Endometrium in in-vitro fertilization cycles: morphological and functional differentiation in the implantation phase

Organoids of the female reproductive tract

Healthy functioning of the female reproductive tract (FRT) depends on balanced and dynamic regulation by hormones during the menstrual cycle, pregnancy and childbirth. The mucosal epithelial lining of different regions of the FRT-ovaries, fallopian tubes, uterus, cervix and vagina-facilitates the selective transport of gametes and successful transfer of the zygote to the uterus where it implants and pregnancy takes place. It also prevents pathogen entry. Recent developments in three-dimensional (3D) organoid systems from the FRT now provide crucial experimental models that recapitulate the cellular heterogeneity and physiological, anatomical and functional properties of the organ in vitro. In this review, we summarise the state of the art on organoids generated from different regions of the FRT. We discuss the potential applications of these powerful in vitro models to study normal physiology, fertility, infections, diseases, drug discovery and personalised medicine.

Reduced live birth rates in frozen versus fresh single cleavage stage embryo transfer cycles: A cross -sectional study

Background

Studies have suggested that embryo-endometrial developmental asynchrony caused by slow-growing embryos can be corrected by freezing the embryo and transferring it back in a subsequent cycle. Therefore, we hypothesized that live birth rates (LBR) would be higher in frozen embryo transfer (FET) compared with fresh embryo transfers.

Objective

To compare LBR between fresh and FET cycles.

Materials and Methods

A cross-sectional analysis of 10,744 single autologous embryo transfer cycles that used a single cleavage stage embryo was performed. Multivariate analysis was performed to compare LBR between FET and fresh cycles, after correcting for various confounding factors. Sub-analysis was also performed in cycles using slow embryos.

Results

Both LBR (19.13% vs 14.13%) and clinical pregnancy (22.48% vs 16.25%) rates (CPR) were higher in the fresh cycle group (p < 0.00). Multivariate analysis for confounding factors also confirmed that women receiving a frozen-thawed embryo had a significantly lower LBR rate compared to those receiving a fresh embryo (OR 0.76, 95% CI 0.68-0.86, p < 0.00). In the sub-analysis of 1,154 cycles using slow embryos, there was no statistical difference in LBR (6.40% vs 6.26%, p = 0.92) or CPR (8.10% vs 7.22%, p = 0.58) between the two groups.

Conclusion

This study shows a lower LBR in FET cycles when compared to fresh cycles. Our results suggest that any potential gains in LBR due to improved embryo-endometrial synchrony following FET are lost, presumably due to freeze-thaw process-related embryo damage.

Keratan Sulphate in the Tumour Environment

Keratan sulphate (KS) is a bioactive glycosaminoglycan (GAG) of some complexity composed of the repeat disaccharide D-galactose β1→4 glycosidically linked to N-acetyl glucosamine. During the biosynthesis of KS, a family of glycosyltransferase and sulphotransferase enzymes act sequentially and in a coordinated fashion to add D-galactose (D-Gal) then N-acetyl glucosamine (GlcNAc) to a GlcNAc acceptor residue at the reducing terminus of a nascent KS chain to effect chain elongation. D-Gal and GlcNAc can both undergo sulphation at C6 but this occurs more frequently on GlcNAc than D-Gal. Sulphation along the developing KS chain is not uniform and contains regions of variable length where no sulphation occurs, regions which are monosulphated mainly on GlcNAc and further regions of high sulphation where both of the repeat disaccharides are sulphated. Each of these respective regions in the KS chain can be of variable length leading to KS complexity in terms of chain length and charge localization along the KS chain. Like other GAGs, it is these variably sulphated regions in KS which define its interactive properties with ligands such as growth factors, morphogens and cytokines and which determine the functional properties of tissues containing KS. Further adding to KS complexity is the identification of three different linkage structures in KS to asparagine (N-linked) or to threonine or serine residues (O-linked) in proteoglycan core proteins which has allowed the categorization of KS into three types, namely KS-I (corneal KS, N-linked), KS-II (skeletal KS, O-linked) or KS-III (brain KS, O-linked). KS-I to -III are also subject to variable addition of L-fucose and sialic acid groups. Furthermore, the GlcNAc residues of some members of the mucin-like glycoprotein family can also act as acceptor molecules for the addition of D-Gal and GlcNAc residues which can also be sulphated leading to small low sulphation glycoforms of KS. These differ from the more heavily sulphated KS chains found on proteoglycans. Like other GAGs, KS has evolved molecular recognition and information transfer properties over hundreds of millions of years of vertebrate and invertebrate evolution which equips them with cell mediatory properties in normal cellular processes and in aberrant pathological situations such as in tumourogenesis. Two KS-proteoglycans in particular, podocalyxin and lumican, are cell membrane, intracellular or stromal tissue-associated components with roles in the promotion or regulation of tumour development, mucin-like KS glycoproteins may also contribute to tumourogenesis. A greater understanding of the biology of KS may allow better methodology to be developed to more effectively combat tumourogenic processes.

