Dissociation of oocyte nuclear and cytoplasmic maturation by the addition of insulin in cultured bovine antral follicles.

Follicles of 4-8 mm in diameter were dissected from ovaries and cultured in Waymouth culture medium in the presence or absence of insulin (5 mug/ml) at 39 degrees C in a humidified atmosphere of 45% O2, 5% CO2 and 50% N2 for 24 h. Following follicle culture, the oocytes were collected and examined for developmental potential, total protein profile and ultrastructural aspects. Oocytes aspirated directly from follicles of the same size were used as controls. Addition of insulin to the follicle culture medium significantly reduced expression of the low molecular weight insulin-like growth factor-binding proteins (IGFBPs) in the follicular fluid, and significantly reduced the cleavage rate of subsequently matured and fertilised oocytes (0.52 vs 0.61). However, there were no differences in the proportion of cleaved embryos which developed to the blastocyst stage (0.30 vs 0.28), nor embryo quality as assessed by total cell number (137 +/- 8.53 vs 124.6 +/- 6.95). The total protein profiles of immature oocytes recovered after 24 h of follicle culture were compared by PAGE. There were marked differences between the two groups, unmatured oocytes recovered from the insulin-positive follicle group showed a protein pattern similar to that of matured oocytes. In addition, examination of ultrastructural features by transmission electron microscopy indicated that oocytes from follicles cultured in the presence of insulin undergo many of the cytoplasmic changes associated with oocyte maturation. In conclusion, follicle culture in the presence of insulin is beneficial for follicular survival and significantly reduces cleavage but has no detrimental effects on the development of cultured embryos. However, many of the cytoplasmic changes associated with oocyte maturation occur prior to the induction of nuclear maturation.