Hum Reprod Sci. 2014;7(2):130–5.
60. Wang L, Qiao J, Li R, Zhen X, Liu Z. Role of endometrial blood flow assessment with color Doppler energy in predicting pregnancy outcome of IVF-ET cycles. Reprod Biol Endocrinol RBE. 2010;8:122.
61. Lesny P, Killick SR. The junctional zone of the uterus and its contractions. BJOG Int J Obstet Gynaecol. 2004;111(11):1182–9.
62. Díaz-Gimeno P, Ruíz-Alonso M, Blesa D, Simón C. Transcriptomics of the human endometrium. Int J Dev Biol. 2014;58(2–4):127–37.
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Authors: Javier del Río, Noemi Díaz and Belén Gómez
What is cryopreservation?
The first successful in vitro fertilization (IVF) treatment was in 1978. Since that, there have been a remarkable number of advances in assisted reproductive technologies (ART). Initially, all available embryos were transferred in IVF treatments owing to its low success rate. However, improvements on clinical and laboratory aspects led not only to increased pregnancy rates, but also to increased risk of multiple pregnancies. To prevent this, fewer embryos are transferred and leftover embryos are cryopreserved for potential future cycles use (1).
The first pregnancy resulting from transferring a thawed cryopreserved human embryo was reported in 1983 in Australia (2), and the first live birth following embryo cryopreservation was reported in 1984 in The Netherlands (3). Subsequently, the need for an effective cryopreservation program arose from rapid development and improvements of assisted reproductive technology protocols (1).
Cryopreservation is a method that requires cells and embryos to be exposed to non-physiological ultra-low temperatures (from -20°C to -196°C) (Fig.2). It aims to achieve “cryogenic suspension of life” through multiple steps, although this puts the elements at risk of damage or “cryoinjury” during temperature changes and phase transitions. These damages could be chilling injury or ice crystal formation, for instance, as a result of the water exchange between the intra- and extracellular compartments, consequence of dramatic changes in osmotic potential (osmotic shock). Therefore, vitrification requires the use of cryoprotectants to avoid the formation of ice crystals in the cells. Two types of cryoprotectants are necessary: permeating and non-permeating. Mixing both at different relative concentrations reduces intracellular ice formation by removing water from inside the cell. Additionally, it creates an osmotic gradient that helps restrict water movement across the cell membrane, thereby preventing osmotic shock (4).
There are two typical methods used for cryopreservation: slow freezing and rapid freezing to achieve vitrification.
Vitrification is a term used to describe the transformation of a solution into glass by a dramatic increase in viscosity. This method requires to minimize the time for the sample to be exposed to temperature ranges associated with chilling injury and ice crystal formation. As slow freezing, vitrification causes cell dehydration using cryoprotectants. However, unlike that, there is no attempt to maintain equilibrium on both sides of cell membrane (4).
The time frame required to reach ultralow temperatures by vitrification is very brief, almost instantaneous. But, the main concern is the need for using high concentrations of cryoprotectant solutions. These might lead to osmotic shock and it could be toxic to cells, affecting embryo survival. Nevertheless, it is possible to limit toxicity by mixing different cryoprotectants, thereby decreasing their relative concentration and the exposure time of embryos to the solution (5).
How efficient is the vitrification?
This technique seems to be more attractive than slow freezing because it does not require expensive equipment. It uses small amount of liquid nitrogen and it is a simpler technique to perform once the embryologist has gained enough experience in it (6).
A recent research performed by Viladimoiv et al. suggests advantages arising from the freezing and thawing process; the authors hypothesize a theory about “cryo-treatment of the embryo”. According to these authors, as a result of freezing or thawing of the embryos there is a decrease in reactive oxygen species levels, in the rate of mitochondrial DNA mutation and cells detoxification is carried out. Also, the authors describe another mechanism involved in restoring the mitochondrial activity (“jumping effect”) which is part of the physiological process of implantation. However, current available data cannot confirm the hypothesis yet (7).
Advantages and disadvantages of fresh and frozen cycles
Nowadays, fresh embryo transfers (ET) are the most common choice in IVF cycles (8). Nevertheless, in the last years, controlled ovarian stimulation has increased the uncertainty on the possible adverse effects of the ovarian hyperstimulation syndrome (OHSS), and also on possible deleterious effects on the endometrium and implications in obstetric and perinatal results (9).
In spite of this, recent developments in cryopreservation of oocytes and embryos have led to substantial improvement in IVF outcomes. This resulted in a significant increase in the number of cycles with frozen embryo transfer (FET), which subsequently led to the enhancement of live births rate (10).
What are the advantages of a frozen cycle?
Ovarian hyperstimulation syndrome
The first strong argument for FET strategy is the prevention of OHSS, that results from an increase in vascular permeability (11,8). OHSS is a medical condition affecting the ovaries of some women who take fertility medication to stimulate oocyte growth. OHSS arguably remains a major cause of morbidity in IVF treatment (10).
During a fresh cycle, a woman has to undergo hormonal treatment to regulate her menstrual period, to stimulate the development of multiple oocytes (superovulation), and to encourage their maturation (11, 12). However, in a frozen cycle (FC) the patient does not have to go through ovarian stimulation or egg retrieval depending on their circumstances (13). Many people find that FETs are less stressful than fresh cycles because they do not have to worry about oocytes production or whether there will be viable embryos, since those procedures have already been done (9).
Deleterious effects on the embryo
The optimization of vitrification protocols has reduced the deleterious effects that this process may produce in embryos. Also, it have been observed similar survival and embryo development in FCs compared to fresh cycles (10). Moreover, best quality embryos, morphologywise, can be stored and transferred in a future cycle in better conditions. These data have allowed for an increment of success rates and the confidence of sanitary personnel and patients over FCs (5).
The implantation process, one of the crucial steps in the success of ART, requires a reciprocal interaction between the embryo and the endometrium during a small period of time called window of implantation. This interaction involves the embryo, along with its inherent molecular program of cell growth and differentiation, as well as differentiation of endometrial cells into an adequate uterine receptivity (11). Some patients may find easier to turn to FCs, since dealing with the whole process of medication during a normal cycle for ovarian stimulation may result psychologically and emotionally overwhelming. In this regard, FC may also provide a better outcome (3).
The importance of an adequate endometrial environment in ART is highlighted in those patients who resort to oocyte donation, where there must be a synchronization between donor and recipient in fresh cycles. Those cases that require an improvement in endometrial receptivity to stimulate implantation of these donor oocytes seem to obtain better results in frozen cycles or in the next fresh cycle (8).
Multiplet pregnancies are one of the major safety concerns of IVF due to the increased risk of neonatal and maternal complications. To achieve good results, to would be ideal to select the optimal single embryo to be transferred. Elective single embryo transfer (eSET) is the most effective way to reduce those risky pregnancies (14).
How can cryopreservation damage embryos?
Upon analyzing some ART studies and results, embryos are able to adapt and develop in a large range of culture media, showing different gene expression models in different environments. Cryopreservation causes stress in embryos and it is known as “hormesis”(5) (Fig.3).
However, if the conditions are too unfavorable or toxic, mitochondrial activity is suppressed below the threshold necessary for the development of the embryo, so that implantation in the endometrium will be affected (5).
Results of embryo transfer in fresh cycles vs. frozen cycles
The main current objective of IVF professionals is to improve pregnancy rates in both fresh and frozen-thawed cycles. It is clear that embryo and endometrial receptivity are important factors to promote pregnancy rate. Recently, many researches showed FET can enhance the embryo utilization rate and improve the success rate in contrast to other research lines (15).
In Roque et al. systematic meta‐analysis for 633 cycles in women aged 27-33 years old showed that FET resulted in a statistically significant increase in the ongoing pregnancy rate and clinical pregnancy compared with the fresh transfer group (8). Interestingly, the fresh group showed a higher miscarriage rate, but no statistical difference was found when compared with the frozen group. According to these data, it seems that the results of IVF-ICSI cycles can be improved by performing the FET especially in patients with normal or high follicular response. This advantage could be explained thanks to a more physiological preparation of endometrium. Several studies have also shown good results with cryopreservation of all embryos and subsequent FET in those patients most susceptible to develop OHSS (8, 16-19).
In contrast, Shavit et al. found lower rates of clinical pregnancy and live births in the vitrified-warmed blastocyst group. The difference in implantation and pregnancy rates could be attributed to a higher proportion of good-quality embryos in the fresh blastocysts transfer group. They suggest that in fresh cycles highest quality blastocyst is selected for transfer and the rest are usually vitrified. Thus, vitrified-warmed blastocysts may have slightly poorer grade after warming and prior to transfer (20).
In addition, it is necessary to take into account those cycles with frozen oocytes. Braga et al. found that warmed oocytes transferred in endometrial prepared cycles yield better clinical outcomes than fresh ETs. Indeed, they found that fertilization rate, embryo quality, and developmental competence was decreased in embryos derived from vitrified oocytes (12). Conversely, previous studies have suggested that the results of oocyte vitrification followed by ICSI are not inferior with regard to fertilization, embryo developmental competence, pregnancy rates, and live birth (21, 22, 23).
An interesting point found in Braga et al. research is that even with lower embryo developmental quality, warmed oocytes transferred in endometrial prepared cycles resulted in higher pregnancy and implantation rates compared with transfer in fresh cycles. This finding strongly suggests that controlled ovarian stimulation impacts endometrial receptivity, which may be a possible cause of implantation failure after ovarian stimulation (12). Indeed, some studies have suggested that pregnancy rate is inversely related to serum progesterone levels on the day of hCG administration (24-27). It has been demonstrated that elevated progesterone levels on hCG trigger day negatively influence the pregnancy, regardless of the oocyte quality. Raised concentrations of progesterone in the late follicular phase are likely to influence the secretory changes of the endometrium, leading to an asynchrony between embryo and endometrial dialogue, which may result in reduced implantation rate (12).
