Tissue Repair 2 PDF

Tissue Repair -2  LOs Wound and types Healing and repair Regeneration of cells and tissues Growth factors Prototype: Healing of skin wound Factors effecting tissue repair  Extracellular Matrix (ECM) Network surrounding cells constitutes a...

Tissue Repair -2  LOs Wound and types Healing and repair Regeneration of cells and tissues Growth factors Prototype: Healing of skin wound Factors effecting tissue repair  Extracellular Matrix (ECM) Network surrounding cells constitutes a significant proportion of any tissue Regulates proliferation, movement and differentiation of cells in it. ECM occurs in two basic forms: Interstitial matrix present in spaces between cells in connective tissue, between epithelium, supportive vascular and smooth muscle structures synthesized by mesenchymal cells Basement membrane Highly organized interstitial matrix around epithelial cells, endothelial cells, and smooth muscle cells. lies beneath the epithelium and is synthesized by overlying epithelium and underlying mesenchymal cells  Components of ECM Fibrous structural proteins: collagens and elastins, which confer tensile strength and recoil Water-hydrated gels: proteoglycans and hyaluronan - permit resilience and lubrication Adhesive glycoproteins: connect the matrix elements to one another and cells  Components of ECM Collagen Fibrous structural proteins that confer tensile strength About 30 collagen types identified Fibrillar collagens: Type I,II,III and V form major proportion of the connective tissue in healing wounds and particularly in scars. Nonfibrillar collagens: Type IV may form basement membrane Type IX: form components of structures such as intervertebral discs Type VII: form dermal-epidermal junctions Elastin Fibrous protein Elastic tissue confers tissues the ability to recoil and return to a baseline structure after physical stress.  Components of ECM Proteoglycans form highly hydrated compressible gels consisting of long polysaccharides - glycosaminoglycans (examples: Dermatan sulfate, Heparan sulfate) linked to protein backbone. confer resilience, lubrication and compressibility to tissue serve as reservoirs for growth factors secreted into the ECM (e.g. FGF and HGF). Hyaluronan huge molecule composed of many disaccharide repeats without a protein core Because of its ability to bind water, it forms a viscous, gelatin-like matrix.  Components of ECM Adhesive Glycoproteins and Adhesion Receptors structurally diverse molecules, involved in 1. cell-to-cell adhesion 2. linkage between cells and ECM, 3. binding between ECM components. Fibronectin major component of interstitial ECM synthesized by fibroblasts, monocytes, and endothelium. have specific domains that bind to wide spectrum of ECM components (e.g. collagen, fibrin, heparin and proteoglycans) and can attach to cell integrins Laminin major constituent of basement membrane. connects cells to underlying ECM components such as type IV collagen and heparan sulfate.  Components of ECM Integrins family of transmembrane glycoproteins composed of α and β chains Main cellular receptors for ECM components, such as fibronectins and laminins. Integrins are present in the plasma membrane of most animal cells, with the exception of red blood cells.  Cell and tissue regeneration cell renewal occurs continuously in labile tissues, such as bone marrow, gut epithelium, and skin Damage to epithelia or an increased loss of blood cells can be corrected by proliferation and differentiation of stem cells Best example - liver. Adrenal,thyroid, pancreas ,lungs have limited regeneration capacity  Repair by connective tissue Severe or chronic tissue injury results in damage to parenchymal cells, epithelia and stromal framework. injury to non dividing cells, repair cannot be accomplished by regeneration alone. Repair begins within 24 hours of injury by emigration of fibroblasts and induction of fibroblast and endothelial cell proliferation. By 3 to 5 days, a specialized type of tissue that is characteristic of healing is apparent - granulation tissue. Term derived from pink, soft, granular gross appearance seen beneath scab of skin wound  Repair by connective tissue Microscopically : characterized by proliferation of fibroblasts and new thin-walled, delicate capillaries in loose ECM. Variable inflammatory infiltrate (mostly mononuclear, with some PMN's Granulation tissue progressively accumulates connective tissue matrix, eventually resulting in scar formation which may remodel over time A. Granulation tissue showing numerous blood vessels, edema, and a loose ECM containing occasional inflammatory cells. Collagen is stained blue by trichrome stain; minimal mature collagen seen. B, Trichrome stain of mature scar, showing dense collagen with only scattered vascular channels.  