Human embryo implantation: current knowledge and clinical implications in assisted reproductive technology

Update on the role of leukemia inhibitory factor in assisted reproduction

PURPOSE OF REVIEW

To review the recent literature on the involvement and importance of leukemia inhibitory factor (LIF) in the human implantation process, and the attempts using LIF-based interventions to improve assisted reproductive technologies (ARTs) outcome in women with recurrent implantation failure.

RECENT FINDINGS

High LIF expression is an indicator of receptive endometrium in fertile women. However, in infertile individuals, the data on endometrial LIF expression and secretion are controversial. Even after ruling out other causes of infertility, such as tubal, endocrine, male factor, and endometriosis, LIF-only detection is not sufficient for assessment of implantation potential in women with unexplained infertility. This is obviously in contrast to evidence of the crucial role of LIF in mouse endometrial physiology. In a large multicenter study, recombinant human LIF failed to improve the outcome of IVF treatment in women with recurrent implantation failure.

SUMMARY

A better comprehension of the mechanisms underlying endometrial receptivity and implantation should guide clinicians through proper management and treatment of infertility and implantation failure, and may eventually enable widespread adherence to single embryo transfer practices.

Microarray evaluation of endometrial receptivity in Chinese women with polycystic ovary syndrome

Patients with polycystic ovary syndrome (PCOS) have lower pregnancy and higher miscarriage rates, possibly due to decreased endometrial receptivity. In this study, endometrium was processed for RNA extraction and hybridization of chemically fragmented, biotinylated, complementary RNA on high-density oligonucleotide microarrays, and screened for 21,571 genes. Real-time polymerase chain reaction (PCR) was used to verify the result. Genes found to be down-regulated in the endometrium during the implantation window in PCOS patients included those whose activity was integral to membrane function, adhesion, invasive growth and the cytoskeleton. Among these genes, some have previously been associated with endometrial receptivity (by microarray research or other methods) and some have never previously been associated with endometrial receptivity. Using real-time PCR, expression of transmembrane 4 superfamily member 4 (TM4SF4) and matrix metalloproteinase 26 (MMP26) was found to be significantly decreased during the implantation window in patients with PCOS (P= 0.003). TM4SF4 has been demonstrated to be associated with adhesion; MMP26 has been shown to be related to degradation of extracellular matrix. It is suggested the down-regulated gene expression during the implantation window in patients with PCOS indicates differential gene expression in the endometrium between PCOS and normal women during the implantation window. This might affect endometrial receptivity.

Association of progesterone receptor polymorphisms with recurrent implantation failure after in vitro fertilization and embryo transfer

INTRODUCTION

Progesterone is the hormone of pregnancy and is required for its initiation. The actions of progesterone are mediated by the progesterone receptor. Polymorphic variants of human progesterone receptor genes have been implicated in implantation failure.

MATERIALS AND METHODS

We, therefore, investigated the prevalence of H770H (C/T genotype), V660L polymorphism and a 306 bp Alu insertion in exon 7 of the progesterone receptor among women with history of recurrent implantation failure to determine whether any of these polymorphisms may serve as a risk factor for implantation failure. DNA was extracted from the buccal swabs obtained from 66 women experiencing implantation failure and 75 fertile control women. PCR amplification of fragments was purified and the DNA sequenced to identify the polymorphism. The frequencies for the three variants were 27% for H770H, 21% for V660L and 0% for the 306 bp Alu insertion in exon 7 among women with implantation failure compared with control women of 25% for H770H and 24%for V660L and 0% for the 306 bp Alu insertion in exon 7.

DISCUSSION

No significant differences in the overall allelic frequency of progesterone receptor variants was seen when women experiencing recurrent implantation failure were compared with control women.

CONCLUSION

We conclude that the H770H and V660L and PROGINS progesterone receptor polymorphisms are not markers that can identify women at risk for recurrent implantation after IVF/ET.

