Dynamics of filamentous actin organization in the sea urchin egg cortex during early cleavage divisions: implications for the mechanism of cytokinesis.

We have used confocal laser scanning microscopy in conjunction with BODIPY-phallacidin staining of filamentous actin to investigate changes in the quantity and organization of cortical actin during the first two cell cycles following fertilization in eggs of the sea urchin Strongylocentrotus purpuratus. Quantification of fluorescent phallacidin staining reveals that the amount of filamentous actin (F-actin) in the cortex undergoes cyclical increases and decreases during early cleavage divisions, peaking near the beginning of the cell cycle and decreasing to a minimum at cytokinesis. Changes in the content of cortical F-actin are accompanied by the growth and disappearance of rootlet-like bundles of actin filaments which extend from the bases of microvilli that cover the surface of the egg. Actin rootlets reach their maximum degree of development by 20 min postfertilization, and then gradually decrease in number and length over the next 40 min. Small actin rootlets persist until cleavage, disappear during cytokinesis, and reform following division. The formation of actin rootlets requires cytoplasmic alkalization and is inhibited by cytochalasin D. Cytochalasin D washout experiments demonstrate that assembly of the cortical actin cytoskeleton can be blocked until 5 min before the onset of cleavage and still allow normal cytokinesis. These results illustrate the dynamic nature of cortical actin organization during early development and demonstrate that cytokinesis occurs at the point of minimum cortical F-actin content. They further demonstrate that cytokinesis can occur in embryos in which the normal developmental sequence of changes in cortical actin organization has been blocked by treatment with cytochalasin D, suggesting that these changes do not function in the establishment of the contractile apparatus for cytokinesis, but rather serve other developmental functions. Cell Motil. Cytoskeleton 36:30-42, 1997.