The role of the insulin-like growth factor (IGF) system in zebrafish (Danio rerio) ovarian development

The presence of an ovarian IGF system in teleosts suggests a distinct role in reproductive physiology. This study investigates the role of the ovarian IGF system in oocyte maturation, the acquisition of maturational competence and steroidogenesis in the zebrafish (Danio rerio). Recombinant human IGF-I and IGF-II stimulated germinal vesicle breakdown (GVBD) in early vitellogenic (EV; 0.35-0.44 mm), midvitellogenic (MV; 0.45-0.56 mm) and full grown (FG; 0.57-0.65 mm) follicles incubated in vitro. By comparison, the maturation inducing steroid 17alpha,20beta-dihydroxy-4-pregnen-3-one (17,20beta-P) only induced GVBD in MV and FG follicles. Collectively these studies suggest that IGF is involved in oocyte maturation and that follicles become responsive to IGFs at an earlier stage compared to 17,20beta-P. IGF-I also increased the responsiveness of the follicle to 17,20beta-P, suggesting a role in promoting maturational competence. IGF-I alone and in combination with human chorionic gonadotropin (hCG) stimulated the production of 17,20beta-P by ovarian follicles incubated in vitro. However, IGF-I had no effect on the production of 17beta-estradiol (E(2)) or the expression of genes involved in steroidogenesis (20beta-hydroxysteroid dehydrogenase; 20beta-HSD and P450c17-II). These results provide evidence that the IGF system plays an important role in the promotion of oocyte maturation and ovarian development in the zebrafish.