Reproduction Abstracts

Introduction: Recent studies have revealed the contribution of steroid hormones to in vitro growth of oocytes in different species. However, exactly how these steroid hormones support oocyte growth is not well understood. This study was conducted to examine the effect of various steroid hormones on in vitro growth of bovine oocytes and to determine the contribution of steroid hormones to oocyte growth by assessing the connections between oocytes and granulosa cells, transzonal projections (TZPs).

Materials and methods: Oocyte-granulosa cell complexes (OGCs, oocyte: 94.3±0.1 μm) were collected from bovine early antral follicles (0.4–0.7 mm in diameter) and cultured for 14 days with 17β-estradiol (E₂), androstenedione (A₄), testosterone (T) and dihydrotestosterone (DHT) either alone or in combination. After 14 days of culture, oocytes were subjected to in vitro maturation culture or fluorescence staining of TZPs.

Results and discussion: During growth culture, OGCs cultured with E₂ showed high survivability. Meanwhile, oocytes grown with androgens matured to metaphase II at high rates in maturation culture. When OGCs cultured with combinations of E₂ and androgens (A₄ or DHT), OGC survivability and maturation rate were increased compared to that cultured with one steroid hormone, and these oocytes grew to the similar size (120.0±0.6 or 118.8±0.7 μm) as in vivo grown oocytes. Moreover, the number of TZPs was maintained in these oocytes after growth, while oocytes grown without steroid hormones had few TZPs. The results suggest that steroid hormone support oocyte growth by maintaining oocyte-granulosa cell connections during growth culture. The results also indicate the different roles of steroid hormones in oocyte growth.