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Savoian, M.S; Khodjakov, A.; Rieder, C.L
doi: 10.1006/cbir.1999.0477pmid: 10772754
Vertebrate somatic cells sometimes form unilateral furrows during cytokinesis that ingress from only one edge of the cell. In some cases after a cell initiates a normal symmetrical circumferential furrow, one of its edges stops furrowing and regresses while the furrow associated with the opposing edge continues across the cell. In cells containing two independent spindles unilateral furrows are sometimes formed that do not follow a linear path but instead sharply change their direction and wander for >40μm through the cell. These observations reveal that the ‘contractile ring’ normally seen during cytokinesis is composed of multiple independent ‘furrowing units’ that are normally coordinated to form a symmetrical furrow around the cell, and that once formed this so‐called contractile band does not function as a ‘purse string’ as commonly envisioned. Individual furrowing units can work independently of one another, and cytokinesis in vertebrates can be consummated by the formation of a single functional furrowing unit in a localized region of the cell cortex that is then propagated across the cell. How this propagation occurs remains an important question for the future.
doi: 10.1006/cbir.1999.0476pmid: 10772755
Chinese hamster ovary cells can be forced to enter mitosis without prior DNA replication by treatment with hydroxyurea and caffeine. Cells treated in this way assemble a spindle that functions normally except that it does not accomplish anaphase spindle elongation (anaphase B). The chromatin detaches from the unreplicated kinetochores, which fragment, but establish microtubule attachments and migrate to the metaphase plate. Partitioning of the kinetochore fragments ensues on the normal schedule. Typical midbodies and cleavage furrows are established and daughter cells of equal size are produced. These results imply that intact chromosomes are not necessary for correct cleavage furrow placement but that kinetochores might be. Further, it is clear that cleavage furrow placement does not depend on anaphase spindle elongation.
Pushkareva, Marina Y.; Janoff, Andrew S.; Mayhew, Eric
doi: 10.1006/cbir.1999.0478pmid: 10772756
1‐ O ‐Octadecyl‐2‐ O ‐methyl‐glycero‐3‐phosphocholine (ET‐18‐OCH3) selectively inhibits the growth of cancer cells. Here we show that in some cell types ET‐18‐OCH3and liposome‐associated ET‐18‐OCH3inhibit cell division without concurrent inhibition of nuclear division, leading to multinucleate cell formation, and cell death through apoptosis. Cell cycle analysis revealed that ET‐18‐OCH3‐treated U‐937 cells continued to move through the cell cycle, but many cells were not able to divide and instead accumulated as tetraploid cells or octaploid cells in the G0/G1 phase of the cell cycle. Inhibition of cytokinesis has been shown to be paralleled by activation of U‐937 cells, including upregulation of some cell‐surface markers, acquisition of phagocytic activity, and secretion of tumor necrosis factor (TNF)‐α (Pushkareva et al., 2000). Furthermore, treatment of cells with ET‐18‐OCH3results in the accumulation of apoptotic cells in time‐ and dose‐dependent manner. It is possible that inhibition of cytokinesis may be related to cytoskeletal effects.
Jensen, Cynthia G.; Watson, Maureen
doi: 10.1006/cbir.1999.0479pmid: 10772757
Using high‐resolution timelapse microscopy, we have followed individual phagocytized fibres through the later stages of division in MeT‐5A human mesothelial cells and LLC‐MK2monkey epithelial cells. The fibres used were crocidolite and chrysotile asbestos, fibrous glass (MMVF), and refractory ceramic fibres (RCF). Long fibres (15–80μm) trapped within the cleavage furrow can partially or completely block cytokinesis. Cells proceed in one of three ways: (1) eventual completion of cytokinesis; (2) incomplete cytokinesis, resulting in two cells joined by a fibre‐containing intercellular channel; or (3) failure of cytokinesis, resulting in a binucleate or trinucleate cell. Two factors associated with fibre‐induced bi/trinucleation are: (1) an initial association between the fibre and the forming daughter nuclei, which is sometimes lost over time, and (2) disintegration of the midbody. The studies suggest that delay of cytokinesis by interzonal fibres can result in bi/trinucleation through the loss of midbody/intercellular bridge proteins that are required for completion of cytokinesis.
Brown, Jason M.; Hardin, Clyde; Gaertig, Jacek
doi: 10.1006/cbir.1999.0480pmid: 10772758
The mechanism responsible for final cell separation at the end of cytokinesis is currently unknown. Knockout strains of the ciliate, Tetrahymena thermophila lacking the kinesin‐II homologous molecular motors, Kin1p and Kin2p are paralyzed due to their complete loss of cilia and undergo frequent cytokinesis failures. Observations of live dividing cells revealed that cleavage furrow ingression is normal in kinesin‐II double knockout cells until the final stage of cell separation (Brown et al., 1999). During closer inspection of dividing cells using video differential interference contrast microscopy, we found that wild‐type cells undergo an extremely complex motile behavior near the end of cytokinesis. This process, which we have named rotokinesis, appears to facilitate the physical separation of daughter cells. Here we present recent work onTetrahymena rotokinesis, and review studies in other organisms which suggest that the use of cell locomotion in the completion of cytokinesis is a general phenomenon of motile cell types.
Numata, Osamu; Fujiu, Kenta; Gonda, Kohsuke
doi: 10.1006/cbir.1999.0481pmid: 10772759
Tetrahymena contains a micronucleus and a macronucleus. The micronucleus divides with typical mitosis, while the macronucleus divides amitotically. Although the mechanism responsible for macronuclear division was previously unknown, we clarified the organization of microtubules during macronuclear division. The macronuclear microtubules dynamically changed their distribution in an organized way throughout the macronuclear division. The macronuclear microtubules and the cytoplasmic microtubules cooperatively carried out the macronuclear division. When the micronuclear division was finished, p85 appeared at the presumptive division plane prior to the cytokinesis. The p85 directly interacted with calmodulin in a Ca2+‐dependent manner, and p85 and CaM colocalized to the division furrow during cytokinesis. Moreover, the Ca2+/CaM inhibitor, W7, inhibited the direct interaction between p85 and CaM, the localization of both proteins to the division plane, and the formation of the division furrow. Thus, Ca2+/CaM and p85 have important roles in initiation and progression of cytokinesis in Tetrahymena.
Sabaneyeva, Elena; Tao, Weihai; Verbelen, Jean‐Pierre
doi: 10.1006/cbir.1999.0482pmid: 10772760
Aphidicolin, a selective inhibitor of DNA polymerase, totally blocks DNA replication in the micronucleus but not in the macronucleus of Paramecium caudatum. The ciliates no longer divide and after 4 days the DNA content of the macronucleus has increased by 64%. Concomitantly the cell volume has increased by 53%.
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