In vitro reconstitution of epigenetic reprogramming in the human germ line.

Epigenetic reprogramming resets parental epigenetic memories and differentiates primordial germ cells (PGCs) into mitotic pro-spermatogonia or oogonia, ensuring sexually dimorphic germ-cell development for totipotency 1 . In vitro reconstitution of epigenetic reprogramming in humans remains a fundamental challenge. Here, we establish a robust strategy for inducing epigenetic reprogramming and differentiation of pluripotent stem cell (PSC)-derived human PGC-like cells (hPGCLCs) into mitotic pro-spermatogonia or oogonia, coupled with their extensive amplification (~>10 10 -fold). Strikingly, bone morphogenetic protein (BMP) signalling is a key driver of these processes: BMP-driven hPGCLC differentiation involves an attenuation of the mitogen-activated protein kinase/extracellular-regulated kinase (MAPK/ERK) pathway and both de novo and maintenance DNA methyltransferase (DNMT) activities, likely promoting replication-coupled, passive DNA demethylation. On the other hand, hPGCLCs deficient in tens-eleven translocation (TET) 1, an active DNA demethylase abundant in human germ cells 2,3 , differentiate into extraembryonic cells, including amnion, with de-repression of key genes bearing bivalent promoters; these cells fail to fully activate genes vital for spermatogenesis and oogenesis, with their promoters remaining methylated. Our study elucidates the framework of epigenetic reprogramming in humans, making a fundamental advance in human biology, and through the generation of abundant mitotic pro-spermatogonia and oogonia-like cells, represents a milestone for human in vitro gametogenesis (IVG) research and its potential translation into reproductive medicine.