WOUND HEALING REPAIR

Objectives This lecture provides an understanding of features of wound healing elaborating on cells involved regenerating capacity of cells Mechanism Complications factors affecting wound healing the process of wound healing in the skin

Learning outcomes At the end of the lecture,student will be able to Describe the process of repair of damaged tissue List the factors influencing the repair process Relate and apply the basic mechanism to the process of wound healing in the skin and bone Discuss the factors affecting and wound healing Recognize the complications of wound healing

HEALING and REPAIR Replacement of dead or damaged cells by new healthy cells

HEALING  Healing is the body response to injury in an attempt to restore normal structure and function  The process of healing involves 2 distinct processes: REGENERATION & REPAIR

REGENERATION Regeneration proliferation of cells and tissues to replace lost structures results in the complete restoration of lost or damaged tissue dependent on the type of normal turnover the original tissue

REPAIR R epair most often consists of a combination of regeneration and scar formation by the deposition of collagen

CELL TYPES based on regenerative capacity Labile: ( continuously dividing tissues cells ) Stable cells (quiescent) : Permanent cells (NON-dividing ):

Types of cells based on regenerative capacity 1.Labile Cells:(continuously dividing tissues cells ) surface epithelia such as stratified squamous epithelia of the skin, oral cavity, vagina, and cervix; lining mucosa of excretory ducts columnar epi; of GIT, uterus,Fall;tubes transitional epi: of urinary tract splenic, lymphoid & haemopoietic cells

2. Quiescent (Stable cells) low level of replication rapid replication in variety of stimuli capable of reconstituting tissue of origin the parenchymal cells of liver, kidneys, and pancreas; mesenchymal cells such as fibroblasts and smooth muscle; vascular endothelial cells; and lymphocytes and other leukocytes

3. NON-dividing (Permanent cells): not undergo mitotic division in post -natal life nerve cells skeletal and cardiac muscle cells

Regeneration requires Presence of stem cells for renewal or tissue cells that are capable to divide in response to growth factors Intact tissue scaffold REGENERATION

To achieve DNA replication and division the cell goes through a tightly controlled sequence of events known as the cell cycle.

The cell cycle consists of  G1 (presynthetic)  S (DNA synthesis)  G2 (premitotic) and  M (mitotic) phases Quiescent cells that not entered the cell cycle are in the G0 state

The replication of cells is stimulated by  growth factors  signaling from ECM components through integrins.

STEM CELL Stem cells are characterized by their self-renewal properties and their capacity to generate differentiated cell lineages ADULT STEM CELL

EMBRYONIC STEM CELL

ADULT STEM CELLS MARROW (HEMOCYTOBLAST) NON-MARROW (RESERVE)

To achieve DNA replication and division the cell goes through a tightly controlled sequence of events known as the cell cycle

The cell cycle consists of  G1 (presynthetic)  S (DNA synthesis)  G2 (premitotic) and  M (mitotic) phases Quiescent cells that not entered the cell cycle are in the G0 state

The replication of cells is stimulated by  growth factors  signaling from ECM components through integrins.

Growth Factors (GFs) The proliferation of many cell types is driven by Growth Factors Polypeptides promote CELL SURVIVALS LOCOMOTION CONTRACTILITY DIFFERENTIATION ANGIOGENESIS

Growth Factors (GFs) Epidermal Transforming (alpha, beta) Hepatocyte Vascular Endothelial Platelet Derived Fibroblast Keratinocyte Cytokines (TNF, IL-1, Interferons)

All growth factors function as ligands that bind to specific receptors, which deliver signals to the target cells These signals stimulate the transcription of genes that may be silent in resting cells, including genes that control cell cycle entry and progression

CELL PLAYERS (source AND targets) Lymphocytes, especially T-cells Macrophages Platelets Endothelial cells Fibroblasts Keratinocytes Mesenchymal cells Smooth muscle cells

Macrophages are key cellular constituents of tissue repair, clearing extracellular debris, fibrin, and other foreign material at the site of repair, and promoting angiogenesis and ECM deposition

