Biology Reference
In-Depth Information
FIGURE 15.3
The junctions of a typical epithelial cell. The diagrams of individual junctions show only a few of
the many proteins that are present at the cytoplasmic face: more information can be gleaned from the reviews cited
in the text.
to the cytokeratin cytoskeleton so that mechanical loads on an epithelium can be shared by all
cells in the tissue.
Cells may also be connected by gap junctions,
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in which connexin protein complexes form
a pore that is continuous between the cytoplasms of adjacent cells although separate from the
intercellular medium. Gap junctions allow the passage of small molecules, which can include
signalling molecules that regulate morphogenesis. For example, some of the signals that set
up left-right asymmetry in vertebrates depend on gap junctions.
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Other cells, particularly
neurons, make great use of gap junction communication.
The interface between the basal surface of the cell and the basement membrane is rich in
integrin-containing junctions such as hemidesmosomes,
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which connect with the cytokeratin
system, and other contacts that connect with actin. These, too, originate intercellular signals
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and are important in preventing programmed cell death due to anoikis (Chapter 24). In
addition to these stable junctions, epithelial cells can form integrin-containing contacts
similar to those of mesenchyme. For example, the laminin receptor
1 integrin can recruit
motile actin structures to allow cell migration, and can signal via PI-3-kinase and small
GTPases of the Rho family to organize the cytoskeleton,
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as described in Chapter 8.
a
6
b
