Regulation of Gap Junctions and Oocyte Maturational Competence by Gonadotropin and Insulin-like Growth Factor-I in Ovarian Follicles of Red Seabream

Abstract

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<0.05). Functional assays confirmed that the hormonal treatments also induced OMC. Naturally competent follicles sampled at the appropriate stage of the reproductive cycle contained high levels of both types of GJ suggesting that the OMC-associated increase in GJ is physiologically relevant. In view of similar, earlier results with the Atlantic croaker, the present results support the concept that an increase in homologous and heterologous GJ is a general developmental event associated with the acquisition of OMC in teleost ovarian follicles. Also, a novel finding of this study is that IGF-I can regulate heterologous and homologous GJ in intact ovarian follicles.