SURFACE SHAPE CHANGES AND CORTICAL ACTIN REORGANIZATION ASSOCIATED WITH THE GROWTH OF MICROVILLI IN THE SEA-URCHIN EGG

This paper investigates the relationship between the assembly of the c ortical cytoskeleton and the reorganization of the egg surface followi ng fertilization in the sea urchin. The surface of the unfertilized eg g is covered with short microvilli, with dimensions of similar to 0.15 x 0.3 mu m. Between 30 and 60 sec after insemination, slender finger- like processes, similar to 2 mu m in length and 0.05 mu m in diameter, grow from the tips of these short microvilli. Over the next 30 sec, c lusters of these processes transform into broad protrusions of the egg surface, containing networks of actin filaments. By 2 min postinsemin ation, bundles of filaments start to form within these networks, begin ning at the tips of the protrusions and growing inward. As the filamen t bundles develop, the plasma membrane encases them to form typical mi crovilli with average dimensions of 0.15 x 2 mu m. The formation of th e network of cortical actin filaments occurs during the transient incr ease in free cytoplasmic Ca++ concentration, while the bundling of act in filaments occurs during the period of cytoplasmic alkalization. Whe n cytoplasmic alkalization is inhibited, actin filament networks fail to reorganize into bundles, but microvilli develop by an identical ser ies of surface shape changes. Eggs fertilized in the presence of cytoc halasin B develop irregular surface protrusions that gradually transfo rm into microvillar-like structures over a period of similar to 30 min without undergoing any definable series of shape changes or assembly of a normal cortical cytoskeleton. These results demonstrate that, whi le the formation of the large surface protrusions requires an intact n etwork of cortical actin filaments, partitioning of these protrusions into microvilli does not depend on the formation of actin filament bun dles. Further, microvillar-like structures are still able to form in e ggs in which the normal sequence of surface shape changes has been blo cked by inhibiting the assembly of the cortical actin filament network . (C) 1997 Wiley-Liss, Inc.