During recovery after a long (up to 12 h) treatment of pig embryo culture cells (PK) with nocodazole at concentrations of 0.02 μg/ml and 0.2 μg/ml all c-metaphase cells divide normally into two daughter cells. During recovery after a short (1-4 h) treatment with 0.6 μg/ml nocodazole only multipolar mitoses (as a rule tripolar) arise. At the ultrastructural level, the increasing nocodazole concentration leads to progressive disruption of the mitotic spindle. At a nocodazole concentration of 0.2 μg/ml kinetochores are not associated with microtubules. At a nocodazole concentration of 0.6 μg/ml there are no microtubules around the centrosomes, and in every cell one of the two diplosomes disintegrates. In tripolar telophase centrioles are distributed among the spindle poles generally in a 2:2:0 pattern. Mother and daughter centrioles are always disoriented but not separated. The centriole-free pole contains a cloud of electron-dense material. During tripolar division two of the three daughter cells mainly fuse shortly after telophase forming one binucleate cell. Thus a multipolar mitosis arises as a result of the uncoupling of mother centrioles and spindle microtubules, but not of the duration of the c-mitotic arrest. Centriole-free poles account for the divergence of chromosomes, but mainly they are unable to ensure the normal cytokinesis of daughter cells.
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