Essential Cell Biology 5th edition

Epithelial Sheets and Cell Junctions
705
Adherens junctions and desmosomes are both built around transmembrane
proteins that belong to the cadherin family: a cadherin molecule in
the plasma membrane of one cell binds directly to an identical cadherin
molecule in the plasma membrane of its neighbor (Figure 20–23). Such
interaction of like-with-like is called homophilic binding. In the case of
cadherins, binding also requires that Ca 2+ be present in the extracellular
medium—hence the name.
plasma membrane
cytoskeletal
filament
At an adherens junction, each cadherin molecule is tethered inside its
cell, via several linker proteins, to actin filaments. Often, the adherens
junctions form a continuous adhesion belt around each of the interacting
epithelial cells; this belt is located near the apical end of the cell,
just below the tight junctions (Figure 20–24). Bundles of actin filaments
are thus connected from cell to cell across the epithelium. This network
of actin filaments also contains myosin filaments and can thus contract,
giving the epithelial sheet the capacity to develop tension and to change
its shape in remarkable ways. By shrinking the apical surface of an epithelial
sheet along one axis, the sheet can roll itself up into a tube (Figure
20–25A and B). Alternatively, by shrinking its apical surface locally along
all axes at once, the sheet can invaginate into a cup and eventually create
a spherical vesicle by pinching off from the rest of the epithelium (Figure
20–25C). Epithelial movements such as these are crucial during embryonic
development, when they create structures such as the neural tube
(see Figure 20–25B), which gives rise to the brain and spinal cord, and the
lens vesicle, which develops into the lens of the eye (see Figure 20–25C).
linker
proteins
cadherin
protein
CELL 1 CELL 2
Figure 20–23 Cadherin proteins mediate
mechanical attachment of one cell to
another. Identical cadherin molecules in the
plasma membranes of adjacent cells bind
to each other extracellularly; inside the cell,
they are attached, via linker proteins, to
cytoskeletal filaments—either actin
filaments or keratin intermediate filaments.
Movie 20.2 shows how, when epithelial
cells in culture touch one another, their
cadherins become concentrated at the
point of attachment, ECB5 e20.24-20.24 leading to the
formation of adherens junctions.
At a desmosome, a different set of cadherin molecules connects to keratin
filaments—the intermediate filaments found specifically in epithelial
cells (see Figure 17−5). Bundles of ropelike keratin filaments criss-cross
the cytoplasm and are “spot-welded” via desmosome junctions to
the bundles of keratin filaments in adjacent cells (Figure 20–26). This
arrangement confers great tensile strength to the epithelial sheet and is
characteristic of tough, exposed epithelia such as the epidermis of the
skin.
actin filaments
inside microvillus
LUMEN
microvilli extending
from apical surface
tight junction
adherens
junction
10 µm
cadherins
bundle of
actin filaments
lateral plasma
membranes of
adjacent
epithelial cells
basal surface
Figure 20–24 Adherens junctions form
adhesion belts around epithelial cells in
the small intestine. A contractile bundle of
actin filaments runs along the cytoplasmic
surface of the plasma membrane near
the apex of each cell. These bundles
are linked to those in adjacent cells via
transmembrane cadherin molecules (see
Figure 20–23).