Unfertilized sea urchin eggs contain a discrete cortical shell of actin that is subdivided into two organizational states.

Changes in the distribution and organizational state of actin in the cortex of echinoderm eggs are believed to be important events following fertilization. To examine the initial distribution and form of actin in unfertilized eggs, we have adapted immunogold-labeling procedures for use with eggs of Strongylocentrotus purpuratus. Using these procedures, as well as fluorescence microscopy, we have revealed a discrete 1-micron-thick concentrated shell of actin in the unfertilized egg cortex. This actin is located in the short surface projections of unfertilized eggs and around the cortical granules in a manner that suggests it is associated with the cortical granule surface. The actin in the short surface projections appears to be organized into filaments. However, most if not all of the actin surrounding the cortical granules is organized in a form that does not bind phalloidin, even though it is accessible to actin antibody. The lack of phalloidin binding is consistent with either the presence of nonfilamentous actin associated with the cortical granules or the masking of actin-filament phalloidin-binding sites by some cellular actin-binding component. In addition to the concentrated shell of actin found in the cortex, actin was also found to be concentrated in the nuclei of unfertilized eggs.