The temporal sequence of changes in oocyte maturation and ovarian steroid hormone production during induced ovulation in the common carp, Cyprinus carpio

Abstract

Oocytes taken from carp sacrificed 3–24 hr after injection with either carp hypophysial homogenate (chh) (primed) or saline (control) were incubated with or without chh for 24 hr at 20°. In oocytes from the primed fish the percentage of peripheral germinal vesicles at sacrifice progressively increased with time from injection. No such change occurred in control fish. After incubation with chh, tissue from primed fish showed a progressive increase in oocyte maturity with increasing time between priming and sacrifice, with germinal vesicle breakdown (GVBD) in 50% of the oocytes in incubations begun 24 hr after priming. In contrast, incubated tissue from control fish showed no variation with time between saline injection and sacrifice, and oocyte maturity did not exceed 50% with peripheral germinal vesicles. GVBD was not significant in these fish. 17-Hydroxyprogesterone, testosterone, and testosterone glucuronide production in vitro showed no major changes with the interval between injection and sacrifice, nor between control and primed fish. In vitro production of 17,20β-dihydroxy-4-pregnen-3-one (17,20βP), however, increased significantly between tissue taken from animals at 3 and at 6 hr after injection of chh, and thereafter remained unchanged. 17,20βP production preceded the increase in GVBD in primed fish but neither this hormone nor GVBD was found in incubations of control fish, supporting the suggestion that 17,20βP is the maturation-inducing hormone in carp. Steroid production was not detectable in the absence of chh in the incubation medium and oocyte maturation showed little advance on that at sacrifice. The results suggest that migration of the germinal vesicle and induction of 17,20βP potential by the priming dose are independent events and they are discussed in the context of the failure of some fish to ovulate despite apparently high initial oocyte maturity.