Repair by connective tissue consists of four sequential processes: 1. Angiogenesis Formation of new blood vessels 2. Migration and proliferation of fibroblasts 3. Scar formation Deposition of ECM 4. Remodeling Maturation and reorganization of fibrous tissue 1. Angiogenesis Blood vessels are assembled by two processes: 1. vasculogenesis: primitive vascular network is assembled from angioblasts (endothelial cell precursors) during embryonic development 2. angiogenesis, or neovascularization, preexisting vessels send out capillary sprouts to produce new vessels A mobilization of bone marrow endothelial precursor cells (EPCs) B. from preexisting vessels at the site of injury  Main steps in angiogenesis from preexisting vessels. 1. Vasodilation in response to nitric oxide and increased permeability of the preexisting vessel induced by vascular endothelial growth factor (VEGF) 2. Migration of endothelial cells toward the area of tissue injury 3. Proliferation of endothelial cells just behind the leading front of migrating cells 4. Inhibition of endothelial cell proliferation and remodeling into capillary tubes 5. Recruitment of periendothelial cells (pericytes for small capillaries and smooth muscle cells for larger vessels) to form the mature vessel  Growth Factors Involved in Angiogenesis Several factors induce angiogenesis. the most important are VEGF and basic fibroblast growth factor (FGF-2) VEGF stimulates both proliferation and motility of endothelial The most important of receptors for angiogenesis is VEGFR-2, which is restricted to endothelial cells new vessels need to be stabilized by the recruitment of pericytes and smooth muscle cells and by the deposition of connective tissue. Angiopoietins 1 and 2 (Ang 1 and Ang 2) and the growth factors PDGF and TGF-β participate in the stabilization process  Growth Factors Involved in Angiogenesis Several factors, most important - VEGF and fibroblast growth factor (FGF-2) VEGF stimulates both proliferation and motility of endothelial most important receptors is VEGFR-2, which is restricted to endothelial cells new vessels need to be stabilized by recruitment of pericytes and smooth muscle cells and by deposition of connective tissue. Angiopoietins 1 &2 and growth factors PDGF &TGF-β participate in stabilization process Ang1 interacts with a receptor on endothelial cells called Tie2 to recruit periendothelial cells. PDGF participates in recruitment of smooth muscle cells TGF-β enhances production of ECM proteins.  2. Migration of Fibroblasts The recruitment and stimulation of fibroblasts is driven by many growth factors. PDGF, FGF-2 , and TGF-β. Source of growth factors: activated endothelium and inflammatory cells. Macrophages are important cellular constituents of granulation tissue. besides clearing extracellular debris and fibrin at the site of injury, they elaborate a host of mediators that induce fibroblast proliferation and ECM production  3. ECM Deposition (Scar Formation) As healing progresses, the number of proliferating fibroblasts and new vessels decreases collagen synthesis by fibroblasts begins early in wound healing (days 3 to 5) and continues for several weeks, depending on the size of the wound. Ultimately, the granulation tissue scaffolding evolves into a 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. TGF-β, PDGF, and FGF 4. ECM and Tissue Remodeling The outcome of repair process is, in part, a balance between ECM synthesis and degradation. after its synthesis and deposition, scar ECM continues to be modified and remodeled The degradation of collagens and other ECM components is accomplished by a family of matrix metalloproteinases (MMPs), 1. Interstitial Collagenases, which cleave fibrillar collagen (MMP-1,-2 and -3); 2. Gelatinases (MMP-2 and 9), which degrade amorphous collagen and fibronectin, 3. Stromelysins (MMP-3, -10, and -11), which degrade a variety of ECM constituents, including proteoglycans, laminin, fibronectin, and amorphous collagen. 4. MMPs are produced by a variety of cell types (fibroblasts, macrophages, neutrophils, synovial cells, and some epithelial cells), and their synthesis and secretion are regulated by growth factors, cytokines, and other agents  Cutaneous Wound Healing This is a process that involves both epithelial regeneration and formation of connective tissue scar Cutaneous wound healing has 3 main phases: (1) inflammation (2) formation of granulation tissue (3) ECM deposition and remodeling Based on nature of wound, the healing of cutaneous wounds can occur by first or second intention  Healing by First Intention uninfected surgical incision approximated by surgical sutures focal disruption of epithelial basement membrane continuity death of a relatively few epithelial and connective tissue cells The narrow incisional space first fills with fibrin-clotted blood, which is rapidly invaded by granulation tissue and covered by new epithelium.  