Dr. Hiba Wazeer Al Zou’bi Tissue repair 2 Dr. Hiba Wazeer Al Zou’bi

Scar formation Repair by connective tissue deposition Repair begins within 24 hours of injury. Components : Formation of new blood vessels (angiogenesis) granulation tissue (3 to 5 days) :Migration and proliferation of fibroblasts, deposition of connective tissue, abundant vessels(new thin-walled, delicate capillaries) and interspersed leukocytes and macrophages. Maturation and reorganization of the fibrous tissue (remodeling) to produce the stable fibrous scar

Granulation Tissue

Angiogenesis Process of new blood vessel development from existing vessels, primarily venules Critical in: - Healing at sites of injury - Development of collateral circulations at sites of ischemia - Allow tumors to increase in size beyond the constraints of their original blood supply.

Angiogenesis involves sprouting of new vessels from existing ones and consists of the following steps : - Vasodilation in response to NO and increased permeability induced by VEGF - Separation of pericytes from the abluminal surface - Migration of endothelial cells toward the area of tissue injury - Proliferation of endothelial cells just behind the leading front of migrating cells - Remodeling into capillary tubes - Recruitment of periendothelial cells (pericytes for small capillaries and smooth muscle cells for larger vessels) to form the mature vessel - Suppression of endothelial proliferation and migration and deposition of the basement membrane

Growth Factors Involved in Angiogenesis VEGF : Major inducer of angiogenesis after injury and in tumors. Bind to VEGFR-2, which is expressed by endothelial cells. Hypoxia is the most important inducer Stimulates both migration and proliferation of endothelial cells Promotes vasodilation by stimulating the production of NO, and contributes to the formation of the vascular lumen. Antibodies against VEGF are approved for the treatment of some tumors that depend on angiogenesis for their spread and growth FGF (FGF-1 ,FGF-2 ) Angiopoietins

Vasculogenesis The growth of blood vessels during embryonic development Vessels are formed de novo by the coalescence of endothelial precursors called angioblasts (derived from hemangioblasts).

Activation of Fibroblasts and Deposition of Connective Tissue - The laying down of connective tissue in the scar occurs in two steps: (1) Migration and proliferation of fibroblasts into the site of injury (2) Deposition of ECM proteins produced by these cells. Recruitment and activation of fibroblasts driven by many growth factors: PDGF, FGF-2 , and TGF-β. The major source of these factors is inflammatory cells, particularly macrophages.

As healing progresses, the number of proliferating fibroblasts and new vessels decreases; however, the fibroblasts progressively assume a more synthetic phenotype, so there is increased deposition of ECM. Collagen synthesis is critical to the development of strength in a healing wound site. As the scar matures, there is progressive vascular regression: transforms the highly vascularized granulation tissue into a pale, largely avascular scar (composed of largely inactive, spindle-shaped fibroblasts, dense collagen, fragments of elastic tissue, and other ECM components ).

Growth Factors Involved in ECM Deposition and Scar Formation Transforming growth factor-β (TGF-β) Binds to receptors , triggering the phosphorylation of transcription factors called Smads. Many effects: Stimulates the production of collagen, fibronectin, and proteoglycans, and it inhibits collagen degradation by both decreasing proteinase activity and increasing the activity of tissue inhibitors of proteinases known as TIMPs. Anti-inflammatory cytokine Platelet-derived growth factor (PDGF) Fibroblast growth factor (FGF)

Remodeling of Connective Tissue After its synthesis and deposition, the connective tissue in the scar continues to be modified and remodeled. The outcome of the repair process is a balance between synthesis and degradation of ECM proteins. The degradation of collagens and other ECM components is accomplished by a family of matrix metalloproteinases (MMPs)include: Interstitial collagenases: cleave fibrillar collagen Gelatinases : degrade amorphous collagen and fibronectin Stromelysins : degrade proteoglycans, laminin, fibronectin, and amorphous collagen. Activated MMPs can be rapidly inhibited by specific tissue inhibitors of metalloproteinases (TIMPs), produced by most mesenchymal cells. Thus, during scarring, MMPs are activated to remodel the deposited ECM, and then their activity is shut down by the TIMPs.

Factors affecting Healing: LOCAL Infection Poor blood supply Type of tissue Mechanical factors Increase local pressure SYSTEMIC Nutritional Protein deficiency Vitamin C deficiency Systemic diseases Diabetes mellitus Arteriosclerosis Infections (systemic) Corticosteroid treatment Age Immune status

Keloid The accumulation of exuberant amounts of collagen can give rise to prominent, raised scars known as keloids : Heritable predisposition More common in African-Americans.

Keloid

Keloid

Healing of Skin Wounds Healing by First Intention Healing of a clean, uninfected surgical incision approximated by surgical sutures. The incision causes only focal disruption of epithelial basement membrane continuity and death of relatively few epithelial and connective tissue cells. Epithelial regeneration is the principal mechanism of repair. The narrow incisional space first fills with fibrin-clotted blood, which then is rapidly invaded by granulation tissue and covered by new epithelium.

Healing by First Intention Within 24 hours: neutrophils are seen at the incision margin, migrating toward the fibrin clot. Within 24 to 48 hours: epithelial cells from both edges have begun to migrate and proliferate along the dermis yielding a thin continuous epithelial layer. By day 3: neutrophils have been largely replaced by macrophages, and granulation tissue progressively invades the incision space. By day 5: neovascularization reaches its peak. Collagen fibrils become more abundant. Second week: continued collagen accumulation and fibroblast proliferation. The leukocyte infiltrate, edema, and increased vascularity are substantially diminished. End of the first month: scar consists of a cellular connective tissue, largely devoid of inflammatory cells, covered by an essentially normal epidermis. However, the dermal appendages destroyed in the line of the incision are permanently lost.

Healing by First Intention

Healing by Second Intention Cell or tissue loss is more extensive, such as in large wounds, at sites of abscess formation, ulceration, and ischemic necrosis (infarction) in parenchymal organs. Repair process is more complex and involves a combination of regeneration and scarring.

Healing by Second Intention Secondary healing differs from primary healing in several respects: A larger clot Inflammation is more intense Require a greater volume of granulation tissue Wound contraction: Within 6 weeks large skin defects may be reduced to 5% to 10% of their original size, largely by contraction, due to the presence of myofibroblasts, which are modified fibroblasts exhibiting many of the ultrastructural and functional features of contractile smooth muscle cells.

Wound contraction

Wound Strength Carefully sutured wounds have approximately 70% of the strength of normal skin, largely because of the placement of sutures. 1 week (when sutures are removed) :wound strength is approximately 10% of that of unwounded skin, but this increases rapidly over the next 4 weeks. 3 months : Wound strength reaches approximately 70% to 80% of normal, usually does not improve substantially beyond that point.