ANIMAL/VEGETAL DIFFERENCES IN CORTICAL GRANULE EXOCYTOSIS DURING ACTIVATION OF THE FROG EGG

One of the more striking morphological events during egg activation is exocytosis of the cortical granules. In the frog egg, the wave of cortical granule exocytosis takes about 100 sec to traverse the animal half, and travels slower in the vegetal half. We examined cortical granule exoctyosis during activation with respect to this animal/vegetal difference. In eggs which were acquiring the ability to be activated (recovering from CO2 -intoxication or undergoing meiotic maturation), animal half cortical granules became capable of responding to activating stimuli prior to vegetal half ones. Since Ca2+ is involved in exocytosis, we examined the effect of Ca2+ on cortical granule breakdown in vitro. There was no difference in sensitivity to Ca2+ of cortical granules from immature vs. mature eggs, but animal half cortical granules were more sensistive to Ca2+ than vegetal half ones. Finally, we found that prick-activation of eggs at the vegetal pole was frequently unsuccessful but would occur when external Ca2+ was raised. These experiments show that there are regional differences in the frog egg with respect to cortical granule responsiveness, and they suggest that the differences are due to Ca2+ sensitivity.