Eggs in the nest.

The last several years have produced a great deal of evidence supporting the once revolutionary notion that the formation of the ovary is a directed rather than passive process. For example, factors such as follistatin, respondin-1, and Wnt-4 are differentially expressed in the XX gonad and appear to play a pivotal role in differentiation of the bipotential ovary (1). Primordial germ cells are also actively directed along different pathways in the testis and ovary (2). The hormone retinoic acid diffuses from the mesonephros into the gonad to initiate meiosis in germ cell nuclei in the ovary. In contrast, a testis-specific enzyme, Cyp26b1, actively degrades retinoic acid and prevents germ cell entry into meiosis in the testis. These early steps of somatic cell differentiation and the initiation of germ cell fate are soon followed by oogenesis and follicle formation. Pepling and colleagues (3–5) have done much to advance our understanding of this step of the process, with their work resulting in a new paper exploring the direct contribution of maternal steroids to primordial follicle assembly (6). With their contribution, the molecular, cellular, and endocrine basis of ovarian follicle formation is being solidified, and the field is poised to address essential mechanisms underlying this process and its impact on both fertility and infertility. It has been known for some time that germ cell nuclei maintain a shared local environment by undergoing incomplete cytokinesis during their early mitotic proliferation, thus creating clusters variously called germ cell nests, germ cell cysts, or germ cell syncytia (5, 7). The utility of a syncytium is not completely clear, but it has the advantages of shared resources and the potential to respond to the changing maternal/fetal environment in a coordinated manner. In this regard, it has been recognized for some time that similar cytoplasmic sharing occurs during male germ cell maturation (8). Germ cell nests persist until a few days after birth in the mouse, when the syncytium breaks down into individual oocytes that become encased in somatic pregranulosa cells to form primordial follicles. At the same time, the number of oocytes within the ovary is drastically reduced by cell death. Two critical questions in ovarian biology are: 1) what regulates germ cell nest breakdown and 2) how does germ cell apoptosis contribute to, or result from, this process? In this issue, Chen et al. (6) contributed key insights to this process that bring us one step closer to the answers. A number of observations have contributed to the hypothesis that maternal hormones are necessary for maintaining germ cell nests in the fetus and that the abrupt loss of exposure to these hormones at birth triggers nest breakdown and primordial follicle formation. Ovaries from mice exposed to estrogen or estrogenic compounds prenatally or as neonates contain a larger proportion of multioocyte follicles (MOFs) (9–12), suggesting that continued exposure to estrogen preserves germ cell nests, which leads to the assembly of pregranulosa cells around multiple oocytes. Aromatasedeficient mice that lack estrogen production exhibit reduced numbers of primordial follicles in the neonatal ovary, although germ cell nests have not been directly examined (13). Studying the cultured rat ovary, Kezele and Skinner (7) found that both estrogen and progesterone delay nest breakdown and slow the transition from primordial to primary follicles, which is otherwise accelerated in culture. Similarly, using both in vivo systems and an in vitro culture mouse ovary system to mimic the loss of maternal steroids at birth, Chen et al. (6) demonstrated that estrogen (or progesterone) is necessary for germ cell nest breakdown and that breakdown can be arrested if the ovary is reexposed to the steroid. It is useful to note that many normal follicles do form in the presence of these steroids, suggesting that even at the earliest stage of follicle formation, there may be responsive and nonresponsive pools of germ cells or follicles. This phenomenon may be an early indication of the adult restraint on all but a few follicles within the ovarian reserve, leading to a limited number of ovulations per month. How small numbers of follicles are parsed out over time remains an unsolved problem in ovarian biology. The Pepling model, that falling maternal steroids control germ cell nest breakdown, is somewhat challenged by the timing of human follicle formation. Human follicles emerge from the nest during the second trimester of pregnancy, a time of relatively high steroid support (14). One plausible explanation is that circulating steroid binding proteins, such as -fetoprotein, may increase during this time, resulting in reduced steroid exposure. A study published by Pepe and colleagues (15) directly tested this prediction by delivering an aromatase inhibitor to baboons during the time of follicle formation, which resulted in fewer primordial follicles and more follicles retained in a nest. The Pepling model predicts that an acute drop in estradiol initiates follicle release from the nest, but the model does not address what requirements the follicles have after formation. The Pepe model is one of chronic loss of estrogen and there may be a requirement for estrogen once follicles form that would confound the evaluation of these ovaries relative to the process of cyst breakdown. Moreover, the few cases of reported aromatase deficiency in human females reveals a range of ovarian pathology (from ovaries with primary follicles to cystic follicles) but provide no information on the absolute number of follicles nor on the presence of unassembled follicles (16). Not surprisingly, no cases of maternal estrogen loss have been reported. Clearly, precise models testing the role of estrogen in nest breakdown in humans or nonhuman primates are difficult. Even so, it may be possible to use the in vitro system to further evaluate primary and secondary effects of estrogen depletion on follicle formation and persistence in rodent and Abbreviations: ER, Estrogen receptor; MOF, multioocyte follicle.