The Periodic Changes in Microvilli Density in Activated Xenopus Eggs That Correspond to the Cleavage Cycle

In order to obtain the cytological basis for the periodic flattening and rounding-up of activated amphibian eggs, the surface ultrastructure and the cortical microfilament organization were studied in Xenopus laevis. Scanning electron microscopy (SEM) of the egg surface revealed that the density of microvilli at the animal pole region decreased significantly when the periodic flattening started, but increased again concomitantly with the commencement of the rounding-up. Isolated pieces of the cortices stained with rhodamine-phalloidin exhibited the periodic disorganization and reorganization of a meshwork with bright dots probably corresponding to microvilli, in good synchrony with the decrease and increase of the microvilli density. Study of appropriate batches of eggs in which the moving front of surface contraction waves (SCWs; 1) can be localized revealed that the decrease and increase of the microvilli density correspond to SCW-1 and -2, respectively. SEM and the cytochemical examination of the eggs from which the germinal vesicle (GV) had been removed revealed that none of these changes occurred in the enucleated eggs. These observations suggest that the GV-dependent regulation of the microfilament organization in an egg cortex constitutes the cytological basis for the SCWs and for the periodic flattening and rounding-up of denuded eggs.