The Genetic and Biochemical Blueprint of Endometrial Receptivity: Past, Present, and Future Factors Involved in Embryo Implantation Success

In the field of assisted reproductive technology, endometrial receptivity is a crucial aspect that affects implantation rates in in-vitro fertilization procedures; in fact, impaired endometrial receptivity has been identified as the rate-limiting step for favorable pregnancy outcomes once factors regarding embryo quality have been optimized. The endometrium is a dynamic tissue that undergoes proliferative and secretory changes in each menstrual cycle, acquiring a short and transient period of embryo receptivity known as the Window of Implantation. Precise embryo-endometrial synchrony is necessary to achieve a successful pregnancy, and it involves complex and multifactorial processes related to morphological, biochemical, and genetic changes. On that behalf, defining the receptive window of each patient for personalized embryo transfer is a current goal. Here, we review different indicators of endometrial receptivity throughout the menstrual cycle, spotlighting the opening of the window of implantation: classical histological and biochemical markers, genetic factors, leading-edge transcriptomic signatures and miRNA profiles, and novel features such as the microbiome and secretome. Understanding the molecular mechanisms behind endometrial receptivity will facilitate the optimization and improvement of infertility treatments

Aneuploidy Rates Inversely Correlate with Implantation during <em>In Vitro</em> Fertilization Procedures: In Favor of PGT

Aneuploidy, the hold of an abnormal number of chromosomes that differs from the normal karyotype, is a recognized leading cause of miscarriage and congenital disabilities. In human gametes and embryos, aneuploidy rates are prevalent, and these rates increase with advanced maternal age; additionally, it has been suggested that hormonal stimulation for achieving in vitro fertilization (IVF) protocols further increases aneuploidy rates. Although about 65% of chromosomally abnormal embryos culminate in spontaneous miscarriages, there is still evidence of live births harboring crucial aneuploidies. Furthermore, although some frequent aneuploidies are consistent, others differ between countries, making it harder to focus on a specific set of anomalies but vital to focus regionally on those more prevalent. Preimplantation genetic testing (PGT) is a highly endorsed technique in assisted reproductive treatments to evaluate possible embryo aneuploidies, genetic defects, and congenital disorders. On this subject, this study shows that IVF aneuploidy rates in embryo cohorts of high morphological quality are inversely associated with implantation rates. In its entirety, this study reinforces the utility of PGT for embryo evaluation

Generación de un índice mitocondrial cuantitativo en células de la granulosa para evaluar buena calidad embrionaria en medicina reproductiva

Objective: To design, standardize, and verify a quantitative method based on mtDNA content in single-oocyte cumulus cells (CCs) samples to search for a correlation between mtDNA content and embryo quality in patients undergoing assisted reproductive techniques. Design: Cohort study. Materials and Methods: Participating patients signed an informed consent; each patient was submitted to a standard ovarian stimulation protocol for IVF and samples of CCs were taken at the time of egg collection. To assess embryo quality a mitochondrial DNA (mtDNA) quantitative analysis in CCs was designed using onestep qPCR. A portion of the D-loop mitochondrial segment was amplified with respect to the IL6 nuclear gene. Identity of the amplicons was confirmed by direct cloning (CloneJet) and automated sequencing (BigDye). A protocol based on phaseseparation was optimized for total DNA and RNA extraction from TRIzol® samples of CCs. The extraction protocol was modified from Thermo Fisher to allow for suitable concentrations and purities of nucleic acids for samples of CCs bearing a small cell number (about 20,000 cells per egg). Profiles of mtDNA content were analyzed and a mitochondrial index was created that reflected mitochondrial content per cohort. A higher mitochondrial index reflects higher mitochondrial content. All experimental samples (40 analyzed CCs) were analyzed using this index and different correlations between established parameters of embryo-quality and mitochondrial content were explored. Results: The extraction of both RNA and DNA rendered suitable concentrations and purities of nucleic acids for downstream analysis. We found an observational relationship between embryo-quality and mitochondrial content in CCs. CCs from embryos that were transferred possessed higher mitochondrial indexes compared to those belonging to embryos that were fertilized but not transferred and this difference is even more pronounced with CCs coming from oocytes that did not fertilize. \nAdditionally, CCs from embryos that followed through day 3 of development after fertilization, higher mitochondrial indexes than those CCs coming from embryos that only followed through day 2, were observed. Similarly, this difference is even more pronounced with CCs coming from embryos that lasted only until day 1. Based on these results a cut-off value of 0.2 in mitochondrial index was suggested for embryo assessment. Conclusion: We generated a quantitative method based on real-time PCR to measure differences in mtDNA content in CCs samples, with the refinement of a method for nucleic acids extraction based on phase separation of TRIzol® samples and we created a mitochondrial index that seems to reflected embryo quality. We propose that CCs holding higher mitochondrial content correspond to those oocytes with greater quality for embryo transfer. Above all, data support the proposal and further exploration of mtDNA content in CCs as a predictor of embryo quality.Fil: López Luna, Almena Montserrat. Universidad de Buenos Aires. Facultad de Farmacia y Bioquímica. Buenos Aires, Argentin