The contractile ring that produces cell cleavage is composed of an organized cytoskeletal network that includes actin and bipolar myosin-II filaments working together in a sliding action that mimics muscle contraction. These filaments are bound to the cytoplasmic border of the plasma membrane by a matrix of attachment proteins. During anaphase, components of the ring begin to polymerize on the membrane, and initial contraction of the parallel array occurs in early telophase. The bundle of cytoskeletal fibers contains approximately 20 actin filaments, which slowly disperse to maintain a constant thickness as the ring tightens. At the end of cytokinesis, the contractile ring dissociates, leaving behind only a narrow bridge between the daughter cells (the midbody) filled with actin filaments. When observed in the optical microscope, the midbody reveals a dense matrix of tightly packed polar microtubules remaining from the mitotic spindle.