Rivron, N. | Search bioRxiv

bioRxiv2019

Dynamics of Meiotic Sex Chromosome Inactivation and Pachytene Activation in Mice Spermatogenesis

During germ cell development, cells undergo a drastic switch from mitosis to meiosis to form haploid germ cells. Sequencing and computational technologies now allow studying development at the single-cell level. Here we developed a multiplexed trajectory reconstruction to create a high-resolution developmental map of spermatogonia and prophase-I spermatocytes from testes of a Dazl-GFP reporter mouse. We identified three main transitions in the meiotic prophase-I: meiotic entry, the meiotic sex chromosome inactivation (MSCI), and concomitant pachytene activation. We validated the key features of these transitions in vivo using single molecule FISH. Focusing on MSCI, we found that 34% of sex chromosomal genes are induced shortly before MSCI, that silencing time is diverse and correlates with specific gene functions. These highlight a previously underappreciated level of regulation of MSCI. Finally, we found that spermatozoal genes in pachytene are activated in a temporal pattern reflecting the future anatomic and functional order of the sperm cell. Altogether we highlighted how precise and sequential changes in gene expression regulate cellular states in meiotic prophase-I.

genomics↗

bioRxiv2019

Self-organization of post-implantation-like embryonic tissues from blastoids

The early mammalian embryo (blastocyst) contains three co-developing types of stem cells. Two supporting extraembryonic types - the trophectoderm and the primitive endoderm - encase and guide the pluripotent epiblast that eventually forms all body tissues. Unlike embryos, stem cell-based models of the embryo can be generated in large numbers and subjected to high-content screens as a basis for basic and biomedical discoveries (Rivron et al., 2018a; Vrij et al., 2016b). Here, we show that aggregates of naive Embryonic stem cells cultured in chemically-defined conditions and exposed to combinatorial screens of signaling molecules, rapidly (48 hours) and efficiently (80%) set apart PrE-like cells. These cells produce a basal lamina, generate progenitors resembling both visceral and parietal endoderm, and polarize co-developing Epiblast cells to form a proamniotic cavity. In blastoids, stem cell-based models of the early blastocyst (Rivron et al., 2018b), this combination of signals increases the ratio and number of Gata6+/Nanog+ cells and promotes the survival, growth and morphogenesis of a post-implantation-like Epiblast in vitro. Modeling early embryonic development in chemically-defined in vitro conditions shows that the primitive endoderm forms via a specific combination of signaling pathways and sufficient to drive the development of the Epiblast.

developmental biology↗