Ultrastructural Basis of the Tension Increase in Sea-Urchin Eggs Prior to Cytokinesis

Cortices of sea-urchin eggs were studied by electron microscopy to identify the structure responsible for the rise in tension at the egg surface prior to cleavage. During anaphase the tension increased and fine filaments of 70–90 Å in diameter appeared in the cell cortex forming a thin mesh-work beneath the cell membrane. The meshwork spread all around the egg cortex without reference to the mitotic axis and the number of filaments seemed to increase up to telophase. Immediately before appearance of the cleavage furrow, the meshwork in the anticipated furrow region became dense. As the furrow appeared the tension began to decrease and the meshwork disappeared. In the progressing furrow region fine filaments of the same size as that of the meshwork-filament were oriented in a bundle to form a contractile ring. Treatment with cytochalasin B suppressed both the tension increase and the formation of the filamentous meshwork. These results suggest that the component filament of the meshwork is an actin microfilament, and that the tension increase at anaphase is due to formation of a meshwork of actin microfilaments from which a contractile ring is subsequently derived at late telophase.