Surface shape changes and cortical actin reorganization associated with the growth of microvilli in the sea urchin egg

Abstract

This paper investigates the relationship between the assembly of the cortical cytoskeleton and the reorganization of the egg surface following fertilization in the sea urchin. The surface of the unfertilized egg is covered with short microvilli, with dimensions of ∼0.15 × 0.3 μm. Between 30 and 60 sec after insemination, slender finger-like processes, ∼2 μm in length and 0.05 μm in diameter, grow from the tips of these short microvilli. Over the next 30 sec, clusters of these processes transform into broad protrusions of the egg surface, containing networks of actin filaments. By 2 min postinsemination, bundles of filaments start to form within these networks, beginning at the tips of the protrusions and growing inward. As the filament bundles develop, the plasma membrane encases them to form typical microvilli with average dimensions of 0.15 × 2 μm. The formation of the network of cortical actin filaments occurs during the transient increase in free cytoplasmic Ca++ concentration, while the bundling of actin filaments occurs during the period of cytoplasmic alkalization. When cytoplasmic alkalization is inhibited, actin filament networks fail to reorganize into bundles, but microvilli develop by an identical series of surface shape changes. Eggs fertilized in the presence of cytochalasin B develop irregular surface protrusions that gradually transform into microvillar-like structures over a period of ∼30 min without undergoing any definable series of shape changes or assembly of a normal cortical cytoskeleton. These results demonstrate that, while the formation of the large surface protrusions requires an intact network of cortical actin filaments, partitioning of these protrusions into microvilli does not depend on the formation of actin filament bundles. Further, microvillar-like structures are still able to form in eggs in which the normal sequence of surface shape changes has been blocked by inhibiting the assembly of the cortical actin filament network. J. Exp. Zool. 277:230–244, 1997. © 1997 Wiley-Liss, Inc.