Cytokinesis

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A cell that has almost completed cytokinesis. An arrow points to a centrosome that can still be seen.

Cytokinesis is the process whereby the cytoplasm of a single cell is divided to spawn two daughter cells. It usually initiates during the late stages of mitosis, splitting a binucleate cell in two to ensure that chromosome number is maintained from one generation to the next. One notable exception to the normal process of cytokinesis is oogenesis (the creation of an ovum in the ovarian follicle of the ovary), where the ovum takes almost all the cytoplasm and organelles, leaving very little for the resulting polar bodies, which then die. In plant cells, a dividing structure known as the cell plate forms across the centre of the cytoplasm and a new cell wall forms between the two daughter cells.

[edit] Animal Cell Cytokinesis

During normal proliferative division, animal cell cytokinesis begins shortly after the onset of sister chromatid separation in anaphase of mitosis. Also called contractile ring comprised of non-muscle myosin II and actin filaments assembles equatorially at the cell cortex (adjacent to the cell membrane). Myosin II uses the free energy released when ATP is hydrolysed to move along these actin filaments, constricting the cell membrane to form a cleavage furrow. Continued hydrolysis causes this cleavage furrow to ingress (move inwards), a stricking process that is clearly visible down a light microscope. Ingression continues until a so-called midbody structure (composed of electron-dense, proteinaceous material) is formed, and serves to physically pinch one cell into two.

Simultaneous with contractile ring assembly, a microtubule based structure termed the central spindle (or midzone) forms when non-kinetochore microtubule fibres are bundled between the spindle poles. A number of different species including H. sapiens, D. melanogaster and C. elegans require the central spindle in order to efficiently undergo cytokinesis, although the specific phenotype described when it is absent varies from one species to the next (for example, flies are incapable of forming a cleavage furrow without the central spindle, whereas in worm embryos and human tissue culture cells a cleavage furrow is observed to ingress, but then regress before cytokinesis is complete). Seemingly vital for the formation of the central spindle (and therefore efficient cytokinesis) is a tetrameric protein complex called centralspindlin. It is centralspindlin which seems responsible for the bundling of microtubules during anaphase.