What is the contribution of embryo-endometrial asynchrony to implantation failure?

PURPOSE

The synchronized development of a viable embryo and a receptive endometrium is critical for successful implantation to take place. The aim of this paper is to review current thinking about the importance of embryo-endometrial synchrony in in vitro fertilization (IVF).

METHODS

Detailed review of the literature on embryo-endometrial synchrony.

RESULTS

By convention, the time when the blastocyst first attaches and starts to invade into the endometrium has been defined as the 'window of implantation'. The term window of implantation can be misleading when it is used to imply that there is a single critical window in time that determines whether implantation will be successful or not. Embryo maturation and endometrial development are two independent continuous processes. Implantation occurs when the two tissues fuse and pregnancy is established. A key concept in understanding this event is developmental 'synchrony', defined as when the early embryo and the uterus are both developing at the same rate such that they will be ready to commence and successfully continue implantation at the same time. Many different events, including controlled ovarian hyperstimulation as routinely used in IVF, can potentially disrupt embryo-endometrial synchrony. There is some evidence in humans that implantation rates are significantly reduced when embryo-endometrial development asynchrony is greater than 3 days (±1.5 days).

CONCLUSIONS

Embryo-endometrial synchrony is critical for successful implantation. There is an unmet need for improved precision in the evaluation of endometrial development to permit better synchronization of the embryo and the endometrium prior to implantation.

The effect of a high progesterone concentration before oocyte retrieval on the peri-implantation endometrium

In this single-centre, prospective cohort study, the effect of high progesterone level before oocyte retrieval on endometrial morphology and uterine natural killer cell (uKN) count in the peri-implantation period was investigated. A total of 106 women undergoing IVF treatment who did not proceed to fresh embryo transfer were included. Endometrial samples were obtained 7 days after HCG administration. Multiple regression analysis was used to identify factors affecting the results of histological staging and uNK cell count. Progesterone level on the day after HCG administration was the only significant variable associated with the results of histological staging (P = 0.004). Endometrial development in women with high progesterone level was significantly (P < 0.001) more advanced than that of women with normal progesterone; progesterone level on the day of HCG administration was the only significant variable associated with uNK cell count. The median (range) of uNK cell count of 9.6% (2.3-21.6%) in women with high progesterone was significantly (P < 0.001) higher than the median (range) of uNK cell count of 5.7% (1.4-18.7%) in women with normal progesterone. High progesterone level before oocyte retrieval was correlated with advancement in endometrial development as well as increased uNK cell count.

Integrins in the endometrium

The endometrium expresses many of the same integrins displayed by other tissues. Endometrial epithelial cells maintain the ‘classic’ epithelial integrins, including α2, α3, α6, and β4, while the stroma expresses the fibronectin receptor, α5β1. During the menstrual cycle, the endometrium undergoes dynamic changes in morphology in preparation for implantation. With these histological changes are concomitant alterations in integrin expression that appear to ‘frame’ the window of implantation, by the co-expression of glandular αvβ3 and α4β1 during days 20 to 24 of the menstrual cycle. The changes in integrin expression shift from epithelial to stroma predominance late in the menstrual cycle, extending into early pregnancy. Decidual integrins that appear upregulated in pregnancy include α1β1, α3β1, α6β1 and αvβ3. Markers of uterine receptivity hold promise for a better understanding of the implantation process and may help to explain many different types of infertility. These markers will be essential for monitoring and improving infertility therapies. The importance of integrins in the human endometrium now seems well established and promises to be an area of great clinical and basic science activity in the future.

