Three waves of ovarian follicles in juvenile mice

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 scRNAseq to clarify their contributions to offspring and to 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 medullar-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 wave 2 follicles differentially express multiple genes, including Wnt4 and Igfbp5. SIGNIFICANCE STATEMENTWe studied the ovaries of juvenile mice 2-6 weeks (wk) of age using transcriptomics and cell lineage to characterize the earliest developing groups of follicles. Wave 1 follicles begin growing at birth in the ovarian medulla, but few produce mature oocytes. Instead, most undergo partial atresia and expand steroidogenic cells expressing androgenic genes at puberty. A newly identified primordial follicle subset located at the medulla-cortex boundary are the earliest to activate and grow. Their oocytes give rise to most early offspring, while no more than a few derive from surviving wave 1 follicles. Our results highlight the importance of spatially localized follicle subgroups and show that follicular waves aid reproduction in distinct ways, some of which involve substantial atresia and cellular remodeling.