Induction of fetal primary oocytes and the meiotic prophase from mouse pluripotent stem cells.

Meiosis is a key mechanism that ensures sexual reproduction and creates genetic diversity. Here we describe a method that induces fetal oocytes and the prophase of the first meiotic division from mouse pluripotent stem cells (PSCs) under defined conditions. PSCs are induced into epiblast-like cells (EpiLCs), which are in turn induced into primordial germ cell-like cells (PGCLCs). PGCLCs are expanded robustly in the presence of forskolin and rolipram, which elevate intracellular cyclic AMP levels. The expanded PGCLCs comprehensively erase their DNA methylome in a manner that recapitulates genome-wide DNA demethylation in germ cells in vivo, and are in turn induced efficiently into the oogenic pathway and the prophase of the first meiotic division up to the pachytene stage in response to bone morphogenetic protein and retinoic acid. This in vitro strategy provides a powerful foundation for exploring the mechanisms of initiation and progression of mammalian oogenesis and meiosis.