Distinct waves of ovarian follicles contribute to mouse oocyte production.

The earliest growing mouse follicles, wave 1, rapidly develop in the ovarian medulla, while the great majority, wave 2, are stored for later use as resting primordial follicles in the cortex. Wave 1 follicles are known to mostly undergo atresia, a fate sometimes associated with the persistence of steroidogenic theca cells, but this connection is poorly understood. We characterized wave 1 follicle biology using tissue clearing, lineage tracing, and scRNA-seq to clarify their contributions to offspring and steroidogenesis. Wave 1 follicles, lineage-marked by E16.5 Foxl2 expression in granulosa cells, reach preantral stages containing theca cell layers by 2 weeks. Atresia begins about a week later, during which 80-100% of wave 1 follicles degrade their oocytes, turn over most granulosa cells, but retain theca cells which expand in number together with interstitial gland cells in the medulla. During puberty (5 weeks), these cells ultrastructurally resemble steroidogenic cells and highly express androgen biosynthetic genes. Unexpectedly, the Foxl2 lineage tag also marked about 400 primordial follicles, located near the medullary-cortical boundary, that become the earliest activated wave 2 follicles. These 'boundary' or 'wave 1.5' follicles generate 70-100% of the earliest mature oocytes, while fewer than 26 wave 1 follicles with oocytes survive. Consistent with their largely distinct fates in steroid or oocyte production, granulosa cells of antral wave 1 and 2 follicles differentially express multiple genes, including Wnt4 and Igfbp5.