Laterally mobile, cortical tension elements can self-assemble into a contractile ring.

Animal cells cleave by the progressive constriction of an equatorial region. The plane of the constricting equator is orthogonal to the mitotic spindle axis and is positioned so that the spindle is bisected, thus ensuring equipartitioning of the chromosomes. Constriction is achieved by the active contraction of a circumferential band of actomyosin-containing filaments. The mitotic apparatus is necessary for the formation of the contractile ring and specifies its position and orientation; however, it takes no active part in furrowing. One of the central questions that has to be answered to understand cytokinesis is: how does the mitotic apparatus act to organize the contractile ring? It has been suggested that the mitotic apparatus acts to modulate locally the force generated by the cortical contractile filaments. The resultant gradients of cortical tension cause filaments to be pulled into regions of higher tension. The geometry of the stimulus from the mitotic apparatus is such that the highest levels of tension occur in the equatorial regions. The geometric distortions that take place when contractile filaments are pulled into this region cause them to become partially oriented circumferentially. When furrowing commences, mechanical forces act to align the filaments further and concentrate them in a narrow band. The furrow therefore becomes self-sharpening. Similar lateral flows of contractile filaments may occur during cell locomotion and growth cone extension.