Essential Cell Biology 5th edition

Cytokinesis
637
a fine glass needle) as soon as the furrow appears, the furrow will disappear
and a new one will develop at a site corresponding to the new
spindle location and orientation. Once the furrowing process is well
under way, however, cleavage proceeds even if the mitotic spindle is artificially
sucked out of the cell or depolymerized using the drug colchicine.
How does the mitotic spindle dictate the position of the cleavage furrow?
The mechanism is still uncertain, but it appears that, during anaphase, the
overlapping interpolar microtubules that form the central spindle recruit
and activate proteins that signal to the cell cortex to initiate the assembly
of the contractile ring at a position midway between the spindle poles
(Figure 18−32). Because these signals originate during anaphase, this
mechanism also contributes to the timing of cytokinesis in late mitosis.
When the mitotic spindle is located centrally in the cell—the usual
situation in most dividing cells—the two daughter cells will be of equal
size. During embryonic development, however, there are some instances
in which the dividing cell moves its mitotic spindle to an asymmetrical
position, and, consequently, the furrow creates two daughter cells that
differ in size. In most of these asymmetric divisions, the daughters also
differ in the molecules they inherit, and they usually develop into different
cell types.
The Contractile Ring of Animal Cells Is Made of Actin
and Myosin Filaments
The contractile ring is composed mainly of an overlapping array of actin
filaments and myosin filaments (Figure 18−33). It assembles at anaphase
and is attached to membrane-associated proteins on the cytosolic face of
the plasma membrane. Once assembled, the contractile ring is capable
of exerting a force strong enough to bend a fine glass needle inserted into
the cell before cytokinesis. Much of this force is generated by the sliding
of the actin filaments against the myosin filaments. Unlike the stable
association of actin and myosin filaments in muscle fibers, however, the
contractile ring is a dynamic and transient structure: it assembles to carry
out cytokinesis, gradually becomes smaller as cytokinesis progresses,
and disassembles completely once the cell has been cleaved in two.
Cell division in many animal cells is accompanied by large changes in
cell shape and a decrease in the adherence of the cell to its neighbors, to
the extracellular matrix, or to both. These changes result, in part, from
the reorganization of actin and myosin filaments in the cell cortex, only
one aspect of which is the assembly of the contractile ring. Mammalian
fibroblasts in culture, for example, spread out flat during interphase, as a
result of the strong adhesive contacts they make with the surface they are
growing on—called the substratum. As the cells enter M phase, however,
they round up. This change in shape takes place, in part, because some
of the plasma membrane proteins responsible for attaching the cells to
the substratum—the integrins (discussed in Chapter 20)—become phosphor-ylated
and thus weaken their grip. Once cytokinesis is complete, the
daughter cells reestablish their strong contacts with the substratum and
Figure 18−32 Position of the cleavage
furrow is dictated by the central spindle.
In this model, the interpolar microtubules
ECB5 m17.45/18.31.5
recruit proteins that generate a signal (red
arrows) that activates a protein called RhoA
in the cell cortex. RhoA, a member of the
Rho family of GTPases discussed in Chapter
17, controls the assembly and contraction
of the contractile ring midway between the
spindle poles.
(A)
interpolar
microtubules
site of
cleavage furrow
remaining interpolar microtubules
from central spindle
contractile ring of actin and
myosin filaments in cleavage furrow
Figure 18−33 The contractile ring divides the cell in two.
(A) Schematic diagram of the midregion of a dividing cell showing the
contractile ring beneath the plasma membrane and the remains of
the two sets of interpolar microtubules. (B) In this dividing sea urchin
embryo, the contractile ring is revealed by staining with a fluorescently
labeled antibody that binds to myosin. (B, courtesy of George von
Dassow.)
(B)
50 µm