Osteopontin and alphavbeta3 integrin expression in the endometrium of infertile and fertile women

Osteopontin and its receptor alpha(v)beta(3) integrin have recently been proposed as a major complex to promote embryo attachment, and thus they would be useful as markers of endometrial receptivity. In the current study alpha(v)beta(3) integrin and osteopontin expression and co-expression in in-phase and out-of-phase endometrial biopsies from normal healthy women (n = 12) and infertile patients (n = 107) were investigated. Two endometrial biopsies (post-ovulatory day +6 to +8, and 4 days later) were performed during a single menstrual cycle in each subject. Oestradiol and progesterone serum concentrations were quantified on the same days as endometrial sampling. No statistically significant difference regarding alpha(v)beta(3) integrin and osteopontin expression and their coexpression was found between fertile controls and infertile patients irrespective of endometria being in-phase or out-of-phase, infertility factors detected or whether patients became spontaneously pregnant or not. Although a co-ordinate high concentration of both glycoproteins on post-ovulatory day 8 onwards was observed, there was an evident lack of temporal co-expression of these markers during the implantation window. It is concluded that the functional significance of the osteopontin:alpha(v)beta(3) integrin complex as a marker of endometrial receptivity and implantation potential in women seems to be untenable.

Effects of ovarian stimulation on endometrial integrin β3 and leukemia inhibitory factor expression in the peri-implantation phase

OBJECTIVE

To investigate the effects of ovarian stimulation on endometrial receptivity in the peri-implantation phase.

DESIGN

Comparison of integrin beta3 and leukemia inhibitory factor (LIF) expression in endometrial biopsies from moderate responders and high responders with temporally matched natural cycles.

SETTING

Clinical and experimental research.

PATIENT(S)

Patients with cancelled embryo transfer were evaluated. Moderate responders were defined as E2 5,000-15,000 pmol/L on the day of hCG administration (group B, n = 7). High responders were defined as E2 >15,000 pmol/L on the day of hCG administration (group C, n = 8). Healthy and fertile volunteers were used as natural controls (group A, n = 10).

INTERVENTION(S)

Endometrial biopsy on the day of LH+7/hCG+7.

MAIN OUTCOME MEASURE(S)

Immunohistochemistry of integrin beta3 and LIF and imaging analysis.

RESULT(S)

Endometrial integrin beta3 and LIF expressions in the peri-implantation phase were significantly lower in stimulated cycles (including both moderate and high responders) compared to natural controls. Expression of LIF in glandular epithelium in high responders was lower than that in moderate responders.

CONCLUSION(S)

These data demonstrated that suggested markers of endometrial receptivity were reduced in stimulated cycles compared with natural cycles, and more so in high response cycles compared with moderate response cycles.

Serial uterine artery Doppler velocity parameters and human uterine receptivity in IVF/ICSI cycles

OBJECTIVE

To evaluate the predictive value of serial uterine artery Doppler ultrasound for embryo implantation in in-vitro fertilization (IVF)-intracytoplasmic sperm injection (ICSI) cycles.

METHODS

This was a prospective observational study at the VU University Medical Center, Amsterdam. Patients with an indication for IVF or IVF-ICSI according to departmental protocol underwent controlled ovarian hyperstimulation followed by IVF or IVF-ICSI and embryo transfer and had serial Doppler ultrasound performed during this treatment cycle. Patient and cycle characteristics, number of conceptions and ongoing pregnancies and pulsatility index (PI) of both uterine arteries on different cycle days were assessed and results were compared between patients who conceived and those who did not.

RESULTS

Of the 102 patients enrolled into the study, 83 underwent embryo transfer. Of these, 41 became pregnant and 42 did not (Group 1). Of the 41 pregnancies, 30 were ongoing (Group 2) and 11 miscarried (Group 3). Between Groups 1, 2 and 3, linear regression revealed no significant difference between any of the variables examined except in the quality of transferred embryos. There was no significant difference in the mean PI of the left and right uterine arteries on any day of the cycle, or in the change in PI during the cycle. Receiver-operating characteristics curves derived to determine the performance of PI to predict pregnancy outcome supported our findings that the uterine artery PI is not a suitable marker for identifying patients with implantation failure. Multivariate analysis showed no relationship between pregnancy and PI between groups, but it did show a relationship between pregnancy and some patient and cycle characteristics.

CONCLUSION

In an unselected group of patients undergoing IVF or IVF-ICSI and embryo transfer, serial Doppler ultrasound examination of the uterine artery does not discriminate between cycles resulting in ongoing pregnancy, miscarriage and no pregnancy.

