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

Abstract

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 to traverse the animal half, and travels slower in the vegetal half. Cortical granule exoctyosis was examined 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, the effect of Ca2+ on cortical granule breakdown was examined 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 sensitive to Ca2+ than vegetal half ones. Prick-activation of eggs at the vegetal pole was frequently unsuccessful, but would occur when external Ca2+ was raised. There are regional differences in the frog egg with respect to cortical granule responsiveness; the differences apparently are due to Ca2+ sensitivity.