Evidence for follicular renewal in the postnatal mouse ovary

Abstract

Proliferation and maturation of germ cells in the fetal ovary establishes a finite, non-growing pool of primordial follicles (PF) at birth. Believed to be non-renewable, the supply of PF declines postnatally through folliculogenesis via atresia, or survival to ovulation. Recent studies suggest that germline stem cells replenish oocytes forming new follicles in the mouse ovary. We quantitated all healthy follicles in postnatal mouse ovaries from day 1–200 using stereology, immunolabeling of oocyte meiotic maturation (GCNA, germ cell nuclear antigen) and cell proliferation (PCNA, proliferating cell nuclear antigen) and electronmicroscopy. Day 1 ovaries had 7924 ± 1564 (SEM) PF and 1986 ± 204 on day 7. Oocytes were ejected from ovaries up to day 25 with 200–800 oocytes per ovary (confirmed by ultrastructure) exiting on day 7 and 12. Discarded and some subtunical oocytes were GCNA-positive suggesting their late meiotic maturation. Total PF (range 1976 ± 306 to 2317 ± 289) and total follicles per ovary (range 2332 ± 349 to 3077 ± 322) were not significantly different from day 7 to 100. Total volumes (x106μm3) of PF oocytes per ovary were not significantly different from day 12 (7.37 ± 0.77) to day 100 (4.35 ± 0.45). PF oocytes were PCNA-negative although occasional mitotic figures enclosed by squamous cells were noted in the ovarian cortex on day 1, 7 and 12. The data provide further evidence for germline stem cells that replenish PF, thus contributing to an unabated supply of growing follicles during puberty and early adult life. Supported by Monash University and NHMRC Australia.