Morphological changes during maturation of starfish oocytes: surface ultrastructure and cortical actin.

The cell surface and extracellular investments of oocytes of the starfish Pisaster ochraceus are analyzed by Nomarski differential interference contrast microscopy and by scanning electron microscopy. The investing coats include a thin sheet of follicle cells, a jelly coat, and a vitelline layer; their morphologies are described. Methods are outlined for systematically removing them without altering the behavior of the oocyte so that the cell surface can be examined directly. The topography of denuded oocytes changes dramatically when they are treated with the maturation-inducing hormone, 1-methyladenine. The major topographical change is the early and transient formation of prominent surface spikes. These structures arise due to the rapid, reversible polymerization of actin into stout bundles. Polymerization and subsequent depolymerization of cortical actin is monitored by epifluorescence microscopy of oocytes stained with NBD-phallacidin, a stain which is specific for polymerized actin. Based on scanning electron microscopy, spikes apparently utilize preexisting plasma membrane of microvilli, and plasma membrane is apparently lost when spikes collapse. Long after microvilli are eliminated due to spike formation, the number of microvilli is somewhat restored, especially around the animal pole where the polar body forms. A chronology of events observed during oocyte maturation is discussed with reference to the possible mechanisms and implications of polymerization and depolymerization of cortical actin.