Contractile ring

Fig. 3. Schematic illustrations of distinct steps in cell division show the central role of contractile motor action in the process of mitosis. (A) to (C) replication (prophase) (D) formation of the mitotic spindle (metaphase) (E) and (F) chromosome migration (anaphase) and building of the nuclear envelopes, and (G) formation of the contractile ring containing actin and myosin, forming the cleavage furrow and eventually two separate daughter cells. CE, centriole pair A, aster of microtubules N, nucleus M, microtubules C, chromosomes K, kinetochore NR, remnant of nuclear envelope NE, nucelar envelope reforming CR, contractile ring CM, cell membrane]. From Squire (1986).

Emoto, K., and Umeda, M., 2000, An essential role for a membrane phosphohpid in cytokinesis Regulation of contractile ring disassembly by redistribution of phosphatidylethanolamine. J. Cell Biol., 149 1215-1224. 

Cytokinesis begins some time in anaphase. A cleavage furrow begins to form in the plasma membrane in the same plane as the metaphase plate. It is not known how the furrow is positioned. The furrow is created and deepens due to the actions of the contractile ring which is an actomyosin network assembled specifically for this purpose and dispersed afterward. Thus, the stability of this network is quite different from its analog in muscle. The dynamics of these networks are also different (see Prob. 5.16). 

Many assemblies of actin and myosin in cells are temporary structures. One example is the beltlike contractile ring that appears during cell division just below the plasma membrane. As the ring contracts, the center of the cell is constricted until two daughter cells are produced. What interactions and processes must occur for this constriction to take place, noting that the ring remains a constant thickness ... 

To allow the force generated between the actin and myosin to act upon the membrane, the contractile ring must be anchored to the membrane by specific actin-binding proteins. As the cell constricts, the contractile ring must disassemble little by little to remain a constant thickness. 

The final mitosis stage involves separation of two sets of chromosomes via microtubules that are filamentous polymers of tubulin monomers. Compounds that interfere with tubulin polymerization such as the plant-derived compounds colchicine, taxol, vinblastine and vincristine are cell division inhibitors (Table 9.6). The cytokinesis of the daughter cells requires equal division of cytoplasm and an actin-myosin-based contractile ring provides the force to make this separation. Accordingly, compounds such as cytochalasin B that interfere with actin will also interfere with cell division (Table 9.6). 

Membrane traffic a driving force in cytokinesis, Trends Cell Biol. 15, 92-101, 2005 Glotzer, M., The molecular requirements for cytokinesis. Science 307, 1735-1739, 2005 Burgess, D.R., Cytokinesis new roles for myosin, Curr. Biol. 15, R310-R311, 2005 Darenfeld, H. and Mandato, C.A., Wound-induced contractile ring a model for cytokinesis, Biochem. Cell Biol. 83, 711-720, 2005 Konopka, C.A., Scheede, J.B., Skop, A.R., and Bednarek, S.Y., Dynamin and cytokinesis. Traffic 7, 239-247, 2006. 

Figure 5 Force production by the contractile ring in cytokinesis, (a) A ring of actin filaments forms at the plasma membrane and contracts to divide the cell in half, (b) Structure of cytochalasin B, a small molecule that targets actin.

Mabuchi I, Hamaguchi Y, Fujimoto H etal. (1993) A rho-like protein is involved in the organization of the contractile ring in dividing sand dollar eggs. Zygotes 1 325-331. 

The results of experiments In which active myosin II is eliminated from the cell demonstrate that cytokinesis is indeed dependent on myosin II (Figure 19-21). In one type of experiment, antl-myosin II antibodies are microinjected into one cell of a sea urchin embryo at the two-cell stage. In other experiments, expression of myosin II Is Inhibited by deletion of the myosin gene or by antisense Inhibition of myosin mRNA expression. In all cases, a cell lacking myosin II replicates to form a multinucleated syncytium because cytokinesis, but not chromosome separation, Is Inhibited. Without myosin II, cells fail to assemble a contractile ring, although other events in the cell cycle proceed normally. 

The contractile ring, a transient bundle of actin and myosin II, forms In a dividing cell and pinches the cell Into two halves In cytokinesis. 

During cytokinesis, the final step in cell division, the actin and myosin filaments composing the contractile ring slide past each other to form a cleavage furrow of steadily decreasing... 

Figure 4. Optical image and two infrared images of a dividing cell (35 pm x 20 pm) obtained from IR spectromicroscopy. These chemical maps are derived from the strength of absorption bands from approximately 100 infrared transmittance spectra collected over the area of the dividing cell. The maximum in the color scale (yellow) represents the position of the maximum absorption by the cell, while the minimum (blue) represents no absorption by the cell. Strikingly, the intensity map of the amide II absorption band shows two peaks in the center of the two halves of the cell, representing the position of two separate nuclei before the cell division is complete. The intensity map of the C-H stretch absorption bands shows that lipids are concentrated at the contractil ring, where the cleavage furrow is located. [Used by permission of the National Academy of Sciences, U.S.A., from Jamin et al. (1998), Proc Natl Acad Sci, Vol. 95, Fig. 3, p. 4839.]...

The basic propulsive movement of the GIT is peristalsis in which a contractile ring appears around the gut and then moves forward. The usual stimulus for peristalsis is distension of the gut wall, which stimulates the ENS to initiate peristaltic movement thus allowing the food to be propelled analward. 

Yumura, S. (2001) Myosin II dynamics and cortical flow during contractile ring formation in Dictyostelium cells. J. Cell Biol. 154, 137-146. 

Protocol 5.4 describes a fixation procedure (Gunsalus et al. 1995 Hime et al. 1996 Giansanti et al. 1998) that preserves F-actin-containing structures because it does not involve the use of methanol. Thus, it is particularly suitable for contractile ring and ftisome visualization. However, it does not preserve cell morphology and microtubules as efficiently as Protocol 5.2 and does not permit a clear detection of the Y loops. 

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