Generation of germinal-vesicle oocytes from mouse embryonic stem cells under an ovarian soma-free condition.

In vitro oogenesis provides a platform to elucidate the mechanisms of oocyte development and advance reproductive medicine. The prevalent in vitro oogenesis model requires ovarian somatic cells (OSCs) to support oocyte development; yet, complex three-dimensional oocyte-OSC interactions pose difficulties in systems regulation and mechanistic understanding. Here, we present an OSC-free system of in vitro oogenesis: upon optimized provision of retinoic acid and bone morphogenetic protein on feeders, mouse primordial germ-cell-like cells induced from embryonic stem cells propagate robustly, and enter/progress through meiotic prophase I, generating abundant fetal oocyte-like cells at diplotene arrest. With key cytokines, signaling activators, and antioxidants, they show prominent growth and differentiate into cells comparable to germinal-vesicle oocytes in morphology, transcriptome, and histone modification profiles, with competence to resume meiosis with germinal-vesicle breakdown. By reconstituting major phases of oogenesis with minimal components, our study creates a foundation for OSC-free in vitro oogenesis in mammals, including humans.