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 insulin-like growth factor-I (IGF-I) on the formation of oocyte–granulosa cell (heterologous) 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-grown 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. Qualitative 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 follicles treated with hCG or IGF-I. For quantitative analysis, we measured changes in the combined numbers of both GJ types, since discrimination between the two was not always possible. The numbers of GJ along 125 μm of granulosa cell–zona radiata border were counted in three follicles/treatment. GJ markedly increased with time in the presence of hCG or IGF-I but not in control medium (Duncan's multiple range test; P