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

THE EXTRACELLULAR MATRIX OF ANIMALS
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The cells of many animal tissues are coupled by gap junctions, which take the
form of plaques of clustered connexons, which usually allow molecules smaller than
about 1000 daltons to pass directly from the inside of one cell to the inside of the
next. Cells connected by gap junctions share many of their inorganic ions and other
small molecules and are therefore chemically and electrically coupled.
Three additional classes of transmembrane adhesion proteins mediate more
transient cell–cell adhesion: selectins, immunoglobulin (Ig) superfamily members,
and integrins. Selectins are expressed on white blood cells, blood platelets, and
endothelial cells; they bind heterophilically to carbohydrate groups on cell surfaces,
helping to mediate the adhesive interactions between these cells. Ig superfamily proteins
also play a part in these interactions, as well as in many other adhesive processes;
some of them bind homophilically, some heterophilically. Integrins, though
they mainly serve to attach cells to the extracellular matrix, can also mediate cell–
cell adhesion by binding to specific Ig superfamily proteins.
THE EXTRACELLULAR MATRIX OF ANIMALS
Tissues are not made up solely of cells. They also contain a remarkably complex
and intricate network of macromolecules constituting the extracellular matrix.
This matrix is composed of many different proteins and polysaccharides that are
secreted locally and assembled into an organized meshwork in close association
with the surfaces of the cells that produce them.
The classes of macromolecules constituting the extracellular matrix in different
animal tissues are broadly similar, but variations in the relative amounts of
these different classes of molecules and in the ways in which they are organized
give rise to an amazing diversity of materials. The matrix can become calcified to
form the rock-hard structures of bone or teeth, or it can form the transparent substance
of the cornea, or it can adopt the ropelike organization that gives tendons
their enormous tensile strength. It forms the jelly in a jellyfish. Covering the body
of a beetle or a lobster, it forms a rigid carapace. Moreover, the extracellular matrix
is more than a passive scaffold to provide physical support. It has an active and
complex role in regulating the behavior of the cells that touch it, inhabit it, or crawl
through its meshes, influencing their survival, development, migration, proliferation,
shape, and function.
In this section, we describe the major features of the extracellular matrix in
animal tissues, with an emphasis on vertebrates. We begin with an overview of the
major classes of macromolecules in the matrix, after which we turn to the structure
and function of the basal lamina, the thin layer of specialized extracellular
matrix that lies beneath all epithelial cells. In the sections that follow, we then
describe the varied types of cell–matrix junctions through which cells are connected
to the matrix.
The Extracellular Matrix Is Made and Oriented by the Cells Within It
The macromolecules that constitute the extracellular matrix are mainly produced
locally by cells in the matrix. As we discuss later, these cells also help to organize
the matrix: the orientation of the cytoskeleton inside the cell can control the orientation
of the matrix produced outside. In most connective tissues, the matrix
macromolecules are secreted by cells called fibroblasts (Figure 19–30). In certain
specialized types of connective tissues, such as cartilage and bone, however, they
are secreted by cells of the fibroblast family that have more specific names: chondroblasts,
for example, form cartilage, and osteoblasts form bone.
The extracellular matrix is constructed from three major classes of macromolecules:
(1) glycosaminoglycans (GAGs), which are large and highly charged
polysaccharides that are usually covalently linked to protein in the form of proteoglycans;
(2) fibrous proteins, which are primarily members of the collagen family;
and (3) a large class of noncollagen glycoproteins, which carry conventional
asparagine-linked oligosaccharides (described in Chapter 12). All three classes of
macromolecule have many members and come in a great variety of shapes and
10 µm
Figure 19–30 Fibroblasts in connective
tissue. This scanning electron micrograph
shows tissue from the cornea of a rat.
The extracellular matrix surrounding the
fibroblasts is here composed largely
of collagen fibrils. The glycoproteins,
hyaluronan, and proteoglycans, which
normally form a hydrated gel filling the
interstices of the fibrous network, have
been removed MBoC6 by m19.54/19.31
enzyme and acid
treatment. (Courtesy of T. Nishida.)