Steroidogenesis in Fundulus heteroclitus: II. Production of 17α-hydroxy-20β-dihydroprogesterone, testosterone, and 17β-estradiol by various components of the ovarian follicle

Fundulus heteroclitus prematurational follicles (1.3–1.4 mm) were dissected into various components and cultured in vitro to examine the type of cells involved in the synthesis of steroids upon F. heteroclitus pituitary extract (FPE) stimulation or addition of exogenous precursors (25-hydroxycholesterol or pregnenolone). Culture media and follicular tissue extracts were assayed for 17α-hydroxy-20β-dihydroprogesterone (17α-OH,20β-DHP), testosterone (T), and 17β-estradiol (E 2) content using specific radioimmunoassays. Complete removal of the follicle wall (denuded oocytes) eliminated steroid accumulation induced by FPE treatment. Removal of the theca/epithelium layer (defolliculated oocytes) did not affect the steroidogenic response (17α-OH,20β-DHP, T, E 2 production) of the follicles to FPE or exogeneously added precursors (25-hydroxycholesterol or pregnenolone). Isolated theca/epithelium layers secreted only T. Isolated follicular preparations that did not contain the oocyte (theca/epithelium layers or follicle cells) secreted higher levels of steroids to the culture media than did intact follicles. We conclude from these results that (1) the follicle cells (granulosa cells) are the primary source of the various steroids produced by the F. heteroclitus ovarian follicle in response to FPE stimulation; (2) the synthesis of 17α-OH,20β-DHP and E 2 does not require the involvement of two cell types as shown in other teleosts; (3) the theca/epithelium layer is able to produce T but lacks the aromatase activity necessary for E 2 synthesis; and (4) steroids synthesized in the follicle wall are both secreted to the medium and accumulated in the oocyte.