Cortical reorganization following fertilization of sea urchin eggs: Sensitivity to cytochalasin B

A transient sensitivity to cytochalasin B was detected in freshly fertilized sea urchin eggs. Large membrane-limited protrusions formed when cytochalasin B was added between 1 and 6 min postinsemination. When cytochalasin B was removed from the cultures, eggs continued to develop with protrusions remaining, and cleavage furrows formed through many of the protrusions. Electron microscopy showed that cytochalasin B disrupted the microfilamentous structures that formed in the egg cortex after fertilization. Protrusions also appeared when eggs were activated by the Ca²⁺ ionophore A23187 followed by cytochalasin B addition. Protrusions did not appear when cytochalasin B was added at any time prior to fertilization, during insemination, or after the 6-min period postinsemination. Mechanical compression or incubation in a cortical isolation medium were effective as treatments enhance the cytochalasin B-sensitive response. These results suggest a time frame for postfertilization changes in the cortex of the zygote.