EFFECT OF CUMULUS MATURITY ON SPERM PENETRATION IN THE GOLDEN-HAMSTER

The influence of the cumulus on gamete interaction was investigated in naturally and precociously ovulated eggs from superstimulated (eCG) hamsters. Inseminations were performed in which the absolute gamete ratio (sperm/egg) was held constant, but the relative gamete ratio (sperm x egg-1 x ml-1) was varied 10-fold. Sperm penetration to the perivitelline space was significantly higher at the higher relative gamete ratio, indicating that there is no chemoattractant in the cumulus and that gamete interaction in cumulus-intact eggs is a function of collision frequency, which varies according to the relative gamete ratio. The level of successful gamete interaction as a function of the relative gamete ratio was determined for naturally and precociously ovulated eggs. The level of sperm penetration reached saturation at a relative gamete ratio of 10(3.3) for naturally ovulated eggs and lo'' sperm x egg-1 x ml-1 for precociously ovulated eggs. At suboptimal relative gamete ratios, there was no difference in the level of penetration between partially and fully capacitated sperm. These results indicate that the larger, more mature cumulus of naturally ovulated eggs is successfully penetrated at lower collision frequencies than the denser, less mature cumulus of precociously ovulated eggs. To determine whether the maturity of the cumulus affects the timing of sperm penetration, cumulus-free and cumulus-intact eggs were inseminated at their optimal relative gamete ratios. The rate of sperm penetration of partially capacitated sperm was not different between cumulus-free and cumulus-intact eggs, regardless of ovulation method. However, the period of time between insemination and when half the eggs were penetrated (T50) was longer in cumulus-intact eggs. There was no difference in either the rate or T50 of sperm penetration of cumulus-free and cumulus-intact eggs when fully capacitated sperm were used. These data indicate that the cumulus has a dual role in that it acts as a barrier to partially capacitated sperm, but it has a positive effect on the penetration of fully capacitated sperm.