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The endometrial window of implantation (WOI), the cycle days during which normal embryo implantation can occur, has generally been assumed to begin on cycle day 19 or 20 of an idealized 28 days cycle and last for 4 to 5 days. Noyes et al took the first steps in defining the WOI by establishing a set of morphological criteria to evaluate endometrial development and receptivity, but recent studies have invalidated their use in the routine evaluation of infertility. Based on greater than 10 years of extensive research, our group has developed a molecular diagnostic tool (the endometrial receptivity array [ERA] test) based on the specific transcriptomic signature that identifies the receptive endometrium in natural and artificial (hormonal replacement therapy) cycles. The ERA test has shown that some patients have a delayed WOI, others have an advanced WOI, and others can have unusually short windows of receptivity. This identification and characterization of the WOI allows the personalization of the embryo transfer. In this review, we describe the ERA and our experience with its use in assessment of the endometrial receptivity in patients undergoing assisted reproduction., (Thieme Medical Publishers 333 Seventh Avenue, New York, NY 10001, USA.)

During the mid-secretory phase, the endometrium acquires the receptive phenotype, which corresponds to the only period throughout the endometrial cycle in which embryo implantation is viable. Endometrial receptivity is a crucial process and even more important in Assisted Reproductive Technologies (ART) where embryo-endometrial synchronization is coordinated through embryo transfer timing. Over the last decade, transcriptomic analyses performed on the human endometrium have shown that specific genomic signatures can be used to successfully phenotype different phases of the menstrual cycle including the receptive stage, independently of the histological appereance of the endometrial tissue. In this paper, we review current evidence demonstrating that endometrial transcriptomics objectively identifies the implantation window in a personalized manner, opening the field for the diagnosis of the endometrial factor in ART and moving to stratified medicine at this level, using microarray technology and soon high-throughput next generation sequencing coupled with functional and systems genomics approach.

Objective: To demonstrate the clinical value of the endometrial receptivity array (ERA) in patients with repeated implantation failure (RIF), for guiding their personalized embryo transfer (pET) as a novel therapeutic strategy., Design: Prospective interventional multicenter clinical trial., Setting: University-affiliated infertility and private clinics., Patient(s): Eighty-five RIF patients and 25 comparison patients., Intervention(s): Endometrial sampling and pET guided by ERA., Main Outcome Measure(s): A receptive (R) or nonreceptive (NR) endometrial status according to ERA. Pregnancy (PR) and implantation (IR) rates after pET., Result(s): The ERA test gave an R result of 74.1% in RIF patients versus 88% in control subjects. Clinical follow-up was possible in 29 RIF patients, in whom pET was performed, resulting in 51.7% PR and 33.9% IR. The IRs and PRs in the 6 months after the biopsy showed that pregnancy was not related to the local injury. Twenty-two RIF patients (25.9%) were NR, and in 15 of them a second ERA validated a displacement of the window of implantation (WOI). In eight of them, pET was performed on the day designated by the ERA, resulting in 50.0% PR and 38.5% IR. These results should be considered as preliminary., Conclusion(s): There is an increased percentage of WOI displacement in RIF patients compared with comparison group patients, leading to the concept of pET as a therapeutic strategy. Rescue of NR patients by pET in a displaced WOI results in similar PR and IR., (Copyright © 2013 American Society for Reproductive Medicine. Published by Elsevier Inc. All rights reserved.)

