Cell-surface changes during cytokinesis in a dipteran egg

Abstract. In the Calliphora blastoderm, cytokinesis is preceded, during the final cleavage mitosis, by a radical surface remodelling which leads to the initiation of cytokinetic furrows. The egg is initially covered with oval surface bulges, each of which contains a mitotic nucleus. The shallow furrows between these bulges are then retracted and replaced by smooth membrane areas. Concomitantly, the remnants of the bulges become covered with large numbers of microprojections, and each bulge splits into two new bulges. The new bulges then increase in size, and cytokinetic furrows appear between them. At this point, the nuclei have also divided and reached interphase. During the first 60 min of cytokinesis, the plasma-membrane area of the egg is increased by the growth of surface microprojections; however, the furrows grow very slowly. During the final 30 min of cytokinesis, the surface becomes almost perfectly smooth, and the furrows grow very rapidly, As a result, cytokinesis is almost complete, and a columnar blastodermal epithelium is formed. Thus, surface microprojections play an essential role in cytokinesis. Plasma membrane utilized for furrow extension is apparently provided by the unfolding of these microprojections. In addition, filamentous microprojections may play an active part in the remodelling of the surface.