Uterine receptivity to human embryonic implantation: histology, biomarkers, and transcriptomics

Embryonic implantation is a dynamic process of paracrine interactions between the maternal compartment and the conceptus and involves a receptive endometrium and a developmentally competent blastocyst. Herein, we review histology, clinical approaches, and the promise of transcriptomics in elucidating mechanisms underlying implantation and development of biomarkers of uterine receptivity-with an eye to diagnose and treat implantation-based disorders of miscarriage, fetal growth restriction, pre-eclampsia, and infertility.

Vascular endothelial growth factor level changes during human embryo development in culture medium

OBJECTIVE

Implantation is a complex phenomenon consisting of the first strong contact between embryo and endometrium. Recent studies have demonstrated that this process is dependent not only on the 'readiness' of the endometrium, but also on complex interactions between endometrial and embryonic tissues that cross-talk by means of different molecules (growth factors, cytokines, vasoactive factors). Investigations performed on human blastocysts indicate a role for vascular endothelial growth factor (VEGF) in these processes. The aim of the present study was to investigate VEGF levels at different stages in human embryo culture medium.

STUDY DESIGN

We selected 20 women among patients undergoing assisted reproduction with the in vitro fertilization-blastocyst transfer protocol. The oocytes were inseminated by intracytoplasmic sperm injection. For each patient, approximately two cultures of four microinjected oocytes (and then of four embryos) were performed. Each culture of four oocytes/embryos was placed in one dish to increase the probability to detect small VEGF concentrations.

RESULTS

Results showed significantly higher VEGF levels in the medium at blastocyst stage (12.16 +/- 2.80 pg/ml) compared with embryos at pronuclear stage (13.58 +/- 2.32 pg/ml) and microinjected oocytes (12.80 +/- 3.45 pg/ml).

CONCLUSIONS

An important VEGF synthesis by blastocysts occurs during human embryo development.

Basic aspects of implantation

Implantation, a critical step for establishing pregnancy, requires molecular and cellular events resulting in healthy uterine growth and differentiation, blastocyst adhesion, invasion and placental formation. Successful implantation requires a receptive endometrium, a normal and functional embryo at the blastocyst stage and a synchronized dialogue between maternal and embryonic tissues. In addition to the main role of sex steroids, the complexity of embryo implantation and placentation is exemplified by the number of cytokines and growth factors with demonstrated roles in these processes. Disturbances of the normal expression and action of these cytokines result in absolute or partial failure of implantation and abnormal placental formation in mice and humans. Members of the gp130 cytokine family, interleukin (IL)-11 and leukaemia inhibitory factor, the transforming growth factor-beta superfamily, colony-stimulating factors, and the IL-1 and IL-15 systems are all crucial for successful implantation. In addition, chemokines are important both in recruiting specific cohorts of leukocytes to the implantation site, and in trophoblast trafficking and differentiation. This review provides discussion on embryonic and uterine factors that are involved in the process of implantation in autocrine, paracrine and/or juxtacrine manners at hormonal, cellular, and molecular levels.

The position of transferred air bubbles after embryo transfer is related to pregnancy rate

OBJECTIVE

The possibility to visualize the transfer air bubbles is one of the main benefits of ultrasonographic-guided embryo transfer. The objective of this study was to analyze the relation between the position of the air bubbles and pregnancy rates.

DESIGN

Prospective data-analysis.

SETTING

University fertility clinic.

PATIENT(S)

IVF and intracytoplasmic sperm injection patients.

INTERVENTION(S)

Transabdominal ultrasonographic guidance at embryo transfer.

MAIN OUTCOME MEASURE(S)

Pregnancy rate, length endometrial plate, distance catheter to fundus, distance air bubbles to fundus.

RESULT(S)

Analysis of 367 consecutive ultrasonographic-guided embryo transfers following IVF or intracytoplasmic sperm injection treatment. Both absolute and relative position of the air bubbles were significantly closer to the fundus in patients who became pregnant compared with patients who did not become pregnant. When the relative position of the air bubbles was in the fundal half of the endometrial plate pregnancy rates were significantly higher compared with the lower half of the endometrial plate, 43.0% and 24.4%, respectively, P=.002. Multiple regression analysis revealed the relative position as an independently associated determinant for pregnancy.

CONCLUSION(S)

The position of the air bubbles after embryo transfer is related to pregnancy rate; the highest pregnancy rates are found when the air bubbles end up closer to the fundus.

