Ontogeny of steroidogenic enzyme gene expression in ovarian theca-interstitial cells in the rat: regulation by a paracrine theca-differentiating factor prior to achieving luteinizing hormone responsiveness.

The theca cells (TC) first become identifiable in preantral follicles after the granulosa cells (GC) begin to divide. It remains unknown when the TC first respond to LH and acquire the capacity to produce androgens. The signal initiating TC differentiation is also unknown since pre-theca cells do not contain LH receptors. Since the first wave of follicle development in the rat occurs postnatally, we correlated the function of dispersed ovarian cells from 4-, 5-, 6-, 7-, and 10-day-old rats with the morphological differentiation of TC. The largest follicles in ovaries from 4-day-old rats were primary follicles without associated TC. These cells were unable to produce cAMP or steroids in vitro in response to hCG. At 5 days, the first theca were associated with follicles containing 2-3 layers of GC. These cells were responsive to hCG, producing cAMP, progesterone, androstenedione, and androsterone. Responses to hCG increased progressively through 10 days of age. Cholesterol side-chain cleavage (P450scc), 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), and 17 alpha-hydroxylase/C17-20 lyase (P450(17 alpha)) enzymes were localized exclusively to the theca interna. Messenger RNAs for LH receptor, P450scc, 3 beta-HSD, and P450(17 alpha) were expressed prior to the time the TC become responsive to LH or morphologically differentiated. To determine the source of the signal regulating TC differentiation, dispersed cells from 4-day-old rat ovaries that were unresponsive to LH were treated with preantral follicle-conditioned medium containing thecal differentiating factor (TDF) activity. The TDF activity stimulated androgen production and expression of LH receptor, P450scc, 3 beta-HSD, and P450(17 alpha) mRNAs. These data demonstrate that a paracrine signal from the preantral follicle can initiate TC differentiation prior to expression of LH receptors. TC become responsive to LH and capable of producing androgens coincident with morphological differentiation.