155
CHAPTER FIVE
Tissues
Macrophages
(mak
ro-fa
¯jez), or histiocytes, originate
as white blood cells (see chapter 14, p. 532) and are almost
as numerous as F broblasts in some connective tissues. They
are usually attached to F
bers but can detach and actively
move about. Macrophages are specialized to carry on phago-
cytosis. They function as scavenger cells that can clear for-
eign particles from tissues, so macrophages are an important
defense against infection
(f g. 5.15)
. They also play a role in
immunity (see chapter 16, p. 630).
Mast cells
are large and widely distributed in connective
tissues, where they are usually near blood vessels
(f g. 5.16)
.
They release
heparin,
a compound that prevents blood clotting.
Mast cells also release
histamine,
a substance that promotes
some of the reactions associated with infl ammation and aller-
gies, such as asthma and hay fever (see chapter 16, p. 639).
Release of histamine stimulates inF
ammation by dilating the small
arterioles that feed capillaries, the tiniest blood vessels. The resulting
swelling and redness is inhospitable to infectious bacteria and viruses
and also dilutes toxins. Inappropriate histamine release as part of an
allergic response can be most uncomfortable. Allergy medications
called antihistamines counter this misplaced inF
ammation.
Connective Tissue Fibers
±ibroblasts produce three types of connective tissue F
bers:
collagenous fibers, elastic fibers, and reticular fibers. Of
these, collagenous and elastic F
bers are the most abundant.
Collagenous
(kol-laj
e
˘-nus)
f
bers
are thick threads of the
protein
collagen
(kol
ah-jen), the major structural protein of
the body. Collagenous F bers are grouped in long, parallel bun-
dles, and they are fl exible but only slightly elastic
(f g. 5.17)
.
More importantly, they have great tensile strength—that is,
they can resist considerable pulling force. Thus, collagenous
F bers are important components of body parts that hold struc-
tures together, such as
ligaments
(which connect bones to
bones) and
tendons
(which connect muscles to bones).
FIGURE 5.14
A scanning electron micrograph of a ±
broblast (4,000×).
Cell being
engulfed
Macrophage
FIGURE 5.15
Macrophages are scavenger cells common in
connective tissues. This scanning electron micrograph shows a
number of macrophages engul±
ng parts of a larger cell (3,330×).
Tissue containing abundant collagenous F bers is called
dense connective tissue.
Such tissue appears white, and for
this reason collagenous F bers of dense connective tissue are
sometimes called white F bers.
Loose connective tissue,
on the
other hand, has sparse collagenous F bers. Clinical Application
5.2 describes disorders that result from abnormal collagen.
When skin is exposed to prolonged and intense sunlight, connective
tissue ±
bers lose elasticity, and the skin sti²
ens and becomes leath-
ery. In time, the skin may sag and wrinkle. Collagen injections may
temporarily smooth out wrinkles. However, collagen applied as a
cream to the skin does not combat wrinkles because collagen mol-
ecules are far too large to penetrate the skin.
Elastic f
bers
are composed of a springlike protein called
elastin.
These F
bers branch, forming complex networks in
various tissues. They are weaker than collagenous F bers but
elastic. That is, they are easily stretched or deformed and will
resume their original lengths and shapes when the force act-
ing upon them is removed. Elastic F bers are common in body
parts normally subjected to stretching, such as the vocal cords
and air passages of the respiratory system. Elastic F bers are
sometimes called yellow F bers, because tissues amply sup-
plied with them appear yellowish (F g. 5.17).
Surgeons use elastin in foam, powder, or sheet form to prevent scar
tissue adhesions from forming at the sites of tissue removal. Elastin
is produced in bacteria that contain human genes that instruct them
to manufacture the human protein. This is cheaper than synthesizing
elastin chemically and safer than obtaining it from cadavers.
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