Oocytes of baboon fetal primordial ovarian follicles express estrogen receptor β mRNA

In fetal ovaries of estrogen-suppressed baboons, we have previously shown that follicle numbers were 50% lower than in estrogen-replete animals and contained oocytes with a reduced number of microvilli. In the baboon fetal ovary, although estrogen receptor (ER)alpha and beta have been detected by immunocytochemistry in granulosa cells, it is not known whether oocytes express ER. Because the actions of estrogen are mediated by interaction with cell-specific receptors, the current study determined whether ER alpha/beta mRNA were expressed in oocytes of baboon fetal ovaries obtained on day 165 (term = day 184) of gestation. Oocyte nuclei and cytoplasm from primordial follicles were isolated by laser capture microdissection and ER alpha, ER beta, GATA-4 (granulosa cell specific marker) mRNAs, and 18S rRNA determined by RT-PCR and products verified by sequencing. ER beta mRNA was expressed in oocytes of 5 of 5 fetuses. In contrast, fetal oocytes did not express ER alpha mRNA. Although 18S rRNA was expressed in all oocytes, GATA-4 mRNA was not detected in oocytes and only detected in granulosa cells confirming purity of oocytes sampled. We conclude that oocytes of the fetal baboon ovary express ER beta mRNA, thereby providing a mechanism by which estrogen regulates oocyte function, e.g. microvillus development.