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

Ultrasound assessment of the peri-implantation uterus: a review

Emerging evidence suggests that early embryo implantation is a more active maternal process than hitherto appreciated, involving active encapsulation of the implanting blastocyst by maternal decidual cells and coordinated changes in the underlying inner myometrium, known as the junctional zone. These concepts raise the possibility that early ultrasound markers predictive of adverse pregnancy outcome could be identified. In this review we assess the role of ultrasound in predicting the likelihood of different pregnancy outcomes and highlight potential novel markers that could be tested.

The genomics of the human endometrium

The endometrium is a complex tissue that lines the inside of the endometrial cavity. The gene expression of the different endometrial cell types is regulated by ovarian steroids and paracrine-secreted molecules from neighbouring cells. Due to this regulation, the endometrium goes through cyclic modifications which can be divided simply into the proliferative phase, the secretory phase and the menstrual phase. Successful embryo implantation depends on three factors: embryo quality, the endometrium's state of receptivity, and a synchronised dialogue between the maternal tissue and the blastocyst. There is a need to characterise the endometrium's state of receptivity in order to prevent reproductive failure. No single molecular or histological marker for this status has yet been found. Here, we review the global transcriptomic analyses performed in the last decade on a normal human endometrium. These studies provide us with a clue about what global gene expression can be expected for a non-pathological endometrium. These studies have shown endometrial phase-specific transcriptomic profiles and common temporal gene expression patterns. We summarise the biological processes and genes regulated in the different phases of natural cycles and present other works on different conditions as well as a receptivity diagnostic tool based on a specific gene set profile. This article is part of a Special Issue entitled: Molecular Genetics of Human Reproductive Failure.

Loss of WT1 expression in the endometrium of infertile PCOS patients: a hyperandrogenic effect?

CONTEXT

In fertile patients the endometrial Wilms tumor suppressor gene (WT1) is expressed during the window of implantation. Polycystic ovary syndrome (PCOS) patients suffer from hyperandrogenemia and infertility and have elevated endometrial androgen receptor (AR) expression. WT1 is known to be down-regulated by AR. Therefore, the expression of WT1 and its targets may be altered in PCOS endometrium.

OBJECTIVE

The objective of the study was to assess the expression and regulation of WT1 and selected downstream targets in secretory endometrium from ovulatory PCOS (ovPCOS) and fertile women.

DESIGN AND PATIENTS

Endometrial samples were obtained from 25 ovPCOS and 25 fertile patients.

MAIN OUTCOME MEASURE

Endometrial expression of WT1 and selected downstream targets were assessed by immunohistochemistry and RT-PCR. The androgen effect on WT1 expression was determined in vitro by immunoblots and RT-PCR. The expression of WT1 and its targets was quantified in fertile and ovPCOS stromal cells in the presence of androgens by RT-PCR. Caspase-3/7 activity was measured to evaluate sensitivity to drug-induced apoptosis.

RESULTS

WT1 expression was down-regulated in secretory-phase ovPCOS endometrium. Stromal expression of Bcl-2 and p27 was higher, and epidermal growth factor receptor was lower in ovPCOS than in fertile patients. Endometrial stromal expression of WT1, Bcl-2, Bcl-2-associated X protein, and β-catenin was regulated by androgens. Apoptosis levels were reduced in ovPCOS samples and androgen-treated fertile samples.

CONCLUSION

WT1 expression is down-regulated in ovPCOS endometrium during the window of implantation. Androgens regulate the expression of WT1 and its targets during endometrial decidualization. The altered balance between WT1 and AR in the endometrium of PCOS patients may jeopardize the success of decidualization and endometrial receptivity.

Endometrial receptivity and human embryo implantation

PROBLEM

The pre-requisite of successful implantation involves an intricate cascade of molecular interactions which plays a crucial role in preparing receptive endometrium and implanting blastocyst.

METHOD OF STUDY

Data are hereby presented for a better understanding of endometrial receptivity in women, hoping to provide a comprehensive picture of the process and identify new areas of basic and translational research in the biology of blastocyst implantation.

RESULTS

Timely regulation of the expression of a number of complex molecules like hormones, cytokines and growth factors, and their crosstalk from embryonic and maternal endometrial side play a major role in determining the fate of the embryo. The molecular basis of endometrial receptivity and the mechanisms by which the blastocyst first adheres to the luminal epithelium and then penetrates into the stroma are only just beginning to be resolved.

CONCLUSION

Advances in the development of implantation models and 'omics' technologies, particularly proteomics and metabolomics, are set to have a major impact on the development of this field.

Research resource: interactome of human embryo implantation: identification of gene expression pathways, regulation, and integrated regulatory networks

A prerequisite for successful embryo implantation is adequate preparation of receptive endometrium and the establishment and maintenance of a viable embryo. The success of implantation further relies upon a two-way dialogue between the embryo and uterus. However, molecular bases of these preimplantation and implantation processes in humans are not well known. We performed genome expression analyses of human embryos (n = 128) and human endometria (n = 8). We integrated these data with protein-protein interactions in order to identify molecular networks within the endometrium and the embryo, and potential embryo-endometrium interactions at the time of implantation. For that, we applied a novel network profiling algorithm HyperModules, which combines topological module identification and functional enrichment analysis. We found a major wave of transcriptional down-regulation in preimplantation embryos. In receptive-stage endometrium, several genes and signaling pathways were identified, including JAK-STAT signaling and inflammatory pathways. The main curated embryo-endometrium interaction network highlighted the importance of cell adhesion molecules in the implantation process. We also identified cytokine-cytokine receptor interactions involved in implantation, where osteopontin (SPP1), leukemia inhibitory factor (LIF) and leptin (LEP) pathways were intertwining. Further, we identified a number of novel players in human embryo-endometrium interactions, such as apolipoprotein D (APOD), endothelin 1 (END1), fibroblast growth factor 7 (FGF7), gastrin (GAST), kringle containing trnasmembrane protein 1 (KREMEN1), neuropilin 1 (NRP1), serpin peptidase inhibitor clade A member 3 (SERPINA3), versican (VCAN), and others. Our findings provide a fundamental resource for better understanding of the genetic network that leads to successful embryo implantation. We demonstrate the first systems biology approach into the complex molecular network of the implantation process in humans.

Variation in stability of housekeeping genes in endometrium of healthy and polycystic ovarian syndrome women

BACKGROUND

The use of housekeeping genes (HKG) as internal controls for real-time qPCR studies of gene expression is based on the assumption of their inherent stability. However, it is unclear whether this stability is maintained in disease states. In order to test this, the present study investigated the expression of specific HKG in the endometrium of healthy and polycystic ovarian syndrome (PCOS) women.

