A second possibility, called the central spindle stimulation model, is that the spindle midzone, or central spindle, generates a furrow-inducing signal that specifies the site of furrow formation at the cell cortex (see Figure 17–45). The overlapping interpolar microtubules of the central spindle associate with numerous signaling proteins, including proteins that may stimulate RhoA (Figure 17–47). Defects in the functions of these proteins (in Drosophila mutants, for example) result in failure of cytokinesis.
A third model proposes that, in some cell types, the astral microtubules promote the local relaxation of actin–myosin bundles at the cell cortex. According to this astral relaxation model, the cortical relaxation is minimal at the spindle equator, thus promoting cortical contraction at that site (see Figure 17–45). In the early embryos of Caenorhabditis elegans, for example, treatments that result in the loss of astral microtubules lead to increased contractile activity throughout the cell cortex, consistent with this model.
In some cell types, the site of ring assembly is chosen before mitosis. In budding yeasts, for example, a ring of proteins called septins assembles in late G1 at the future division site. The septins are thought to form a scaffold onto which other components of the contractile ring, including myosin II, assemble. In plant cells, an organized band of microtubules and actin filaments, called the preprophase band, assembles just before mitosis and marks the site where the cell wall will assemble and divide the cell in two, as we now discuss.
both nuclei cleavage occurs both between the centrosomes enter mitosis linked by mitotic spindles and between the two centrosomes that are simply adjacent, and four daughter cells are formed
Figure 17–45 Three current models of how the microtubules of the anaphase spindle generate signals that influence the positioning of the contractile ring. no single model explains all the observations, and furrow positioning is probably determined by a combination of these mechanisms, with the importance of the different mechanisms varying in different organisms. see text for details.
Figure 17–46 An experiment demonstrating the influence of the position of microtubule asters on the subsequent plane of cleavage in a large egg cell. If the mitotic spindle is mechanically pushed to one side of the cell with a glass bead, the membrane furrowing is incomplete, failing to occur on the opposite side of the cell. subsequent cleavages occur not only at the midzone of each of the two subsequent mitotic spindles (yellow arrowheads), but also between the two adjacent asters that are not linked by a mitotic spindle—but in this abnormal cell share the same cytoplasm (red arrowhead). apparently, the contractile ring that produces the cleavage furrow in these cells always forms in the region midway between two asters, suggesting that the asters somehow alter the adjacent region of cell cortex to induce furrow formation between them.
The phragmoplast guides Cytokinesis in Higher plants
In most animal cells, the inward movement of the cleavage furrow depends on an increase in the surface area of the plasma membrane. New membrane is added at the inner edge of the cleavage furrow and is generally provided by small membrane vesicles that are transported on microtubules from the Golgi apparatus to the furrow.