Three waves of ovarian follicles in juvenile mice

We characterized 3 waves of ovarian follicles in juvenile mice using tissue clearing, lineage tracing and scRNAseq. Wave 1 follicles grow with little delay, and express steroidogenic cell inducers by 2 weeks, as thecal and interstitial gland cells arise. Beginning at 3 weeks, >80% of wave 1 follicles degrade their oocytes, turn over most granulosa cells, and cluster together in the medulla. Thecal cells increase further around holes left by oocytes, and during puberty (5 wk), highly express androgen biosynthetic genes. "Boundary follicles" at the medullar-cortical boundary, comprise the earliest-activated wave 2 follicles. They express Foxl2 as early as E14.5, and generate >70% of the first mature oocytes. Our results indicate the medulla's broad importance in initial follicular development, and argue that gamete waves, a common animal strategy, serve specific purposes. Wave 1 follicle derivatives bring about high androgen expression during puberty that likely impacts sex-specific neurodevelopment, while boundary and wave 2 follicles support early and lifelong fertility. Consistent with their distinct roles, granulosa cells of antral wave 1 and wave 2 follicles differentially express multiple genes, including Wnt4 and Igfbp5.