Establishment of the mechanism of cytokinesis in animal cells.

The division mechanism is fixed in the surface during anaphase or about 4 minutes before furrowing begins in cylindrical cells. Under experimental conditions, the minimum time that the mitotic apparatus must act upon the surface is about 1 minute. The stimulus period is followed by a latent period of 2-3 minutes. The time of furrow formation can be advanced or delayed by manipulating the surface and the mitotic apparatus. Since furrows can be elicited long after normal division would have been completed, it is suggested that the brevity of the normal interaction period is not a consequence of the constitution of the interactants. The component of the mitotic apparatus that establishes the furrow moves from the region of the mitotic axis to the surface at 6-8 microns/minute, The components of the mitotic apparatus that are essential for furrow establishment are confined to the achromatic regions. In spherical cells with large asters, the spindles are not required, although the spindle's ability to establish furrows in spherical cells can be demonstrated by changing the cell's geometry. In nonspherical cells with small asters, the spindle is probably the normal active agent. Although the ability of the mitotic apparatus to establish furrows can be diminished or abolished by measures that reduce its overall size, there are no decisive data concerning which of its ultrastructural components play essential roles in cytokinesis. The effect of changing the geometrical relation between the mitotic apparatus and the surface differs according to the region affected. Division can be blocked or impeded only by changing the relation between the equatorial surface and the mitotic apparatus. The ability of the mitotic apparatus to establish furrows is diminished by increasing the distance between the astral centers and also by increasing the distance between the mitotic axis and the equatorial surface. The cleavage block that results from reduction in size of the mitotic apparatus can be reversed only by decreasing the distance from the mitotic axis to the equatorial surface. Artificial constrictions imposed in other regions are ineffective. The normal distance relation between the astral centers and the equatorial and polar surfaces in spherical eggs is not required for division. Cleavage can occur when the dimensional relations are reversed. Both the surface and the mitotic apparatus can interact to establish furrows after exposure to measures that disrupt their normal organization. Single, isolated asters can cause furrow-like constrictions. Their immediate effect is to cause local contraction in nearby surface.(ABSTRACT TRUNCATED AT 400 WORDS)