A shift in steroidogenesis occurring in ovarian follicles prior to oocyte maturation

Abstract

Gonadotropins (GTHs; FSH and LH) require two major steroidal mediators, estradiol-17β (E2) and 17α,20β-dihydroxy-4-pregnen-3- one (17α,20β-DP) to act as critical hormones to execute oocyte growth and maturation, respectively. A two-cell type model has been proposed, where the theca cells provide the precursor steroids, and the granulosa cells produce the two steroidal mediators under the direct influence of FSH and LH. A distinct shift in steroidogenesis, i.e. from E2 to 17α,20β-DP as well as the steroidogenic enzyme genes from ovarian cytochrome P450 aromatase (oP450arom) to 20β-hydroxysteroid dehydrogenase (20β-HSD), occurs in the granulosa layers of ovarian follicles prior to oocyte maturation. The triggering of the steroidogenic shift by GTHs in granulosa cells occurs through the subjugation of Ad4BP/SF-1 expression in respect of oP450arom, followed by an over-expression of 20β-HSD probably through the CREB. © 2003 Elsevier Ireland Ltd. All rights reserved.