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The differentiation of primordial germ cells (PGCs) into functional oocytes is important for the continuation of species. In mammals, PGCs begin to differentiate into oocytes during embryonic development. Oocytes develop in clusters called germ line cysts. During fetal or neonatal development, germ cell cysts break apart into single oocytes that become surrounded by pregranulosa cells to form primordial follicles. During the process of cyst breakdown, a subset of cells in each cyst undergoes cell death with only one-third of the initial number of oocytes surviving to form primordial follicles. The mechanisms that control cyst breakdown, oocyte survival, and follicle assembly are currently under investigation. This review describes the mechanisms that have been implicated in the control of primordial follicle formation, which include programmed cell death regulation, growth factor and other signaling pathways, regulation by transcription factors and hormones, meiotic progression, and changes in cell adhesion. Elucidation of mechanisms leading to formation of the primordial follicle pool will help research efforts in ovarian biology and improve treatments of female infertility, premature ovarian failure, and reproductive cancers.

Mouse oocytes develop in clusters of interconnected cells called germline cysts. Shortly after birth, the majority of cysts break apart and primordial follicles form, consisting of one oocyte surrounded by granulosa cells. Concurrently, oocyte number is reduced by two-thirds. Exposure of neonatal females to estrogenic compounds causes multiple oocyte follicles that are likely germline cysts that did not break down. Supporting this idea, estrogen disrupts cyst breakdown and may regulate normal oocyte development. Previously, the CD-1 strain was used to study cyst breakdown and oocyte survival, but it is unknown if there are differences in these processes in other mouse strains. It is also unknown if there are variations in estrogen sensitivity during oocyte development. Here, we examined neonatal oocyte development in FVB, C57BL/6, and F2 hybrid (Oct4-GFP) strains, and compared them with the CD-1 strain. We found variability in oocyte development among the four strains. We also investigated estrogen sensitivity differences, and found that C57BL/6 ovaries are more sensitive to estradiol than CD-1, FVB, or Oct4-GFP ovaries. Insight into differences in oocyte development will facilitate comparison of mice generated on different genetic backgrounds. Understanding variations in estrogen sensitivity will lead to better understanding of the risks of environmental estrogen exposure in humans.

It is believed that the endowment of primordial follicles in mammalian ovaries is finite. Once follicles are depleted, infertility ensues. Thus, the size of the initial endowment has consequences for fertility and reproductive longevity. Follicular endowment is comprised of various processes that culminate with the incorporation of meiosis-arrested oocytes into primordial follicles. Apoptosis is prominent during follicular endowment, and apoptosis regulatory genes are involved in its regulation. Conflicting data exist with regard to the role of the proapoptotic Bcl-2 associated X protein (BAX) in follicular endowment. Therefore, we investigated the role of BAX during follicular endowment in embryonic and neonatal ovaries. We found that BAX is involved in regulating follicular endowment in mice. Deletion of Bax yields increased oocyte numbers in embryonic ovaries and increased follicle numbers in neonatal ovaries when compared with wild-type ovaries. Increased follicular endowment in Bax−/−ovaries is not due to enhanced germ cell viability. Further, it is not due to an increased primordial germ cell (PGC) allotment, a delay in the onset of meiosis, or altered proliferative activity of oogonia. Instead, our data suggest that the regulatory activity of BAX in follicular endowment likely occurs during PGC migration, prior to PGC colonization of the gonad.