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In a landmark study published in 1972, Thomas Schroeder provided the first ultrastructural description of the contractile ring during cell cleavage in sea urchin eggs. He observed that the contractile ring was composed of microfilaments that were similar to muscle actin filaments. Formation of the contractile ring coincided with the onset of cytokinesis, its spatial location coincided with that of the cleavage furrow and the filaments disappeared when cytokinesis was over, which indicated that the contractile ring might be responsible for cytokinesis.
To test this hypothesis, Schroeder treated cells with the drug cytochalasin B, which was later found to inhibit actin polymerization. The microfilaments rapidly depolymerized, the contractile ring disassembled and the cleavage furrow regressed. By contrast, microtubules of the mitotic apparatus were unaffected. So, the contractile ring drives cytokinesis independently of mitosis. Schroeder also showed that the volume of the contractile ring declined as the furrow constricted, and proposed that the filaments disassemble as the ring contracts.
A year later, Schroeder confirmed the presence of actin in the contractile ring by treating cells with heavy meromyosin (HMM), which is a tryptic fragment of muscle myosin that interacts with actin filaments. He also observed that the contractile ring contained another component, besides the actin microfilament, which was not marked by HMM, and proposed that it could be an oligomeric form of myosin. Indeed, in 1976, using myosin-specific antibodies that were coupled with fluorescent dyes, Fujiwara and Pollard demonstrated that myosin-II was abundant in the contractile ring.
...showed that cytokinesis depends on a myosin-powered contraction of a ring of actin filaments.
Tom Pollard
It was therefore reasonable to postulate that actin and myosin-II interact to produce the force of ring constriction, and the first demonstration of this came in 1977, with the work of Mabuchi and Okuno. They microinjected antibodies against starfish egg myosin-II into eggs, and found that this blocked cytokinesis but did not interfere with the function of the mitotic spindle. So, during cytokinesis, myosin-II powers the 'belting up' of the contractile ring composed of actin filaments.
