Human embryo implantation is a three-stage process (apposition, adhesion and invasion) involving synchronized crosstalk between a receptive endometrium and a functional blastocyst. This ovarian steroid-dependant phenomenon can only take place during the window of implantation, a self-limited period of endometrial receptivity spanning between days 20 and 24 of the menstrual cycle. Implantation involves a complex sequence of signalling events, consisting in the acquisition of adhesion ligands together with the loss of inhibitory components, which are crucial to the establishment of pregnancy. Histological evaluation, now considered to add little clinically significant information, should be replaced by functional assessment of endometrial receptivity. A large number of molecular mediators have been identified to date, including adhesion molecules, cytokines, growth factors, lipids and others. Thus, endometrial biopsy samples can be used to identify molecules associated with uterine receptivity to obtain a better insight into human implantation. In addition, development of functional in vitro systems to study embryo–uterine interactions will lead to better definition of the interactions existing between the molecules involved in this process. The purpose of this review was not only to describe the different players of the implantation process but also to try to portray the relationship between these factors and their timing in the process of uterine receptivity.

Repeated implantation failure (RIF) is a major problem encountered in IVF. We have previously reported that RIF-IVF patients have a different endometrial gene expression profile during the window of implantation. Considering microRNA (miRNA) function in post-transcriptional regulation of gene expression, the aim of the study was to evaluate the involvement of miRNA in defects of endometrial receptivity. We used TaqMan miRNA array cards to identify the miRNAs differentially expressed in the secretory endometrium of RIF-IVF patients when compared with fertile women, and bioinformatics tools to identify their predicted targets and the molecular networks they may affect. Comparing miRNA expression profiles, we identified 13 miRNAs, differentially expressed in RIF endometrial samples, that putatively regulate the expression of 3800 genes. We found that 10 miRNAs were overexpressed (including miR 145, 23b and 99a) and 3 were underexpressed. Using our previous gene expression analysis, we paralleled miRNA–mRNA expression profiling. By this means, we identified novel and previously characterized miRNA-regulated molecular pathways such as adherens junctions, cell adhesion molecules, Wnt-signaling, p53 signaling and cell cycle pathways. Consistent with the miRNA-predicted targets, mRNA levels of N-cadherin, H2AFX, netrin-4 and secreted frizzled-related protein-4, belonging to the cell adhesion molecules, Wnt signaling and cell cycle pathways were lower in RIF-IVF patients. To our knowledge, this is the first study to evaluate the differential expression of miRNAs in the secretory endometrium of RIF-IVF patients. We suggest that the RIF-associated miRNAs could be exploited as new candidates for diagnosis and treatment of embryo implantation failures.

Repeated implantation failure (RIF) is a severe obstacle in human assisted reproduction treatment. Aiming to identify global gene profile in RIF patients, gene-array analyses were performed on endometrial samples collected on day 21 of the cycle from fertile women (n = 12) and from RIF patients (n = 20). Validation of cyclin E2, Slug, dickkopf homolog 1 (DKK1), lymphoid enhancer-binding factor 1 (LEF1) and secreted frizzled-related protein 1 (SFRP1) was carried out by real-time PCR. Gene-array analysis revealed 313 genes exhibiting modified expression levels in RIF patients. Of these, 288 genes (92%) were down-regulated and only 25 genes (8%) were up-regulated. Classification of the down-regulated genes to biological pathways revealed cell cycle, Wnt signaling and cellular adhesion pathways. Real-time PCR validation of cyclin E2, SFRP1 and LEF1 showed significantly lower expression levels in RIF–IVF patients as compared with fertile women. In addition, two up-regulated genes, Slug and DKK1, were also validated. Interestingly, about 8% of the down-regulated genes were estrogen-dependent. Western blot of estrogen receptor α revealed low expression of this protein in the RIF group. The evaluation of the endometrium of RIF patients by gene array analysis demonstrates that the expression of various genes is altered, including those belonging to the cell cycle, Wnt signaling and cellular adhesion pathways.