Comprehensive comparison of female germ cell developmentin vitroandin vivoidentifies epigenetic gene regulation crucial for oocyte development and embryonic competence

Abstract

Germ cells are the origin of new individuals. Hence, specifying germ cell identity is crucial for reproduction. The recent establishment of in vitro culture systems for generating oocytes from mouse pluripotent stem cells provides a basis for progress in studies of oogenesis and reproductive technology. However, currently the developmental competence of in vitro generated oocytes is low compared to in vivo grown oocytes. The causes underlying poor oocyte quality remain to be determined. By reconstituting germ cell development in culture from different developmental starting points within gametogenesis, we show that the differentiation of primordial germ cells (PGCs) and primordial germ cell-like cells (PGCLCs) to growing oocytes (GROs), as well as the subsequent growth of follicles are critical culture steps for specifying competence of fully-grown oocytes (FGOs) for preimplantation development. A systematic comparison of transcriptomes of single oocytes having undergone different in vitro culture trajectories identifies genes normally upregulated during oocyte growth to be susceptible for mis-regulation during in vitro oogenesis. Many of such genes have been described as targets of Polycomb repressive complexes (PRCs). Deregulation of Polycomb repression therefore likely perturbs the accumulation of cytoplasmic factors and/or setting of chromatin states in FGOs that are required for embryonic development after fertilization. Conversely, in vitro derived oocytes often displayed failure of zygotic genome activation (ZGA) and abnormal acquisition of 5-hydroxymethylcytosine (5hmC) on maternal chromosomes after activation. In addition, subcellular delocalization of pyruvate dehydrogenase (PDH) and of STELLA were observed suggesting new molecular markers for defective oocyte development. Our study identifies epigenetic regulation at an early stage of oogenesis as crucial for developmental competence and suggests specific in vitro culture steps as targets for improving oocyte quality.

Genetic and Epigenetic Regulation of Drosophila Oocyte Determination

“…While mammalian in vitro gametogenesis has shown remarkable progress in replicating female gametogenesis [ 142 , 143 ], the continued use of invertebrate model organisms, such as Drosophila and Caenorhabditis elegans , can still provide valuable insights into the mechanisms of germ cell development and differentiation. For instance, by combining Drosophila’s exceptional genetics with ongoing research on distinct invertebrate model organisms (arthropod and non-arthropod), whose available genetic tools maybe be more limited, there is a unique opportunity for highly collaborative studies focused on gaining a molecular understanding of the evolution of female gametogenesis and its contribution to distinct reproductive strategies.…”