(a) a drawing of the cleavage furrow in a dividing cell. (B) an electron micrograph of the ingrowing edge of a cleavage furrow of a dividing animal cell. (C) Fluorescence micrographs of a dividing slime mold amoeba stained for actin (red) and myosin II (green). Whereas all of the visible myosin II has redistributed to the contractile ring, only some of the actin has done so; the rest remains in the cortex of the nascent daughter cells. (B, from H.W. Beams and
R.g. kessel, Am. Sci. 64:279–290, 1976. With permission from sigma Xi; C, courtesy of Yoshio Fukui.) be placed between the two sets of daughter chromosomes, thereby ensuring that each daughter cell receives a complete set. The correct timing and positioning of cytokinesis in animal cells are achieved by mechanisms that depend on the mitotic spindle. During anaphase, the spindle generates signals that initiate furrow formation at a position midway between the spindle poles, thereby ensuring that division occurs between the two sets of separated chromosomes. Because these signals originate in the anaphase spindle, this mechanism also contributes to the correct timing of cytokinesis in late mitosis. Cytokinesis also occurs at the correct time because dephosphorylation of Cdk substrates, which depends on cyclin destruction in metaphase and anaphase, initiates cytokinesis. We now describe these regulatory mechanisms in more detail, with an emphasis on cytokinesis in animal cells.
Studies of the fertilized eggs of marine invertebrates first revealed the importance of spindle microtubules in determining the placement of the contractile ring. After fertilization, these embryos cleave rapidly without intervening periods of growth. In this way, the original egg is progressively divided into smaller and smaller cells. Because the cytoplasm is clear, the spindle can be observed in real time with 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—supporting the idea that signals generated by the spindle induce local furrow formation.
How does the mitotic spindle specify the site of division? Three general mechanisms have been proposed, and most cells appear to employ a combination of these (Figure 17–45). The first is termed the astral stimulation model, in which the
Figure 17–43 The midbody. (a) a scanning electron micrograph of a cultured animal cell dividing; the midbody still joins the two daughter cells. (B) a conventional electron micrograph of the midbody of a dividing animal cell. Cleavage is almost complete, but the daughter cells remain attached by this thin strand of cytoplasm containing the remains of the central spindle. (a, courtesy of guenter albrecht-Buehler; B, courtesy of J.m. mullins.) assembly and contraction of actin–myosin ring
Figure 17–44 Regulation of the contractile ring by the GTPase RhoA.
Like other Rho family gTpases, Rhoa is activated by a RhogEF protein and inactivated by a Rho gTpase-activating protein (Rhogap). The active gTpbound form of Rhoa is focused at the future cleavage site. By binding formins, activated Rhoa promotes the assembly of actin filaments in the contractile ring. By activating Rho-activated protein kinases, such as Rock, it stimulates myosin II filament formation and activity, thereby promoting contraction of the ring.
astral microtubules carry furrow-inducing signals, which are somehow focused in a ring on the cell cortex, halfway between the spindle poles. Evidence for this model comes from ingenious experiments in large embryonic cells, which demonstrate that a cleavage furrow forms midway between two asters, even when the two centrosomes nucleating the asters are not connected to each other by a mitotic spindle (Figure 17–46).