Nuclear maturation of ovine oocytes in cultured preantral follicles

Small (150-250 μm in diameter) and large (251-400 μm in diameter) preantral follicles (PFs) in sheep were cultured for 6 days in four different concentrations of transforming growth factor-alpha (TGF-α), epidermal growth factor (EGF), FSH and LH. Proportions of follicles exhibiting growth, antrum formation and increase in follicular and oocyte diameter were the initial indicators of development. The ability of the oocytes isolated from these cultured follicles to mature to metaphase II (MII), after 24 h culture in a known in vitro maturation medium was the final criterion of success. TGF-α 2.5 ng ml−1, EGF 50 ng ml−1 and FSH 1 and 2 μg ml−1 supported good initial growth of the PFs. Thirty and seventeen percent of the oocytes from the large PFs cultured in TGF-α 2.5 ng ml−1 and FSH 2 μg ml−1 respectively, matured to the MII stage. These proportions for oocytes from small PFs were 11 and 6%, respectively. Oocytes from follicles cultured in EGF did not mature to the MII stage. LH at all concentrations tested and TGF-α, EGF and FSH above 5, 50 ng ml−1 and 2 μg ml−1, respectively, induced degeneration of the PFs. It was concluded that (i) TGF-α 2.5 ng ml−1 supports development of large PFs in sheep to obtain meiotically competent oocytes, (ii) PFs > 250 μm in initial diameter develop better in vitro, and (iii) in vitro development of sheep PFs could be obtained independent of gonadotropin stimulation.