Is ovarian stimulation detrimental to the endometrium?

Ovarian stimulation in assisted reproduction technology produces lower implantation rates per embryo transferred than natural and ovum donation cycles, suggesting suboptimal endometrial development due to the abnormal concentrations of hormones used to recruit more oocytes. After the publication of several studies on the gene expression profile of endometrial receptivity in the natural cycle using microarray technology, researchers have investigated the impact of ovarian stimulation on the gene expression pattern of the endometrium. Ovarian stimulation cycles that use gonadotrophin-releasing hormone (GnRH) agonists and antagonists have been analysed in detail during the window of implantation to establish differences compared with the natural cycle. This paper reviews results obtained in different studies to elucidate the changes induced by the different protocols used in clinics. At the morphological level, no relevant alteration was observed in endometrial development in the early and mid-luteal phases in women undergoing ovarian stimulation following GnRH antagonist treatments. However, the gene expression pattern of the endometrium showed some differences. In addition, the endometrial development after GnRH antagonist mimics the natural endometrium more closely than after GnRH agonist at both the morphological (no relevant differences) and molecular level (only 23 genes dysregulated at high dose). Clinical implications of these differences should be analysed in more detail.

Wide genomic analysis of human endometrial receptivity: new times, new opportunities

Microarray technology has broadened the insight into many research fields allowing scientists to analyse the expression of many genes in quick and efficient experiments aimed at translating these findings into clinical applications. In reproductive medicine, researchers have exploited microarrays to increase understanding of the molecular mechanisms involved in endometrial receptivity and how a possible therapeutic translation can be feasible. In the last 4 years, several studies have focused on the genomics of the human endometrium in different physiological and pathological conditions, and these studies have generated a large amount of information about the regulation and dysregulation of the window of implantation (WOI) genes in fertile, subfertile and refractory conditions. However, the key molecules/mechanisms in endometrial receptivity remain to be elucidated. In this comprehensive review, we have analysed the available results obtained in our own and other laboratories, defining the genomic profile of the receptive endometrium in different situations and its possible clinical application.

Effect of controlled ovarian hyperstimulation in IVF on endometrial gene expression profiles

Controlled ovarian hyperstimulation (COH) used in IVF produces lower implantation rates per embryo transferred compared to natural cycles utilized in ovum donation, suggesting a suboptimal endometrial development. Endometrial receptivity has recently been investigated in natural menstrual cycles with the aid of microarray technology. The aim of this study is to investigate the impact of COH using urinary gonadotrophins with a long protocol with GnRH agonists without progesterone supplementation (similar to the natural cycle) on endometrial gene expression profiles during the window of implantation by comparing the profiles at day hCG + 7 of COH versus LH + 7 of a previous natural cycle in the same women. For this purpose we have used microarray technology by Affymetrix (GeneChip HG_U133A), which allows more than 22,000 genes to be tested simultaneously. Results were validated by semi-quantitative PCR and quantitative PCR experiments. We found that more than 200 genes showed a differential expression of more than 3-fold when COH and normal cycles were compared at hCG + 7 versus LH + 7. We simultaneously re-analysed the LH + 2 versus LH + 7 endometrial gene expression profiles in previous natural cycles in the same subject using this specific GeneChip, the results obtained were consistent with our own published results. This is the first time that gene expression profiles of the endometrium during COH are reported. The large degree of gene expression disturbance is surprising and highlights the need for further efforts to optimize COH protocols.

