The Microtubules of the Mitotic Spindle Determine the Plane of Animal Cell Division
The mitotic spindle in animal cells not only separates the daughter chromosomes, it also specifies the location of the contractile ring, and thereby the plane of cell division. The contractile ring invariably forms in the plane of the metaphase plate, at right angles to the long axis of the mitotic spindle, thereby ensuring that division occurs between the two sets of separated chromosomes. The part of the spindle that specifies the division plane varies depending on the cell type: in some cells, it is the astral microtubules; in others, it is the overlapping antiparallel microtubules in the central spindle.
The relationship between the spindle microtubules and the placement of the contractile ring has been studied by manipulating fertilized eggs of marine invertebrates. After fertilization, these embryos undergo a series of rapid cleavage divisions, without intervening periods of growth. In this way, the original egg is progressively divided up into smaller and smaller cells. During cytokinesis, the cleavage furrow appears suddenly on the surface of the cell and deepens rapidly (Figure 18-30). Because the cytoplasm is clear, the spindle can be observed in real time through a microscope. If the spindle is tugged into a new position with a fine glass needle in early anaphase, the incipient cleavage furrow disappears, and a new one develops in accord with the new spindle site.
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How does the mitotic spindle control the plane of division? Ingenious experiments in large embryonic cells demonstrate that a cleavage furrow forms midway between the asters originating from the two centrosomes, even when the two centrosomes are not connected to each other by a mitotic spindle (Figure 18-31). Thus, in these cells, the microtubule asters—not the chromosomes or other parts of the spindle—signal to the cell cortex to specify where the contractile ring should assemble. In other cells, the central spindle, rather than the astral microtubules, is apparently responsible for this specification. In either case, it has been speculated that the overlapping microtubules may provide tracks for motor proteins to deliver contractile ring regulators, and perhaps new membrane, to the appropriate region of the dividing cell. But, in fact, the molecular mechanism by which the spindle positions the cleavage furrow remains a mystery.
In some cells, the site of ring assembly is chosen before mitosis, according to a landmark placed in the cortex during a previous cell cycle. In budding yeasts, for example, a ring of proteins called septins assembles before mitosis, adjacent to a bud scar left on the cell surface as the mother and daughter cells separated in the previous division. The septins are thought to form a scaffold onto which other components of the contractile ring, including myosin II, assemble. As we discuss later, in plant cells, an organized band of microtubules and actin filaments assembles just before mitosis and marks the site where the cell wall will assemble and divide the cell in two.
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