STIMULATORY EFFECT OF INSULIN-LIKE GROWTH-FACTOR-I AND EPIDERMAL GROWTH-FACTOR ON THE MATURATION OF RABBIT OOCYTES IN-VITRO

The effects of different combinations of epidermal growth factor (EGF) and insulin-like growth factor I (IGF-I) on cumulus expansion and mei otic maturation were examined in rabbit oocytes. Selected rabbit folli cular oocytes were matured in vitro and were classified as cumulus-ooc yte complexes or denuded oocytes. They were cultured in TCM 199, and w ere treated with growth factors at different concentrations: EGF at 0, 1, 10, 50 and 100 ng ml(-1), IGF-I at 0, 50, 100 and 200 ng ml(-1) an d EGF plus IGF-I at 10 + 50; 10 + 100; 50 + 50 and 50 + 100 ng ml(-1), respectively. After 6 h of culture, the oocytes were assessed for nuc lear maturation and after 16 h of culture, for cumulus expansion and m aturation stage. After culture for 6 h, the incidence of germinal vesi cle breakdown was higher (P < 0.05) in all of the growth factor treatm ents tested compared with controls. After culture for 16 h, EGF enhanc ed the incidence of cumulus expansion at all of the concentrations tes ted. Cumulus expansion was greatest with 50 mg EGF ml(-1) plus 100 ng IGF-I ml(-1) (72.0% versus 2.4% in controls). Treatment with IGF-I sig nificantly increased (P < 0.05) the incidence of metaphase II stage, a nd maximum stimulation occurred at 100 ng IGF-I ml(-1) (84.5% versus 3 1.1% in controls). However, IGF-I did not affect cumulus expansion. Wh en denuded oocytes were used, no positive effects on nuclear maturatio n rates were observed for any treatment. These results suggest that: ( I) EGF, either alone or with IGF-I, stimulates cumulus expansion; (2) the addition of IGF-I or EGF plus IGF-I significantly enhances nuclear maturation in immature rabbit oocytes; and (3) this effect is mediate d by the presence of cumulus cells.