Apport de la biopsie d’endomètre dans l’évaluation de la réceptivité utérine

The histological changes underwent by endometrium during a natural menstrual cycle were described more than 50 years ago. Dating yields several methodological flaws, with intra- and inter-observer variability and shows questionable relationship to endometrial receptivity. The best studied endometrial factors involved in implantation include the formation of luminal epithelial "pinopodes", expression of adhesion molecules and of cytokines. Ovarian stimulation for IVF is known to affect luteal phase function. Factors influencing the endometrial receptivity in such cycles are poorly understood. Studies comparing the endometrium in IVF cycles with natural cycle controls have shown premature secretory changes in the postovulatory and early luteal phase of IVF cycles. These findings suggest a profound modification of luteal endometrial development in stimulated cycles. Studies exploring the endometrium within the cycle of embryo transfer have shown a deleterious effect of severe periovulatory maturation advancement exceeding 3 days, as no clinical pregnancies were observed in this condition.

Ovulation induction disrupts luteal phase function

Abnormalities in the luteal phase have been detected in virtually all the stimulation protocols used in in vitro fertilization, on both the hormonal and endometrial levels. Supraphysiological follicular or luteal sex steroid serum concentrations, altered estradiol: progesterone (E2/P) ratio, and disturbed luteinizing hormone pituitary secretion leading to corpus luteum insufficiency or a direct drug effect have been postulated as the main etiologic factors. Luteinizing hormone supports corpus luteum function, and low LH levels have been described after human menopausal gonadotropin treatment, after gonadotropin-releasing hormone (GnRH)-agonist treatment, or after GnRH-antagonist treatment. These low luteal LH levels may lead to an insufficient corpus luteum function and consequently to a shortened luteal phase or to the low luteal progesterone concentrations frequently described after ovulation induction. A direct effect of the GnRH agonist or GnRH antagonist on human corpus luteum or on human endometrium and thus on endometrial receptivity cannot be excluded, as GnRH receptors have been described in both compartments. Endometrial histology has revealed a wide range of abnormalities during the various stimulation protocols. In GnRH-agonist cycles, mid-luteal biopsies have revealed increased glandulo-stromal dyssynchrony and delay in endometrial development, strong positivity of endometrial glands for progesterone receptors, decreased alphavbeta3-integrin subunit expression, and earlier appearance of surface epithelium pinopodes. These factors suggest a shift forwards of the implantation window. Progesterone supplementation improves endometrial histology, and its necessity has been well established, at least in cycles using GnRH agonists.

The optimum time for exogenous human chorionic gonadotropin to rescue the corpus luteum

PURPOSE

Our purpose was to study the optimum time to administer exogenous human chorionic gonadotropin (hCG) to rescue the human corpus luteum during the luteal phase of normal menstrual cycles.

METHODS

Groups of normally cycling women were given 4-day regimes of exogenous hCG by daily injection beginning 4 (Group A), 8 (Group B), and 12 (Group C) days after the midcycle luteinizing hormone surge. The hCG regime used was designed to mimic hCG levels following a spontaneous implantation. All subjects acted as their own controls in a preceding normal menstrual cycle.

RESULTS

Group A subjects exhibited patterns and levels of salivary progesterone concentration similar to those seen in the control cycles throughout the normal luteal phase. In contrast, subjects in both Group B and Group C demonstrated a rapid and sustained increase in progesterone production following the hCG injections. Furthermore, subjects in Group B achieved the highest mean peak progesterone concentrations and the total amount of salivary progesterone secreted was significantly higher than in the control cycles (P < 0.05). Although the mean luteal-phase length was greatest in Group C, the response of the corpus luteum was suboptimal, with a delayed rise in salivary progesterone.

CONCLUSIONS

These data show that the qualitative and quantitative response of corpus luteum to an early pregnancy-type hCG signal is maximal around the midluteal phase, coincident with the time of implantation.

Endometrial estrogen and progesterone receptor and pinopode expression in stimulated cycles of oocyte donors

OBJECTIVE

To study the effect of controlled ovarian hyperstimulation and the ovarian response on several features of endometrial morphology simultaneously.

DESIGN

Prospective controlled study.

SETTING

Academic infertility center.

PATIENT(S)

Twenty-five oocyte donors undergoing COH and 10 ovulatory controls.

INTERVENTION(S)

Endometrial biopsies during the luteal phase and measurement of serum E2 and progesterone levels on days 12, 13, and 18-20.

