Essential Cell Biology 5th edition

696 CHAPTER 20 Cell Communities: Tissues, Stem Cells, and Cancer
the matrix are scattered within it like raisins in a pudding (Figure 20–8);
the tensile strength—whether great or small—is chiefly provided not by a
polysaccharide, as it is in the cell wall of plants, but by fibrous proteins,
principally collagens. The various types of connective tissues owe their
specific characters to the type of collagen that they contain, to its quantity,
and, most importantly, to the other molecules that are interwoven
with it in varying proportions. These other molecules include the rubbery
protein elastin, which gives the walls of arteries their resilience as blood
pulses through them, as well as a host of specialized polysaccharide molecules,
which we discuss shortly.
100 µm
Figure 20–8 Extracellular matrix is
plentiful in connective tissue such as
bone. This micrograph shows a cross
section of bone in which the cells have been
lost during preparation. The spaces where
the cells had been appear as small, dark,
antlike shapes ECB5 e20.08/20.08
in the bone matrix, which
occupies most of the volume of the tissue
and provides all its mechanical strength.
The alternating light and dark bands are
layers of matrix, consisting almost entirely
of oriented fibrils of type I collagen (made
visible with the help of polarized light).
Calcium phosphate crystals (not visible)
fill the interstices between the collagen
fibrils, strengthening the bone matrix and
hardening it like reinforced concrete.
Collagen Provides Tensile Strength in Animal Connective
Tissues
The collagens are a family of proteins that come in many varieties.
Mammals have over 40 different collagen genes coding for the various
collagens that support the structure and function of different tissues.
Collagens are the chief proteins in bone, tendon, and skin (leather is
pickled collagen), and they constitute 25% of the total protein mass in a
mammal—more than any other type of protein. Type I collagen, which is
the most abundant, accounts for 90% of the body’s collagen.
The characteristic feature of a typical collagen molecule is its long,
stiff, triple-stranded helical structure, in which three collagen polypeptide
chains are wound around one another in a ropelike superhelix (see
Figure 4−29A). Some types of collagen molecules in turn assemble into
ordered polymers called collagen fibrils, which are thin cables 10–300 nm
in diameter and many micrometers long; these can pack together into still
thicker collagen fibers (Figure 20–9). Other types of collagen molecules
single collagen
polypeptide chain
N
C
triple-stranded
collagen molecule
1.5 nm
QUESTION 20–1
Cells in the stem of a seedling that
is grown in the dark orient their
microtubules horizontally. How
would you expect this to affect the
growth of the plant?
collagen fibril
0.5–3 µm
10–300 nm
Figure 20–9 Collagen fibrils are organized
into bundles. The drawings show the
steps of collagen assembly, from individual
polypeptide chains to triple-stranded
collagen molecules, then to fibrils and,
finally, fibers. The electron micrograph
shows fully assembled collagen fibers in the
connective tissue of embryonic chick skin.
The fibrils are bundled into fibers, some
running in the plane of the section, others
approximately at right angles to it. The
cell in the micrograph is a fibroblast, which
secretes collagen and other extracellular
matrix components. (Photograph from
C. Ploetz, E.I. Zycband, and D.E. Birk,
J. Struct. Biol. 106:73–81, 1991. With
permission from Elsevier.)
collagen fibers
1 µm