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

1036 Chapter 19: Cell Junctions and the Extracellular Matrix
1
epithelial
tissue
basal
lamina
mechanical stresses
are transmitted from
cell to cell by cytoskeletal
filaments anchored to
cell–matrix and cell–cell
adhesion sites
2
connective
tissue
extracellular matrix
directly bears mechanical
stresses of tension and
compression
collagen
fibers
adhesion site to adhesion site. The cytoskeleton of epithelial cells is also linked to
the basal lamina through cell–matrix junctions.
Figure 19–2 provides a closer view of epithelial cells to illustrate the major
types of cell–cell and cell–matrix junctions that we will discuss in this chapter. The
diagram shows the typical arrangement of junctions in a simple columnar epithelium
such as the lining of the small intestine of a vertebrate. Here, a single layer
of tall cells stands on a basal lamina, with the cells’ uppermost surface, or apex,
MBoC6 m19.01/19.01
free and exposed to the extracellular medium. On their sides, or lateral surfaces,
the cells make junctions with one another. Two types of anchoring junctions link
the cytoskeletons of adjacent cells: adherens junctions are anchorage sites for
actin filaments; desmosomes are anchorage sites for intermediate filaments. Two
additional types of anchoring junctions link the cytoskeleton of the epithelial cells
to the basal lamina: actin-linked cell–matrix junctions anchor actin filaments to
the matrix, while hemidesmosomes anchor intermediate filaments to it.
Figure 19–1 Two main ways in which
animal cells are bound together. In
connective tissue, the main stress-bearing
component is the extracellular matrix. In
epithelial tissue, it is the cytoskeletons
of the cells themselves, linked from cell
to cell by adhesive junctions. Cell–matrix
attachments bond epithelial tissue to the
connective tissue beneath it.
APICAL
TIGHT JUNCTION
tight junction seals gap between
epithelial cells
CELL–CELL
ANCHORING JUNCTIONS
junctional
complex
adherens junction connects actin
filament bundle in one cell with
that in the next cell
desmosome connects intermediate
filaments in one cell to those in
the next cell
CHANNEL-FORMING
JUNCTION
CELL–MATRIX
ANCHORING JUNCTIONS
gap junction allows the passage
of small water-soluble molecules
from cell to cell
BASAL
actin-linked cell–matrix junction
anchors actin filaments in cell
to extracellular matrix
hemidesmosome anchors intermediate
filaments in a cell to extracellular matrix
Figure 19–2 A summary of the various cell junctions found in a vertebrate epithelial cell, classified according to their primary functions.
In the most apical portion of the cell, the relative positions of the junctions are the same in nearly all vertebrate epithelia. The tight junction occupies
the most apical position, followed by the adherens junction (adhesion belt) and then by a special parallel row of desmosomes; together these form
a structure called a junctional complex. Gap junctions and additional desmosomes are less regularly organized. Two types of cell-matrix anchoring
junctions tether the basal surface of the cell to the basal lamina. The drawing is based on epithelial cells of the small intestine.