ABSTRACT
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.
SIGNIFICANCE STATEMENT We 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 (central region), but most do not produce mature oocytes. Instead they remodel to generate cells synthesizing androgens at puberty, which likely affect sex-specific neurodevelopment. A newly recognized group, "boundary follicles" activate next at the medulla-cortex boundary; their oocytes mature and give rise to most early offspring. The remaining "wave 2" follicles constitute the quiescent ovarian reserve and activate in small groups throughout later life. Our results highlight the importance of specific spatially-localized follicle subgroups and show that they aid reproduction in distinct ways, some of which involve extensive cellular remodeling.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Competing interests: The authors declare that no competing interests exist.
The manuscript was rewritten to improve clarity, and some new data and figure panels were added. Additional scRNAseq analysis is now included that allows us to compare granulosa cell gene expression in wave 1 and wave 2 antral follicles. We find that there are differences in gene expression, such as high expression of Wnt4 in antral granulosa cells of wave 1 at 2 wk, compared to >10X lower expression in corresponding cells of wave 2 antral follicles at 5wk or 6 wk.
Data availability
The scRNAseq data have been deposited at the NIH GEO database under accession number GSE268466.
