Steroid production by amphibian (Rana nigromaculata) ovarian follicles at different developmental stages

Rana ovarian follicles at different developmental stages were obtained from frogs collected at different seasons. Isolated follicles were sorted into five different classes (Class I–V) by size and cytoplasmic pigmentation. They were incubated for 6 hours in amphibian Ringer in the presence or absence of frog pituitary homogenate (FPH, 0.05 pituitary/ml) and changes in steroid production by follicles were assessed by measuring estradiol (E2), testosterone (T), and progesterone (P4) in media and follicles by radioimmunoassay. Small follicles (size range 0.22–0.7 mm) secreted very low or non‐detectable levels of the three steroids in the presence or absence of FPH. Medium‐sized vitellogenic follicles (0.70–1.11 mm) actively secreted all the three types of steroids tested (E2, P4, and T). Larger follicles (1.15–1.58 mm), isolated during September–February, secreted much more T than E2 and P4. In contrast, fully grown follicles (> 1.50 mm) collected late in the hibernation period secreted more P4 and less T than the large follicles and very low levels of E2; however, follicles (Class V) collected in breeding season (May) secreted solely P4 and negligible amounts of T and E2. All types of follicles produced very low levels of the steroids without FPH stimulation. Intrafollicular levels of steroids essentially reflected those which accumulated in the medium. Oocyte responsiveness to exogenous hormone, as evidenced by germinal vesicle breakdown (GVBD), increased as the breeding season approached. The data indicate that only the medium‐sized follicles produced considerable amounts of E2 while larger follicles produced mainly T and full‐grown follicles collected during breeding season produced only P4.