Another issue to consider is the obstetric and perinatal outcomes of frozen-thawed cycles. Maheshwari et al. quantified in a meta-analysis the obstetric and perinatal risks for singleton pregnancies after FET and compared it with those after fresh embryo transfer (28). They indicated better perinatal outcomes in singleton pregnancies after the transfer of frozen‐thawed embryos when compared to fresh IVF embryos. This could be explained by antepartum hemorrhage, very preterm birth (delivery at <32 weeks), preterm delivery (delivery at <37 weeks), small for gestational age, low birth weight (birth weight <2500 g), and perinatal mortality significantly lower in women who received frozen embryos than those transferred with fresh embryos (29, 28).
It is important to note that most studies comparing perinatal outcome of singleton births conceived after fresh and cryopreserved ETs included both single and multiple ETs. Therefore, part of the adverse perinatal outcome may be attributed to the vanishing twin phenomenon, which occurs in up to 10% of multiple ETs resulting in a singleton live birth (20).
What can we conclude?
Elective embryo cryopreservation followed by single FET has attracted increasing attention and has been regarded as a potential innovation of IVF treatment. Choosing the well-selected embryo could further increase the chance of live birth of a eSET, which is of high clinical significance. However, there are great gaps in the literature about the risk/benefit ratio of this strategy, which includes multiple steps of treatment (30).
The good outcomes in FC might be associated with having a well‐balanced embryo‐endometrium interaction in FC, and also with lacking controlled ovarian hyperstimulation, which may adversely affect endometrial receptivity during fresh IVF cycles. In addition, when hormone replacement cycles were applied in FETs, estrogen and progesterone were given in physiological doses to mimic natural cycles, while supraphysiological doses of gonadotropins were given in fresh cycles (31).
On the other hand, other authors find fresh cycles as the best choice, especially in patients who resort to oocyte donation. In fact, it seems that there is a higher proportion of good-quality embryos in fresh blastocysts compared to vitrified-warmed blastocysts, which may have slightly poorer grade after warming and prior to transfer. (8, 20).
In conclusion, each case must be individualized in relation to clinical characteristics of the patients and to oocyte, seminal and embryo quality. By doing so, results will be optimized in each cycle and the chances of achieving a live birth will be highly improved.
1. Wong KM, Mastenbroek S, Repping S. Cryopreservation of human embryos and its contribution to in vitro fertilization success rates. Fertil Steril. 2014;102(1):19-26.
2. Trounson A, Mohr L. Human pregnancy following cryopreservation, thawing and transfer of an eight-cell embryo. Nature. 1983;305(5936):707-9.
3. Zeilmaker GH, Alberda AT, van Gent I, Rijkmans CM, Drogendijk AC. Two pregnancies following transfer of intact frozen-thawed embryos. Fertil Steril. 1984; 42(2):293-6.
4. Sparks AE. Human embryo cryopreservation-methods, timing, and other considerations for optimizing an embryo cryopreservation program. Semin Reprod Med. 2015;33(2):128-44.
5. Konc J, Kanyó K, Kriston R, Somoskői B, Cseh S. Cryopreservation of embryos and oocytes in human assisted reproduction. Biomed Res Int. 2014;2014:307268.
6. Loutradi KE, Kolibianakis EM, Venetis CA, Papanikolaou EG, Pados G, Bontis I, et al. Cryopreservation of human embryos by vitrification or slow freezing: a systematic review and meta-analysis. Fertil Steril. 2008;90(1):186-93.
7. Vladimirov IK, Tacheva D, Diez A. Theory about the Embryo Cryo-Treatment. Reprod Med Biol. 2017;16:118–125.
8. Roque M, Lattes K, Serra S, Solá I, Geber S, Carreras R, Checa MA. Fresh embryo transfer versus frozen embryo transfer in in vitro fertilization cycles: a systematic review and meta-analysis. Fertil Steril. 2013;99(1):156-62.
9. Gurbuz AS, Gode F, Ozcimen N, Isik AZ.Gonadotrophin-releasing hormone agonist trigger and freeze-all strategy does not prevent severe ovarian hyperstimulation syndrome: a report of three cases. Reprod Biomed Online 2014;29:541-544.
10. Lattes K, Prat M, Robles A, Carreras R, Brassesco M, Checa MA. Ciclos de criopreservación y vitrificación de ovocitos y embriones: indicaciones y transferencia diferida. Guía 21 de Práctica Clínica de la SEF y de la SEGO.
11. Lessey BA. Endometrial receptivity and the window of implantation. Baillieres Best Pract Res Clin Obstet Gynaecol. 2000;14(5):775-88.
12. Braga D, Setti A, Figueira R, Azevedo M, Iaconelli A, Lo Turco E et al. Freeze-all, oocyte vitrification, or fresh embryo transfer? Lessons from an egg-sharing donation program. Fertil Steril. 2016;106(3):615-622.
13. Shapiro BS, Daneshmand ST, Garner FC, Aguirre M, Hudson C. Clinical rationale for cryopreservation of entire embryo cohorts in lieu of fresh transfer. Fertil Steril. 2014;102:3-9.
14. Tobias T, Sharara FI, Franasiak JM, Heiser PW, Pinckney-Clark E. Promoting the use of elective single embryo transfer in clinical practice. Fertil Res Pract. 2016;2(1):1-9.
15. Shen C, Shu D, Zhao X, Gao Y. Comparison of clinical outcomes between fresh embryo transfers and frozen-thawed embryo transfers. Iran J Reprod Med. 2014. Jun;12(6):409–14.
16. Griesinger G, von Otte S, Schroer A, Ludwig AK, Diedrich K, Al-Hasani S, et al. Elective cryopreservation of all pronuclear oocytes after GnRH agonist triggering of final oocyte maturation in patients at risk of developing OHSS: a prospective, observational proof-of-concept study. Hum Reprod. 2007;22(5):1348-1352.
17. D'Angelo A. Ovarian hyperstimulation syndrome prevention strategies: cryopreservation of all embryos. Semin Reprod Med. 2010;28(6):513-518.
18. Griesinger G, Schultz L, Bauer T, Broessner A, Frambach T, Kissler S. Ovarian hyperstimulation síndrome prevention by gonadotropin-releasing hormone agonist triggering of final oocyte maturation in a gonadotropin-releasing hormone antagonist protocol in combination with ‘‘freeze-all’’ strategy: a prospective multicentric study. Fertil Steril. 2011;95(6):2029-2033.
19. Devroey P, Polyzos NP, Blockeel C. An OHSS-Free Clinic by segmentation of IVF treatment. Hum Reprod. 2011;26(10):2593-2597.
20. Shavit T, Oron G, Weon-Young S, Holzer H, Tulandi T. Vitrified-warmed single-embryo transfers may be associated with increased maternal complications compared with fresh single-embryo transfers. Reprod Biomed Online. 2017;35(1):94-102.
21. Trokoudes KM, Pavlides C, Zhang X. Comparison outcome of fresh and vitri- fied donor oocytes in an egg-sharing donation program. Fertil Steril. 2011; 95:1996-2000.
22. Herrero L, Pareja S, Aragones M, Cobo A, Bronet F, Garcia-Velasco JA. Oocyte versus embryo vitrification for delayed embryo transfer: an observational study. Reprod Biomed Online. 2014;29:567-72.
23. Rienzi L, Romano S, Albricci L, Maggiulli R, Capalbo A, Baroni E, et al. Embryo development of fresh ‘versus’ vitrified metaphase II oocytes after ICSI: a prospective randomized sibling-oocyte study. Hum Reprod. 2010;25:66-73.
24. Xu, B., Li, Z., Zhang, H., Jin, L., Li, Y., Ai, J. et al, Serum progesterone level effects on the outcome of in vitro fertilization in patients with different ovarian response: an analysis of more than 10,000 cycles. Fertil Steril. 2012;97 (1321-7.e1-4).
25. Wu, Z., Li, R., Ma, Y., Deng, B., Zhang, X., Meng, Y. et al, Effect of HCG-day serum progesterone and oestradiol concentrations on pregnancy outcomes in GnRH agonist cycles. Reprod Biomed Online. 2012;24:511–520.
26. Bosch, E., Labarta, E., Crespo, J., Simon, C., Remohi, J., Jenkins, J. et al, Circulating progesterone levels and ongoing pregnancy rates in controlled ovarian stimulation cycles for in vitro fertilization: analysis of over 4000 cycles. Hum Reprod. 2010;25:2092–2100.
27. Hamdine, O., Macklon, N.S., Eijkemans, M.J., Laven, J.S., Cohlen, B.J., Verhoeff, A. et al, Elevated early follicular progesterone levels and in vitro fertilization outcomes: a prospective intervention study and meta-analysis. Fertil Steril. 2014;102:448–454.e1.
28. Maheshwari A, Pandey S, Shetty A, Hamilton M, Bhattacharya S. Obstetric and perinatal outcomes in singleton pregnancies resulting from the transfer of frozen thawed versus fresh embryos generated through in vitro fertilization treatment: a systematic review and meta-analysis. Fertil Steril. 2012;98:368–77.e1.
29. Qiao J, Zhang L, Yan L, Zhi X, Yan J. Female Fertility: Is it Safe to "Freeze?". Chin Med J (Engl). 2015;128(3):390.
30. Wei D, Sun Y, Liu J, Liang X, Zhu Y et al. Live birth after fresh versus frozen single blastocyst transfer (Frefro-blastocyst): study protocol for a randomized controlled trial. Trials 2017; 18(253): 1-7.
31. Zhang L, Yan LY, Zhi X, Yan J, Qiao J. Female Fertility: Is it Safe to “Freeze?” Chin Med J. 2015;128 (3):390-7.
Authors: Shuyana Deba and Roberto de la Fuente
In assisted laboratories, semen samples are usually obtained after masturbation and the ejaculate is collected into a container. However, semen recovered from non-spermicidal condoms can be of higher quality because of the time producing the sample influences in the amount of seminal emission before ejaculation (2). Moreover, according to the World Health Organization criteria, laboratory manual for the examination and processing of human semen, the results of laboratory measurements of semen quality will depend on: completed collection of the sample, products of the accessory glands (that will dilute the concentration of epididymal spermatozoa), time past since last sexual activity, abstinence period and the size of the testis. All these factors, among others, will have an influence on the total sperm count per ejaculate (2).
