Perinatal expression of estrogen receptor (ER) protein and mRNA and the influence of FSH on this process were examined by immunoflourescence and RT-PCR using ovaries from fetal (d 13-15 of gestation) and postnatal [postnatal d 1-15 (P1-P15)] hamsters and from 8-d-old hamsters exposed in utero to an anti-FSH serum on d 12 of gestation and saline or equine chorionic gonadotropin (eCG) on P1. A few somatic cells expressing ERα immunoreactivity appeared first on d 14 of gestation and increased markedly by P8-P15 in the interstitial cells and granulosa cells of primordial follicles. In contrast, appreciable ERβ immunoreactivity was localized on d 13 gestation, and more cells expressed ERβ immunoreactivity by P1-P8. By P7, ERβ immunoreactivity was present in cells adjacent to the oocytes, and by P8, ERβ was preferentially localized in the granulosa cells. Receptor immunoreactivities decreased markedly in P8 ovaries exposed in utero to the FSH antiserum but were reversed with postnatal eCG replacement. Oocytes and somatic cells expressed ERα and ERβ mRNA, and levels of ER mRNA in the ovary increased by P7-P8, corresponding to the appearance of primordial follicles. Thereafter, only ERβ mRNA levels incraesed progressively with postnatal ovary development. Similar to ER protein, mRNA levels decreased significantly in FSH antiserum-treated ovaries but were restored by eCG. These results indicate that both ER subtypes are expressed in undifferentiated somatic cells and the oocytes during perinatal ovary development in the hamster; however, ERβ expression segregates with the differentiation of granulosa cells. Furthermore, ER expression and differentiation of somatic cells to granulosa cells depend on perinatal FSH action.
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