RELATIVE CHANGES IN F-ACTIN DURING THE FIRST CELL-CYCLE - EVIDENCE FOR 2 DISTINCT POOLS OF F-ACTIN IN THE SEA-URCHIN EGG

The cortical actin cytoskeleton undergoes dramatic rearrangements duri ng fertilization of sea urchin eggs. To characterize these changes fur ther, we quantified the relative changes in filamentous actin (F-actin ) during fertilization and the first cell cycle in both intact eggs an d in isolated cortices by quantitative fluorescence microscopy. The le vel of F-actin in the intact egg decreased after fertilization and con tinued to decrease throughout the first cell cycle. By 60 min after fe rtilization, the level of F-actin had decreased to 50% of the unfertil ized sea urchin egg. By cytokinesis, the level of F-actin had decrease d to 30% of the unfertilized egg. After completion of cell division, i ndividual blastomeres had 10% of the F-actin in the unfertilized egg. In contrast, there was an increase in cortical F-actin to 370% of the level in the unfertilized egg after fertilization. This increase corre sponded to the formation of microvilli. There was Little change in the level of cortical. F-actin during the first eel cycle. We draw parall els to other systems that increase the amount of F-actin in the Triton -insoluble cytoskeleton by recruiting actin from a Triton-solubIe pool of F-actin. (C) 1996 Wiley-Liss, Inc.