MAIN OUTCOME MEASURE(S)

Endometrial morphology as judged by histologic dating, pinopode expression, and estrogen and progesterone receptor content.

RESULT(S)

Controlled ovarian hyperstimulation caused the early expression of endometrial features as judged by histologic dating criteria, estrogen and progesterone receptor expression, and the timing of pinopode expression in many of the subjects. A significant correlation within subjects with regard to their particular result on any one measure (e.g., histologic examination) and the others (e.g., estrogen and progesterone receptors, pinopodes) was observed. Those with higher levels of progesterone the day after hCG administration exhibited the most prematurity of morphologic features.

CONCLUSION(S)

Many controlled ovarian hyperstimulation cycles are associated with synchronous early expression of the expected pattern of histologic features, estrogen and progesterone receptors, and pinopodes. The most predictive feature of this premature expression was the level of progesterone the day after hCG administration.

In premature luteinization, progesterone induces secretory transformation of the endometrium without impairment of embryo viability

OBJECTIVE

To determine the effect of serum P on endometrial histology in stimulated cycles.

DESIGN

Prospective clinical study.

SETTING

Community hospital-based donor oocyte program.

PATIENT(S)

Fertile young oocyte donors and infertile donor oocyte recipients.

INTERVENTION(S)

Oocyte donors underwent gonadotropin stimulation after midluteal pituitary suppression. Endometrial biopsies were obtained at the time of oocyte retrieval.

MAIN OUTCOME MEASURE(S)

Endometrial histology and serum P levels in oocyte donors. Pregnancy and implantation rates in oocyte recipients.

RESULT(S)

Thirteen biopsy specimens (52.0%) showed in-phase mixed proliferative pattern (days 14 to 15), whereas 12 (48.0%) were secretory (days 16 to 17). On the day of hCG, subjects with secretory endometrium had higher P of 1.7 ng/mL (5.4 nmol/L) than women with the mixed pattern (0.8 ng/mL [2.5 nmol/L]). Progesterone > or = 0.9 ng/mL had a 78.6% positive predictive value for secretory transformation. In 75.0% of cycles with secretory endometrium, P was > or = 0.9 ng/mL, (2.9 nmol/L) as early as 2 days before hCG. Both mixed and secretory patterns were associated with similar clinical pregnancy rates (57.1% and 60.0%, respectively) and delivery rates (38.1% and 50.0%, respectively) in recipients.

CONCLUSION(S)

Subtle elevation of P induced secretory endometrial transformation without reduction in embryo viability.

Delayed embryo implantation following in vitro fertilization and embryo transfer (IVFET)

PURPOSE

Our goal was to compare serum human chorionic gonadotropin (hCG) levels in singleton pregnancies achieved following IVFET with those achieved following spontaneous conception.

RESULTS

The mean serum hCG level of patients who became pregnant following IVFET lagged 1.5 days behind that of patients who became pregnant spontaneously.

CONCLUSIONS

The use of gonadotropin releasing hormone analogue as part of the stimulation protocol leading to egg retrieval and IVFET results in a delay in embryo implantation.

Estrogen biosynthesis--regulation, action, remote effects, and value of monitoring in ovarian stimulation cycles

OBJECTIVE

To review current knowledge regarding estrogen biosynthesis, its regulation and action, specifically concerning local as opposed to remote effects of this hormone, and to examine the effectiveness and prognostic value of monitoring hormone concentrations and endometrial response in cycles of controlled ovarian hyperstimulation.

DATA IDENTIFICATION AND SELECTION

Studied that relate specifically to estrogen biosynthesis, enzymatic pathways, estrogen receptor physiology, and the clinical aspects of estrogen monitoring were identified through literature and Medline searches.