Contrary to what is found in natural fertilization after ejaculation, there are no actual barriers to enhance semen capacitation in vitro. As a consequence, in assisted reproduction laboratories sperm must be separated from the seminal plasma within one hour after ejaculation. The purpose of semen processing is to increase the concentration of high quality spermatozoa, and the method choice will depend on the nature of the sperm, that will in turn determine which reproductive procedure will be performed (2).
Reports over the last two decades have made emphasis on the need for the optimal method for sperm selection to compile as many as possible of the following conditions: non-toxicity, ease to perform, inexpensiveness, suitability for high-throughput sample processing, accuracy in selecting the best subpopulation of sperm and ability to discard other cell types and seminal plasma as well as other substances that may harm the sperm (cryoprotectants, bioactive complements, motility enhancers) (3). Unfortunately, no known method to date successfully meet all the above, and so the selection of the most appropriate method normally depends on the specific procedure to follow.
“Conventional” techniques like the nowadays common washing swim-up (WSU) and differential density gradient centrifugation (DDGC) procedures are normally used for a variety of assisted reproductive procedures, although it is clear that not all of them will have comparable efficiency in selecting high quality sperm. Some techniques are better indicated for certain conditions. For instance, high concentrations of highly motile sperm from normospermic men will show a good performance after WSU, and this will be suitable for regular IVF. On the contrary, severe oligospermic or asthenospermic individuals will require semen selection by using other techniques. The introduction of ICSI in the regular clinical practice enhanced the fertilization rates in the 90’s (4), but did not significantly increase live birth rates because of failures in early development, mostly due to the high incidence of structural chromosomal aberrations (5). This is a direct consequence of the inability of ICSI to specifically detect good quality spermatozoa.
In the following paragraphs we discuss the appropriateness of different techniques for distinct assisted reproduction treatments, namely: in vitro fertilization (IVF), intrauterine insemination (IUI) and intracytoplasmic sperm injection (ICSI).
Sperm preparation techniques for IVF
In the last few years several studies have discussed about the best technique for sperm capacitation in vitro. Recently, Volpes and collaborators compared the effects of four methods used for sperm preparation, namely, direct swim-up, pellet swim-up, density gradient and density gradient followed by swim-up (6). The authors evaluated levels of DNA fragmentation using the sperm chromatin dispersion test for samples meeting the following criteria: minimum volume of 2 mL, minimum sperm concentration of 10 million/mL and a minimum motility of 35%. The study showed lower DNA fragmentation after pellet swim-up and after density gradient followed by swim-up. However, the study highlighted the limitation that clinical outcomes from IVF/ICSI procedures were not correlated with DNA fragmentation in the sperm (6).
A different study compared sperm processing by WSU and DDGC in normospermic individuals, analyzing sperm motility, concentration, and morphology recovery rates (7). Conclusions were that, on the one hand, DGC was appropriate for males with low sperm concentration, since it yielded higher sperm concentration than the WSU technique. On the other hand, the latter facilitated a more efficient morphology-based sperm selection. Nevertheless, the main limitation of the study was the small number of patients and the fact that all of them were normospermic (7).
In 2016, Yamanaka and coauthors attempted to determine the efficiency of combining both DGC and WSU techniques in reducing the number of sperm with abnormal nuclear morphology under the microscope (8). Results showed that combination of the two approaches was better than one alone; both DNA fragmentation levels and sperm motility were improved compared to results after just DGC. Data also demonstrated that the combination of both techniques was efficient in enriching the sample with sperm with normal head and flagellum morphology (8).
Intrauterine Insemination (IUI)
A study by Karamahmutoglu on the most effective sperm preparation technique for IUI compared WSU vs. DGC (9). Even though data showed higher IUI success rates after having performed the DGC approach, no significant difference was found in the "mild male factor" subfertile group (with sperm count in the range of 5-15 million/mL). Moreover, other factors were affecting fecundity success rate, such as female age, number of cycles and type of infertility (9). These observations on the efficacy of DGC and WSU methods for IUI have recently been confirmed in a similar study by Butt and Chohan (10).
On the particularities of ICSI
Sperm selection for ICSI is commonly carried out by the embryologist’s own judgement based on morphological criteria. This results in inconsistent decision-making and often selection of poor quality sperm, since semen samples are considered morphologically normal with just 4% of normal-looking spermatozoa (2). Therefore functional, physiological and molecular traits of spermatozoa cannot be evaluated by ICSI, and so unnoticed DNA abnormalities (even specific causes of sterility) might be passed on to the offspring by the selected spermatozoon. Thus it is easy to understand why the embryology and reproduction community has been trying to develop new strategies to successfully select the best spermatozoa regarding phenotype, functional characteristics and genetic and molecular integrity.
Advanced methods for sperm preparation
There are currently three main groups of methods to facilitate selection of high quality sperm, based on morphology, electrical charge and sperm surface maturity and organization (11, 12):
A simpler and cheaper technique uses the electrokinetic potential of the sperm (19); the electric potential difference of the sperm membrane decreases with capacitation, which is used to pipette washed sperm into positively charged tubes, so that negatively charged (mature) sperm can be retrieved afterwards. However, the total sperm recovered by using this technique is low, which represents an important limitation (11).
PS externalization to the outer sperm membrane is a typical apoptotic feature, which allows the cell to bind to magnetic beads conjugated with Annexin-V. This made it possible to develop a magnetic-activated cell sorting system (MACS) (25); the sperm suspension is incubated with the microbeads so that those apoptotic spermatozoa will bind to the beads, which will be subsequently retained within the MACS column in a magnet. Non-apoptotic sperm will then flow freely to be collected. Even though this technique enriches the sample in healthy sperm, it does not discard leukocytes or germ cells, and thus it must be combined with DDGC (26). A variation of the approach known as Annexin V glass wool (annexin V-GW) eliminates potential side effects of free magnetic beads (27), but still needs to be combined with repeated DGC cycles, which is actually not appropriate for oligospermic individuals (11).
The approach followed for semen preparation and selection for every patient/couple needs to be chosen upon a series of factors that mainly refer to the cause of infertility. The wide range of situations found regarding this topic makes it necessary to adopt a specific strategy every time. This will ultimately define the treatment to be applied and the techniques to be carried out subsequently.
Recent advances have developed new and better ways to detect good quality spermatozoa, minimizing DNA fragmentation and optimizing the rates of good morphology or motility, for instance. However, it is important to remember that there exists no particular strategy that always relates to the optimal clinical outcome. On the contrary, each situation must be considered in the light of the patient’s needs and characteristics, and so the technique for sperm preparation must be chosen accordingly.
Authors: Inés Abad, Roberto de la Fuente and Sara Sanz
Fertility treatments are more and more common in our days, reason why it is important to perform these procedures accurately resembling in vivo conditions. Additionally, male factor may oftentimes be underrated, and yet it is 50% of the treatment. The following text aims to establish an updated comparison between in vivo and in vitro semen preparation methods. In the first part a general description of the processes of maturation and capacitation of sperm are presented.
Where does sperm maturation take place?
Once spermatogenesis is completed in the seminiferous epithelium, immature spermatozoa migrate towards the epididymis, the organ in which sperm maturation and storage take place. The epididymis is usually divided into three different parts: caput (head), corpus (body) and cauda (tail) (2).
Typical changes in sperm during maturation
1. Acquisition of progressive motility. Even though immature sperm have functional movement machinery, motility of these cells begins in the caput segment. Whereas beating intensity is similar throughout the whole epididymis, flagellar amplitude is modified within this path. This is due to changes on the sperm surface, such as acquisition of new proteins and molecular changes involving inactivation of Ser/Thr phosphatases (3, 4).
2. Migration of the cytoplasmic droplet (CD). This droplet migrates from the neck towards the annulus of the mammalian spermatozoa (in humans, the CD is more proximal, located at the neck as opposed to the distal position of the annulus). The role of this droplet is to regulate ion homeostasis. It contains K+, Cl- and water channels, which have been suggested to work in regulation of sperm volume during the different regions of the epididymis. It also accumulates Ca2+, which has a biphasic role controlling phosphorylation pathways in sperm cells. In immature spermatozoa, it has been hypothesized that high Ca2+ levels found in the CD maintain low levels of tyrosine phosphorylation (5).
3. Changes in sperm protein and lipid profile.
- Protein and lipid content
Even though changes in these profiles are not well understood, there are three complementary mechanisms that participate in completing maturation:
Perhaps the most important among these changes is the significant reduction in cholesterol content of the sperm membrane. This reduction involves a decrease in the cholesterol/phospholipid ratio that facilitates protein trafficking from and onto the membrane and enhances its fluidity, which will eventually play a role in triggering capacitation and fertilisation (8).
- Post-translational protein changes
Additionally, certain post-translational modifications of proteins have also been hypothesised to occur during sperm maturation. This is the case for oxidation of thiol groups, which promotes the formation of disulphide bonds (S-S) and stabilises components of both the head and flagellum (9).
Fig.2. Schematic representation of the main items during sperm maturation. (A) Principal functional and morphological aspects in immature caput spermatozoa. (B) Molecular characteristics of immature spermatozoa. (C) Main morphofunctional traits of cauda mature spermatozoa and (D) their molecular features (4) (*) Ability to fertilize the egg will ultimately depend on completing capacitation.
What is the composition of the ejaculate?
In mammals, semen is composed of two different phases:
Following ejaculation, semen is deposited to the anterior wall of the vagina, adjacent to the ectocervical tissues. From here on, for sperm to progress towards the egg through the oviduct or Fallopian tubes, semen must undergo liquefaction. This process usually takes about 20-30 minutes (11). Semen goes through the cervix and reaches the distal portion of the Fallopian tube, where sperm is stored and maintained by interacting with the endosalpingeal epithelium (12, 13).