Endometrial expression of IL-1RtI in patients undergoing miscarriage or unsuccessful IVF cycles

This study sought to compare the endometrial expression of interleukin-1 receptor type I in patients with a history of spontaneous recurrent miscarriage and patients with unsuccessful IVF cycles. Eight patients who had undergone two or more consecutive unsuccessful IVF cycles (group I), in which at least one good quality embryo was transferred, and 18 patients with at least three spontaneous late miscarriages (group II) were included in the study. Endometrial and follicular development were evaluated by ultrasonography. Plasma concentrations were evaluated for FSH and prolactin on cycle day 3 and for progesterone on day 7 after ovulation, when an endometrial biopsy was performed; samples were cryopreserved for interleukin-1 receptor type I immunohistochemistry and embedded in paraffin for endometrial dating. Patients with no ovulation or any anomalies in hormone concentrations, uterine cavity or endometrial histology were excluded. Interleukin-1 receptor type I was expressed in the lumenal epithelium of both groups. There was a significant difference (P = 0.0357, two-tailed Fisher's exact test) in the glandular epithelium expression of interleukin-1 receptor type I between groups I (87.5% of patients) and II (38.9% of patients). Endometrial expression of interleukin-1 receptor type I does not seem to be involved in embryo implantation in IVF patients, but may play a role in spontaneous recurrent miscarriage.

Pregnancy prediction models and eSET criteria for IVF patients--do we need more information?

PURPOSE

The purpose of the present study was to evaluate statistical prediction models and simple allocation criteria, based on predictors for pregnancy, as tools to identify a good prognosis group in a possible eSET setting.

METHODS

A pregnancy prediction model based on logistic regression models was generated by analysis of 1675 DET treatment cycles. The model was evaluated and compared to simple eSET allocation criteria.

RESULTS

Embryo quality, patient age, and basal FSH were identified as significant predictors (at 5% significance level) of pregnancy. Although comparable to previously generated models, the predictive ability of the present model was relatively poor and practically similar to simple allocation criteria based on age and embryo quality.

CONCLUSIONS

Existing prediction models, or simple allocation criteria, are limited in identifying good prognosis patients. Future studies of the applicability of improved pregnancy prediction models will need very comprehensive and detailed patient and embryo information.

Presence of hydrosalpinx correlated to endometrial inflammatory response in vivo

OBJECTIVE

To evaluate and compare the inflammatory response and mediators in the endometrium of patients with hydrosalpinges compared with normal controls.

DESIGN

Retrospective case-control study.

SETTING

Urban medical center.

PATIENT(S) AND INTERVENTION(S)

Hysterectomy samples were identified as being affected by hydrosalpinx or salpingitis (n = 30) and were age-matched with control samples (n = 30).

INTERVENTIONS

Fallopian tube and endometrial slides were analyzed for leukocytes and immunohistochemical techniques performed for cytokines (interleukin-2 [IL-2]).

MAIN OUTCOME MEASURE(S)

Evaluate and compare the endometrial inflammatory response (leukocytes and cytokines) from samples affected and non-affected by hydrosalpinx and salpingitis.

RESULT(S)

Examination of tubal and endometrial slides with hydrosalpinx demonstrated a statistically significant increase in the number of overall inflammatory cells. High-intensity immunohistochemical staining for IL-2 was demonstrated in 7.4% of controls versus 65% of cases.

CONCLUSION(S)

A defined, identifiable, local response to hydrosalpingeal fluid has been demonstrated in the endometrium. This response consists of statistically significant elevations of leukocytes and IL-2. An inflammatory endometrial response may be an independent contributor to the decreased reproductive outcome observed in patients with hydrosalpinges.

Ultrasonographic-guided embryo transfer does not enhance pregnancy rates compared with embryo transfer based on previous uterine length measurement

OBJECTIVE

To compare pregnancy rates (PRs) after ultrasound-guided embryo transfer and embryo transfer based on ultrasonographic length measurement.

DESIGN

Prospective intervention group combined with retrospective control group.

SETTING

University fertility clinic.

PATIENT(S)

Patients undergoing IVF and intracytoplasmic sperm injection.

INTERVENTION(S)

Transabdominal ultrasonographic guidance at embryo transfer.

MAIN OUTCOME MEASURE(S)

Pregnancy and implantation rates.