METHODS

Endometrial tissue samples were taken from women with PCOS (n= 9) and controls (n= 10). The stability of nine candidate reference genes in the endometrial tissues were evaluated; four encode mitochondrial proteins [ATP5B, succinate dehydrogenase complex subunit A (SDHA), cytochrome c-1, glyceraldehyde-3-phosphatedehydrogenase], two encode ribosomal protein genes (18s ribosomal RNA, ribosomal protein L13A), one for cell structure (SDHA), one for cell signalling (beta actin, ACTB) and one involved in DNA repair (topoisomerase I, TOP1). The expression stability of these HKGs was calculated using geNORM qbasePLUS software, with stability defined by M-values, where higher M-value indicating less stability. In addition, changes in their cycle threshold values were analysed to determine direction of change between groups, and a Mann-Whitney U-test was used to determine statistical differences in expression.

RESULTS

The most stable HKGs observed across both PCOS endometrium were found to be YWHAZ, CYC1 and ACTB. Further TOP1 demonstrated higher gene expression in the endometrium from PCOS women compared with those from healthy women.

CONCLUSIONS

Of the nine HKGs examined, only YWHAZ, CYC1 and ACTB were stable in both control and PCOS endometrium: these should therefore be used as internal controls for quantitative reverse transcription-polymerase chain reaction analysis. Published discrepancies between endometrial gene expression studies may therefore be due in part to in the inappropriate HKG selection, and future gene expression studies should be based on HKG of known stability in both the disease and healthy states to avoid erroneous interpretation of results.

Site-specific endometrial injury improves implantation and pregnancy in patients with repeated implantation failures

BACKGROUND

To test whether a site-specific hysteroscopic biopsy-induced injury in the endometrium during the controlled ovarian hyperstimulation cycle improves subsequent embryo implantation in patients with repeated implantation failure, a total of 30 patients who have had good responses to controlled ovulation stimulation but have failed to achieve pregnancy after two or more transfers of good-quality embryos were recruited in this prospective study.

METHODS

A single, site-specific hysteroscopic biopsy-induced injury was generated on the posterior endometrium at midline 10-15 mm from the fundus during the D4-D7 period of the ongoing controlled ovarian hyperstimulation cycle in six patients.

RESULTS

Patients received endometrial biopsy protocol achieved a pregnancy rate of 100%. By contrast, only 46% of patients with similar clinical characteristics (N = 24) achieved pregnancy without the hysteroscopic biopsy-induced endometrium injury (p < 0.05).

CONCLUSIONS

Our proof-of-concept study demonstrates that a site-specific hysteroscopic endometrium injury performed during the ongoing in vitro fertilization (IVF) cycle, instead of injuries received during prior cycles, significantly improves clinical outcomes in patients with repeated implantation failure.

Deregulation of the serum- and glucocorticoid-inducible kinase SGK1 in the endometrium causes reproductive failure

Infertility and recurrent pregnancy loss (RPL) are prevalent but distinct causes of reproductive failure that often remain unexplained despite extensive investigations. Analysis of midsecretory endometrial samples revealed that SGK1, a kinase involved in epithelial ion transport and cell survival, is upregulated in unexplained infertility, most prominently in the luminal epithelium, but downregulated in the endometrium of women suffering from RPL. To determine the functional importance of these observations, we first expressed a constitutively active SGK1 mutant in the luminal epithelium of the mouse uterus. This prevented expression of certain endometrial receptivity genes, perturbed uterine fluid handling and abolished embryo implantation. By contrast, implantation was unhindered in Sgk1-/- mice, but pregnancy was often complicated by bleeding at the decidual-placental interface and fetal growth retardation and subsequent demise. Compared to wild-type mice, Sgk1-/- mice had gross impairment of pregnancy-dependent induction of genes involved in oxidative stress defenses. Relative SGK1 deficiency was also a hallmark of decidualizing stromal cells from human subjects with RPL and sensitized these cells to oxidative cell death. Thus, depending on the cellular compartment, deregulated SGK1 activity in cycling endometrium interferes with embryo implantation, leading to infertility, or predisposes to pregnancy complications by rendering the feto-maternal interface vulnerable to oxidative damage.

Contemporary genetic technologies and female reproduction

BACKGROUND

The Fifth Evian Annual Reproduction (EVAR) Workshop Meeting discussed knowledge regarding contemporary genetics in female reproduction.

METHODS

Specialist reproductive medicine clinicians and geneticists delivered presentations based on published literature and current research. The content of this report is based on the expert presentations and subsequent group discussions that took place during this Workshop.

RESULTS

Numerous ovarian genes with a role in infertility have been identified. Future challenges for genetic screening of patients, such as those with polycystic ovary syndrome, primary ovarian insufficiency or endometriosis, include the identification of high-throughput strategies and how to apply these findings to infertile patients. The identification of high-quality embryos in IVF using objective technologies remains a high priority in order to facilitate single-embryo transfer. Gene expression profiling of cumulus cells surrounding the oocyte, and proteomic and metabolomic approaches in embryo culture media may significantly improve non-invasive embryo quality assessment.

CONCLUSIONS

The way forward in advancing the knowledge of genes involved in reproduction was considered to be through genome-wide association studies involving large numbers of patients. Establishing international collaboration is required to enable the application of such technologies in sufficient numbers of patients.

Ovarian stimulation modulates steroid receptor expression and spheroid attachment in peri-implantation endometria: studies on natural and stimulated cycles

OBJECTIVE

To compare the effect of high serum E(2) levels on endometrial steroid receptors in gonadotropin-stimulated cycles (hCG + 7) and natural cycles (LH + 7), and to study its effect on spheroid attachment.

DESIGN

Observational.

SETTING

University hospital.

PATIENT(S)

Infertile patient with normal menstrual cycles undergoing IVF treatment.

INTERVENTION(S)

Gonadotropin stimulation and endometrial biopsy; trophoblast spheroid (embryo surrogate, Jeg-3)-endometrial cell (Ishikawa) coculture assay.

MAIN OUTCOME MEASURE(S)

Steroid receptor expression by quantitative polymerase chain reaction and immunohistochemistry; spheroid attachment rate.

