Essential Cell Biology 5th edition

704 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
LUMEN
tracer
molecule
interacting
plasma membranes
CELL
1
CELL
2
CELL
3
tight
junction
tight
junction
intercellular
space
(A)
(B)
CELL
1
CELL
2
CELL
1
CELL
2
0.5 µm 0.5 µm
sealing
strands of
occludin and
claudin
proteins
Figure 20–22 Tight junctions allow epithelial cell sheets to serve as barriers to
solute diffusion. (A) Schematic drawing showing how a small, extracellular tracer
molecule (yellow) added on one side of an epithelial cell sheet cannot traverse the
tight junctions that seal adjacent cells together. (B) Electron micrographs of cells in an
epithelium where a small, extracellular tracer molecule (dark stain) has been added to
either the apical side (on the left) or the basolateral side (on the right); in both cases,
the tracer is stopped by the tight junction. (C) A simplified model of the structure
of a tight junction, showing how the cells are sealed together by branching strands
of transmembrane proteins (green), called claudins and occludins, in the plasma
membranes of the interacting cells. Each type of protein binds to the same type in
the apposed membrane (not shown). (B, courtesy of Daniel Friend, by permission of
E.L. Bearer.)
(C)
CELL
1
CELL
2
0.3 µm
that water-soluble molecules cannot easily leak between them. If a small
tracer molecule is added to one side of an epithelial cell sheet, it will usually
not pass beyond the tight junction (Figure 20–22A and B). The tight
junction is formed from proteins called claudins and occludins, which
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are arranged in strands along the lines of the junction to create the seal
(Figure 20–22C). Without tight junctions to prevent leakage, the pumping
activities of absorptive cells such as those in the gut would be futile,
and the composition of the extracellular fluid would become the same on
both sides of the epithelium.
Tight junctions also play a key part in maintaining the polarity of the
individual epithelial cells in two ways. First, the tight junctions around
the apical region of each cell prevent diffusion of proteins in the plasma
membrane and so keep the contents of apical domain of the plasma
membrane separate—and different—from the basolateral domain (see
Figure 11−32). Second, in many epithelia, the tight junctions are sites
of assembly for the complexes of intracellular proteins that govern the
apico-basal polarity of the cell interior.
Cytoskeleton-linked Junctions Bind Epithelial Cells
Robustly to One Another and to the Basal Lamina
The cell junctions that hold an epithelium together by forming strong
mechanical attachments are of three main types. Adherens junctions and
desmosomes bind one epithelial cell to another, while hemidesmosomes
bind epithelial cells to the basal lamina. All of these junctions provide
mechanical strength to the epithelium by the same strategy: the proteins
that form the junctions span the plasma membrane and are linked inside
the cell to cytoskeletal filaments. In this way, the cytoskeletal filaments
are tied into a network that extends from cell to cell across the whole
expanse of the epithelial sheet.