Ontogeny of granulosa cells in the ovary: Lineage-specific expression of transforming growth factor β2 and transforming growth factor β1

The differentiation of ovarian somatic cells into granulosa, interstitial, and thecal lineages, and the ovarian growth factor(s) potentially associated with the cytodifferentiation of granulosa cells during the perinatal period were investigated using cell type-specific protein markers. Ovaries collected from prenatal (Days 12-15 of pregnancy) and postnatal (Days 1-30) female hamsters were processed for immunohistochemical localization of transforming growth factor (TGF) β (TGFβ1, β2, and β3). Plasma levels of FSH, LH, progesterone, and estradiol-17β were detected by RIA. Prenatal hamster ovaries contained numerous mitotic oocytes and a few somatic cells. Only a fraction of somatic cells expressed barely detectable TGFβ2 activity. Plasma FSH levels were quite detectable on postnatal Day 1 and increased gradually to reach a peak on Day 25, whereas LH did not increase until Day 12 and reached a plateau by postnatal Day 20. On postnatal Day 1, TGFβ2 immunoreactivity was localized only in certain cells closely apposed to primordial oocytes. On postnatal Day 1, flattened, TGFβ2-positive cells encircled individual oocytes, forming the very first cohort of primordial follicles. By Days 7 and 8, primary and early secondary follicles with intense TGFβ2-positive cuboidal granulosa cells appeared. Subsequently, in large preantral follicles TGFβ2 was expressed only in mural granulosa cells. On Day 13, TGFβ1 and β2 immunoreactivities appeared for the first time in the interstitial cells. TGFβ1 was localized in cells closely apposed to follicles, but TGFβ2 activity was restricted to scattered cell clusters. Subsequently, the entire interstitium was positive for TGFβ1 protein. These results suggest that differentiation of somatic cells into granulosa cells is the first event in ovarian morphogenesis; once the finite number of granulosa cells is selected, the residual cells differentiate into interstitium. Whether ovarian TGFβ2 and TGFβ1 are physiologically important in granulosa and interstitial cell differentiation needs further evaluation.