RESULT(S)

Endometrial biopsies from natural (n = 12) and stimulated (n = 23) cycles were obtained. The expression of estrogen receptor α (ERα) but not ERβ or progesterone receptor (PR) transcript was significantly reduced in stimulated cycles compared with natural cycles. Glucocorticoid receptor (GR) transcript was significantly increased in the excessive responders of the stimulated cycle. There was no difference in ERα immunoreactivity in endometrial stroma, but a higher immunoreactivity was seen in endometrial glands of stimulated cycles. The endometrium of stimulated cycles had a lower expression of PR protein in glands, but a higher expression in stroma. Although no GR protein was detected in glands, GR protein expression was significantly up-regulated in stroma of the stimulated cycles. Endometrial cells treated with high steroid concentrations had a reduced spheroid attachment rate compared with the controls.

CONCLUSION(S)

High serum E(2) level affects the expression of steroid receptors in the endometrial cells and suppresses spheroid attachment.

Endometrial gene expression in the window of implantation is altered in obese women especially in association with polycystic ovary syndrome

OBJECTIVE

To determine whether luteal phase endometrial transcriptome is altered in obese women during the window of implantation (WOI), considering the presence of infertility, fat distribution and association with polycystic ovary syndrome (PCOS).

DESIGN

Prospective study.

SETTING

University-affiliated infertility clinic, between May 2007 and March 2009.

PATIENT(S)

One control group of women with normal weight (n=4), and four study groups of obese women (n=6 each one) according to the association with infertility, PCOS, and ovarian stimulation.

INTERVENTION(S)

The endometrium was biopsied 7 days after LH surge or hCG administration in 28 women.

MAIN OUTCOME MEASURE(S)

Endometrial gene expression during the WOI.

RESULT(S)

One hundred and fifty-one genes were dysregulated in obese groups compared with controls. This dysregulation was more pronounced when infertility was associated. The biologic processes of these genes belonged mainly to development and regulation of different biological functions such as transcription and biosynthesis. The molecular functions overrepresented were transcription and peptide receptor activity. The endometrium of obese women with PCOS showed dysregulated genes related to biologic processes such as development, morphogenesis, and the immune system, as well as different molecular functions such as protein binding, binding, growth factor activity, and carboxylic acid transmembrane transporter activity. Some of these genes have been previously related to implantation and unexplained infertility.

CONCLUSION(S)

Obese women present a different endometrial gene expression than controls during the WOI, which is more pronounced when infertility or polycystic ovary syndrome are associated.

Relationship between adenomyosis and uterine polyps

OBJECTIVE

To evaluate whether adenomyosis is associated with uterine polyps.

STUDY DESIGN

To perform an ethical human study, we planned a large retrospective observational study covering the entire patient population of the ULSS 17-Veneto district who underwent resectoscopy in 2007 and 2008 (959 patients). We enrolled all the patients underwent resectoscopy with resection of uterine polyps, with resection of myomas, with endometrial ablations by slicing, and with endometrial biopsies in both the presence and absence of hysteroscopically visible lesions. The diagnosis of adenomyosis was made by pathological examination of tissue specimens obtained using a resectoscope loop. Multivariate logistic regression was performed to determine if adenomyosis was one of the various risk factors for the presence and number of uterine polyps. Statistical Package for the Social Sciences (SPSS 16.0) was employed, and p≤0.05 was the minimum for significance.

RESULTS

Age (p=0.005), adenomyosis (p=0.013), high fasting glucose levels (p=0.004), and hypertension (p=0.045) were significantly associated with endometrial polyps. The presence of multiple endometrial polyps seemed to be associated with the presence of adenomyosis (p=0.016). The presence of cervical polyps was significantly associated only with presence of adenomyosis (p=0.002). The presence of multiple cervical polyps did not seem to be influenced by any of the variables considered.

CONCLUSIONS

The results clearly demonstrate an association between adenomyosis and uterine polyps. The pathogenetic role of adenomyosis in the development of polyps should therefore be investigated further.

Molecular evidence for differences in endometrium in severe versus mild endometriosis

Women with stage III/IV versus stage I/II endometriosis have lower implantation and pregnancy rates in natural and assisted reproduction cycles. To elucidate potential molecular mechanisms underlying these clinical observations, herein we investigated the transcriptome of eutopic endometrium across the menstrual cycle in the setting of severe versus mild endometriosis. Proliferative (PE), early secretory (ESE), and mid-secretory (MSE) endometrial tissues were obtained from 63 participants with endometriosis (19 mild and 44 severe). Purified RNA was subjected to microarray analysis using the Gene 1.0 ST Affymetrix platform. Data were analyzed with GeneSpring and Ingenuity Pathway Analysis and subsequently validated. Comparison of differentially regulated genes, analyzed by cycle phase, revealed dysregulation of progesterone and/or cyclic adenosine monophosphate (cAMP)-regulated genes and genes related to thyroid hormone action and metabolism. Also, members of the epidermal growth factor receptor (EGFR) signaling pathway were observed, with the greatest upregulation of EGFR in severe versus mild disease during the early secretory phase. The extracellular matrix proteoglycan versican (VCAN), which regulates cell proliferation and apoptosis, was the most highly expressed gene in severe versus mild disease. Upregulation of microRNA 21 (MIR21) and DICER1 transcripts suggests roles for microRNAs (miRNAs) in the pathogenesis of severe versus mild endometriosis, potentially through regulation of gene silencing and epigenetic mechanisms. These observed differences in transcriptomic signatures and signaling pathways may result in poorly programmed endometrium during the cycle, contributing to lower implantation and pregnancy rates in women with severe versus mild endometriosis.

Glycodelin in endometrial flushing fluid and endometrial biopsies from infertile and fertile women

OBJECTIVE

To investigate in the natural cycle just before IVF, whether glycodelin levels in endometrial flushing fluid obtained days LH+1 and LH+7 can be used in predicting pregnancy in the following IVF cycle, and whether there are differences in women with tubal factor infertility compared to women with unexplained infertility and fertile controls.

STUDY DESIGN

A prospective observational multicentre study of 21 fertile and 75 infertile women (25 showed abnormal tubes with no signs of hydrosalpinges, 18 had uni- or bi-lateral hydrosalpinges, 17 were salpingectomised because of hydrosalpinges, and 15 women had unexplained infertility). Endometrial flushing at days LH+1 and LH+7, endometrial biopsy, and blood sampling at day LH+7 were performed before down-regulation for IVF. Glycodelin levels in endometrial flushing fluids (EFF), biopsies, and plasma samples were related to tubal pathology, endometrial dating and IVF outcome. Furthermore, total protein concentration was measured in EFF to investigate the influence of normal endometrial maturation on protein concentrations from days LH+1 and LH+7.

