Essential Cell Biology 5th edition

698 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
Figure 20–11 Incorrect collagen assembly can cause the skin to
be hyperextensible. James Morris, “the elastic skin man,” from a
photograph taken in about 1890. Abnormally stretchable skin is part
of a genetic syndrome that results from a defect in collagen assembly.
In some individuals, this condition arises from a lack of an enzyme that
converts procollagen to collagen; in others, it is caused by a defect in
procollagen itself.
intervening collagen becomes organized into a dense band of aligned fibers
that connect the two explants (Figure 20–13). The fibroblasts migrate
out from the explants along the aligned collagen fibers. In this way, the
fibroblasts influence the alignment of the collagen fibers, and the collagen
fibers in turn affect the distribution of the fibroblasts. Fibroblasts presumably
play a similar role in generating long-range order in the extracellular
matrix inside the developing body—in helping to create tendons, for
example, and the tough, dense layers of connective tissue that ensheathe
and bind together most organs. Fibroblast migration is also important for
healing wounds (Movie 20.1).
ECB5 e20.12/20.12
Integrins Couple the Matrix Outside a Cell to the
Cytoskeleton Inside It
Cells are able to interact with the collagen in the extracellular matrix
thanks to a family of transmembrane receptor proteins called integrins.
The extracellular domain of an integrin binds to components of the
matrix, while its intracellular domain interacts with the cell cytoskeleton.
This internal mooring provides a strong and stable point of attachment;
without it, integrins would be easily torn from the flimsy lipid bilayer, and
cells would be unable to anchor themselves to the matrix.
Integrins do not, however, interact directly with collagen fibers in the
extracellular matrix. Instead, another extracellular matrix protein,
fibronectin, provides a linkage: part of the fibronectin molecule binds to
collagen, while another part forms an attachment site for integrins.
When the extracellular domain of the integrin binds to fibronectin, the
intracellular domain binds (through a set of adaptor molecules) to an
actin filament inside the cell (Figure 20–14). For many cells, it is the formation
and breakage of these attachments on either end of an integrin
molecule that allows the cell to crawl through a tissue, grabbing hold of
the matrix at its front end and releasing its grip at the rear (see Figure
17−33). Integrins coordinate these “catch-and-release” maneuvers by
undergoing remarkable conformational changes. Binding to a molecule
on one side of the plasma membrane causes the integrin molecule to
stretch out into an extended, activated state so that it can then latch
onto a different molecule on the opposite side—an effect that operates
in either direction across the membrane (Figure 20–15). Thus, an intracellular
signaling molecule can activate the integrin from the cytosolic
side, causing it to reach out and grab hold of an extracellular structure.
Similarly, binding to an external structure can switch on a variety of intracellular
signaling pathways by activating protein kinases that associate
with the intracellular end of the integrin. In this way, a cell’s external
attachments can help regulate its behavior—and even its survival.
5 µm
Figure 20–12 Collagen fibrils in the skin of some animals are
arranged in a plywoodlike pattern. The electron micrograph shows
a cross section of tadpole skin. Successive layers of fibrils are laid
down nearly at right angles to each other (see also Figure 20–9). This
arrangement is also found in mature bone and in the cornea, but not in
mammalian skin. (Courtesy of Jerome Gross.)