SIGNALING MECHANISMS IN CELL GROWTH Receptor-mediated signal transduction  Autocrine signaling  Paracrine signaling  Endocrine signaling

Extracellular Matrix and Cell-Matrix Interactions various functions of ECM include: Mechanical support Control of cell growth Maintenance of cell differentiation Scaffolding for tissue renewal Establishment of tissue microenvironments Storage and presentation of regulatory molecules (For example, growth factors like FGF and HGF are secreted and stored in the ECM in some tissues)

Three groups of macromolecules constitute the ECM Fibrous structural proteins Collagen, Fibrillins Adhesive glycoproteins Cadherin, Integrins, Immunoglobulin family, Selectins Proteoglycans and Hyaluronic Acid

Healing by Repair, Scar Formation and Fibrosis If tissue injury is severe or chronic the main healing process is Repair by deposition of collagen and other ECM components, causing the formation of a scar

Repair by connective tissue deposition includes the following basic features:  Haemostasis  inflammation (to remove dead and damaged tissue)  Angiogenesis &formation of granulation tissue  migration and proliferation of fibroblasts  Tissue remodeling  Wound contraction  Increase in wound strength (scar = fibrosis)

WOUND HEALING PRIMARY UNION ( HEALING BY FIRST INTENTION) clean surgical incision no significant bacterial contamination minimal loss of tissue clot, scab formation 24 hr: neutrophils, mitotic activity of basal layer, thin epithelial layer

day 3: macrophages, granulation tissue Granulation tissue consists of newly formed blood vessels, macrophages, fibroblasts and loose ECM framework day 5: collagen bridges the incision, epidermis thickens 2nd week: continued collagen and fibroblasts, blanching End of 1st month: scar (cellular connective tissue, intact epidermis, lost appendages)

WOUND HEALING 2. SECONDARY UNION (healing by 2nd intention) Occurs when there is : more extensive loss of cells and tissue: infarction inflammatory ulceration abscess formation surface wound with large defect large tissue defect that must be filled Differs from primary healing: 1. inflammatory reaction is more intense 2. larger amounts of granulation tissue are formed 3. wound contraction (5 to 10%), ?myofibroblasts

Contraction of wounds The wound starts contracting after 2-3 days and the process is completed by the 14th day. During this period, the wound is reduced by approximately 80% of its original size Contracted wound results in rapid healing since lesser surface area of the injured tissue has to be replaced

WOUND HEALING 1 st INTENTION Edges lined up 2 nd INTENTION Edges NOT lined up More granulation More epithelialization MORE FIBROSIS

Injury leads to accumulation of platelets and coagulation factors. Coagulation results in fibrin formation and release of PDGF and TGF-b and other inflammatory mediators by activated platelets. This leads to more Neutrophil recruitment which signals the beginning of inflammation (24 h). After 48 h macrophages replace neutrophils. Neutrophils and macrophages are responsible for removal of cellular debris and release growth factors to reorganize the cellular matrix. At 72 hours the proliferation phase begins as recruited fibroblasts stimulated by FGF and TFG-b begin to synthesize collagen. Previously formed fibrin forms initial matrix for fibroblasts Collagen cross-linking and reorganization occurs following months after injury in the remodeling phase of repair. Wound contraction follows in large surface wounds and is facilitated by actin-containing fibroblasts (myofibroblasts) The phases of cutaneous wound healing

Complications of wound healing Deficient scar formation Wound dehiscence Ulceration Excessive formation of scar tissue Keloid (excessive collagen deposition) Exuberant granulation (proliferation of fibroblasts that inhibits re-epithelialization) Desmoid (aggressive fibromatosis, semi-malignant) Formation of contractures results in deformities of the wound and the surrounding tissues

Wound dehiscence Wound ulceration Wound dehiscence

Keloid

Contracture

Complications of Wound Healing 1.Infection wound is portal of entry to organisms ; delays healing and stops it completely if severe increased tissue destruction increased fibrous tissue -> ugly scar

2.Wound Dehiscence: Burst abdomen after laparotomy High mortality due to inappropriate sutures or poor surgical technique hypoproteinemia, Vit C deficiency, poor nutrition local factors – cough, vomiting, ileus, peritonitis

