DNA replication in mammals is temporally bimodal. "Housekeeping" genes, which are active in all cells, replicate during the first half of the S phase of cell growth. Tissue-specific genes replicate early in those cells in which they are potentially expressed, and they usually replicate late in tissues in which they are not expressed. Replication during the first half of the S phase is, therefore, a necessary but not sufficient condition for gene transcription. A change in the replication timing of a tissue-specific gene appears to reflect the commitment of that gene to transcriptional competence or to quiescence during ontogeny. Most families of middle repetitive sequences replicate either early or late. These data are consistent with a model in which two functionally distinct genomes coexist in the nucleus.

To determine if thecal cells of rat preovulatory (PO) follicles become functionally luteinized, theca from small antral (SA) and PO follicles were isolated before and 8 h after iv injection of an ovulatory dose (10 IU) of hCG. Thecal explants were cultured for 30 days in Dulbecco's Modified Eagle's Medium-Ham's F-12 medium containing 1% fetal calf serum (FCS) with or without 5 ng/ml ovine LH or 10 microM forskolin. Whereas theca from SA, hCG-treated SA, and PO follicles were dependent on LH or forskolin to maintain progesterone (greater than 10 ng/ml) and androstenedione (greater than 10 ng/ml) accumulation, luteinizing theca (hCG-treated PO) accumulated more than 10 ng/ml progesterone and more than 2 ng/ml androstenedione with or without LH or forskolin for 30 days. Granulosa cells were isolated from these same follicles and cultured under similar conditions, including 10 ng/ml testosterone and 25 ng/ml ovine FSH. Only granulosa cells isolated from luteinizing follicles (hCG-treated PO) maintained progesterone (greater than 20 ng/ml) and estradiol (10 ng/ml) accumulation with or without FSH or forskolin for 30 days. Basal concentrations of cAMP were 5 to 10-fold higher in thecal and granulosa cells from luteinizing follicles than in these tissues isolated from SA or PO follicles. We conclude that thecal cells as well as granulosa cells of rat PO follicles respond to the LH/hCG surge by becoming functionally luteinized, less dependent on LH, and capable of maintaining an increased accumulation of basal cAMP. Furthermore, the data suggest that one luteinizing thecal explant produces a similar amount of progesterone as one follicle equivalent of luteinizing granulosa cells. Thus, luteinized theca have the potential of contributing significantly to progesterone secretion by the mature rat corpus luteum.

Regulation of 9.5-kb androgen receptor mRNA concentrations in Sertoli and peritubular cells from 20-day-old rats was studied by Northern blot analysis. Treatment of cells in vitro for 1-7 days with 300 ng/ml FSH increased androgen receptor mRNA up to 4-fold in Sertoli cells but not in peritubular cells. Testosterone (100 ng/ml) had no effect or slightly decreased androgen receptor mRNA in Sertoli and peritubular cells. Androgen receptor mRNA concentrations in Sertoli and peritubular cells from rats killed 15 days after hypophysectomy were elevated 4-5-fold over those in cells from intact rats. The androgen receptor mRNA concentration was decreased in both Sertoli and peritubular cells isolated from hypophysectomized animals treated with 500 micrograms/day testosterone propionate in vivo and subsequently with 100 ng/ml testosterone in vitro. FSH treatment (100 micrograms/day in vivo, followed by 300 ng/ml in vitro) did not increase androgen receptor mRNA over that in cells from hypophysectomized controls but rather decreased its concentration to varying degrees in Sertoli and peritubular cells. The rise in androgen receptor mRNA in both Sertoli and peritubular cells isolated from hypophysectomized animals is attributable, at least in part, to the absence of the inhibitory influence of testosterone. Other data in the literature suggest positive regulation of Sertoli cell androgen receptor protein by FSH and androgens. Consequently, complex mechanisms involving transcriptional, translational, and post-translational regulation probably control androgen receptor concentrations in the cells of the rat seminiferous tubule.

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