Structural Analysis of the Sea Urchin Egg Cortex Isolated on a Substratum

Summary

A method for the isolation of the sea urchin egg cortex appropriate for preservation of the cortical structure was sought. In the cortex of an egg at the streak stage isolated by homogenization, the cortical vacuoles tended to be situated apart from the plasma membrane to some extent, making a room which well-developed actin filament meshworks occupied. However, in the intact egg at the streak stage, which was rapid-frozen and freeze-substituted, the vacuoles were observed close to the plasma membrane. Homogenization may have induced the formation of the meshwork. On the other hand, the isolation on a protamine-coated substratum was found to preserve the cortical structure in situ and used for the electron microscopic analysis of the structural changes of the cortical cytoskeleton. Short actin filaments in microvilli of the unfertilized egg cortex were demonstrated clearly for the first time. They were attached to the plasma membrane at their barbed ends and cross-linked with each other by thin strands. A dramatic actin polymerization which occurs soon after fertilization was observed as a formation of a filamentous layer underneath the plasma membrane. Subsequently, this layer reduced in thickness and microvillar actin filament bundles become prominent structures of the cortical cytoskeleton. These results supported previous studies using fluorescence microscopy.