In 1951, Austin and Chang individually observed that a certain period of time in the female tract was required for sperm before fertilization could take place (14, 15). Later, observations in multiple mammalian species confirmed these first notions, and certain studies showed a delay of at least 2 hours before sperm entry into the egg. This supports the hypothesis of sperm maturation before becoming fertile after ejaculation (16). Nowadays, such process is known as capacitation, as opposed to maturation in the male tract explained above.
Capacitation and fertility
Some of the factors involved in sperm capacitation are steroid hormones such as oestrogens and progesterone, both produced by the follicle. These steroids play different roles: they act as chemoattractants, facilitate triggering of hyperactivation, regulate trafficking of cGMP or modulate the potential for completing acrosome reaction (17-19).
Semen liquefaction following ejaculation is mainly modulated by prostate derived peptidase KLK3. In females, KLKs 5–8, 10–11, and 13–15 are expressed at very high levels in the cervix and vagina compared to other adult tissues (20, 21). Moreover, KLK1 and KLK3 transcripts are expressed at the highest level in human endometrium when circulating estradiol (E2) is elevated. These findings suggest that KLKs are expressed in the human reproductive tracts and that some of the KLKs in the uteri are regulated by E2. Abnormal E2 signalling in the female reproductive tract leads to semen liquefaction defects, associated with defective SEMG cleavage and sperm transport, which may result in some cases of infertility.
It is known that mice lacking ESR1 (one of the oestrogen receptors) in the epithelial cells are infertile (22), partly due to a reduction in the number of sperm able to reach the oviduct (23). However, the effect of ESR1 loss in the epithelial cells on sperm transport in the uterus has not yet been investigated. Similarly, other potential research lines could investigate liquefaction defects caused by diminished KLK activity in females or regulation of KLKs by molecular signalling in the female tract.
Once semen trespasses the cervix, sperm are known to achieve capacitation in an asynchronous fashion during the interaction with the epithelium, which results in a continuous flow of fertile spermatozoa towards the Graafian follicle (24). The ability to bind to the epithelium, in turn, may be indirectly related to the sperm DNA integrity, and so DNA fragmentation levels would be indicative of the fertility potential of the sperm (25).
Elements involved in sperm capacitation
Even though capacitation had traditionally been regarded as a two-step process, through which changes in the cell membrane would lead to the acrosome reaction (AR) (26), capacitation is currently considered as a continuous process that culminates in the AR. It would be difficult to describe all capacitation-related events separately because all of them are connected to each other in time. However, the most important changes in the sperm during the process are (27-37):
Sperm capacitation is a complex process with multitude of interconnected and highly regulated molecular pathways. One of the first events is the alteration of the permeabilization of the sperm plasma, so that the influx and intracellular concentration of certain ions are increased. The main molecules involved are probably Ca2+ and HCO3-; the net intake by the sperm cell triggers alkalinisation of the pH and the concomitant activation of the soluble adenylyl cyclase (sAC) (38, 39). As an immediate consequence, cAMP levels increase followed by activation of the protein kinase A (PKA) (40, 41).
The rise of cAMP causes redistribution of certain phospholipids and proteins of the membrane, and so exposing cholesterol, which accumulates in lipid rafts (42). The organization of these rafts promotes the removal of cholesterol and its translocation to extracellular acceptors like albumin (43). Also, increased cAMP activates PKA, which in turn activates SRC kinase (44).
Eventually, SRC kinase activity triggers tyrosine phosphorylation, which results and a wide range of proteins been modified and relocated in capacitating sperm. This has been described in several species, including humans (45). The end result of capacitation is the acrosome reaction (AR), the process by which the content of the acrosome is released to the extracellular environment. In natural conditions, this environment is actually the cummulus cells, whose connections will be broken by the chemical reactions of the acrosomal content, mainly proteases like acrosin and hyaluronidase, also exposed to the right membrane domains during lipid redistribution (46). It is not surprising that mutations affecting any of these processes will result in multiple causes for infertility (47).
Findings like the one regarding post-ejaculated liquefaction, mutations on acrosome protease-encoding genes or other molecular mechanisms of sperm capacitation are crucial to progress in the field of reproductive medicine, and can lead to: (i) potential diagnostic tools for unexplained infertility cases, (ii) the development of a novel contraception technology to entrap sperm (48), (iii) or even revolutionary new methods for human sperm capacitation in the laboratory (49), which could significantly improve live birth rates for fertility treatments.
In the following post... different methods for sperm selection in the laboratory will be explained, paying attention to advantages and disadvantages under different circumstances. The importance of different sperm features like DNA fragmentation or morphology will be discussed in relation to the best sperm selection method to achieve optimal clinical outcomes.
Authors: Paula Brígido, Roberto de la Fuente and Javier Del Río
Assisted reproduction technology (ART) can help fertile couples to achieve successful pregnancies. Sometimes, reproductive desires of these couples are affected by the presence of a genetic disease in either partner. In such cases, couples are at a reproductive risk and find themselves in the need of assistance that only ART can provide.
Preimplantation genetic diagnosis (PGD) provides an alternative to prenatal diagnosis to detect the specific genetic condition or disease they suffer from, and allows them to avoid passing it on their offspring (2). It requires the analyses of the embryos generated by ART in the in vitro fertilization (IVF) laboratory, by means of accurate and sensitive methodologies such as embryo biopsy, genetics, single cell genomics and, of course, background on prenatal diagnosis and counselling from experts.
Clinical application of PGD dates back to the late 60’s, when blastocysts of research animals could be sexed (3) (note that this was already possible ten years before Louis Brown, the first IVF baby, was born in the UK in 1978). At the beginning of the 90’s, early human embryos were sexed before implantation and the first genetic analyses were performed to avoid children inheriting Mendelian diseases. By the end of the century, other nowadays considered basic genetic methodologies were routinely used for preimplantation diagnosis and PGD was applied as a normal procedure to guarantee healthy babies (4).
In the present post we aim to give an account of the importance of PGD and the current view of the main clinical approaches for its application.
WHEN IS PGD INDICATED?
Indications for PGD are multiple and emerge from different motivations. Firstly, the patient may have suffered from a number of terminations due to the embryo having inherited the genetic condition. It could also be motivated by the parents already having a child with a severe genetic disease. In this case they might be willing to avoid passing it on the next one or even looking for a suitable treatment, if possible. However, one of the parents (or both) may be worried about their family history, being aware of the presence of a specific genetic condition, regardless of the type of inheritance.
If the parents are carriers of any genetic disease, either an autosomal-dominant disorder like Huntington disease or an autosomal-recessive one like cystic fibrosis, they are at reproductive risk because the resulting embryo may be affected (the probability depending on the specific disorder itself and the way it is inherited) (see  for details on inherited conditions). But there are even cases in which motivation is not based on biological but in ethical or religious reasons. Certain families might have serious concerns about going on for abortion of an affected embryo. In such cases, application of PGD may circumvent this kind of ethical conflicts.
Broadly speaking, steps for PGD are as follows (2):
PGD vs. PGS
Preimplantation genetic screening (PGS) is the general term for a compound of approaches that aim to evaluate the genetic content of the cell, in contrast to genetic tests whose goals are to determine whether an embryo is affected by a specific genetic condition (PGD). Originally termed PGD-AS (preimplantation genetic diagnosis for aneuploidy screening), PGS was developed to confirm the ploidy status of the embryo, searching for possible aneuploidies. Available data suggest that most of miscarriages occurred during the first trimester are a consequence of some sort of aneuploidies (5), and that mainly selected chromosomes were involved in these structural abnormalities (6). Thus, the main approach developed for PGS was the fluorescence in situ hybridization (FISH) for such chromosomes.
Types of approaches for PGD in the laboratory
Current technical methodologies for preimplantational genetic analyses mainly lie in one of the following:
WHEN TO PERFORM BIOPSY
Typical biopsies for PGD (and PGS) are as described as follows:
DAY 3. CLEAVAGE STAGE BIOPSY
There is a controversy regarding utility of this type of biopsy. In the cleavage stage biopsy, embryos are biopsied at day 3 when individual cells can be differentiated. This technique entails aspiration of one to two blastomeres to obtain the embryonic genetic material for PGD analysis (13). Following genetic diagnosis, embryo transfer may be performed on blastocyst stage. Embryos are usually selected for biopsy based on morphological criteria. Unfortunately, these do not predict the development potential of the embryo, and so it could fail to progress until blastocyst stage. This would compromise the advantages of using the day-3 approach (14). On the other hand, performing biopsy on the cleavage stage allows embryos to be cultured in vitro until they reach the blastocyst. This means they can be fresh transferred (15), whereas embryos biopsied on day 5 must be vitrified and transferred in a subsequent cycle.
How many cells should be removed?
The number of cells to be removed in the biopsy is still a controversial issue. Aspirating one cell reduces the cellular mass extracted but it can imply the presence of mosaicism. Conversely, aspirating two cells can reduce the risk of mosaicism, but removing such cellular mass could have consequences on the implantation rate (14).
Reported data have shown a dramatic reduction of 39% in the implantation rate in cleavage stage biopsy (16). The authors related it with proportion of the embryo total cellular removed. Whereas around five cells pulled out of the embryo in the trophectoderm biopsy represent 2-3% of the total cell content (expanded blastocyst has 200-220 cells approximately), extraction of a single cell from an eight cell embryo supposes 13% of the total content (16).
What do experts say?
Cleavage stage biopsy produces different opinions among embryologists because of the presence of mosaicism and the possibility of self-correction of aneuploidies from cleavage to blastocyst stage (17). On the contrary, studies using array-comparative genomic hybridization (array-CGH) technology to analyse genetic abnormalities in day-3 blastomeres and confirming it in trophectoderm biopsy showed concordance between day 3 diagnosis and day 5 reanalysis; Treff and coauthors showed more reliable results for SNP-microarray (96% vs. 83%) and also a lower mosaicism degree (31%) for SNP-microarray samples in a study comparing array technology versus FISH technique (18). These data would support the suggestion of some authors, who proposed that the incidence of mosaicism may have been overestimated in previous studies due to technical inconsistency of the FISH technique (17, 18, 19). At present, this matter remains controversial.
