Cleavage inhibition by cell shape change

The division mechanism of animal cells is positioned by the mitotic apparatus. The mitotic apparatus determines the position by a pattern of regional inequality of its effect on the surface and cortex. Because the effect of the asters decreases with distance, the location of the division plane can be manipulated geometrically. It follows that cytokinesis should be preventable by imposition of a cell shape that prevents the normal pattern of unequal aster effect. The shape that inhibits division reveals something of the normal pattern of aster effect as well as the nature of the immediate response of the surface to the aster. In 13 of 13 experiments, sand dollar (Echinarachnius parma) eggs fail to cleave when the polar and subpolar regions are extruded in the mitotic axis, although some surface contraction may develop in the extruded region. Extrusion was accomplished by sucking the appropriate regions into opposed pipets. A computer model of the relative aster effect in this circumstance indicates that the minimum is at the equator, and the maximum is in the extruded regions. These experiments indicate that furrows do not form where the aster effect is minimal. Previous experiments showed that cells divide when cells are artificially constricted in the equatorial plane where the aster effect is maximal. Together, the geometrical experiments strongly suggest that furrowing activity occurs in maximally affected areas. These results are consistent with the equatorial contraction theory of cell division, but inconsistent with the aster relaxation theory. © 1992 Wiley-Liss, Inc.