RESULT(S)

In 367 ultrasound-guided embryo transfers clinical PR, ongoing PR, and implantation rate were 35.1%, 31.1%, and 24.3%, respectively. In 363 embryo transfers based on previous ultrasonographic length measurement, the rates were 33.9%, 29.5%, and 24.2%, respectively. There were no statistical significant differences between the groups.

CONCLUSION(S)

Ultrasonographic guidance does not show any benefit in terms of PR and implantation rate compared to previous ultrasonographic length measurement, an other precise and atraumatic transfer technique.

Factors determining early pregnancy loss in singleton and multiple implantations

BACKGROUND

The incidence of first trimester pregnancy loss is much lower in IVF twin pregnancies than in IVF singleton pregnancies. The objective of this study was to determine which embryonic and maternal factors contribute to this finding.

METHODS

Retrospective data analysis of the outcome of 1593 pregnancies after day 3 double-embryo transfer (DET) after IVF or ICSI treatment.

RESULTS

Of 1148 single implantations at 6 weeks, 936 (81.5%) were ongoing pregnancies. Of 445 multiple implantations at 6 weeks, 354 (79.6%) were ongoing multiple pregnancies, 80 (17.9%) were ongoing singleton pregnancies and 11 (2.5%) ended in a spontaneous abortion. Total pregnancy loss was 18.5 and 2.5% (P < 0.001) in singleton and twin gestations, respectively. Loss per gestational sac was 18.5 and 11.46% (P < 0.001), respectively. Determinants contributing to the continuation of gestation beyond 6 weeks were young maternal age, possibility to cryopreserve embryos and short GnRH agonist flare-up stimulation protocol. Whereas factors promoting multiple implantation at 6 weeks of gestation were young maternal age, high cumulative embryo score (CES), male infertility, long stimulation protocol and thick endometrium.

CONCLUSIONS

Although multiple implantation at 6 weeks is predominantly determined by (morphological) embryo quality, the continuation of pregnancy beyond 6 weeks becomes more dependent on the combination of genetic and developmental potential of the embryo(s) and an optimal uterine milieu.

The human endometrium as a fertility-determining factor

Intensive research work has been performed to better understand the regulation of the endometrium and its clinical implications to improve implantation. Although many proteins and molecules may influence endometrial development, their co-ordinated contribution to the implantation process is still poorly understood and a translation into clinical use has not sufficiently been performed. Clinical evaluation of the endometrium by ultrasound and other techniques, like endometrial biopsy and analysis of uterine secretions, has been intensively studied and therapeutic options to improve endometrial function have been suggested and tested. Systemic treatment with heparin, aspirin or corticosteroids did not result in improved implantation rates. Gene therapy and cervical treatment, e.g. with seminal plasma, are still in the phase of clinical research. Therefore, this review focuses on different aspects of endometrial research, which all contribute to the diagnosis, evaluation and therapy of endometrial function and dysfunction. First, the endometrial development towards a receptive milieu is described. Second, the actual clinical evaluation of endometrial receptivity, possible therapeutic strategies and in particular, the evaluation of endometrial function in the non-natural situation of hormonal stimulation is critically evaluated. In conclusion, the endometrium shall be considered as an important fertility-determining factor and therapeutic options should be developed in near future.

Ovarian stimulation with GnRH agonist, but not GnRH antagonist, partially restores the expression of endometrial integrin 3 and leukaemia-inhibitory factor and improves uterine receptivity in mice

BACKGROUND

The impact of different ovarian stimulation (OS) protocols on endometrial receptivity remains controversial. In this study, the effects of different OS on the expression of endometrial integrin beta3 subunit and leukaemia-inhibitory factor (LIF) during the implantation window and the implantation rate in mice were investigated.

METHODS

Three OS protocols were used, involving either pregnant mare's serum gonadotrophin (PMSG) alone, PMSG plus GnRH agonist or PMSG plus GnRH antagonist. Uterus samples were collected at 48 h after OS or ovulation and were detected with immunohistochemistry, Western blot and RT-PCR analyses. Normal embryos at gestation day 4 were transferred into the uteri of mice in the control and OS groups.