RESULTS

At day LH+1, EFF glycodelin levels were higher in infertile women with abnormal tubes compared to fertile women, particularly in women conceiving after the following IVF. For women with unexplained infertility, a higher level at day LH+1 was present only in women not conceiving after the following IVF. ROC curve analysis showed that at day LH+1 EFF glycodelin levels had no predictive value for IVF outcome. At day LH+7, glycodelin levels in endometrial flushing fluids and biopsies depended on endometrial dating.

CONCLUSIONS

At day LH+1, glycodelin concentration is increased in endometrial flushing fluid from infertile women with abnormal tubes compared to fertile controls without being a valuable predictor of subsequent pregnancy. At day LH+7 the glycodelin level depends on endometrial dating.

Bioinformatic detection of E47, E2F1 and SREBP1 transcription factors as potential regulators of genes associated to acquisition of endometrial receptivity

BACKGROUND

The endometrium is a dynamic tissue whose changes are driven by the ovarian steroidal hormones. Its main function is to provide an adequate substrate for embryo implantation. Using microarray technology, several reports have provided the gene expression patterns of human endometrial tissue during the window of implantation. However it is required that biological connections be made across these genomic datasets to take full advantage of them. The objective of this work was to perform a research synthesis of available gene expression profiles related to acquisition of endometrial receptivity for embryo implantation, in order to gain insights into its molecular basis and regulation.

METHODS

Gene expression datasets were intersected to determine a consensus endometrial receptivity transcript list (CERTL). For this cluster of genes we determined their functional annotations using available web-based databases. In addition, promoter sequences were analyzed to identify putative transcription factor binding sites using bioinformatics tools and determined over-represented features.

RESULTS

We found 40 up- and 21 down-regulated transcripts in the CERTL. Those more consistently increased were C4BPA, SPP1, APOD, CD55, CFD, CLDN4, DKK1, ID4, IL15 and MAP3K5 whereas the more consistently decreased were OLFM1, CCNB1, CRABP2, EDN3, FGFR1, MSX1 and MSX2. Functional annotation of CERTL showed it was enriched with transcripts related to the immune response, complement activation and cell cycle regulation. Promoter sequence analysis of genes revealed that DNA binding sites for E47, E2F1 and SREBP1 transcription factors were the most consistently over-represented and in both up- and down-regulated genes during the window of implantation.

CONCLUSIONS

Our research synthesis allowed organizing and mining high throughput data to explore endometrial receptivity and focus future research efforts on specific genes and pathways. The discovery of possible new transcription factors orchestrating the CERTL opens new alternatives for understanding gene expression regulation in uterine function.

Predictive value of endometrial thickness, pattern and sub-endometrial blood flows on the day of hCG by 2D doppler in in-vitro fertilization cycles: A prospective clinical study from a tertiary care unit

AIMS AND OBJECTIVES

To evaluate the role of endometrial thickness, pattern and sub-endometrial blood flows measured by 2D power Doppler ultrasound to predict pregnancy during in-vitro fertilization (IVF) treatment.

STUDY DESIGN

Prospective, non-randomized clinical study.

MATERIALS AND METHODS

This was a prospective observational study. A total of 101 infertile women were recruited from our IVF-ET program from January to December, 2009. Women with tubal factor, male factor and unexplained infertility were included in the study.

RESULTS

The mean age was 35 years and mean duration of infertility was 8 years. Seventy five (74.25%) patients had primary infertility and 26 (25.74%) had secondary infertility. The mean endometrial thickness was 8.1 mm and endometrial blood flow was in Zone I in 18 patients, 28 patients had blood flow in Zone II and 54 had in Zone III. Overall, 27 (26.73%) patients conceived and in these women the endometrial thickness was between 6 and 12 mm.

CONCLUSIONS

With a thin endometrium (≤7 mm) and no-triple-line endometrial pattern coexisting in an in-vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) candidate, cryopreservation should be recommended. With a thin endometrium and a good texture (triple-line), other prognostic factors, such as embryo quality, should be taken into account. The endometrial vascularity has a useful predictive value on the implantation rate in IVF cycles irrespective of the morphological appearance of the endometrium. However, further study is needed to make a definitive conclusion.

Variation in hyaluronan-binding protein 2 (HABP2) promoter region is associated with unexplained female infertility

We set up to analyze polymorphisms in hyaluronan-binding protein 2 (HABP2) gene in healthy fertile women (n = 158) and in women with unexplained infertility (n = 116) and to investigate the potential role of HABP2 in receptive endometrium. Minor rs1157916 A and the major rs2240879 A alleles together with AA genotypes were significantly less frequent in infertile women than in controls. Immunohistochemistry analysis of endometrial HABP2 expression at the time of implantation identified significantly lower HABP2 protein level in infertile women in stroma and vessels than in fertile women. Migration assay analysis of cultured trophoblast and endothelial cells toward HABP2 protein referred to the function of HABP2 in endometrial endothelial cells. In conclusion, our results indicate that polymorphisms in the regulatory region of HABP2 gene could influence gene expression levels in the receptive endometrium and could thereby be one reason for infertility complications in women with unexplained infertility. Additionally, HABP2 protein involvement in endometrial angiogenesis is proposed.

Human endometrial CD98 is essential for blastocyst adhesion

Background

Understanding the molecular basis of embryonic implantation is of great clinical and biological relevance. Little is currently known about the adhesion receptors that determine endometrial receptivity for embryonic implantation in humans.

Methods and Principal Findings

Using two human endometrial cell lines characterized by low and high receptivity, we identified the membrane receptor CD98 as a novel molecule selectively and significantly associated with the receptive phenotype. In human endometrial samples, CD98 was the only molecule studied whose expression was restricted to the implantation window in human endometrial tissue. CD98 expression was restricted to the apical surface and included in tetraspanin-enriched microdomains of primary endometrial epithelial cells, as demonstrated by the biochemical association between CD98 and tetraspanin CD9. CD98 expression was induced in vitro by treatment of primary endometrial epithelial cells with human chorionic gonadotropin, 17-β-estradiol, LIF or EGF. Endometrial overexpression of CD98 or tetraspanin CD9 greatly enhanced mouse blastocyst adhesion, while their siRNA-mediated depletion reduced the blastocyst adhesion rate.

Conclusions

These results indicate that CD98, a component of tetraspanin-enriched microdomains, appears to be an important determinant of human endometrial receptivity during the implantation window.

