Follicles were dissected by hand or enzymatically from the ovary of the proestrous hamster at 0900 h and classified into 10 stages: stages 1–4, follicles with 1–4 layers of granulosa cells and no theca; stages 5–8, preantral follicles with 5 or more layers of granulosa cells and theca to small antral follicles; stage 9, intermediate-sized atretic antral follicles; and stage 10, healthy preovulatory antral follicles. Follicles were then incubated for 2 h with [3H]thymidine ([3H]Tdr) in the absence or presence of gonadotropins and with incorporation of radionuclide into DNA as the end point. FSH (25 ng) significantly stimulated [3H]Tdr incorporation in all stages of follicular development with a latency of 2 h, and this effect was inhibited by 2 μg unlabeled Tdr. While FSH and PRL (25 and 100 ng) stimulated [3H]Tdr incorporation in all stages, LH (0.2–5 ng) action began from stage 5 onward, when definitive thecal cells and LH receptors started appearing. LH (5 ng) also suppressed 25 ng FSH-induced DNA synthesis in stages 5μ10; however, stages 1–4 were unaffected. Significant increases in both intraand extracellular cAMP levels occurred in follicles at stages 2–10 after FSH administration. In contrast, LH was active in stages 5–10, whereas PRL was ineffective. Follicular DNA synthesis increased markedly when stimulated by 8-bromo-cAMP (0.01–2 mM). These results show that gonadotropins act directly as a primary stimulus at the level of small primary and secondary follicles to regulate DNA synthesis and, thus, perhaps the growth and differentiation of granulosa and thecal cells; cAMP functions as one of the possible intracellular mediators of gonadotropin action in initiating DNA replication.
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