INDUCTIVE AND INHIBITORY ACTIONS OF A LOW MOLECULAR WEIGHT SERUM FACTOR ON IN VITRO MATURATION OF OOCYTES OF THE MEDAKA

Abstract

A low molecular weight factor in chicken and rabbit steroid-free sera (LMS) can induce in vitro maturation of intrafollicular oocytes of the fish Oryzias latipes by a two-step incubation procedure, but not that of follicular cell-free oocytes. In vitro maturation of intrafollicular oocytes induced by gonadotropin and 17α,20β-didihydroxy-4-pregnen-3-one (DHP) was reversibly inhibited by LMS. The inhibitory, as well as the inductive effect, of the rabbit LMS was biphasic and dosedependent, with an optimum concentration at 0.5 mg/ml. Cyclic AMP production was also stimulated by a similar concentration of LMS. Addition of LMS to the culture medium enhanced secretion of estradiol-17β and DHP by granulosa cells. Further incubation after removal of LMS from culture medium resulted in increased secretion of DHP and decreased secretion of estradiol-17β. The present findings suggest that LMS is available as a tool for investigation of the mechanism of maturation of medaka oocytes.