The molecular basis of recurrent pregnancy loss: impaired natural embryo selection

Recurrent pregnancy loss (RPL) is a common and distressing disorder. Chromosomal errors in the embryo are the single most common cause, whereas uterine factors are invariably invoked to explain non-chromosomal miscarriages. These uterine factors are, however, poorly defined. The ability of a conceptus to implant in the endometrium is normally restricted to a few days in the menstrual cycle. A limited 'window of implantation' ensures coordinated embryonic and endometrial development, thereby minimizing the risk of late implantation of compromised embryos. In this paper, we review emerging evidence, indicating that RPL is associated with impaired differentiation of endometrial stromal cells into specialized decidual cells. From a functional perspective, this differentiation process, termed decidualization, is not only critical for placental development but also signals the end of the implantation window and bestows on the endometrium the ability to recognize, respond to and eliminate implanting compromised embryos. Thus, we propose that spontaneous decidualization of the human endometrium, which inevitably causes menstrual shedding in the absence of a viable conceptus, serves as functional 'window for natural embryo selection'. Conversely, impaired decidualization predisposes to late implantation, negates embryo quality control and causes early placental failure, regardless of the embryonic karyotype. This pathological pathway also explains the common observation that many RPL patients seem exceptionally fertile, often conceiving within one or two cycles. Thus, as the clinical correlate of inappropriate uterine receptivity, 'superfertility' should be considered as a genuine reproductive disorder that requires targeted intervention.

Proteomics and the search for biomarkers of female reproductive diseases

Over the past decade, high-throughput proteomics technologies have evolved considerably and have become increasingly more commonly applied to the investigation of female reproductive diseases. Proteomic approaches facilitate the identification of new disease biomarkers by comparing the abundance of hundreds of proteins simultaneously to find those specific to a particular clinical condition. Some of the best studied areas of female reproductive biology applying proteomics include gynaecological cancers, endometriosis and endometrial infertility. This review will discuss the progress that has been made in these areas and will highlight some of the emerging technologies that promise to contribute to better understanding of the female reproductive disease.

Expression of E- and N-cadherin and CD44 in endometrium and hydrosalpinges from infertile women

In this prospective comparative study, compared with fertile control subjects (n = 12), infertile patients with hydrosalpinx (n = 18) had lower E-cadherin and a trend toward decreased N-cadherin H-scores in the endometrium (3.6 ± 0.6 vs. 2.4 ± 0.8 and 0.57 ± 1.0 vs. 0.52 ± 0.5, respectively). In hydrosalpinx, epithelial N-cadherin expression was discontinuous and disappeared in atrophic patches.

Natural selection of human embryos: decidualizing endometrial stromal cells serve as sensors of embryo quality upon implantation

Background

Pregnancy is widely viewed as dependent upon an intimate dialogue, mediated by locally secreted factors between a developmentally competent embryo and a receptive endometrium. Reproductive success in humans is however limited, largely because of the high prevalence of chromosomally abnormal preimplantation embryos. Moreover, the transient period of endometrial receptivity in humans uniquely coincides with differentiation of endometrial stromal cells (ESCs) into highly specialized decidual cells, which in the absence of pregnancy invariably triggers menstruation. The role of cyclic decidualization of the endometrium in the implantation process and the nature of the decidual cytokines and growth factors that mediate the crosstalk with the embryo are unknown.

Methodology/Principal Findings

We employed a human co-culture model, consisting of decidualizing ESCs and single hatched blastocysts, to identify the soluble factors involved in implantation. Over the 3-day co-culture period, approximately 75% of embryos arrested whereas the remainder showed normal development. The levels of 14 implantation factors secreted by the stromal cells were determined by multiplex immunoassay. Surprisingly, the presence of a developing embryo had no significant effect on decidual secretions, apart from a modest reduction in IL-5 levels. In contrast, arresting embryos triggered a strong response, characterized by selective inhibition of IL-1β, -6, -10, -17, -18, eotaxin, and HB-EGF secretion. Co-cultures were repeated with undifferentiated ESCs but none of the secreted cytokines were affected by the presence of a developing or arresting embryo.

Conclusions

Human ESCs become biosensors of embryo quality upon differentiation into decidual cells. In view of the high incidence of gross chromosomal errors in human preimplantation embryos, cyclic decidualization followed by menstrual shedding may represent a mechanism of natural embryo selection that limits maternal investment in developmentally impaired pregnancies.

Local injury of the endometrium induces an inflammatory response that promotes successful implantation

OBJECTIVE

To study whether an injury-induced inflammation might be the mechanism underlying the favorable effect of endometrial biopsy on the implantation rate in in vitro fertilization (IVF) patients.

DESIGN

Controlled clinical study.

SETTING

A medical center IVF unit and a research institute.

PATIENT(S)

Women undergoing IVF who had previous failed treatment cycles.

INTERVENTION(S)

Endometrial samples were collected from two groups of patients on day 21 of their spontaneous menstrual cycle. The experimental, but not the control group underwent prior biopsy treatment on days 8 or/and 11 to 13 of that same cycle.

MAIN OUTCOME MEASURE(S)

Abundance of immune cells, cytokines/chemokines level, correlation between these parameters and pregnancy outcome.

RESULT(S)

A statistically significantly higher amount of macrophages/dendritic cells (HLA-DR+ CD11c+ cells) and elevated proinflammatory cytokines, tumor necrosis factor-α (TNF-α), growth-regulated oncogene-α (GRO-α), interleukin-15 (IL-15), and macrophage inflammatory protein 1B (MIP-1B), were detected in day-21 endometrial samples of the experimental group. A direct stimulatory effect of TNF-α on MIP-1B, GRO-α, and IL-15 messenger RNA (mRNA) expression was demonstrated. A positive correlation was found between the levels of macrophages/dendritic cells, MIP-1B expression, and TNF-α expression and the pregnancy outcome.

CONCLUSION(S)

A biopsy-induced inflammatory response may facilitate the preparation of the endometrium for implantation. Increased MIP-1B expression could possibly serve for prediction of implantation competence.

Cytokine array analysis of peritoneal fluid between women with endometriosis of different stages and those without endometriosis

Cytokines are key mediators of intercellular communication and are likely to promote the development and progression of endometriosis. Previous studies provided evidence that endometriosis develops as a result of the pathogenetic factors in the peritoneal environment, especially the peritoneal fluid (PF). We determined different cytokine expression in peritoneal fluid between women with minimal/mild and moderate/severe endometriosis and those without endometriosis using the cytokine array. As a result, 78 cytokines were found to have a threefold change, including 74 increases and four decreases in endometriosis compared with the control group; 96 cytokines had a threefold change including 91 increases and five decreases in minimal and mild endometriosis compared with the control group; 83 cytokines had a threefold change including 14 increases and 69 decreases in moderate and severe endometriosis compared with minimal and mild endometriosis. The cytokine networks were produced by Pathway Studio software and revealed that most cytokines are involved in cell binding, interaction and protein synthesis and transportation regulation. Among them activin A, Smad7 and beta-nerve growth factor are the most interesting as they may be involved in the pathogenesis of endometriosis. These results suggest that cytokines are very important factors in the development of endometriosis. The findings of differentially expressed cytokines improves our knowledge of the pathogenesis and development of endometriosis and these findings warrant further studies to develop potential targets for the diagnosis and treatment of endometriosis.

