Organization of the Body
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:
replaces destroyed tissue with the same kind
, ﬁbrous connective tissue proliferates to form
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
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
. ±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.
Organization restores the blood supply.
Even while the in-
ﬂammatory process is going on, the ﬁrst phase of tissue re-
, begins. During organization the
blood clot is replaced by
, 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.
Regeneration and ﬁbrosis effect permanent repair.
ing organization, the surface epithelium begins to
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
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.
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
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
line the thoracic
wall and cover the lungs; the
encloses the heart;
encloses the abdominopelvic viscera.
Check Your Understanding
What type of membrane consists of epithelium and
connective tissue, and lines body cavities open to the
What type of membrane lines the thoracic walls and covers
the lungs, and what is it called?
For answers, see Appendix H.
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
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
is a relatively nonspeciﬁc reac-
tion that develops quickly wherever tissues are injured, while
is extremely speciﬁc, but takes longer to
swing into action. We consider the inﬂammatory and immune
responses in detail in Chapter 21.