Overripening of ovulated eggs in goldfish, Carassius auratus: II. Possible involvement of postovulatory follicles and steroids.

Changes in steroid hormone levels in the serum and ovarian fluid during overripening were studied in goldfish. Ovulated eggs retained in the ovarian cavity became overripe at around 12h after ovulation based on loss of fertilizability, with advanced degeneration by 24h. Blood and ovarian fluid were taken at 0, 3, 6, 12, 18 and 24h after ovulation. Both serum and ovarian fluid progesterone (P) showed a highly significant decline by 18h with a further decline by 24h; P levels were higher in the ovarian fluid. Serum 17α,20β-dihydroxy-4-pregnen-3-one(17,20β-P) levels showed a progressive and more rapid decline, decreasing significantly by 12h with further decreases by 18h and 24h. Serum testosterone (T) levels increased significantly at 3h and remained high till 18h, thereafter they declined to the 0h level. No significant changes in estradiol-17β (E2) levels were observed in the serum, except for a significant difference between 6 and 24h. There were no significant changes in ovarian fluid E2, T or 17,20β-P levels.The postovulatory follicles (POFs) showed degenerative changes which corresponded to the decline in P and 17,20β-P. The hypothesis that overripening may be associated with declining levels of P or 17,20β-P was tested in vivo by immersing just ovulated females (0h) in P or 17,20β-P solutions for 12h, and in vitro by immersing just-ovulated eggs (0h) in ovarian fluid with an anti-serum against P (Anti-P). In the former, P at 0.05 ppm significantly enhanced the fertilization rate of ovulated eggs stripped from the females at 12h while 17,20β-P did not produce a significant effect at the tested concentrations (0.025 and 0.05 ppm) on the fertilization rate. In the latter, anti-P significantly lowered the fertilization rate of 0h ovulated eggs after 6h incubation. The evidence suggests a role of P in the maintenance of viability of ovulated eggs.