Estradiol, Progesterone, and Genistein Inhibit Oocyte Nest Breakdown and Primordial Follicle Assembly in the Neonatal Mouse Ovary in Vitro and in Vivo

Abstract

In developing mouse ovaries, oocytes develop as clusters of cells called nests or germ cell cysts. Shortly after birth, oocyte nests dissociate and granulosa cells surround individual oocytes forming primordial follicles. At the same time, two thirds of the oocytes die by apoptosis, but the link between oocyte nest breakdown and oocyte death is unclear. Although mechanisms controlling breakdown of nests into individual oocytes and selection of oocytes for survival are currently unknown, steroid hormones may play a role. Treatment of neonatal mice with natural or synthetic estrogens results in abnormal multiple oocyte follicles in adult ovaries. Neonatal genistein treatment inhibits nest breakdown suggesting multiple oocyte follicles are nests that did not break down. Here we investigated the role of estrogen signaling in nest breakdown and oocyte survival. We characterized an ovary organ culture system that recapitulates nest breakdown, reduction in oocyte number, primordial follicle assembly, and follicle growth in vitro. We found that estradiol, progesterone, and genistein inhibit nest breakdown and primordial follicle assembly but have no effect on oocyte number both in organ culture and in vivo. Fetal ovaries, removed from their normal environment of high levels of pregnancy hormones, underwent premature nest breakdown and oocyte loss that was rescued by addition of estradiol or progesterone. Our results implicate hormone signaling in ovarian differentiation with decreased estrogen and progesterone at birth as the primary signal to initiate oocyte nest breakdown and follicle assembly. These findings also provide insight into the mechanism of multiple oocyte follicle formation.

Follicular assembly: mechanisms of action

“…Subsequently, neonatal progesterone treatment was shown to reduce primordial follicle assembly in rats supporting the idea that progesterone, like estrogen, acts during neonatal oocyte development (Kezele & Skinner 2003). Progesterone treatment of neonatal mouse ovaries reduced cyst breakdown (Chen et al 2007). Since progesterone can be converted to estrogen, its effect could be exerted either directly or via conversion to estrogen.…”

“…One model is that normally, high levels of E 2 in the fetal ovary keeps oocytes in cysts and that late in fetal development, E 2 levels drop resulting in cyst breakdown (Chen et al 2007). When oocytes are exposed to estrogens, cyst breakdown is (Heldring et al 2007).…”

“…When neonatal oocytes are exposed to estrogens, cyst breakdown is inhibited but it is unclear which estrogen receptor is used (Chen et al 2007). During cyst breakdown, ESR1 is expressed in pregranulosa cells and ESR2 in some oocyte nuclei (Chen et al 2009).…”

Premature ovarian failure in nobox-deficient mice is caused by defects in somatic cell invasion and germ cell cyst breakdown

“…Thus, it is possible that the downregulation of certain genes, such as Fgf8, results in the inability of Nobox deficient oocytes to properly communicate with ovarian somatic cells. Recent studies on hormonal regulation of ovarian development indicate that germ cell cyst breakdown correlates with the drop of maternal estradiol level [20], and that estradiol, progesterone and genistein inhibit cyst breakdown in vivo and in vitro [21]. There is also a study demonstrating that gonadotropin induces the changes in Nobox gene expression in mouse ovary [22].…”

Soy, phytoestrogens and their impact on reproductive health

“…These include alterations in ovarian development (increased percentage of multi-oocyte follicles (MOFs)), the timing of vaginal opening, estrous cyclicity, ovarian function, HPG axis, subfertility and an increased incidence of uterine adenocarcinoma (Chen et al, 2007;Delclos et al, 2001Delclos et al, , 2009Jefferson et al, 2006Jefferson et al, , 2002Jefferson et al, , 2005Kouki et al, 2003;Lewis et al, 2003;Nagao et al, 2001;Newbold et al, 2001;Nikaido et al, 2004;NTP, 2008). For example, in mice neonatal injection of genistein at doses of 0.5, 5 and 50 mg/kg/day on postnatal days 1-5 leads to prolonged estrous cycles with a dose-dependent and age-related increase in severity (Jefferson et al, 2002).…”