28
U N I T 1
Cell and Tissue Function
Two main classes of extracellular macromolecules
make up the extracellular matrix. The first is an amor-
phous gel-like material called
ground substance
. Ground
substance is composed of polysaccharide chains of pro-
teins called
glycosaminoglycans
(GAGs), which are usu-
ally found linked to protein as proteoglycans. The second
type consists of the fibrous proteins (i.e., collagen and
elastin) and the fibrous adhesive proteins (i.e., fibronectin
and laminin) that are found in the basement membrane.
Members of each of these two classes of extracellular
macromolecules come in a variety of shapes and sizes.
Ground Substance.
The proteoglycan and GAG mol-
ecules form a highly hydrated, gel-like substance, or tis-
sue gel, in which the fibrous proteins are embedded. The
polysaccharide gel resists compressive forces; the collagen
fibers strengthen and help organize the matrix; the rub-
berlike elastin adds resilience; and the adhesive proteins
help cells attach to the appropriate part of the matrix.
Polysaccharides in the tissue gel are highly hydrophilic,
and they form gels even at low concentrations. They
also produce a negative charge that attracts cations such
as sodium, which are osmotically active, causing large
amounts of water to be sucked into the matrix. This cre-
ates a swelling pressure, or turgor, that enables the matrix
to withstand extensive compressive forces. For example,
the cartilage matrix that lines the knee joint can support
pressures of hundreds of atmospheres by this mechanism.
Fibrous Proteins.
Three types of fibers are found in the
extracellular space: collagen, elastin, and reticular fibers.
Collagen
is the most common protein in the body. It is
a tough, nonliving, white fiber that serves as the struc-
tural framework for skin, ligaments, tendons, and many
other structures.
Elastin
acts like a rubber band; it can
be stretched and then returns to its original form. Elastin
fibers are abundant in structures subjected to frequent
stretching, such as the aorta and some ligaments.
Reticular
fibers
are extremely thin fibers that create a flexible net-
work in organs subjected to changes in form or volume,
such as the spleen, liver, uterus, or intestinal muscle layer.
Cell Adhesion Molecules
Important classes of extracellular macromolecules are the
cell adhesion molecules (CAMs). Cell adhesion molecules
can be cell-to-cell or cell-to-matrix adhesion molecules.
There are four major classes of CAMs: cadherins, selec-
tins, integrins, and the immunoglobulin (Ig) superfamily
of proteins. These proteins are located on the cell surface
where they function as receptors, or they can be stored in
the cytoplasm. As receptors, CAMs can bind to similar or
Continuous
tight junction
(zonula occludens)
Microfilament bundle
(marginal band)
Adhesion belt
(zonula adherens)
Tonofilament
bundle
Basement
membrane
Desmosome
(macula adherens)
Gap junction
Hemidesmosome
Channel
FIGURE 1-20.
Three types of intercellular junctions found in epithelial tissue: the continuous
tight junction (zonula occludens); the adhering junction, which includes the adhesion belt (zonula
adherens), desmosomes (macula adherens), and hemidesmosomes; and the gap junction.