Proteomics of embryonic implantation

Implantation is a complex process involving an intricate cascade of molecular interactions between the implanting blastocyst and the receptive endometrium. The molecular basis of endometrial receptivity and the mechanisms by which the blastocyst first adheres to the luminal epithelium and then penetrates into the stroma are only just beginning to be resolved. Advances in "omics" technologies, particularly proteomics and metabolomics, are set to have a major impact on the development of this field. In the wake of this information, novel targets for contraceptive intervention may become apparent.

Plexin-B1, glycodelin and MMP7 expression in the human fallopian tube and in the endometrium

BACKGROUND

To study the expression of Plexin-B1, Glycodelin, and MMP7 during the menstrual cycle in the endometrium and in the fallopian tube.

METHODS

The research included women undergoing hysterectomy, tubal sterilization or salpingo-oophoerectomy. Total RNA from endometrial and fallopian tube tissues was extracted using a total RNA isolation kit. Semi-quantitative RT-PCR was performed to examine mRNA relative expression.

RESULTS

Plexin-B1 expression in the endometrium was significantly higher on days 19 - 23 compared to days 12 - 14 (1.166 +/- 0.42 versus 0.523 +/- 0.299), P < 0.005. In the fallopian tube the level of plexin-B1 did not change significantly throughout the menstrual cycle. Glycodelin expression was significantly higher on days 19 - 23 compared with days 12-14, both in the endometrium (0.819 +/- 0.564 versus 0.072 +/- 0.343, P < 0.05) and the fallopian tube (0.796 +/- 0.196 versus 0.329 +/- 0.398, P < 0.05). Although the level of MMP7 secretion was the highest in the secretory phase the difference from the proliferative phase did not reach statistical significance, neither in the endometrium nor in the fallopian tube. This could result from a lack of power.

CONCLUSIONS

In the endometrium, both Glycodelin and Plexin-B1 are exhibiting a cyclic pattern suggesting a possible steroid regulation and a role in endometrial receptivity.

Proteomic analysis of endometrium from fertile and infertile patients suggests a role for apolipoprotein A-I in embryo implantation failure and endometriosis

Pregnancy is dependent upon the endometrium acquiring a receptive phenotype that facilitates apposition, adhesion and invasion of a developmentally competent embryo. Surface-enhanced laser desorption/ionization time-of-flight mass spectrometry of mid-secretory endometrial biopsies revealed a 28 kDa protein peak that discriminated highly between samples obtained from women with recurrent implantation failure and fertile controls. Subsequent tandem mass spectroscopy unambiguously identified this peak as apolipoprotein A-I (apoA-I), a potent anti-inflammatory molecule. Total endometrial apoA-I levels were, however, comparable between the study and control group. Moreover, endometrial apoA-I mRNA expression was not cycle-dependent although there was partial loss of apoA-I immunoreactivity in luminal and glandular epithelium in mid-secretory compared with proliferative endometrial samples. Because of its putative anti-implantation properties, we examined whether endometrial apoA-I expression is regulated by embryonic signals. Human chorionic gonadotrophin (hCG) strongly inhibited apoA-I expression in differentiating explant cultures but not when established from eutopic endometrium from patients with endometriosis. Pelvic endometriosis was associated with elevated apoA-I mRNA levels, increased secretion by differentiating eutopic endometrial explant cultures and lack of hCG-dependent down-regulation. To corroborate these observations, we examined endometrial apoA-I expression and its regulation by hCG in a non-human primate model of endometriosis. As in humans, hCG strongly inhibited endometrial apoA-I mRNA expression in disease-free baboons, but this response was entirely lost upon induction of pelvic endometriosis. Together, these observations indicate that perturbations in endometrial apoA-I expression, modification or regulation by paracrine embryonic signals play a major role in implantation failure and infertility.

Endometrial gene expression analysis at the time of embryo implantation in women with unexplained infertility

Successful embryo implantation depends on the quality of the embryo, as well as on the receptivity of the endometrium. The aim of this study was to investigate the endometrial gene expression profile in women with unexplained infertility in comparison with fertile controls at the time of embryo implantation in order to find potential predictive markers of uterine receptivity and to identify the molecular mechanisms of infertility. High-density oligonucleotide gene arrays, comprising 44 000 gene targets, were used to define the endometrial gene expression profile in infertile (n = 4) and fertile (n = 5) women during the mid-secretory phase (day LH + 7). Microarray results were validated using real-time PCR. Analyses of expression data were carried out using non-parametric methods. Hierarchical clustering and principal component analysis showed a clear distinction in endometrial gene expression between infertile and fertile women. In total we identified 145 significantly (>3-fold change) up-regulated and 115 down-regulated genes in infertile women versus controls. Via Database for Annotation, Visualization and Integrated Discovery functional analysis we detected a substantial number of dysregulated genes in the endometria of infertile women, involved in cellular localization (21.1%) and transport (18.8%) and transporter activity (13.1%) and with major localization in extracellular regions (19.2%). Ingenuity Pathways Analysis of the gene list showed dysregulation of gene pathways involved in leukocyte extravasation signalling, lipid metabolism and detoxification in the endometria of infertile women. In conclusion, endometrial gene expression in women with unexplained infertility at the time of embryo implantation is markedly different from that in fertile women. These results provide new information on genes and pathways that may have functional significance as regards to endometrial receptivity and subsequent embryo implantation.

