An Asynchronous Production Line of Meiotic Prophase I in the Mouse Fetal Ovary

The initiation of meiosis in the mammalian female germline has long been described as a synchronous event, occurring within a narrow developmental window. Here, we challenge this paradigm through a systematic, quantitative analysis of meiotic entry and progression in the mouse fetal ovary. Using dynamic expression profiling of key regulators Stra8, Sycp1, and Sycp3 alongside proliferation markers, we demonstrate that germ cells enter meiosis asynchronously and continuously between embryonic days E12.5 and E16.5. During this extended period, mitotic proliferation persists, indicating that germ cells are progressively recruited into the meiotic pathway rather than halting division simultaneously. Homologous chromosome synapsis, marked by Sycp1/Sycp3 co-localization, initiates at E14.5 and is completed prenatally by E18.5. Integrating these data into a continuous-time Markov chain model, we derive a "meiotic clock" that predicts fixed temporal intervals from meiotic entry to synapsis completion ([~]72 hours) and to meiotic arrest ([~]91 hours), regardless of entry time. This model provides a predictive framework for germ cell development and establishes that female meiotic initiation is a prolonged, asynchronous production line rather than a synchronized transition, reshaping our understanding of oogenesis and its regulatory logic. TEASERThe initiation of female meiosis in mice is redefined as an asynchronous, continuous production line from E12.5 to E16.5, supported by a predictive "meiotic clock" model with fixed temporal intervals for synapsis and arrest.