contractile ring mah cytokinetic ring A cytoskeletal structure composed of filamentous protein that forms beneath the membrane of many cells or organelles, in the plane of cell or organelle division. Ring contraction is associated with centripetal growth of the membrane that divides the cytoplasm of the two daughter cells or organelles. GO:0070938 constriction ring cellular_component 2009-09-22T02:41:32Z cellular anatomical structure