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lüll Contractile ring formation in Xenopus egg and fission yeast Noguchi T; Arai R; Motegi F; Nakano K; Mabuchi ICell Struct Funct 2001[Dec]; 26 (6): 545-54How actin filaments (F-actin) and myosin II (myosin) assemble to form the contractile ring was investigated with fission yeast and Xenopus egg. In fission yeast cells, an aster-like structure composed of F-actin cables is formed at the medial cortex of the cell during prophase to metaphase, and a single F-actin cable(s) extends from this structure, which seems to be a structural basis of the contractile ring. In early mitosis, myosin localizes as dots in the medial cortex independently of F-actin. Then they fuse with each other and are packed into a thin contractile ring. At the growing ends of the cleavage furrow of Xenopus eggs, F-actin at first assembles to form patches. Next they fuse with each other to form short F-actin bundles. The short bundles then form long bundles. Myosin seems to be transported by the cortical movement to the growing end and assembles there as spots earlier than F-actin. Actin polymerization into the patches is likely to occur after accumulation of myosin. The myosin spots and the F-actin patches are simultaneously reorganized to form the contractile ring bundles. The idea that a Ca signal triggers cleavage furrow formation was tested with Xenopus eggs during the first cleavage. We could not detect any Ca signals such as a Ca wave, Ca puffs or even Ca blips at the growing end of the cleavage furrow. Furthermore, cleavages are not affected by Ca-chelators injected into the eggs at concentrations sufficient to suppress the Ca waves. Thus we conclude that formation of the contractile ring is not induced by a Ca signal at the growing end of the cleavage furrow.|Actins/*metabolism[MESH]|Animals[MESH]|Calcium/metabolism[MESH]|Cell Division/*physiology[MESH]|Cytoskeleton/metabolism[MESH]|Myosin Type II/*metabolism[MESH]|Oocytes/*cytology/metabolism[MESH]|Schizosaccharomyces pombe Proteins/metabolism[MESH]|Schizosaccharomyces/*physiology[MESH]|Xenopus Proteins/metabolism[MESH]|Xenopus laevis/*physiology[MESH] |