RESULTS

All OS groups showed a significant decrease in the expression of both the endometrial integrin beta3 subunit and LIF during the implantation window and the implantation rate. Among the three OS groups, GnRH agonist-treated mice showed a higher endometrial integrin beta3 subunit and LIF expression and a higher implantation rate. No significant difference was found in the measured indices between the GnRH antagonist and PMSG groups.

CONCLUSIONS

OS may inhibit the expression of endometrial integrin beta3 subunit and LIF and impair endometrial receptivity in mice. OS with GnRH agonist, but not GnRH antagonist, may partially restore the endometrial physiological secretion and improve uterine receptivity.

p53 tumour suppressor gene polymorphism is associated with recurrent implantation failure

Implantation failure is the most frequent cause of lack of pregnancy after IVF and embryo transfer. Successful implantation requires trophoblastic growth into the endometrium to stimulate its own blood supply. The p53 tumour suppressor gene has been shown to regulate cell growth and induce angiogenesis. Therefore, the prevalence of p53 codon 72 polymorphism was investigated among women with a history of recurrent implantation failure to determine whether this polymorphism may serve as a risk factor for implantation failure. DNA was extracted from the buccal swabs obtained from 70 women experiencing implantation failure and polymerase chain reaction amplification of p53 arginine (Arg) 72 and proline (Pro) 72 variants were performed. The frequency of homozygous Arg and Pro or heterozygous Arg/Pro genotypes was compared with frequencies among 205 women experiencing recurrent pregnancy loss and 20 control women. The frequency of Pro72 was significantly higher (P=0.003) among women experiencing recurrent implantation failure compared with women experiencing recurrent miscarriage and with control women. It is concluded that p53 codon 72 polymorphisms may serve as a susceptibility factor affecting the chances of recurrent implantation after IVF or embryo transfer.

Cytokine expression in the endometrium of women with implantation failure and recurrent miscarriage

One potential cause of reproductive failure such as infertility and recurrent miscarriage may be an endometrial defect. Numerous studies in mice have suggested the importance of various different cytokines in successful pregnancy outcome. This article reviews the literature available on the role of T helper cytokines and IL-1, IL-11, LIF, IL-12 and IL-18 in infertility and recurrent miscarriage, with particular emphasis on the role that endometrial cytokines may play. Although there are numerous studies on cytokines in recurrent miscarriage, much less has been reported on their role in infertility with or without failure after IVF. There is also considerable variation in the results obtained from various different studies, which may be due to different populations studied, the different timing of the sample collection, and whether the cytokines were measured in whole tissue or a specific cell population. The presence of complicated networks of cytokines and their overlapping biological activities means that alteration of one cytokine is likely to affect others and this also makes the study of their role in implantation failure very difficult. There is an urgent need to re-examine the role played by various cytokines in reproductive failure through carefully planned and vigorously designed studies and to compare the different types of reproductive failure.

Defective implantation and placentation: laying the blueprint for pregnancy complications

Normal implantation and placentation is critical for pregnancy success. Many pregnancy-related complications that present late in gestation (such as pre-eclampsia and preterm labour) appear to have their origins early in pregnancy with abnormalities in implantation and placental development. Implantation is characterized by invasion of the maternal tissues of the uterus by fetal trophoblast, and the degree to which trophoblast invades these tissues appears to be a major determinant of pregnancy outcome. Excessive invasion can lead to abnormally firm attachment of the placenta to the myometrium (placenta accreta) with increased maternal and perinatal morbidity. Inadequate invasion, specifically restricted endovascular invasion, has been implicated in the pathophysiology of such conditions as pre-eclampsia (gestational proteinuric hypertension), preterm premature rupture of membranes, preterm labour, and intrauterine growth restriction. The molecular and cellular mechanisms responsible for implantation remain enigmatic. This review will include an overview of implantation followed by a discussion of a number of molecular mechanisms implicated in defective implantation and placentation including the role of decidual prostaglandins and haemorrhage in regulating trophoblast invasion. An improved understanding of the molecular mechanisms responsible for abnormal implantation and placentation will likely improve clinicians' abilities to treat disorders that occur along this continuum, including infertility, recurrent pregnancy loss, pre-eclampsia, and preterm birth.