Regarding pregnancy rates, in both types of biopsies higher pregnancy rates are obtained comparing with the control group, in which no biopsy was performed (14, 19).
To sum up:
DAY 5. TROPHECTODERM BIOPSY
The blastocyst stage is currently supposed to be an optimal time to perform biopsies for PGD/PGS. The combination of improved blastocyst culture, trophectoderm (TE) biopsy, refined cryopreservation techniques, and molecular assays, such as array comparative genomic hybridization that allows for 24-chromosome screening, have led to a renaissance of PGS. TE biopsy will not detect every circumstance in which the embryo is at risk of aneuploidy, but it will detect mosaicism more reliably than cleavage-stage biopsy (which cannot be relied on at all for this purpose) (20, 21).
Moreover, when diagnosing monogenic disorders in single blastomere cells using PCR-based protocols, there is a high risk of PCR failure due to either no amplification (allele dropout) or preferential amplification of one of the alleles, potentially resulting in a reduced number of unaffected embryos available for transfer. Increasing the amount of starting DNA template should in principle increase the sensitivity and reliability of genetic diagnosis. Therefore, the biopsy of multiple trophectoderm cells from the blastocyst rather than a single cell from cleavage stage embryos should potentially lead to improved PGD outcome for patients (14).
How many cells should be removed?
Research to determine the appropriate number of biopsied TE cells in blastocyst biopsies are limited. The exact number of biopsied TE cells is hard to count visually because cells are small and usually remain as a clump. In most studies using comparative genome hybridization or single-nucleotide polymorphism array technology for genetic testing, biopsied TE cells were used for genome amplification and their number was impossible to know. Moreover, some studies showed that removing four to five cells leads to better results. Therefore, the biopsied cell number should be higher in the blastocysts with better TE quality than those with worse characteristics (22, 23).
Can biopsies affect blastocyst development and its implantation?
Whereas it remains possible that biopsy of cleavage-stage embryos can critically arrest further development through reduction of cell mass, the low miscarriage rates and high term birth rates in the present series, as well as data presently under analysis, suggest that this is not the case for TE biopsy. It can be speculated that the damage to blastocyst development potential caused by TE biopsy would be less for blastocysts with a greater number of TE cells (21, 22).
Some experts assured that TE biopsy at the blastocyst stage had no meaningful impact on the developmental competence of the embryo as measured by implantation and delivery rates. This contrasts with the information above-mentioned on the significant reduction in the probability for an embryo to implant and progress up to delivery (16). When combined with TE biopsy and blastocyst vitrification, SNP microarray has resulted in high implantation and low miscarriage rates for some IVF patients (15, 16, 24).
Are there any limitations?
Owing to the limitations of genetic analysis, most of the biopsied blastocysts need to be cryopreserved by vitrification, and blastocysts with normal results would be transferred in the next frozen cycle. In addition, biopsy of numerous cells from blastocysts with grade B or C may cause damage to the embryo, leading to either its arrest or implantation failure. However, 1-5 cells may be the appropriate biopsied TE cell number to maintain the implantation potential (15, 22).
Also, the personnel experience of different embryologists is an influencing factor in this technique. The number of biopsied cells in the blastocyst biopsy is hard to quantify and largely dependent on the experience of embryologist (22).
To sum up:
WHAT CAN WE CONCLUDE?
The availability of new embryology and molecular techniques allow preimplantation genetic diagnosis laboratories to offer patients at genetic risk the transfer of developmentally competent embryos, unaffected by genetic disease. Cleavage stage biopsy allows for fresh embryo transfer after genetic diagnosis. However, there are reports of high levels of mosaicism when the biopsy is performed on day 3. Trophectoderm biopsy, in turn, provides sufficient material for an effective and more reliable diagnosis in embryos compared to those on cleavage stage. Moreover, it seems that it does not compromise embryo implantation and pregnancy rates in PGD cycles. The drawback for this option is the usual need for cryopreservation and transfer in a different cycle.
The offer of PGD in fertility centres has increased over the last decade, primarily due to the progress on the application of diagnostic methods. The choice for either development stage relates to successful outcomes in the clinic, which mainly depend on technical challenges and timing of the developing embryo. For the embryologists, both day-3 and day-5 approaches are supported by evidence, but it will be essential to consider every single aspect of them to evaluate the best option for the laboratory.
Authors: Shuyana Deba, Isabel Sánchez and Sara Sanz
Every procedure carried out in an IVF laboratory, from ovarian puncture and semen capacitation to the embryo transfer, must be performed under specific safety conditions. These standards must be followed to avoid a decrease in gamete/embryo viability (2).
From the in vitro culture, gametes and embryos are exposed to diverse artificial situations that do not take place in nature. In vivo, both fertilization and embryo development in the female reproductive tract occur in the complete absence of light. In this environment, other conditions include oxygen (O2) concentration of 2-8%, pH 7.2-7.4, 37⁰C and gradients of diverse nutrients to which the embryo is exposed (3). Changes in temperature, CO2 and O2 pressure, light exposure or volatile organic compounds may adversely affect embryo quality.
Also, manipulation of embryos by embryologists is as important as air quality and culture conditions. Each human being is covered by about 1012 bacteria (4), which could contaminate embryo cultures if embryologists do not use the necessary clothing, such as laboratory cap, footwear and mask.
Therefore, a daily quality control at different levels should be carried out to obtain good results in IVF cycles. We are going to analyze the effect of some of the elements that can affect germ cells and embryos.
In vivo, mammalian germ cells and embryos are not exposed to light, which might explain why they do not exhibit any protection mechanism against this factor (5,6). In addition, sperm do not have the capacity to repair DNA, unlike oocytes and embryos, which do present some mechanisms for DNA repair (7).
Light variables to be considered are intensity, duration and wavelength. It seems clear that photooxidation increases along with light intensity and duration. However, what type of wavelength would be the most harmful for embryos and germ cells? Energy increases when wavelength is shorter (8). Accordingly, artificial cool white fluorescent light has been demonstrated as the most stressful in mouse embryos. Incandescent light, in turn, seems to be less harmful, and the best outcomes are achieved when warm white fluorescent light is used (7).
How can light affect the quality of these cells?
Indirect effect: Culture and oil photooxidation can affect embryo development (8). In this case, modified components will damage the lipid membranes. Also, if HEPES- or riboflavin-containing media is exposed to light, it results in the formation of hydrogen peroxide, a highly cytotoxic substance (9). Additionally, light can heat up both the plasticware and the oil, resulting in more toxic and damaging components (8,9).
Direct effect: Light can potentially compromise the quality of gametes and embryos, by activating stress-related genes or by ionisation, which may also damage the DNA. This phenomenon would cause DNA fragmentation and mutation, as well as an increase in the apoptotic index and change in the number of mitochondria levels (10).
How can we avoid this effect? (7)
1) Reducing the exposure time.
2) Using warm white fluorescent light in the lab and green filters on microscopes.
3) Adding antioxidants in the media in order to mitigate damages from ROS.
4) Avoiding riboflavin, which is responsible of the phototoxicity in the media.
VOLATILE PARTICLES EFFECT
Since the 1990s, IVF laboratory indoor air quality has taken a high relevance. Thence, focused on creating an optimal environment, laboratories have become clean rooms where filtration of particles is performed by using high-efficiency particle arresting (HEPA) filters, and successful chemical air filtration is achieved by removing volatile organic compounds (VOCs) with solid-phase filtration (e.g., potassium permanganate-impregnated, carbon filters) (11).
Focusing on VOCs (hydrocarbon-based compounds that are emitted by industries, cleaning products, computers, and microscopes among others), several studies have demonstrated their harmful effect on embryos, initially reported by Boone laboratory on mouse embryo development (11). Moreover, VOCs have been shown to increase DNA fragmentation in human sperm, and they can also have detrimental effects on pregnancy rates (12).
Recently, a retrospective study by Munch et al. concluded that, without solid carbon filtration, fertilization, cleavage, and blastocyst conversion rates declined in fresh IVF cycles. Even more, results were found to be even worse in ICSI cycles, probably due to the lack of protective barrier provided by the cumulus cells (13). However, the authors did not observe the same results when embryos had been cryopreserved in an environment with carbon solid filtration but thawed in a laboratory deprived of such systems. The absence of significant changes in cleavage and blastocyst conversion rates, as well as in the proportion of good quality blastocyst developed after thawing suggests that embryos are affected in the peri-fertilization period (13).
Also, products like cosmetics emit VOCs, especially perfumes, colognes, and deodorants. They are highly toxic to embryo development in vitro, primarily due to evaporation of their solvent bases (14,15). After analyzing the results of studies determining the toxicity of VOCs, ideal levels should be below 0.2 ppm but preferably zero (12). Personnel must understand the principles of air quality control, including the function of airflows and airlocks, hygiene, dress code and the use of cleaning agents (16).
pH level depends on bicarbonate concentration of culture media and the CO2 concentration of the incubator. However, other factors like altitude and composition of culture media could affect the pH level, too (17,18). Embryos are able to develop over a range of media pH, considering that they possess an intracellular mechanism to regulate its internal level (17,18). However, it is important to control pH variations because they affect development (17). To control pH level outside the incubator some culture media contain buffers like HEPES or MOPS, but long exposure of embryos to these buffers is not recommended (17). Thawed denuded oocytes and embryos are specially sensitive to pH variations because they do not have an inner system to regulate pH (17). So, an increase in the pH of the medium can affect the physiology and development of oocytes and embryos. Thus, acidification of the medium can even affect the fetal weight and size (18).
As previously mentioned, CO2 is necessary to control pH level of culture media (17,18). The importance of CO2 was demonstrated in 1985, in a study carried out on hamsters (19). The authors cultured hamster embryos in different CO2 concentrations (5% and 10%). They found a higher rate of blastocysts in those cultured at 10% compared to 5%, which demonstrated differences in embryonic development. This results showed that CO2 level is an important factor for embryo culture (19). The capacity of CO2 to get through cell membranes allows for regulation of the inner pH levels in blastomeres. In other studies, it has been shown that the required CO2 concentration to achieve the optimal pH varies in different species. For instance, the required CO2 level in rats is 7.5%, whereas for humans it is 6.5% (19).
