Molecular and Cellular Mechanisms in Preimplantation IVF-Embryo Development

The aims of this Frontiers Research Topic were to further our understanding of how the preimplantation embryo develops, and to consider current techniques used to evaluate embryo viability. If we can deepen our understanding of the molecular and cellular events taking place during preimplantation embryo development, we will be closer to identifying new potential molecular markers of embryo viability, which may one day be used to increase the success of in vitro fertilization (IVF).Much of what is known about human embryo development at the cell and molecular level was discovered in mouse embryos or other animal models. Therefore, understanding the similarities and differences between human embryo development and other animal models is essential in clarifying the molecular steps taking place during human embryo development (reviewed by Bissiere et al. 2023). Researchers often rely on cultured cell lines and stem cells to understand these molecular and cellular events (White and Plachta, 2021). For example, Oh et al. report deriving lineage specific embryonic cell lines from porcine blastocysts to assess gene expression.For patients undergoing IVF, the embryo selection process for transfer has traditionally relied 28 on morphology metrics, and more recently, genetic analysis of such preimplantation 29 genetic testing for (PGT-A) the trophectoderm (reviewed by Harris et al. 30 2021). Interestingly, advancements imaging specifically time-lapse imaging throughout 31 the culture period of preimplantation embryos, have provided new avenues for morphology 32 studies. For example, several recent reports on the presence of cytoplasmic strings (via time-33 lapse imaging) in preimplantation embryos conceived via IVF have been published that try to 34 link this morphological structure with blastocyst quality and pregnancy outcomes (Joo et al. 35 2023; Ma et al. 2022). Arguably the most cutting-edge research associated with embryo time-36 lapse imaging is the application of artificial intelligence (AI) for embryo screening. Using AI as 37 a means of non-invasive embryo screening is a promising area of research yet there is still more 38 to learn before we will see AI routinely used in the clinic (reviewed by Jiang et al. 2023). 39 Some efforts have focused on using molecular markers from spent media and blastocoel fluid 40 to assess the viability of the preimplantation embryo. Cell-free DNA and the contents of 41 exosomes found in spent embryo culture media have all been studied to try and understand 42 their specific origin from within the preimplantation embryo Other molecules in the spent embryo culture media can also be detected and potentially serve 49 as markers for embryo viability. For example, Meng et al. report the use of a Raman 50 spectroscopy method to assess the metabolome present in spent embryonic culture media. 51 However, there is still much that is unknown regarding preimplantation embryonic 52 development and measurable indicators of successful embryo transfer. 53 Xu et al. reported the identification of two genetic variants of the PADI6 gene in two patients 54 who experienced infertility. The PADI6 protein is part of a multi-protein complex that is 55 required for embryo development, specifically zygote gene activation. While this study used a 56 very small sample size, it still highlights the need for further identification of genetic mutations 57 that that are associated with genes regulating embryo development. Therefore, identified 58 mutations in a patients may allow for different counselling for women seeking Assisted 59 Reproductive Technologies (ART), as these mutations may predict poor IVF outcomes. If 60 specific poor-IVF-outcome-gene-mutations can be identified, a step could be 61 finding to these mutations in the preimplantation embryo. Yaghoobi et al. 62 propose a strategy to take the molecular knowledge we have regarding embryo implantation to 63 alter gene expression in the trophoblast layer of the developing blastocyst via CRISPR/dCas9 64 exosome system. 65Having a more thorough understanding of the changes taking place in this stage of 66 development may allow for better use and interpretation of biomarkers of embryo viability, 67 therefore increasing IVF success rates. 68 69