Effect of follicular cells, IGF-I and tyrosine kinase blockers on oocyte maturation.

The aim of this study was to test the following hypotheses: (i) that oocyte maturation is controlled by surrounding follicular cells; (ii) that a meiosis-regulating factor of follicular origin is not species-specific; (iii) that one of the follicular regulators of oocyte maturation is IGF-I; and, (iv) that Cumulus oophorus and tyrosine kinase-dependent intracellular mechanisms do not mediate IGF-I action on oocytes. It was found that co-culture of cumulus-enclosed bovine oocytes with isolated bovine ovarian follicles or with isolated porcine ovarian follicles significantly increased the proportion of matured oocytes (at metaphase II of meiosis) after culture. Porcine oocytes without cumulus investments had lower maturation rates than cumulus-enclosed oocytes. Co-culture with isolated porcine ovarian follicles resulted in stimulation of maturation of both cumulus-free and cumulus-enclosed porcine oocytes. These observations suggest that follicular cells (whole follicles or Cumulus oophorus) support bovine and porcine oocyte maturation, and that follicular maturation-promoting factor is not species-specific. The release of significant amounts of IGF-I by cultured bovine and porcine isolated follicles and granulosa cells was demonstrated. Addition of IGF-I to culture medium at 10 or 100 (but not 1000) ng/ml stimulated meiotic maturation of both cumulus-enclosed and cumulus-free porcine oocytes. Neither of the tyrosine kinase blockers, genistein or lavendustin (100 ng/ml medium), changed the stimulating effect of IGF-I on porcine oocytes. The present data suggest that at least one of the follicular stimulators of oocyte nuclear maturation is IGF-I, and that its effect is probably not mediated by cumulus investment or by tyrosine kinase-dependent intracellular mechanisms.