RESULTS

Folliculogenesis is the basic unit of ovarian activity, which has a dual purpose: oocyte maturation and steroid production. Steroidogenic granulosa and theca cells cooperate under gonadotropin control to produced estrogens by stimulating synthesis of steroidogenic enzyme messenger RNAs. Steroid synthesis is amplified further by local growth factors and follicular cell multiplication. Estrogen synthesis is directed by FSH, and only small amounts of LH are needed to amplify the follicular estrogenic potential. However, the growth of preovulatory follicles can proceed without LH, under FSH regulation only, even in the presence of low peripheral estrogen levels. Oocyte maturation and fertilization may proceed independently of ambient estrogen levels, leading to the assumption that estrogen exerts a minimal autocrine-paracrine function. The notable effect of follicular estrogen production is to promote adequate receptive endometrium for embryo implantation. Clinical treatment cycles may be monitored more effectively by evaluating end-organ response to estrogen rather than by evaluating absolute serum E2 concentrations or sonographic follicular measurements.

CONCLUSION

Follicular estrogen production is regulated by a complex set of signals that synergize to produce optimal steroidogenesis. Most importantly, the effect of estrogen is truly an endocrine effect, as it prepares the endometrium for implantation. Therefore, the goal of effective treatment and monitoring strategies should focus on direct assessment of the biologic activity of estrogen as it optimizes endometrial receptivity in anticipation of subsequent implantation.

Endometrial differentiation in the peri-implantation phase of women with recurrent miscarriage: a morphological and immunohistochemical study

OBJECTIVES

To study endometrial differentiation in the peri-implantation phase of women with recurrent miscarriage and to compare the results with endometrium of normal fertile women.

DESIGN

A prospective study of endometrial specimens precisely timed from the LH surge, using traditional histologic dating (Noyes' criteria), quantitative histologic measurement (morphometric analysis), and immunohistochemical techniques.

RESULTS

Fifteen of 25 (60%) subjects in the recurrent miscarriage group had retarded endometrial development in the peri-implantation period as monitored by morphometry. The recurrent miscarriage group showed reduced levels of four mucin-related secretory epitopes, and greater reductions were associated with morphological retardation. Normal differentiation was observed in all of the 14 subjects in the control group.

CONCLUSIONS

Women with idiopathic recurrent pregnancy loss may be divided into two distinct subgroups on the basis of their endometrial response in the peri-implantation period. Precisely timed endometrial biopsy should be incorporated in the investigation of recurrent miscarriage.

High dosages of gonadotropins are associated with poor pregnancy outcomes after in vitro fertilization-embryo transfer

OBJECTIVE

To compare the pregnancy outcomes during IVF-ET when different dosages of hMG are used after follicular phase suppression with leuprolide acetate (LA).

DESIGN

Retrospective chart review.

SETTING

Hospital-based IVF-ET program.

PATIENTS

From January 1990 to December 1992, 264 cycles reached ET after LA downregulation and gonadotropin stimulation.

RESULTS

Higher doses of gonadotropins, as measured by both average daily dose and total dose per cycle, were associated with lower clinical pregnancy rates. This effect was independent of age, basal FSH level, endometrial thickness, maximal E2 levels, number of eggs retrieved, and embryos transferred.

CONCLUSION

High dosages of exogenous gonadotropins are associated with lower pregnancy rates in IVF-ET.

Endometrial structure after superovulation: a prospective controlled study

OBJECTIVE

To demonstrate the effect of superovulation using a GnRH agonist (GnRH-a) and hMG and hCG on endometrial structure.

DESIGN

Prospective, case-controlled study.

SETTING

Tertiary referral assisted reproduction unit in an academic department.

PATIENTS

Eleven women undergoing GIFT or IVF, without ET.

INTERVENTIONS

All women were treated with a long stimulation regimen using the depot GnRH-a Goserelin (ICI, Macclesfield, United Kingdom) and hMG and hCG.

MAIN OUTCOME MEASURES

Comparison of endometrial biopsy specimens taken 4 days after ovulation in an unstimulated cycle with specimens taken 4 days after oocyte recovery, using standard dating criteria and morphometric analysis.

RESULTS

There was no difference in endometrial glandular development as assessed by either standard criteria or morphometric analysis.

CONCLUSIONS

Superovulation preceded by pituitary down regulation is not associated with abnormal endometrial glandular development, even though supraphysiological levels of E2 and P are induced.