Some studies have compared different values of O2 concentration in the incubator and they show that a low level (5-6%) improves results when compared to an ambient level (21%). It has been shown that low O2 levels increase implantation, pregnancy and live birth rates (17,20). It seems that a low O2 level reduces ROS in the culture and the presence of volatile particles in the air, although the exact mechanism of action is still unknown (17).
Standard temperature generally used in IVF laboratories is 37⁰C (17,18). However, optimal temperature is unknown because in the female reproductive tract it could be slightly lower, about 36⁰C. On average, temperature of the Fallopian tube is about 1.5⁰C less, whereas the follicular liquid temperature can reach 2-3⁰C lower than core body temperature (17). It is important to control and prevent temperature variations because it can affect meiotic spindle stability and alter embryonic metabolism. It has been shown that an increase of 2⁰C during 20 minutes potentially alters the integrity of the meiotic spindle, which cannot be completely repaired when temperature is set back to 37⁰C. As a consequence of this increase in temperature, embryos express some stress-response genes that compromise development (18). Interestingly, a small decrease in temperature does not have any effect on oocytes, whereas a large difference can be severely harmful for the meiotic spindle (18).
CULTURE MEDIA EFFECT
Nowadays, there exist two kinds of culture media: one-step media and sequential media (with different compositions for days 0-3 and 3-6) (17,21). All culture media are similar in composition; they contain energy substrates like glucose, pyruvate or lactate, and both organic and inorganic salts, which must be balanced accordingly. Culture media also contain amino acids in different proportions. The exact composition of amino acids in culture media is unknown. One of the most important problems related to the presence of amino acids is the ammonium generated as a product of metabolism. Ammonium has negative effects on embryo and fetal development. To avoid this problem, some culture media contain glutamine, which reduces ammonium production (17,21,22). Also, culture media can be supplemented with macromolecules and other components like HSA, α and β globulins, growth factors, vitamins, lipids, nucleotides, cytokines and hormones (17,22).
What can we conclude?
There are many parameters that should be kept in mind in order to maintain the optimal conditions for both gamete and embryo development in an IVF laboratory. In vitro, cells and embryos are exposed to different stress situations that must be minimized. Therefore, a routine control at different levels needs to be performed, so that the environment in the laboratory is adapted to resemble the reproductive tract and the intrauterine conditions.
Authors: Paula Brígido, Shuyana Deba, Javier Del Río and Isabel Sánchez
Implantation is one of the key steps in human reproduction, and hatching of the blastocyst is a critical point in the sequence of physiological events that lead to implantation (2). It has been estimated that only 15-20% of embryo transfers culminate in implantation, and as a consequence clinical pregnancy and live birth rates are quite low (3, 4). The potential of an embryo to implant is related to its own nature, the quality of gametes and the endometrial receptivity. But failure in blastocyst hatching is also an important factor that prevents implantation (2, 3, 4). With the aim of solving this problem and in order to improve both implantation and pregnancy rates after Assisted Reproductive Technology (ART) procedures, scientists developed assisted hatching (AH) (2, 3, 4). AH consists of an artificial alteration of the zona pellucida (ZP) either by slimming or breakage. This technique was first described on 1988 by Cohen et al., who reported the first pregnancy after AH (4), and successive works have since shown its efficiency (2, 3).
Because AH does not seem to present clear advantages to all patients, it should not be applied, in principle, to all of them as a rule (3). Some studies have shown that AH has effectively improved ART outcomes in patients with bad prognosis, like those with a history of 2+ implantation failures, bad embryo quality, aged patients (38+), patient with frozen embryos or those who have oocytes with a thick ZP (2, 3, 4, 6).
ART have some actual consequences on gametes and embryos, one of them being hardening of the ZP by the use of certain culture media or by cryopreservation. These and other negative effects may hamper blastocyst hatching, which might be solved through AH. Overall, this approach might be useful considering the early implantation window in women treated with exogenous gonadotrophin stimulation, compared to a natural cycle. Additionally, benefits from AH include the possibility of ZP breakage improving the embryo-endometrium communication (5).
In order to help the embryo hatch through the ZP before implantation, a variety of techniques over the years have been developed (7). As a commonality for all of them, it is important to minimize the time of the embryo being outside the incubator and to optimize the methodologies to reduce both pH and temperature variations (3).
MECHANICAL ASSISTED HATCHING (PARTIAL ZONA DISSECTION)
This method does not require any new equipment, and it can be easily performed by an embryologist handling a micromanipulator (8). During this procedure, the embryo is held by the suction exerted by the holding pipette, while the microneedle is passed through the zona pellucida at the largest visible portion of the perivitelline space and introduced in a tangential trajectory to the embryo. Then, in order to tear the ZP apart, the embryo is released from the holding pipette and rubbed against it (7).
However, one of the main disadvantages of this technique is the difficulty of creating a hole of significant size without abruptly damaging the embryo. Moreover, it is not a fast procedure, which means embryos need to remain outside the incubator for a long time, and so this may affect development of the transferred embryos (7, 9).
ACID TYRODE'S ASSISTED HATCHING
For this procedure, first the embryo is stabilized with the holding pipette. Second, a pipette containing acid Tyrode is approached to the embryo, oriented at the 3 o`clock position, next to an area of empty perivitelline space. Finally, the acidic solution is gently expelled over a small area (~30 µm Ø) and immediately washed away (7). This technique allows the formation of a hole of significant size; nevertheless, the exposure to acid is problematic, since this compound may be potentially embryotoxic. Furthermore, this technique requires practice and expertise from the operator (7, 10).
Laser-assisted hatching (LAH) is designed for easy positioning of the embryos, focus and shooting. It can be executed with a single click of the mouse controller. The diameter of the drilled holes vary between 5 and 10 µm according to how many shots are performed and the irradiation time (7). From the technical point of view, this method is easier, better controlled and more precise. Without physical contact with other embryos, the procedure can be completed faster than other methods. So, time of the embryos out of the incubator is shorter than for other techniques (9).
Even though the equipment may result expensive, the laser-assisted technique seems to represent the lowest potential risk for the embryo, and it is relatively simple to perform with consistency between operators (3).
EFFICACY OF AH
Although it has been performed for more than 20 years, up to date results are still inconclusive. Ma and co-authors performed one of the first trials to determine the overall effect of this technique. They concluded that implantation rates could be enhanced by performing ICSI along with AH, but differences were not statistically significant (11).
Two recent meta-analyses evaluating potential benefits of this technique have reported significant heterogeneity among results (12, 13), suggesting that effects of AH may differ depending on specific patient features (14).
Most researchers support the hypothesis that this technique improves clinical pregnancy rates in patients with previous failed IVF cycles or poor prognosis. However, there is insufficient evidence to affirm that AH improves live-birth rates in these populations, and so it remains uncertain whether AH is beneficial to other patients (6, 14, 16, 17).
IS THERE AN INCREASE IN MULTIPLE GESTATION RATE?
Even though certain reports associate artificial manipulation of the zona pellucida with multiple pregnancies, there is actually insufficient evidence to support an increased risk of monozygotic twinning after AH. In fact, the overall rate of monozygotic twin pregnancy in IVF with AH is less than 1% (6, 16).
MISCARRIAGE AND CONGENITAL MALFORMATIONS RATES
It has been reported that this technique may enhance implantation of abnormal embryos. Thus, the lower live birth rate observed, which is related to the high number of pregnancies ending in early miscarriage, may be due to chromosomal abnormalities of the embryos (17). However, Ma et al. performed cytogenetic tests on miscarriaged embryos and umbilical cord blood from newborn infants, where they found a similar incidence of major congenital malformations in ICSI-born patients compared to the general population (11), thus ruling out any direct relationship of AH and miscarriage.
EFFECTS OF ASSISTED HATCHING ON FROZEN CYCLES
Data show that cryopreservation may induce zona hardening as well as advanced female age and in vitro culture conditions (18). The answer to this problem could be AH, but results obtained on implantation and pregnancy rates after AH for frozen embryo transfer cycles are controversial, as well as for fresh IVF cycles (19). These discrepancies may be attributed to the type of AH, the extent of ZP microdissection, the number of patients and criteria for their selection, or even the quality and stage of embryos selected for AH performance.
In an earlier study, Primi et al. (2004) were unable to show any specific advantage of LAH in cryopreserved embryos. In this study, no embryo selection was reported, so this could explain why the implantation rates observed were lower than those achieved when embryos were selected (20).
Ng et al. (2005) also found a negative effect of LAH on frozen-thawed embryos in their randomised study (implantation rates: 9.0% vs. 12.5%; pregnancy rates: 6.8% vs. 15%). Although a subgroup analysis showed a higher implantation rate, differences were not statistically significant when LAH was performed on embryos with zona thickness of 1.6 mm (21).
In contrast, other studies using similar techniques of LAH were able to show improvement in implantation and pregnancy rates in the LAH group. Such was the case in a study by Balaban and co-authors (2006), in which their data revealed that implantation rates (20.1% vs. 9.9%) and pregnancy rates (40.9% vs. 27.3%) were significantly higher in the group in which embryos had been subjected to LAH before transfer, as compared to those from the control group (22). Valojerdi et al. (2008) in turn showed that LAH increased significantly both implantation and pregnancy rates in embryo cryopreservation cycles (23). These conclusions resemble those by Kanyo and co-authors (2016), who compared clinical pregnancy rates after using LAH technique on day-3 frozen-thawed embryos, and found a higher pregnancy rate after applying the LAH procedure (24).
Reports on patient populations are found that show benefits from AH, which include patients whose embryos present a thick zona, with elevated FSH, over 38 years old and cryopreserved cycles. It seems that the laser-based method is the most used among professionals and the most effective, although differences between are found between studies. However, no significant data show any increase in clinical pregnancy rates that could be translated into an encouragement of this technique to be routinely performed.