3.Keloid formation: excessive collagen -raised area of scar tissue due to repeated trauma, F.B irritation, hair, keratin more in Blacks, young girls, TB patients neck region, especially after burns 4.Proud Flesh; exuberant granulation tissue

5.Implantation ( epidermoid) cysts:  epithelial cells persist in wound -> proliferate -> epidermoid cyst 6.Pigmentary changes:  colored particles introduced into wound -> tattooing effect  healed chronic ulcers have color due to hemosiderin

7.Painful Scars: local or referred pain when neuroma is included in the scar 8.Weak Scars: scar subjected to continuous strain -> stretching -> incisional hernia

9.Cicatrisation: reduction of scar size in severe skin burns contracture hollow viscera -> stenosis with stricture liver cirrhosis -> venous obstruction -> portal hypertension 10.Neoplasia:  squamous cell CA may develop in scars

Systemic factors Malnutrition Protein deficiency Vitamin C deficiency (inhibition of collagen synthesis) Metabolic status e.g Diabetes mellitus Consequence of microangiopathy Cortisone treatment inhibits inflammation and collagen synthesis Circulatory status Inadequate blood supply due to ateriosclerosis Varicose veins (retarded venous drainage) Factors that influence wound healing

Factors that influence wound healing (continue) Local Factors Infection (single most important reason for delayed wound healing) Foreign bodies suture material, bone and wood splinters …. Mechanical factors Early movement Pressure Size, location, and type of wound Wounds in richly vascularized areas, such as the face, heal faster than those in poorly vascularized ones, such as the foot

Healing in Bone ( Fracture Healing) Stage I : Haematoma Formation - immediately after injury -> bleeding from torn vessels and periosteum -> blood extend to muscles -> can be organised and ossified -> myositis ossificans

Stage 2 : Traumatic Inflammation -tissue damage -> inflammation -> more fibrin in clot - increased blood flow and PMNs -> reduced density of bone ends -periosteum loosened -> fusiform hematoma

Stage 3 : Demolition - macrophages & osteoclasts invade the clot &remove fibrin, rbcs, exudate & debris. -bone fragments -> necrosis -> scavenged by macrophages -red fatty marrow -> necrosis -> release of fat globules -> FBGC reaction or fat emboli

Stage 4: Formation of Granulation Tissue - capillaries & mesenchymal cell grows in from periosteum and endosteum -periosteal cells and blood vessels -> granulation t/s -fracture neck of femur, if periosteum damaged -> necrosis of head -long bone fracture -> periosteum less important

Stage 5: Woven Bone & Cartilage Formation - mesenchymal osteoblasts may form woven bone or cartilage -callus = unites bone ends -soft callus = granulation tissue -hard callus =bone or cartilage

Woven bone formation: - osteoblasts -> collagen and osseomucin -> osteoid -collagen –> irregular bundles with no lamellar structure -osteoid -> calcification -> woven bone - after 10 days -> alkaline tide -> calcium depositon -ALP takes part in calcification -> bone ends united by woven bone

Cartilage Formation: - chondroblasts -> form cartilage -matrix undergoes calcification and cartilage cells die. - seen in fractures with incomplete immobilisation e.g fracture ribs

Stage 6 : Formation of Lamellar Bone -dead cartilage disintegrate -> invaded by osteoblasts and blood vessels -woven bone removed by osteoclasts -initial callus removed -> osteoid calcifies -collagen bundles in lamellar fashion - Haversian systems form

Stage 7 : Remodelling - continued osteoclastic removal -osteoblastic bone formation increases External callus -> removed Intermediate callus -> compact bone Internal callus -> marrow cavity

Abnormalities of Fracture Healing 1.Repair or Fibrous Union 2. Non-Union 3. Delayed Union

Types of Fracture 1. Traumatic fracture 2. Pathological Fracture: -osteogenesis imperfecta, bone cysts, GCT, secondary bone tumours

Healing of a bone fracture The haematoma at the fracture site gives a framework for healing It is replaced by a fracture callus which is subsequently replaced by lamellar bone which is then remodelled to restore the normal trabecular pattern of the bone