means the ligaments have been overly stretched), you
know that complete healing may take several months.
An irregular type of fibrous connective tissue forms
the dermis of the skin and the fasciae (membranes)
around muscles. Although the collagen fibers here are
not parallel to one another, the tissue is still strong.
The dermis is different from other fibrous connective
tissue in that it has a good blood supply (see also Box
4–3: Cosmetic Collagen).
ELASTIC CONNECTIVE TISSUE
As its name tells us, elastic connective tissueis pri-
marily elastin fibers. One of its locations is in the walls
of large arteries. These vessels are stretched when the
heart contracts and pumps blood, then they recoil, or
snap back, when the heart relaxes. This recoil helps
keep the blood moving away from the heart, and is
important to maintain normal blood pressure.
Elastic connective tissue is also found surrounding
the alveoli of the lungs. The elastic fibers are stretched
during inhalation, then recoil during exhalation to
squeeze air out of the lungs. If you pay attention to
your breathing for a few moments, you will notice that
normal exhalation does not require “work” or energy.
This is because of the normal elasticity of the lungs.BONE
The prefix that designates bone is “osteo,” so bone
cells are called osteocytes. The matrix of boneis
made of calcium salts and collagen and is strong, hard,78 Tissues and Membranes
BOX4–2 VITAMIN C AND COLLAGEN
Many people take extra vitamin C, for various rea-
sons. Vitamin C has several functions, and an im-
portant one is the synthesis of collagen.
Imagine the protein collagen as a ladder with
three uprights and rungs that connect adjacent
uprights. Vitamin C is essential for forming the
“rungs,” without which the uprights will not stay
together as a strong unit. Collagen formed in the
absence of vitamin C is weak, and the effects of
weak collagen are dramatically seen in the disease
called scurvy.In 1753 James Lind, a Scottish surgeon, recom-
mended to the British Navy that lime juice be taken
on long voyages to prevent scurvy among the
sailors. Scurvy is characterized by bleeding gums
and loss of teeth, poor healing of wounds, fractures,
and bleeding in the skin, joints, and elsewhere in
the body. The lime juice did prevent this potentially
fatal disease, as did consumption of fresh fruits and
vegetables, although at the time no one knew why.
Vitamin C was finally isolated in the laboratory in
1928.BOX4–3 COSMETIC COLLAGEN
system as foreign tissue. More seriously, an autoim-
mune response may be triggered in some individu-
als, and the immune system may begin to destroy
the person’s own connective tissue.
In an effort to avoid these problems, some cos-
metic surgeons now use the person’s own collagen
and fat, which may be extracted from the thigh,
hip, or abdomen. The long-term consequences and
outcomes of such procedures have yet to be evalu-
ated. We might remember that for many years the
use of silicone injections had been considered safe.
Silicone injections are now banned by the FDA,
since we now know that they carry significant risk
of serious tissue damage.Collagen is the protein that makes tendons, liga-
ments, and other connective tissues strong. In 1981,
the Food and Drug Administration (FDA) approved
the use of cattle collagen by injection for cosmetic
purposes, to minimize wrinkles and scars. Indeed,
collagen injected below the skin will flatten out
deep facial wrinkles and make them less prominent,
and many people have had this seemingly simple
cosmetic surgery.
There are, however, drawbacks. Injected colla-
gen lasts only a few months; the injections must be
repeated several times a year, and they are expen-
sive. Some people have allergic reactions to the cat-
tle collagen, which is perceived by the immune