Ultrastructural Dynamics of Human Reproduction, from Ovulation to Fertilization and Early Embryo Development1

This study describes the updated, fine structure of human gametes, the human fertilization process, and human embryos, mainly derived from assisted reproductive technology (ART). As clearly shown, the ultrastructure of human reproduction is a peculiar multistep process, which differs in part from that of other mammalian models, having some unique features. Particular attention has been devoted to the (1) sperm ultrastructure, likely "Tygerberg (Kruger) strict morphology criteria"; (2) mature oocyte, in which the MII spindle is barrel shaped, anastral, and lacking centrioles; (3) three-dimensional microarchitecture of the zona pellucida with its unique supramolecular filamentous organization; (4) sperm-egg interactions with the peculiarity of the sperm centrosome that activates the egg and organizes the sperm aster and mitotic spindles of the embryo; and (5) presence of viable cumulus cells whose metabolic activity is closely related to egg and embryo behavior in in vitro as well as in vivo conditions, in a sort of extraovarian "microfollicular unit." Even if the ultrastructural morphodynamic features of human fertilization are well understood, our knowledge about in vivo fertilization is still very limited and the complex sequence of in vivo biological steps involved in human reproduction is only partially reproduced in current ART procedures.

Human implantation: the last barrier in assisted reproduction technologies?

Implantation processes are highly complex involving the actions of numerous hormones, immunoglobulins, cytokines and other factors in the endometrium. They are also essential matters for the success of assisted reproduction. The nature of early embryonic development is of equal significance. It involves ovarian follicle growth, ovulation, fertilization and preimplantation growth. These processes are affected by imbalanced chromosomal constitutions or slow developmental periods. Post-implantation death is also a significant factor in cases of placental insufficiency or recurrent abortion. Clearly, many of these matters can significantly affect birth rates. This review is concerned primarily with the oocyte, the early embryo and its chromosomal anomalies, and the nature of factors involved in implantation. These are clearly among the most important features in determining successful embryonic and fetal growth. Successive sections cover the endocrine stimulation of follicle growth in mice and humans, growth of human embryos in vitro, their apposition and attachment to the uterus, factors involved in embryo attachment to uterine epithelium and later stages of implantation, and understanding the gene control of polarities and other aspects of preimplantation embryo differentiation. New aspects of knowledge include the use of human oocyte maturation in vitro as an approach to simpler forms of IVF, and new concepts in developmental genetics.

Recurrent implantation failure in assisted reproduction: how to counsel and manage. A. General considerations and treatment options that may benefit the couple

Recurrent implantation failure is a distressing phenomenon, both for the infertile couple and for the physician responsible for their treatment. Aetiology is often not clear and treatment options are vague. Particularly when transferred embryos are of good quality, recurrent implantation failure may be attributed to less than optimal embryo transfer technique, pathological lesions of the uterine cavity, the presence of hydrosalpinges, fibroids and endometriosis. Poor embryo quality, especially when repetitive, is a major impediment to successful implantation and cannot be corrected at the present time. Molecular abnormalities at the endometrial level and abnormal embryo-endometrium dialogue may be responsible for some cases of recurrent implantation failure. Furthermore, there may be over- or under-expressed genes that may be related to successful implantation. At the present time, the physician confronted with a couple presenting with recurrent implantation failure should discuss openly the potential causes of this phenomenon, with special emphasis on correctable causes, and offer remedies that are evidence based.

Significance of inhibin in reproductive pathophysiology and current clinical applications

The human reproductive process is regulated by complex mechanisms that involve many organs, including the brain, gonads and endocrine system. It has been more than 70 years since the name 'inhibin' was used to describe a substance produced in the gonads that negatively regulates pituitary secretion. Inhibin B controls FSH secretion via a negative feedback mechanism. It is a glycoprotein hormone secreted by the Sertoli cells of the testis and granulosa and theca cells of the ovary. Serum inhibin B concentrations are positively related to testicular volume and sperm counts. Current understanding of inhibin physiology and pathology in the human suggests that inhibin B may be of importance as a marker of Sertoli cell function in men with infertility and as a prognostic indicator in women undergoing ovulation induction therapy. Inhibin concentrations are elevated in patients with granulosa cell tumours and in post-menopausal women with mucinous ovarian cancers. Immunoreactivity against the inhibin alpha-subunit was identified in all cases of adrenal cortical adenoma and carcinoma, and levels are suppressed in the malignant prostate disease. This article discusses the structure, regulation and clinical use of inhibin and other related substances.