Essential Cell Biology 5th edition

Epithelial Sheets and Cell Junctions
703
The apical and basal faces of an epithelium are different: each contains a
different set of molecules that reflect the polarized organization of the individual
epithelial cells: each has a top and a bottom, with different properties
and functions. This polarity is crucial for epithelial function. Consider, for
example, the simple columnar epithelium that lines the small intestine of
a mammal. It mainly consists of two intermingled cell types: absorptive
cells, which take up nutrients, and goblet cells (so called because of their
shape), which secrete the mucus that protects and lubricates the gut lining
(Figure 20–21). Both cell types are polarized. The absorptive cells import
food molecules from the gut lumen through their apical surface and export
these molecules from their basal surface into the underlying tissues. To
do this, absorptive cells require different sets of membrane transport proteins
in their apical and basal plasma membranes (see Figure 12−17).
The goblet cells also have to be polarized, but in a different way: they
have to synthesize mucus and then discharge it from their apical end
only (see Figure 20–21); their Golgi apparatus, secretory vesicles, and
cytoskeleton are all polarized so as to bring this about. For both types
of epithelial cells, polarity depends on the junctions that the cells form
with one another and with the basal lamina. These cell junctions in turn
control the arrangement of an elaborate system of membrane-associated
intracellular proteins that create the polarized organization of the cytoplasm,
as we discuss next.
Tight Junctions Make an Epithelium Leakproof and
Separate Its Apical and Basolateral Surfaces
Epithelial cell junctions can be classified according to their function.
Some provide a tight seal to prevent the leakage of molecules across
the epithelium through the gaps between its cells; some provide strong
mechanical attachments; and some provide for an intimate type of intercytosolic
exchange. In most epithelia, all these types of junctions are
present. As we will see, each type of junction is characterized by its own
class of transmembrane proteins.
In vertebrates, the barrier function of epithelial sheets is made possible
by tight junctions. These junctions seal neighboring cells together so
overlying epithelial cells
underlying
connective tissue
20 µm
Figure 20–20 The basal lamina supports a
sheet of epithelial cells. Light micrograph
of the epithelial sheet that lines the small
intestine. The sheet of columnar cells sits
on a thin mat-like structure, the basal lamina
(red arrowheads), which is woven from type
IV collagen and laminin proteins. A network
of other collagen
ECB5 e20.21/20.21
fibrils and fibers in the
underlying connective tissue interacts with
the lower face of the lamina. (Jose Luis
Calvo/Shutterstock.)
GUT LUMEN
microvilli
mucus in
secretory
vesicles
basal lamina
5 µm
goblet
(mucus)
cell
absorptive
cell
Figure 20–21 Different types of
functionally polarized cell types line the
intestine. Absorptive cells, which take up
nutrients from the intestine, are mingled
in the gut epithelium with goblet cells
(brown), which secrete mucus into the gut
lumen. The absorptive cells are often called
brush-border cells, because of the brushlike
mass of microvilli on their apical surface;
the microvilli serve to increase the area of
apical plasma membrane for the transport
of small molecules into the cell. The goblet
cells owe their gobletlike shape to the mass
of mucus-containing secretory vesicles
that distends the cytoplasm in their apical
region. (Adapted from R. Krstić , Human
Microscopic Anatomy. Berlin: Springer, 1991.
With permission from Springer-Verlag.)