Abstract
Recent studies in mice suggest that androgens are important for normal follicle development. However, there have been few reports concerning the action of androgens in the growth of oocytes from large animals. The purpose of this study was to determine the roles of androgens in bovine oocyte growth in vitro. Oocyte-granulosa cell complexes (OGCs) collected from 0.4-0.7 mm early antral follicles were cultured for 14 days with 17β-estradiol (E2) and a non-aromatizable androgen, dihydrotestosterone (DHT). We also examined the ability of an androgen receptor (AR) inhibitor, hydroxyflutamide, to antagonize the effect of androgens on the oocytes. During growth culture, the OGC structures collapsed in the medium with DHT alone, while in the presence of E2, the OGC structures were maintained. In the medium with both androgens and E2, the mean diameter of oocytes was increased from 95 μm to around 120 μm, larger than those grown with E2 alone (115 μm). Also in the maturation culture, oocytes grown with androgens (A4 or DHT) and E2 showed higher percentages of metaphase II oocytes (63% or 69%, respectively) than those grown with E2 alone (32%). Moreover, these maturation rates were decreased by hydroxyflutamide in a dose-dependent manner. Immunostaining showed that ARs were expressed in oocytes and granulosa cells in early antral follicles, and the nuclei of granulosa cells showed intense AR expression. In conclusion, although E2 supports the OGC structure, additional androgens promote oocyte growth and their acquisition of meiotic competence via AR during in vitro growth culture.