begins almost immediately. Infammation is a gen-
eralized (nonspeciFc) body response that attempts
to prevent ±urther injury. The immune response,
on the other hand, is extremely speciFc and
mounts a vigorous attack against recognized in-
vaders (bacteria, viruses, toxins). These protective
responses are considered in detail in Chapter 12.
Here we will concentrate on the process o± tissue
Tissue repair, or wound healing, occurs in
two major ways: by regeneration and by Fbrosis.
is the replacement o± destroyed
tissue by the same kind o± cells, whereas
involves repair by dense (Fbrous) connective tis-
sue, that is, by the ±ormation o±
occurs depends on (1) the type o± tissue damaged
and (2) the severity o± the injury. Generally
speaking, clean cuts (incisions) heal much more
success±ully than ragged tears o± the tissue.
Tissue injury sets a series o± events into motion.
The capillaries become very permeable.
This allows fuid rich in clotting proteins and
other substances to seep into the injured area
±rom the bloodstream. Then leaked clotting
proteins construct a clot, which stops the loss
o± blood, holds the edges o± the wound to-
gether, and walls o±± the injured area, prevent-
ing bacteria or other harm±ul substances ±rom
spreading to surrounding tissues. Where the
clot is exposed to air, it quickly dries and hard-
ens, ±orming a scab.
Granulation tissue Forms.
is a delicate pink tissue composed largely
o± new capillaries that grow into the damaged
area ±rom undamaged blood vessels nearby.
These capillaries are ±ragile and bleed ±reely,
as when a scab is picked away ±rom a skin
wound. Granulation tissue also contains
phagocytes that eventually dispose o± the
blood clot and connective tissue cells (Fbro-
blasts) that synthesize the building blocks o±
collagen ±ibers (scar tissue) to permanently
bridge the gap.
The surFace epithelium regenerates.
sur±ace epithelium begins to regenerate, it
makes its way across the granulation tissue just
beneath the scab. The scab soon detaches and
the Fnal result is a ±ully regenerated sur±ace
epithelium that covers an underlying area o±
Fbrosis (the scar). The scar is either invisible or
visible as a thin white line, depending on the
severity o± the wound.
The ability o± the di±±erent tissue types to re-
generate varies widely. Epithelial tissues such as
the skin epidermis and mucous membranes regen-
erate beauti±ully. So, too, do most o± the Fbrous
connective tissues and bone. Skeletal muscle re-
generates poorly, i± at all, and cardiac muscle and
nervous tissue within the brain and spinal cord are
replaced largely by scar tissue.
Scar tissue is strong, but it lacks the fexibility
oF most normal tissues. Perhaps even more impor-
tant is its inability to perForm the normal Functions oF
the tissue it replaces. Thus, iF scar tissue Forms in
the wall oF the bladder, heart, or another muscular
organ, it may severely hamper the Functioning oF that