Regulation of gap junctions and oocyte maturational competence by gonadotropin and insulin-like growth factor-I in ovarian follicles of red seabream

The in vitro effects of human chorionic gonadotropin (hCG) and human insuli n-like growth factor-I (IGF-I) on the formation of oocyte-granulosa cell (h eterologous) and granulosa cell-granulosa cell (homologous) gap junctions ( GJ) were determined during the course of development of oocyte maturational competence (OMC) in ovarian follicles of red seabream. Incompetent, full-g rown follicles were incubated in hCG (10 IU/ml), IGF-I (10 nM), or hormone- free medium for 0, 6, and 12 h and examined by electron microscopy. Qualita tive analysis indicated that homologous and heterologous GJ were relatively rare in incompetent follicles and remained rare if they were maintained in control medium. However, both types of GJ were readily observed in follicl es treated with hCG or IGF-I. For quantitative analysis, we measured change s in the combined numbers of both GJ types, since discrimination between th e two was not always possible. The numbers of GJ along 125 mu m of granulos a cell-zona radiata border were counted in three follicles/treatment. GJ ma rkedly increased with time in the presence of hCG or IGF-I but not in contr ol medium (Duncan's multiple range test; P < 0.05). Functional assays confi rmed that the hormonal treatments also induced OMC. Naturally competent fol licles sampled at the appropriate stage of the reproductive cycle contained high levels of both types of GJ suggesting that the OMC-associated increas e in GJ is physiologically relevant. In view of similar, earlier results wi th the Atlantic croaker, the present results support the concept that an in crease in homologous and heterologous GJ is a general developmental event a ssociated with the acquisition of OMC in teleost ovarian follicles. Also, a novel finding of this study is that IGF-I can regulate heterologous and ho mologous GJ in intact ovarian follicles. (C) 1999 Academic Press.