Progesterone and interferon tau-regulated genes in the ovine uterine endometrium: identification of periostin as a potential mediator of conceptus elongation

During early pregnancy in ruminants, progesterone (P4) and interferon tau (IFNT) act on the endometrium to regulate genes hypothesized to be important for conceptus development and implantation. The present study was conducted to verify several candidate genes (actin α-2, smooth muscle, aorta (ACTA2), collagen, type III, α-1 (COL3A1), periostin (POSTN), secreted protein acidic cysteine-rich (SPARC), S100 calcium-binding protein A2 (S100A2),STAT5Aand transgelin (TAGLN)) regulated by pregnancy, P4, and/or IFNT in the endometrium determined using a custom ovine cDNA array.S100A2mRNA was detected primarily in endometrial epithelia and conceptuses.S100A2mRNA increased in endometrial epithelia from days 10 to 16 in cyclic ewes and from days 10 to 14 in pregnant ewes and declined thereafter. The abundance ofS100A2mRNA was less in endometrial luminal epithelium of IFNT-infused ewes receiving P4. Expression ofCOL3A1, SPARC, ACTA2, andTAGLNwas independent of pregnancy, P4, or IFNT.POSTNmRNA was detected primarily in compact stroma of intercaruncular and caruncular endometria, but not in the conceptus. EndometrialPOSTNmRNA increased between days 12 and 14 in pregnant but not cyclic ewes, andPOSTNmRNA was more abundant in uterine stroma of ewes treated with P4. POSTN protein was detected in uterine flushings of pregnant ewes and found to mediate attachment and stimulate migration of ovine trophectoderm cellsin vitro. These results support the ideas that POSTN and S100A2 are regulated by P4and IFNT respectively, and that POSTN is involved in conceptus elongation during early pregnancy.

Novel pathways for implantation and establishment and maintenance of pregnancy in mammals

Uterine receptivity to implantation varies among species, and involves changes in expression of genes that are coordinate with attachment of trophectoderm to uterine lumenal and superficial glandular epithelia, modification of phenotype of uterine stromal cells, silencing of receptors for progesterone and estrogen, suppression of genes for immune recognition, alterations in membrane permeability to enhance conceptus-maternal exchange of factors, angiogenesis and vasculogenesis, increased vascularity of the endometrium, activation of genes for transport of nutrients into the uterine lumen, and enhanced signaling for pregnancy recognition. Differential expression of genes by uterine epithelial and stromal cells in response to progesterone, glucocorticoids, prostaglandins and interferons may influence uterine receptivity to implantation in mammals. Uterine receptivity to implantation is progesterone-dependent; however, implantation is preceded by loss of expression of receptors for progesterone (PGR) so that progesterone most likely acts via PGR-positive stromal cells throughout pregnancy. Endogenous retroviruses expressed by the uterus and/or blastocyst also affect implantation and placentation in various species. Understanding the roles of the variety of hormones, growth factors and endogenous retroviral proteins in uterine receptivity for implantation is essential to enhancing reproductive health and fertility in humans and domestic animals.

HOXA10 signals on the highway through pregnancy

Implantation represents the remarkable synchronization between the development of the embryo and the differentiation of the endometrium. It depends on uterine-dependent and embryo-specific events, which are critically and sequentially coordinated. A plethora of molecules have been identified which play major roles before and after embryo implantation. In recent years HomeoboxA10 (HOXA10) has emerged as one of the most promising candidates which regulate the events occurring in the maternal compartment for successful establishment of pregnancy. HOXA10 is a transcription factor that is crucial for development and patterning of the uterus during embryogenesis. In the adult endometrium, HOXA10 is expressed in a menstrual cycle dependent manner and it is regulated by ovarian steroid hormones and embryonic signals, HOXA10 is required for uterine receptivity and implantation, and is a key regulator of decidualization. In the decidua, HOXA10 is involved in regulation of cell cycle and local immunomodulation. The present review summarizes the events that are regulated by HOXA10 in embryo implantation and decidualization.

Proteomic characterization of midproliferative and midsecretory human endometrium

This study aimed to identify proteins differentially expressed in the human endometrium between the proliferative and secretory phases of normal menstrual cycles by 2D differential in-gel electrophoresis (DIGE). A total of 196 out of 1017 spots were differentially expressed (p < 0.05). Mass spectrometry identified 76 proteins representing 41 different gene products. Immunohistochemistry confirmed the observed changes in 3 representative proteins (Rho-GDIalpha, CLIC1, PGRMC1). Biological pathway analysis identified the Jnk and EGF signaling pathways as key regulators of protein expression in the midsecretory phase of endometrial proteome.

Calcium-binding protein S100P is highly expressed during the implantation window in human endometrium

OBJECTIVE

To investigate S100P expression and localization in human endometrium throughout the menstrual cycle.

DESIGN

Experimental study.

SETTING

University hospital.

PATIENT(S)

Eighty-four women.

INTERVENTION(S)

Complementary DNA (cDNA) microarray analysis was performed on human endometrium from days LH+4, LH+7, and hCG+7. Reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis were used to detect the expression of S100P and of additional S100 family members, S100A4, S100A13, and S100A6. Immunofluorescence was used to detect the localization of S100P protein in LH+7 and LH+4 endometrium.

MAIN OUTCOME MEASURE(S)

Differential gene expression, levels of S100P messenger RNA (mRNA), and protein expression and immunofluorescent localization of S100P.

RESULT(S)

A statistical method, based on hierarchical clustering, identified genes whose expression varied at LH+7 compared with LH+4. We found that S100P was the fourth most up-regulated gene at LH+7. The S100P mRNA and protein levels were quite low during the proliferative phase and LH+4, but were elevated significantly at LH+7. The S100P expression at hCG+7 was lower than that at LH+7. However, the expression of S100A4, S100A13, and S100A6 did not vary throughout the menstrual cycle.

CONCLUSION(S)

S100P was specifically up-regulated during the implantation window. The underlying biological effects of S100P need further exploration.

Gene expression profile of human endometrial receptivity: comparison between natural and stimulated cycles for the same patients

BACKGROUND

The adjunction of exogenous hormones for controlled ovarian stimulation (COS) may alter endometrial receptiveness. In order to identify the genes misregulated under COS, we compared the endometrium gene expression profiles, from the same patients, in a natural cycle and in a subsequent COS cycle.

METHODS

For the same normal-responder patients (n = 21), endometrial biopsies (n = 84) were collected during the pre-receptive (LH + 2) and receptive stages (LH + 7) of a natural cycle and, subsequently, on oocyte retrieval day (hCG + 2) and on transfer day (hCG + 5) of a stimulated cycle. Samples were analyzed using DNA microarrays. Gene expression profiles and biological pathways involved in endometrial receptivity were analyzed.

RESULTS

Although endometrium transition profiles from pre-receptive to receptive phases are similar between patients, COS regimens alter endometrial receptivity in comparison with natural cycle. Under COS conditions, two endometrial profiles were identified and were associated either with a moderately altered receptivity profile for the majority of the patients or a strongly altered profile for a sub-category of patients. The receptive endometrium transcription profile under COS was defective for biological functions such as TGFbeta signaling, leukocyte transendothelial migration and the cell cycle.

CONCLUSIONS

Gonadotrophin treatments in COS cycles led to disruptions of the transcriptional activation of genes involved in normal endometrial receptivity. We propose that when the receptiveness of the endometrium is seriously compromised by the COS protocol, fresh embryo replacement should be cancelled, the embryo frozen and thawed embryo replacement should be performed under natural cycles.