It should also be noted that some studies do not include specific important confounding factors, such as patient socioeconomic status, parity, duration of infertility, number of repeated failed cycles, embryo quality and/or smoking and alcohol intake, which might cause residual variance in the obtained results. Additional limitations of the study include the lack of information on the type of assisted hatching (mechanical, chemical, or laser), which may have changed over time or between clinics.
To learn more about this topic, visit our ‘VIDEOS’ section, where you will find videos about mechanical, chemical and laser hatching.
Authors: Shuyana Deba, Javier Del Río, Isabel Sánchez and Sara Sanz
Fertilization is a sequence of coordinated events that results in the metabolic activation of the ootid (nearly mature oocyte) and triggers cleavage of the zygote (2).
Nowadays, in assisted reproduction laboratories cleavage can be evaluated in vitro and in real time. Once in vitro fertilization (IVF) has been accomplished, early development of the embryo can be recorded by using time-lapse systems (TLP) (3). This approach makes it possible to evaluate morphology, including dynamic parameters, based on the uninterrupted culture of the embryo, which also allows for a better embryo selection, thus rising pregnancy rates (4). Even so, there are still clinics all over the world that select embryos for transfer using light microscopy, which means the use of a conventional incubation method (5).
CRITERIA FOLLOWED FOR EMBRYO CLASSIFICATION
It is known that an international consensus is needed in the way embryos are assessed and described. The following standardized criteria is from Alpha Scientists in Reproductive Medicine and ESHRE Special Interest Group of Embryology, 2011 and includes ‘minimum standards’ for oocyte and embryo morphology scoring (6): the current expected observation for embryo development is 4 cells on day 2 and 8 cells on day 3 after fertilization (day 0). Moreover, embryos with <10% fragmentation, stage-specific cell size and not multinucleated are considered of good quality (6).
According to this consensus, scoring for day 4 (morula stage) regards as good embryos those that enter into a fourth round of cleavage, which implies evidences of compaction that virtually involve the whole volume of the embryo (6). Finally, on day 5 blastocysts are to be observed expanded with: a prominent inner cell mass (ICM) consisting of many cells, compacted and tightly adhered together; and a trophectoderm (TE), forming a cohesive epithelium (6).
Nevertheless, these parameters do not restrict laboratories to annotate further observations in order to select the best embryo for transfer (6).
BEST DAY TO PERFORM EMBRYO TRANSFER
One of the most important aspects that influence the success of ART is embryo transfer from the culture medium to the uterus (7). This has been a controversial subject that still generates quite some doubts. Morphological evaluation of embryos is sometimes a subjective process, and it provides limited information on the possible genetic abnormalities that embryos may have (8). Currently, there exists a great controversy on the optimal moment to carry out embryo transfer.
IN WHICH CASE DOES THIS TRANSFER USUALLY TAKE PLACE?
Day 2 transfer is usually indicated in cases of poorly responding patients. Indeed, it is also indicated when the sperm, oocyte and/or embryos are also of low quality and/or number (9, 10, 11).
WHAT DO EXPERTS SAY?
Several retrospective studies have compared embryo transfer on day 2, day 3 and day 5 after oocyte recovery, all of which presented conflicting results. A study performed by Mahdavi et al. among poor responder patients revealed no clinical differences between day-2 and -3 embryo transfer (10). However, this study found that pregnancy rates per oocyte retrieval and embryo transfer were significantly higher in the day-2 embryo transfer group compared to day 3 group. It is worth mentioning that other investigators did not find significant differences in pregnancy outcomes when they compared embryo transfer on day 2 and day 3 (11, 12).
Additional results from other studies have revealed higher clinical and ongoing pregnancy rates after embryo transfer on day 2 than on day 3 in poor responders. This suggests that the occurrence of miscarriage can be reduced by restricting embryo culture to only 2 days, which could also provide an alternative for managing poorly responding patients (11). That is the reason why embryo transfer on day 2 is still performed in many IVF centres; there is an actual risk of compromising the viability of embryos by prolonged in vitro culture in sub-optimal conditions, with an increased risk of obtaining no blastocysts to replace on day 5 (9, 13, 14).
Even though there seems to be a large number of benefits for these patients, certain disadvantages that may potentially occur must also be taken into account, as it can be seen below (Table 1).
PATIENTS IN WHICH TRANSFER D+3 SHOULD BE PERFORMED
There exists no criterion to select patients who should be transferred on D+3. Traditionally, embryo transfer has been performed on cleavage stage, so the chosen day was D+3 of embryo development (7). Generally speaking, embryo transfer was carried out on this day in all patients, until a culture medium was developed that allowed to keep embryos in the laboratory for 5-6 days, and with the exception of the cases previously mentioned (11).
SCIENTIFIC LITERATURE TO SUPPORT D+3 AS THE BEST DAY FOR EMBRYO TRANSFER
Many studies show contradictory results on whether it is better to transfer on D+2 or D+3. However, there are no significant differences as for implantation, clinical pregnancy or live birth rates when comparing transfer on these days. A study by Modares et al. (15) with patients under 40 years old showed a slight improvement in these results when transferring on D+3, although differences were not significant. The authors also showed embryo quality to be worse when the transfer was performed on D+3 than on D+2. Thus, implantation rate has been observed to be higher in D+3, because extending embryo culture for one day allows to discard those embryos that stop their development from D+2 to D+3 (16, 17).
Furthermore, it is necessary to consider that there are other external factors that affect embryo development and, consequently, the selection of the best day to transfer. Quinn et al. (18) determined that one of these factors is culture media. Thus, in sub-optimal lab conditions, it would be interesting to transfer on D+2 rather than D+3, in order to spend the shorter time possible in the media.
Regarding D+5 transfer, some studies have shown higher implantation rates in embryos transferred on the blastocyst stage compared to those transferred on D+3 (cleavage stage). However, it is necessary to consider that only 25% of embryos reach the blastocyst stage (15); this implies that the number of embryos transferred and vitrified in a cycle is lower than for D+2 and D+3. As a consequence, when considering cumulative pregnancy rates no significant differences are found between transferring on cleavage stage and blastocyst (7).
Again, benefits for the patients must be considered along with potential disadvantages (Table 2).
It has been observed that transfer on blastocyst stage helps to improve pregnancy rates and reduce the risk of a multiple pregnancy. Why? One reason might be that there is no method to determine whether embryos that initially seem to be of good quality are likely to develop up to blastocyst (19).
WHO ARE THE IDEAL PATIENTS?
1. Those with a large number of embryos (20).
2. Those whose day-3 embryos are of good quality (20).
3. Those in which day-1 embryos exhibit pro-nuclei and present a grading profile (20).
4. Young women with good ovarian response (21).
5. Those whose embryos display an early cleavage (22).
POTENTIAL BENEFITS OF BLASTOCYST-STAGE TRANSFER vs. CLEAVAGE-STAGE TRANSFER
First of all, the new culture media allow us to perform longer incubations in the laboratory, after which the best embryos can be selected with higher accuracy and with lower risk of aneuploidies (23). Moreover, there will exist a better synchronization between the embryo and the mother. Additionally, uterine contractility decreases during the luteal phase (24, 25). The size of these blastocysts is bigger, so some studies have found fewer cases of ectopic pregnancies in comparison to transfers on day 3 (26).
A parallel comparison of benefits vs. disadvantages for this procedure can be seen in below (Table 3).
IS IT BETTER TO TRANSFER ON DAY 5 OR ON DAY 6?
The optimal time for embryo transfer depends on a variety of factors, such as the embryo growth speed. However some studies have revealed both implantation and pregnancy rates to be more successful when embryos are transferred on day 5 compared to day 6. This is due to the fact that viability of embryos expanded on day 5 is higher than for those expanded on day 6 (30).
In conclusion, it seems difficult to define the most appropriate day for embryo transfer to be applied for each patient. Therefore, every single case must be individually analyzed. In addition, several factors should be taken into account when deciding on the day for embryo transfer, such as maternal age, sperm and oocyte quality or physiological response of the woman and/or the available embryos. By doing so, a good decision can always be made in order to improve implantation and pregnancy rates.
Authors: Shuyana Deba, Javier Del Río and Sara Sanz
Special collaboration: Álvaro Martínez Moro
Infertility affects millions of couples all around the world. In spite of the solutions to their problems reproductive technology can achieve, the efficacy is eventually limited by the number and the quality of the oocytes available from the woman. In actuality, such efficiency is determined by the ovarian reserve, the oocyte quality and the maternal age, among the most important factors (2).
Diminished ovarian reserve (DOR)
Since ovarian reserve defines the quantity and quality of the primordial follicle pool, diminished ovarian reserve (DOR) indicates a reduction in quantity in women of reproductive age. Consequently, it represents important cause of infertility in many couples. Moreover, DOR may be associated with low pregnancy rates and high pregnancy loss regardless of age, but further research is needed in order to fully understand its implications (3).
Advanced maternal age
It is well known that women’s fertility declines sharply after age 35 due to several factors, which include specific issues of reproductive organs (uterus and oviducts), general health and decreasing number and quality of oocytes over time. The oocyte pool starts to decline during foetal life and continues within the reproductive life of women. Oocyte quality also decreases as a consequence of the increased rate of aneuploidies observed with age: 74% at the age of 41–42, and up to 93% after the age of 42 (5). Advanced age is too associated with a reduction in the quality of the oocyte cytoplasm (ooplasm), which directly affects oocyte maturation (3).
What are the main reasons for this reduction in ooplasm quality? Mitochondria are one of the most important organelles, which are affected in different ways (6,7):
- Morphological and functional abnormalities
- Mitochondrial swelling
- Alterations in mitochondria's cristae
- Alterations of the membrane potential
- Alterations of the metabolic pathways in cummulus cells, which may result in impaired mitochondria biogenesis during oogenesis.
