Enhancement of cumulus expansion and nuclear maturation during bovine oocyte maturation in vitro by the addition of epidermal growth factor and insulin-like growth factor I

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Abstract
This study was carried out to identify the possible effects of epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I) on cumulus expansion and nuclear maturation in bovine oocytes matured in vitro in serum-free media. Bovine follicular oocytes were matured in vitro and were then classified as cumulus cell-enclosed oocytes or denuded oocytes. They were then cultured for 24 h in TCM-199 (tissue culture medium-199) with EGF (50 ng ml-1), IGF-I (100 ng ml-1) and EGF plus IGF-I (50 and 100 ng ml-1, respectively). At the end of the culture period, the morphology of oocytes was evaluated for cumulus expansion and nuclear stage of maturation. In Expt 1, percentages of oocytes reaching maximum cumulus expansion were: 12.5% (control), 46.5% (EGF), 15.2% (IGF-I) and 52.8% (EGF plus IGF-I). In Expt 2, the respective rates of nuclear maturation were: 35.6%, 52.1%, 45.5% and 61.4% for cumulus cell-enclosed oocytes, and 35.3%, 46.6%, 45.4% and 42.5% for denuded oocytes; the same growth factor treatments were used in both cases. There was no significant effect of IGF-I on cumulus expansion. Maximum rates of cumulus expansion and nuclear maturation were obtained in the presence of both growth factors. These results lead to the following conclusions: (i) EGF, either alone or together with IGF-I, stimulates cumulus expansion; and (ii) both growth factors, acting alone or together, enhance nuclear maturation in oocytes surrounded by compact cumulus cells.

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