Surface and shape changes during cell division.

Rat kangaroo cells (PtK2) were studied with scanning and transmission electron microscopy in order to correlate shape changes during the cell cycle with the presence or absence of microvilli andd stress fibers. During interphase, bundles of actin are prominent in the cytoplasm, and microvilli are localized over and around the centrally positioned nucleus. As mitosis begins, the interphase bundles of actin and the microvilli disappear, but the mitotic cells maintain a flattened shape. At metaphase the cell is still so flat that both the chromosomes and spindle apparatus are visible through the intact cell membrane. Microvilli reappear in late anaphase above the chromosomes and poles. Before cleavage begins, microvilli increase in number until they cover the apical surface of the cell. At the same time, the cell increases in height so that the chromosomes and mitotic apparatus can no longer be detected through the cell membrane. During cleavage, microvilli continue to cover the cell in a uniform manner but become greatly diminished in number after cytokinesis is completed and the cells flatten and enter interphase. It is suggested that the microvilli organize a network of actin filaments which interact with cortical myosin to produce the cell rounding porior to cleavage.