Tissue repair follows three main steps: 1) inflammation sets the stage by causing blood vessel dilation and permeability, allowing immune cells and clotting to occur; 2) organization restores blood supply through granulation tissue and collagen deposition; 3) regeneration and fibrosis effect permanent repair through epithelial thickening and scar tissue formation, restoring the barrier. The regenerative capacity varies by tissue, from high regeneration in epithelia and bone to little regeneration in cardiac muscle and brain.

1. 4.7 Tissue Repair
• When the body’s barriers are compromised, the inflammatory and
immune responses are activated
• Repair starts very quickly
• Repair is the function of the inflammatory process
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2. Steps in Tissue Repair
• Repair can occur in two major ways:
– Regeneration: same kind of tissue replaces destroyed tissue, so
original function is restored
– Fibrosis: connective tissue replaces destroyed tissue, and original
function lost
• Step 1: Inflammation sets stage
– Release of inflammatory chemicals causes:
• Dilation of blood vessels
• Increase in blood vessel permeability
– Clotting of blood occurs
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3. Figure 4.12-1 Tissue repair of a nonextensive skin wound: regeneration and fibrosis.
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Scab Blood clot in
incised wound
Epidermis
Vein
Inflammatory
chemicals
Migrating
white
blood cell
Artery
Inflammation sets the stage:
• Severed blood vessels bleed.
• Inflammatory chemicals are released by
injured tissue cells, mast cells, and others.
• Local blood vessels become more
permeable, allowing white blood cells, fluid,
clotting proteins, and other plasma proteins
to seep into the injured area.
• Clotting occurs; surface exposed to air
dries and forms a scab.
1

4. Steps in Tissue Repair (cont.)
• Step 2: Organization restores blood supply
– Organization begins as the blood clot is replaced with granulation
tissue (new capillary-enriched tissue)
– Epithelium begins to regenerate
– Fibroblasts produce collagen fibers to bridge the gap until
regeneration is complete
– Any debris in area is phagocytized
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5. Figure 4.12-2 Tissue repair of a nonextensive skin wound: regeneration and fibrosis.
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Regenerating
epithelium
Area of
granulation
tissue
ingrowth
Fibroblast
Macrophage
Budding
capillary
Organization restores the blood supply:
• The clot is replaced by granulation tissue,
which restores the vascular supply.
• Fibroblasts produce collagen fibers that
bridge the gap.
• Macrophages phagocytize dead and
dying cells and other debris.
• Surface epithelial cells multiply and
migrate over the granulation tissue.
2

6. Steps in Tissue Repair (cont.)
• Step 3: Regeneration and fibrosis effect permanent repair
– The scab detaches
– Fibrous tissue matures
– Epithelium thickens and begins to resemble adjacent tissue
– Results in a fully regenerated epithelium with underlying scar
tissue, which may or may not be visible
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7. Figure 4.12-3 Tissue repair of a nonextensive skin wound: regeneration and fibrosis.
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Regenerated epithelium
Fibrosed area
Regeneration and fibrosis effect
permanent repair:
• The fibrosed area matures and
contracts; the epithelium thickens.
• A fully regenerated epithelium with
an underlying area of scar tissue
results.
3

8. Regenerative Capacity of Different Tissues
• Tissues that regenerate extremely well include:
– Epithelial tissues, bone, areolar connective tissue, dense irregular
connective tissue, blood-forming tissue
• Tissue with moderate regenerating capacity:
– Smooth muscle and dense regular connective tissue
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9. Regenerative Capacity of Different Tissues
(cont.)
• Tissues with virtually no functional regenerative capacity:
– Cardiac muscle and nervous tissue of brain and spinal cord
– New research shows cell division does occur, and efforts are
underway to coax them to regenerate better
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10. Clinical – Homeostatic Imbalance 4.3
• Scar tissue that forms in organs, particularly the heart, can
severely impair the function of that organ
– May cause the organ to lose volume capacity
– May block substances from moving through organ
– May interfere with ability of muscles to contract or may impair nerve
transmissions
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11. Clinical – Homeostatic Imbalance 4.3
• Scar adhesions may cause organs to adhere to neighboring
structures, preventing normal functions
• Scarring can potentially cause progressive failure of the organ,
particularly the heart
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12. Developmental Aspects of Tissues
• Primary germ layers
– Superficial to deep: ectoderm, mesoderm, and endoderm
– Formed early in embryonic development
– Specialize to form the four primary tissues
• Nerve tissue arises from ectoderm
• Muscle and connective tissues arise from mesoderm
• Epithelial tissues arise from all three germ layers
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13. Developmental Aspects of Tissues
• Tissues function well through youth and middle age if given
adequate diet and circulation and if wounds and infections are
minimal
• As the body ages, epithelia thin, so they are more easily
breached
• Tissue repair is less efficient
• Bone, muscle tissues, and nervous tissues begin to atrophy
• DNA mutations increase cancer risk
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14. Figure 4.13 Embryonic germ layers and the primary tissue types they produce.
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16-day-old embryo
(dorsal surface view)
Ectoderm
Mesoderm
Endoderm
Epithelium
(from all three
germ layers)
Inner lining of
digestive system
(from endoderm)
Nervous tissue
(from ectoderm)
Muscle and
connective
tissue
(mostly from
mesoderm)