MicroRNA signature and regulatory functions in the endometrium during normal and disease states

During the menstrual cycle, human endometrium undergoes extensive cyclic morphologic and biochemical modifications in preparation for embryo implantation. These processes are highly regulated by ovarian steroids and various locally expressed gene products and involve inflammatory reaction, apoptosis, cell proliferation, angiogenesis, differentiation (tissue formation), and tissue remodeling. MicroRNAs (miRNAs) have emerged as key regulators of gene expression, and their altered and/or aberrant expression has been associated with establishment and progression of various disorders, including tumorigenesis. This review highlights the endometrial expression of miRNAs and their potential regulatory functions under normal and pathologic conditions such as endometriosis, dysfunctional uterine bleeding, and endometrial cancer. Given the key regulatory function of miRNAs on gene expression stability, understanding the underlying mechanisms of how endometrial miRNAs are regulated and identifying their specific target genes and their functions might lead to the development of preventive and therapeutic strategies by regulating specific target genes associated with such reproductive disorders.

Non-genomic progesterone actions in female reproduction

BACKGROUND

The steroid hormone progesterone is indispensable for mammalian procreation by controlling key female reproductive events that range from ovulation to implantation, maintenance of pregnancy and breast development. In addition to activating the progesterone receptors (PRs)-B and -A, members of the superfamily of ligand-dependent transcription factors, progesterone also elicits a variety of rapid signalling events independently of transcriptional or genomic regulation. This review covers our current knowledge on the mechanisms and relevance of non-genomic progesterone signalling in female reproduction.

METHODS

PubMed was searched up to August 2008 for papers on progesterone actions in ovary/breast/endometrium/myometrium/brain, focusing primarily on non-genomic signalling mechanisms.

RESULTS

Convergence and intertwining of rapid non-genomic events and the slower transcriptional actions critically determine the functional response to progesterone in the female reproductive system in a cell-type- and environment-specific manner. Several putative progesterone-binding moieties have been implicated in rapid signalling events, including the 'classical' PR and its variants, progesterone receptor membrane component 1, and the novel family of membrane progestin receptors. Progesterone and its metabolites have also been implicated in the allosteric regulation of several unrelated receptors, such as gamma-aminobutyric acid type A, oxytocin and sigma(1) receptors.

CONCLUSIONS

Identification of the mechanisms and receptors that relay rapid progesterone signalling is an area of research fraught with difficulties and controversy. More in-depth characterization of the putative receptors is required before the non-genomic progesterone pathway in normal and pathological reproductive function can be targeted for pharmacological intervention.

Endometrial receptivity and implantation are not affected by the presence of uterine intramural leiomyomas: a clinical and functional genomics analysis

CONTEXT

Uterine leiomyomas are the most frequent benign tumors during reproductive age. Whether intramural leiomyomas cause infertility and should be removed is controversial because no study has addressed the underlying mechanism of infertility.

OBJECTIVE

The objective of the study was to test the effect of intramural leiomyomas on endometrial function by comparing gene during the window of implantation and implantation in an oocyte donation program, in which the quality of the embryos replaced is similar and the endocrine environment of the endometrium is standardized by exogenous steroids.

DESIGN

Human endometria of women with single intramural leiomyomas (group A, <5 cm and group B, > or =5 cm) and controls (group C) were collected on day LH+7 and processed for histology and gene expression analysis, using different methods and validated by quantitative RT-PCR. To compare in vitro fertilization outcome, a total of 1035 cases from our oocyte donation database were included, comprising patients with one fibroid less than 5 cm (A1, n = 532); two leiomyomas less than 5 cm (A2, n = 128); three or more leiomyomas less than 5 cm (A3, n = 125); one fibroid 5 cm or greater (B, n = 22); and two control groups: C1 (n = 93), women with previous myomectomy; and C2 (n = 135), women without uterine pathology treated on the same dates as C1.

RESULTS

There was a strong positive and negative correlation in the expression profile of 69 genes according to the leiomyomas's size, but only three of the 25 genes related to the window of implantation were dysregulated. Term pregnancy rates after oocyte donation were 36.9, 34.1, 39.0, 36.4, 39.2, and 42.6% (P = 0.769) among the established groups. Similarly, no correlation between implantation and miscarriage with leiomyoma number and size was found.

CONCLUSIONS

This study provides evidence that intramural leiomyomas not affecting the endometrial cavity alters the expression pattern of some endometrial genes, but the genes involved in implantation are not affected. This is confirmed by leiomyomas having no effect on oocyte donation outcome when the size and number of leiomyomas are analyzed.

Dynamic changes in messenger RNA profiles of bovine endometrium during the oestrous cycle

During the oestrous cycle, the bovine endometrium exhibits characteristic morphological and functional changes, which are mainly induced by progesterone (P(4)), oestrogens and oxytocin. We studied the response of the endometrium to this changing hormonal environment at the transcriptome level using a custom-made cDNA microarray. Endometrium samples were recovered from Simmental heifers on days 0 (oestrus), 3.5 (metoestrus), 12 (dioestrus) and 18. The latter group was divided into animals with high (late dioestrus) and low P(4) levels (preoestrus). Significance analysis of microarrays revealed 269 genes exhibiting significant changes in their transcript levels during the oestrous cycle in distinct temporal patterns. Two major types of expression profiles were observed, which showed the highest mRNA levels during the oestrus phase or the highest levels during the luteal phase respectively. A minor group of genes exhibited the highest mRNA levels on day 3.5. Gene ontology (GO) analyses revealed GO categories related to extracellular matrix remodelling, transport, and cell growth and morphogenesis enriched at oestrus, whereas immune response and particular metabolic pathways were overrepresented at dioestrus. Generation of gene interaction networks uncovered the genes possibly involved in endometrial remodelling (e.g. collagen genes, TNC, SPARC, MMP2, MEP1B, TIMP1, TIMP2, HTRA1), regulation of angiogenesis (e.g. ANGPTL2, TEK, NPY, AGT, EPAS1, KLF5 ), regulation of invasive growth (e.g. PCSK5, tight junction proteins, GRP, LGALS1, ANXA2, NOV, PLAT, MET, TDGF1, CST6, ITGB4), cell adhesion (e.g. MUC16, LGALS3BP) and embryo feeding (e.g. SLC1A1, SLC11A2, SLC16A1, SEPP1, ENPP1). Localisation of mRNA expression in the endometrium was analysed for CLDN4, CLDN10, TJP1, PCSK5, MAGED1, and LGALS1.