Early Events in Anuran Amphibian Fertilization: An Ultrastructural Study of Changes Occurring in the Course of Monospermic Fertilization and Artificial Activation

In unfertilized frog eggs, the plasma membrane displays an animal vegetal polarity characterized by the presence of short microvilli in the vegetal hemisphere and long microvilli or ridge-like protrusions in the animal hemisphere. The densities of microvilli are similar in the two hemispheres.
The fertilizing sperm always fuses with the animal hemisphere of the egg and induces a wave of exocytosis of cortical granules from its site of penetration. Similar spreading of the cortical reaction is seen on activation by pricking the egg cortex. The integration of the cortical granule membrane with the plasma membrane is rapidly followed by elongation of microvilli, which is progressively realized all over the egg surface from the site of sperm entry or the site of pricking. At this time, the length and shape of the microvilli in the animal and vegetal hemispheres are similar and their densities are the same as in unfertilized eggs.
A "smoothing" wave can be seen on the living egg, 40–60 seconds after pricking, starting around the site of pricking. This wave of microvillar elongation is accompanied by changes in intensity of diffracted light spots observed at the surface of the egg. This pattern might result from rapid and progressive thickening of the cortex that would drive pigment granules into the cytoplasm. The Brownian movement of these granules is thought to be responsible for the observed diffracted light spots.
Electrical stimulus or the ionophore A23187 induced activation reactions similar to those triggered by the sperm or by pricking, except that the cortical reaction began simultaneously in several distinct sites of the cortex.