Within 24 hours, Within 24 hours, neutrophils are seen at the incision margin, migrating toward the fibrin clot. Basal cells at the cut edge of the epidermis begin to show increased mitotic activity.  Within 24 to 48 hours, Within 24 to 48 hours, epithelial cells from both edges have begun to migrate and proliferate along the dermis, depositing basement membrane components as they progress. The cells meet in the midline beneath the surface scab, yielding a thin but continuous epithelial layer  By day 3 By day 3, neutrophils have been largely replaced by macrophages, and granulation tissue progressively invades the incision space. Collagen fibers are now evident at the incision margins, but these are vertically oriented and do not bridge the incision. Epithelial cell proliferation continues, yielding a thickened epidermal covering layer.  By day 5 By day 5, neovascularization reaches its peak as granulation tissue fills the incisional space. Collagen fibrils become more abundant and begin to bridge the incision. The epidermis recovers its normal thickness as differentiation of surface cells yields a mature epidermal architecture with surface keratinization. This is a healing biopsy site on the skin seen a week following the excision, The skin surface has re-epithelialized, and below this is granulation tissue with small capillaries and fibroblasts forming collagen. After a month, just a small collagenous scar will remain.  the second week During the second week, there is continued collagen accumulation and fibroblast proliferation. The leukocyte infiltrate, edema, and increased vascularity are substantially diminished. The long process of "blanching" begins, accomplished by increasing collagen deposition within the incisional scar and the regression of vascular channels  Healing by Second Intention When cell or tissue loss is more extensive, such as in large wounds, abscess formation, and ulceration A larger clot or scab rich in fibrin and fibronectin forms at the surface of the wound. Inflammation is more intense because large tissue defects have a greater volume of necrotic debris, exudate, and fibrin that must be removed. Much larger amounts of granulation tissue are formed. A greater volume of granulation tissue generally results in a greater mass of scar tissue. Secondary healing involves wound contraction.  Wound Strength sutured wounds have approximately 70% of the strength of unwounded skin After suture removal ,usually at 1 week, wound strength is approximately 10% of that of unwounded skin, but this increases rapidly over the next 4 weeks. Wound strength reaches approximately 70% to 80% of normal by 3 months but usually does not substantially improve beyond that point.  Complications/ abnormalities Deficient scar formation mechanical reasons →dehiscence due to pressure by cough, inadequate blood supply or neuropathy ( devoid of sensation) → ulceration Excessive scar formation – excess collagen → hypertrophic scar or keloid – Exuberant (excess) granulation → above epithelial level → block re- epithelialization – Desmoids or aggressive fibromatoses contractures → deformities interfere with mobility – after burns  Pathologic aspects of repair Infections Nutritional deficiency Mechanical variables Foreign bodies Type of tissue injured Keloid formation Exuberant granulation  Delayed healing Prolonged inflammatory phase Infection Inflammatory diseases Chronic inflammation Chronic immune reaction- Persistent stimulus for fibrogenesis – growth factors – cytokines, proteases. Collagen degradation by collagenases – joint destruction  Factors influencing healing Systemic: Nutrition- proteins,vitamin c metabolic status- DM circulatory status- atherosclerosis, venous abnormalities hormones - glucocorticoids antiinflammatory local : infection mechanical e.g. early movements foreign body: sutures, steel, glass, bone size/location/type of wound: richly vascularized area like face, small injury – faster healing  Factors affecting healing Type of inflammation (acute or chronic) Extent of tissue necrosis Regenerative ability of damaged parenchymal cells Immune status Blood glucose levels (impaired white cell function) Hydration status (slows metabolism)  Factors affecting healing Nutrition Blood albumin levels (‘building blocks’ for repair, colloid osmotic pressure - oedema) Oxygen and vascular supply Pain (causes vasoconstriction) Corticosteroids (depress immune function)