STUDY QUESTION Which genes regulate receptivity in the epithelial and stromal cellular compartments of the human endometrium, and which molecules are interacting in the implantation process between the blastocyst and the endometrial cells? SUMMARY ANSWER A set of receptivity-specific genes in the endometrial epithelial and stromal cells was identified, and the role of galectins (LGALS1 and LGALS3), integrin β1 (ITGB1), basigin (BSG) and osteopontin (SPP1) in embryo–endometrium dialogue among many other protein–protein interactions were highlighted. WHAT IS KNOWN ALREADY The molecular dialogue taking place between the human embryo and the endometrium is poorly understood due to ethical and technical reasons, leaving human embryo implantation mostly uncharted. STUDY DESIGN, SIZE, DURATION Paired pre-receptive and receptive phase endometrial tissue samples from 16 healthy women were used for RNA sequencing. Trophectoderm RNA sequences were from blastocysts. PARTICIPANTS/MATERIALS, SETTING, METHODS Cell-type-specific RNA-seq analysis of freshly isolated endometrial epithelial and stromal cells using fluorescence-activated cell sorting (FACS) from 16 paired pre-receptive and receptive tissue samples was performed. Endometrial transcriptome data were further combined in silico with trophectodermal gene expression data from 466 single cells originating from 17 blastocysts to characterize the first steps of embryo implantation. We constructed a protein–protein interaction network between endometrial epithelial and embryonal trophectodermal cells, and between endometrial stromal and trophectodermal cells, thereby focusing on the very first phases of embryo implantation, and highlighting the molecules likely to be involved in the embryo apposition, attachment and invasion. MAIN RESULTS AND THE ROLE OF CHANCE In total, 499 epithelial and 581 stromal genes were up-regulated in the receptive phase endometria when compared to pre-receptive samples. The constructed protein–protein interactions identified a complex network of 558 prioritized protein–protein interactions between trophectodermal, epithelial and stromal cells, which were grouped into clusters based on the function of the involved molecules. The role of galectins (LGALS1 and LGALS3), integrin β1 (ITGB1), basigin (BSG) and osteopontin (SPP1) in the embryo implantation process were highlighted. LARGE SCALE DATA RNA-seq data are available at www.ncbi.nlm.nih.gov/geo under accession number GSE97929. LIMITATIONS, REASONS FOR CAUTION Providing a static snap-shot of a dynamic process and the nature of prediction analysis is limited to the known interactions available in databases. Furthermore, the cell sorting technique used separated enriched epithelial cells and stromal cells but did not separate luminal from glandular epithelium. Also, the use of biopsies taken from non-pregnant women and using spare IVF embryos (due to ethical considerations) might miss some of the critical interactions characteristic of natural conception only. WIDER IMPLICATIONS OF THE FINDINGS The findings of our study provide new insights into the molecular embryo–endometrium interplay in the first steps of implantation process in humans. Knowledge about the endometrial cell-type-specific molecules that coordinate successful implantation is vital for understanding human reproduction and the underlying causes of implantation failure and infertility. Our study results provide a useful resource for future reproductive research, allowing the exploration of unknown mechanisms of implantation. We envision that those studies will help to improve the understanding of the complex embryo implantation process, and hopefully generate new prognostic and diagnostic biomarkers and therapeutic approaches to target both infertility and fertility, in the form of new contraceptives. STUDY FUNDING/COMPETING INTEREST(S) This research was funded by the Estonian Research Council (grant PRG1076); Horizon 2020 innovation grant (ERIN, grant no. EU952516); Enterprise Estonia (grant EU48695); the EU-FP7 Marie Curie Industry-Academia Partnerships and Pathways (IAPP, grant SARM, EU324509); Spanish Ministry of Economy, Industry and Competitiveness (MINECO) and European Regional Development Fund (FEDER) (grants RYC-2016-21199, ENDORE SAF2017-87526-R, and Endo-Map PID2021-127280OB-100); Programa Operativo FEDER Andalucía (B-CTS-500-UGR18; A-CTS-614-UGR20), Junta de Andalucía (PAIDI P20_00158); Margarita Salas program for the Requalification of the Spanish University system (UJAR01MS); the Knut and Alice Wallenberg Foundation (KAW 2015.0096); Swedish Research Council (2012-2844); and Sigrid Jusélius Foundation; Academy of Finland. A.S.-L. is funded by the Spanish Ministry of Science, Innovation and Universities (PRE2018-085440). K.G.-D. has received consulting fees and/or honoraria from RemovAid AS, Norway Bayer, MSD, Gedeon Richter, Mithra, Exeltis, MedinCell, Natural cycles, Exelgyn, Vifor, Organon, Campus Pharma and HRA-Pharma and NIH support to the institution; D.B. is an employee of IGENOMIX. The rest of the authors declare no conflict of interest. [ABSTRACT FROM AUTHOR]

STUDY QUESTION Does the combined oral contraceptive pill (COCP) change endometrial gene expression when used for cycle programming? SUMMARY ANSWER COCP used for scheduling purposes does not have a significant impact on endometrial gene expression related to endometrial receptivity. WHAT IS KNOWN ALREADY Controversy exists around COCP pretreatment for IVF cycle programming, as some authors claim that it might be detrimental to the live birth rate. Microarray technology applied to the study of tissue gene expression has previously revealed the behavior of genes related to endometrial receptivity under different conditions. STUDY DESIGN, SIZE, AND DURATION Proof-of-concept study of 10 young healthy oocyte donors undergoing controlled ovarian stimulation (COS) recruited between June 2012 and February 2013. PARTICIPANTS/MATERIALS, SETTING, AND METHODS Microarray data were obtained from endometrial biopsies from 10 young healthy oocyte donors undergoing COS with GnRH antagonists and recombinant FSH. In group A (n = 5), COCP pretreatment was used for 12–16 days, and stimulation began after a 5-day pill-free interval. Stimulation in group B (n = 5) was initiated on cycle day 3 after a spontaneous menses. Endometrial biopsies were collected 7 days after triggering with hCG. MAIN RESULTS AND THE ROLE OF CHANCE No individual genes exhibited increased or decreased expression (fold change (FC) >2) in patients with prior COCP treatment (group A) compared with controls (group B). However, the results of the functional analysis showed a total of 11 biological processes that were significantly enriched in group A compared with group B (non-COCP). LIMITATIONS, REASONS FOR CAUTION The Endometrial Receptivity Array (ERA) has only been validated on endometrial samples obtained in natural cycles and after hormonal replacement treatment (HRT). Therefore, it was not possible in this study to classify the endometrial samples as receptive or non-receptive. We used the ERA to focus on 238 genes that are intimately related to endometrial receptivity, thus simplifying the analysis and understanding of the data. WIDER IMPLICATIONS OF THE FINDINGS Cycle scheduling is common in IVF units and is used to avoid weekend retrievals and/or to distribute evenly the workload for better efficiency. Our failure to detect any relevant changes in the genes related to the window of implantation when cycles were programmed with COCP pretreatment suggests that, despite controversial clinical results in previous studies, the use of COCPs in this way does not affect uterine receptivity adversely. STUDY FUNDING/COMPETING INTEREST(S) Funding for this study was provided by an unrestricted grant from Merck Sharp & Dohme. C.S. and A.P. are co-inventors (with Patricia Diaz-Gimeno) of the Endometrial Receptivity Array and hold the patent. The other authors have no conflicts of interest to declare. TRIAL REGISTRATION NUMBER EudraCT registration number is 2011-003250-34. [ABSTRACT FROM AUTHOR]