142
UNIT 1
Organization of the Body
4
Steps of Tissue Repair
Tissue repair requires that cells divide and migrate, activities
that are initiated by growth factors (wound hormones) released
by injured cells. Repair occurs in two major ways:
Regeneration
replaces destroyed tissue with the same kind
of tissue.
In
fibrosis
, fibrous connective tissue proliferates to form
scar
tissue
.
Which of these occurs depends on (1) the type of tissue damaged
and (2) the severity of the injury. In skin, the tissue we will use as
our example, repair involves both activities
(Figure 4.12)
.
1
Inflammation sets the stage.
Tissue trauma causes injured
tissue cells, macrophages, mast cells, and others to release
inflammatory chemicals, which cause the capillaries to dilate
and become very permeable. White blood cells (neutrophils,
monocytes) and plasma fluid rich in clotting proteins, an-
tibodies, and other substances seep into the injured area.
±e leaked clotting proteins construct a clot, which stops the
loss of blood, holds the edges of the wound together, and
effectively walls in, or isolates, the injured area, preventing
bacteria, toxins, or other harmful substances from spreading
to surrounding tissues. ±e part of the clot exposed to air
quickly dries and hardens, forming a
scab
. ±e inflammatory
events leave behind excess fluid, bits of destroyed cells, and
other debris, which are eventually removed via lymphatic
vessels or phagocytized by macrophages.
2
Organization restores the blood supply.
Even while the in-
flammatory process is going on, the first phase of tissue re-
pair, called
organization
, begins. During organization the
blood clot is replaced by
granulation tissue
, a delicate pink
tissue composed of several elements. It contains capillaries
that grow in from nearby areas and lay down a new capil-
lary bed. Granulation tissue is actually named for these cap-
illaries, which protrude nublike from its surface, giving it a
granular appearance. ±ese capillaries are fragile and bleed
freely, as we see when someone picks at a scab. Proliferat-
ing fibroblasts in granulation tissue produce growth fac-
tors as well as new collagen fibers to bridge the gap. Some
of these fibroblasts have contractile properties that allow
them to pull the margins of the wound together or to pull
existing blood vessels into the healing wound. As organiza-
tion proceeds, macrophages digest the original blood clot
and collagen fiber deposit continues. ±e granulation tis-
sue, destined to become scar tissue (a permanent fibrous
patch), is highly resistant to infection because it produces
bacteria-inhibiting substances. As a rule, wound healing is
a self-limited response. Once enough matrix has accumu-
lated in the injured area, the fibroblasts either revert to the
resting stage or undergo apoptosis.
3
Regeneration and fibrosis effect permanent repair.
Dur-
ing organization, the surface epithelium begins to
regenerate
,
growing under the scab, which soon detaches. As the fi-
brous tissue beneath matures and contracts, the regenerating
sheet lies directly over a layer of loose connective tissue called
the
lamina propria
(lam
9
ĭ-nah pro
9
pre-ah; “one’s own layer”). In
some mucosae, the lamina propria rests on a third (deeper) layer
of smooth muscle cells.
Mucous membranes are oFen adapted for absorption and
secretion. Although many mucosae secrete mucus, this is not a
requirement. ±e mucosae of both the digestive and respiratory
tracts secrete copious amounts of lubricating mucus, but that of
the urinary tract does not.
Serous Membranes
Serous membranes
, or
serosae
(se-ro
9
se), introduced in
Chapter 1, are the moist membranes found in closed ventral
body cavities (²igure 4.11c). A serous membrane consists of
simple squamous epithelium (a mesothelium) resting on a
thin layer of loose connective (areolar) tissue. ±e mesothe-
lial cells add hyaluronic acid to the fluid that filters from the
capillaries in the associated connective tissue. ±e result is
the thin, clear
serous fluid
that lubricates the facing surfaces
of the parietal and visceral layers, so that they slide across
each other easily.
±e serosae are named according to their site and specific
organ associations. ²or example, the
pleurae
line the thoracic
wall and cover the lungs; the
pericardium
encloses the heart;
and the
peritoneum
encloses the abdominopelvic viscera.
Check Your Understanding
19.
What type of membrane consists of epithelium and
connective tissue, and lines body cavities open to the
exterior?
20.
What type of membrane lines the thoracic walls and covers
the lungs, and what is it called?
For answers, see Appendix H.
Tissue Repair
Outline the process of tissue repair involved in normal
healing of a superficial wound.
±e body has many techniques for protecting itself from unin-
vited “guests” or injury. Intact mechanical barriers such as the
skin and mucosae, the cilia of epithelial cells lining the respira-
tory tract, and the strong acid (chemical barrier) produced by
stomach glands represent three defenses at the body’s external
boundaries.
When tissue is injured, these barriers are penetrated. ±is
stimulates the body’s inflammatory and immune responses,
which wage their battles largely in the connective tissues of the
body. ±e
inflammatory response
is a relatively nonspecific reac-
tion that develops quickly wherever tissues are injured, while
the
immune response
is extremely specific, but takes longer to
swing into action. We consider the inflammatory and immune
responses in detail in Chapter 21.
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