These effects are due to the higher ratio of mutation consequence of the proximity of these organelles to the respiratory chain, the inefficient repair mechanism and the exposure of histories. How these changes affect oocyte quality (8)? First of all, negative effects on chromosome segregation have been observed as a result of a decreasing ATP concentration (9,10). Additionally, defects have been found in different signalling pathways such as Ca2+ signalling, which affects fertilization and the subsequent embryo development (11).
Nevertheless, different mitochondrial haplogroups should be taken into consideration. These have different bioenergetic functions, including production of reactive oxygen species (ROS) and mitochondrial coupling efficiency, aspects that might affect the oocyte longevity (13). Consequently, new techniques are being developed in order to increase the reproductive options in women with oocyte problems. Recently, one of these techniques that have been highly treated in the media is the development of additional viable oocytes from polar body genomes (2).
HOW DOES TRANSFER OF POLAR BODY GENOME WORK?
Originally, the transfer of polar body has been applied to cases of infertility with a genetic cause, such as the presence of mitochondrial diseases. These cases can be treated with the use of donor oocytes in clinical practice. Additionally, another application is the formation of human metaphase II (MII) oocytes, which increases the number of available oocytes for an assisted reproduction cycle (2).
Two specific combined steps are needed. First, the donor oocyte spindle is removed, which requires the utilization of polarized light. Once located, it will be biopsied, obtaining an enucleated oocyte (14,15,16). Secondly, the patient polar body is biopsied, provided elimination of the spindle apparatus has been confirmed.
Once both processes have been performed, the last step is the introduction of the polar body genome inside the enucleated oocyte (17).
FUNCTIONAL HUMAN OOCYTES GENERATED BY TRANSFER OF POLAR BODY GENOMES
Hong Ma and his group have tried to test the efficiency and possible limitations of this technique (ref). The main objective to be achieved was the formation of spindles resembling those typical of MII oocytes, including the appropriate chromosome dosage.
HOW EFFICIENT IS THIS TECHNIQUE?
Although DAPI staining demonstrated that all polar body nuclear transfer (PBNT)-oocytes contained spindle-chromosome complexes, only two of five experimental oocytes formed metaphase spindles similar to intact MII oocytes. This low number may be due to residual meiotic activity in enucleated human MII oocytes, which is sometimes not enough to induce formation of normal MII-like spindles.
For a different cohort of oocytes, the rate of successful fertilization was 76%, still slightly lower than control oocytes. Furthermore, 42% of embryos reached blastocyst stage, indicating that most of the PBNT-oocytes were capable of completing the second meiotic division. Short tandem repeat (STR) analysis revealed that two sampled PBNT-blastocysts contained normal diploid chromosomes, determining that these embryos were completely viable.
WHAT CAN BE CONCLUDED?
• Polar body genome transfer seems to be a significant technique for the improvement of assisted reproductive technology (ART) outcomes and pregnancy rates, particularly for women with decreased ovarian reserve and low response to stimulation.
• The cytoplasm from young donor oocytes may reduce incidences of low cytoplasmic oocyte quality.
• It could provide an additional technique to support mitochondrial replacement therapy.
Nevertheless, this technique is not suitable for women who cannot produce mature oocytes, typical profile of ART patients. Additionally, incidences of aneuploidy resulting from errors in mitosis or in the second meiotic division may still occur because of women advanced age. Larger datasets from this technique are needed to confirm its efficacy and safety. Also, improving preimplantation genetic screening (PGS) is critical before eventual clinical application.
Authors: Shuyana Deba, Javier Del Río and Sara Sanz
Some weeks ago, Dr. Hayashi's group published a study in Nature in which they described how to generate functional mouse eggs from cultured stem cells. To better understand this experiment, it is necessary to explain the natural ovarian cycle. Although the experiment was carried out in mice, we believe these experiments could be of great importance in humans in the near future. Therefore, the following description applies to human ovarian cycle.
GONADAL DEVELOPMENT IN THE EMBRYO
As the yolk sac begins to incorporate into the embryo, germ cells migrate along the dorsal mesentery of the hindgut to the gonadal ridges, which they reach by the end of the fourth or early fifth week of development (2). Around week 7 of gestation, gonadal ridges undergo proliferation to form primitive sex cords, to which primordial germ cells (PGCs) are associated (Fig. 2). In the absence of both expression of the SRY gene product and male sex hormones, differentiation into female organs is determined (3,4). Whereas the origin of the follicular cells of the ovary remains unclear, likely candidates are cells from the coelomic epithelium and the mesonephros. The follicular cells eventually associate with the PGCs to form primordial ovarian follicles (3,4).
ORIGIN OF GERM CELLS
Human oocytes are derived from round PGCs that can be identified in the wall of the yolk sac as early as 24 days after fertilization. Once the gonad develops into an ovary, PGCs become oogonia, and mitotic divisions go on up to the seventh foetal month, and cease at some point shortly before birth (5).
By the 8-9th week after fertilization, some oogonia enter into prophase I of meiosis, thus becoming primary oocytes. Meiosis starts in the deepest part of ovary (medulla) and progresses towards the cortex. Some time after it has been incorporated into a primordial follicle, the oocyte arrests after diplotene, at a particular stage called dictyate or dictyotene. This means the oocyte enters into a long quiescence period that begins before birth and ends up either resuming after the LH surge (leading to ovulation) or in follicular atresia (3).
Even though the experiment by Hayashi's group has just been successfully performed in mice, the purpose of the investigation is eventually applying it to humans.
FOLLICULOGENESIS AFTER BIRTH
Once placental oestrogen disappears, gonadotrophin levels begin to increase after birth. This increment will last 12-24 months postpartum in women. Thus, the ovaries remain inactive until the beginning of pubertal stage, moment when they will start showing response to GnRH pulsatility (6). By this time, ovaries contains germ cells at the primordial follicle phase, which are composed of small immature oocytes arrested at dictyate, and surrounded by a single layer of flat squamous granulosa cells (7).
WHAT HAPPENS AFTER PUBERTY?
Once sexual maturity is reached, multiple primordial follicles are activated from their quiescent state, resuming folliculogenesis by cohorts every menstrual cycle. During follicle maturation, follicles are subjected to activin, AMH, FSH, inhibin, LH, TGFβ and a multitude of other molecules that control the process. At the beginning of this process, as the oocyte resumes meiosis follicular cells proliferate and surround the oocyte, which develops into a primary follicle (7,8,9).
Briefly, transition into the secondary or antral follicle stage involves the alignment of the stroma around the basal lamina and the development of an independent blood supply. Follicular cells of the underlying stroma, the granulosa cells, subsequently differentiate into an inner and an outer theca layers, the latter supporting this blood supply. The production of follicular fluid by these cells promotes the formation of the antrum in pre-antral follicles (10).
Progression through antral follicle development is promoted by FSH and coincides with (7):
- Completion of oocyte growth.
- Acquisition of competence to complete meiosis (dependent on fertilization).
- Continued granulosa cell proliferation.
- Differentiation of cummulus and mural granulosa cells upon formation of the follicular antrum.
Response to FSH causes growth of the follicle by proliferation of follicular cells and coalescence of the astral cavities, thus generating a follicle whose volume keeps increasing. The final stage of folliculogenesis takes place when the Graafian follicle reaches its maximum size (around 20 mm diameter). The LH surge triggers the ovulation of a metaphase II-arrested oocyte (along with the cummulus), capable of being fertilized and able to support embryonic development (7). Although there are notable differences between species, in humans, usually only one follicle is capable of completing growth and maturation up to ovulation (preovulatory), due to its more efficient response to FSH in a "competitive manner" against the rest of developing follicles. The one follicle will progress until subsequent fertilization (dominant follicle), whereas the remaining follicles in the cohort will undergo atresia (8).
ABOUT PLURIPOTENT STEM CELLS
For centuries, scientists have investigated how oocytes and sperm develop in mammals. Most of this work has been performed in non-human species, mainly the mouse. Human pluripotent stem cells (PSCs)-derived gamete research presents notable scientific value and potential, both for understanding basic mechanisms of gamete biology and for overcoming clinical problems (11).
PSCs are currently used to answer important scientific questions such as the role of specific genes in early germ cell development, involved pathways, interaction between germ cells and supporting somatic cells, or partly or entirely in vitro PSCs-derived eggs and sperm (11).
Some of the potential benefits of these new techniques could be (11):
- Additional options for assisted human reproduction.
- New ways to prevent and treat infertility, genetic diseases and some types of cancer.
- Optimization of the number of oocytes obtained from women.
"IN VITRO RECONSTITUTION OF THE ENTIRE CYCLE OF THE MOUSE FEMALE GERMLINE" (1)
The authors have shown a method to reconstruct the entire process of mouse oogenesis in vitro, using embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) in order to obtain fully functional and mature oocytes. Why does this study mean so much?
First of all, this is a big achievement in the field of regenerative medicine, through which it is possible to study the function of certain genes involved in oogenesis. Furthermore, it sets the ground for future similar studies and culture systems in other specie, and as such, it may result in a great advance in reproductive medicine.
However, which are the limitations for this approach?
The culture system requires somatic cells, which are obtained from embryos. So far, this is an ethical and legal issue in humans. A feasible solution would be to obtain the gonadal somatic cell-like cells from PSCs. The authors found that the differentially expressed genes (DEGs) were down-regulated in in vitro MII oocytes, whereas in vivo MII oocytes showed an increase in the number of transcripts. This maybe a consequence to the fact that, during in vitro growth (IVG) and in vitro maturation (IVM), oocyte development was jeopardized within a subset of (or perhaps all) the oocytes. Why? Misregulation of such genes, related to mitochondrial functions, might attenuate the potential of the in vitro MII oocytes. This problem could be solved by an analysis of the metabolic pathway, which could in turn provide information to refine both IVG and IVM.
RESULTS, "PROBLEMS" AND "ACHIEVEMENTS" OF THE EXPERIMENTS (1)
As it can be seen concluded from above, it seems clear that further tests are still needed, which would help measure some aspects of the viability and functionality of this in vitro generated gametes.
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