Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

892 Chapter 16: The Cytoskeleton
Figure 16–2 Diagram of changes in
cytoskeletal organization associated
with cell division. The crawling fibroblast
drawn here has a polarized, dynamic
actin cytoskeleton (shown in red) that
assembles lamellipodia and filopodia to
push its leading edge toward the right. The
polarization of the actin cytoskeleton is
assisted by the microtubule cytoskeleton
(green), consisting of long microtubules
that emanate from a single microtubuleorganizing
center located in front of
the nucleus. When the cell divides, the
polarized microtubule array rearranges
to form a bipolar mitotic spindle, which
is responsible for aligning and then
segregating the duplicated chromosomes
(brown). The actin filaments form a
contractile ring at the center of the cell
that pinches the cell in two after the
chromosome segregation. After cell
division is complete, the two daughter
cells reorganize both the microtubule and
actin cytoskeletons into smaller versions of
those that were present in the mother cell,
enabling them to crawl their separate ways.
blood cell, chases and engulfs bacterial and fungal cells that accidentally gain
access to the normally sterile parts of the body, as through a cut in the skin. Like
most crawling cells, neutrophils advance by extending a protrusive structure filled
with newly polymerized actin filaments. When the elusive bacterial prey moves in
a different direction, the neutrophil is poised to reorganize its polarized protrusive
structures within seconds (Figure 16–3).
The Cytoskeleton Determines Cellular Organization and Polarity
MBoC6 m16.02/16.02
In cells that have achieved a stable, differentiated morphology—such as mature
neurons or epithelial cells—the dynamic elements of the cytoskeleton must also
provide stable, large-scale structures for cellular organization. On specialized epithelial
cells that line organs such as the intestine and the lung, cytoskeletal-based
cell-surface protrusions including microvilli and cilia are able to maintain a constant
location, length, and diameter over the entire lifetime of the cell. For the
actin bundles at the cores of microvilli on intestinal epithelial cells, this is only a
few days. But the actin bundles at the cores of stereocilia on the hair cells of the
inner ear must maintain their stable organization for the entire lifetime of the animal,
since these cells do not turn over. Nonetheless, the individual actin filaments
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Figure 16–3 A neutrophil in pursuit of
bacteria. In this preparation of human
blood, a clump of bacteria (white arrow) is
about to be captured by a neutrophil. As
the bacteria move, the neutrophil quickly
reassembles the dense actin network at
its leading edge (highlighted in red) to
push toward the location of the bacteria
(Movie 16.1). Rapid disassembly and
reassembly of the actin cytoskeleton in
this cell enables it to change its orientation
and direction of movement within a few
minutes. (From a video recorded by
David Rogers.)