HEALING AND REPAIR in the human body system.pptx

HEALING AND REPAIR in the human body system

1. HEALING

2. OBJECTIVES At the end of the lecture, you should be able to: Define and explain the meaning of the process of healing Describe cells according to their proliferative potential State the components of the ECM and discuss their functions Discuss the main processes involved in healing State and describe the types of wound healing State and discuss factors affecting wound and/or bone healing and complications that can arise Discuss control of healing

3. DEFINITION Healing is a response to injury following the inflammatory process and comprising of a number of sequentially orderly steps resulting in replacement of dead tissue with regenerated cells and/or fibrous (scar) tissue and a complete ( resolution ) or partial restitution ( organization ) of organ structure and function Note: Understanding the healing process is important to be able to facilitate those factors that promote wound healing, minimize those factors that inhibit wound healing and anticipate complications that can arise as a result of healing at specific sites

4. Healing b egins soon after the acute inflammatory response induced by the initial injury and/or tissue death, which prepares the grounds for healing to commence. Involves: Regeneration – regrowth of the original tissue (usually applied to epithelial cells) returning to its original state Repair or organization , comprising: Migration and proliferation of connective tissue cells Synthesis of extracellular matrix Remodeling Maturation (collagenization) and attainment of wound strength

5. The process of healing and the acute inflammatory response induced by injury to a tissue or organ depend on: The nature and magnitude of the injurious agent Ability of various surviving cells to proliferate and repopulate the damaged area (regeneration) Preservation of supporting stroma to allow orderly replacement The inflammatory cells recruited during the inflammatory response begin the healing process by breaking down and removing necrotic debris, plasma proteins present in the injured area and connective tissue fragments.

6. TISSUE PROLIFERATIVE ACTIVITY Healing involves proliferation of cells Cells are divided into 3 groups depending on their proliferative potential: Static cell populations (permanent cells) – have no proliferative capacity in adult life e.g. neurons, striated and cardiac muscle. Healing in such sites is basically by repair Renewing cell populations (labile cells) – continue to proliferate throughout adult life. These cells are continuously in the cell cycle and replacement of damaged cells is relatively fast

7. Conditionally renewing cell populations (stable cells) – normally show a very low rate of cell proliferation but are capable of rapid proliferation after cell loss. These cells are in the Go phase of the cell cycle and are induced or stimulated to re-enter the cycle and proliferate

8. EXTRACELLULAR MATRIX Repair begins with migration of fibroblasts from margins of viable tissue into defect caused by injury Fibroblasts proliferate and synthesize and secrete extracellular matrix proteins, which provide the framework for proliferating endothelial and parenchymal cells. ECM critically influences cell growth, movement and differentiation. Composed of macromolecules outside the cell Secreted locally and assembled into a meshwork that surrounds cells

9. Cells grow, move, and differentiate in intimate contact with the ECM Functions include: Sequestration of water that provides turgor to soft tissue and minerals that give rigidity to bones Reservoir for growth factors regulating cell proliferation Cell-to-cell interactions Providing bedrock for cells to adhere, migrate and proliferate Modulates cell form and function Synthesis and degradation of ECM accompanies morphogenesis, wound healing, chronic fibrotic processes and tumour invasion and metastases Made up of three groups of macromolecules: Fibrous Structural Proteins – Collagens and elastins

10. Adhesive Glycoproteins – Fibronectin and laminin Proteoglycans Collagens The most common protein in the animal world Provide the extracellular framework, structural support and tensile strength for almost all tissues and organs Composed of a triple helix of three polypeptide  chains About 27 different types encoded by 41 genes Types I, II, III and V are fibrillar – called interstitial collagens Type IV is non-fibrillar, forming sheets, is the main component of basement membrane

11. Fibrillar collagen is synthesized from procollagen derived from preprocollagen Three procollagen chains align in phase to form the triple helix after hydroxylation of lysine and proline residues followed by glycosylation of lysine in RER Vitamin C is required for hydroxylation In extracellular space, secreted procollagen is cleaved by proteases to form the basic collagen units Oxidation of specific lysine and hydroxylysine residues by lysyl oxidase results in cross-linking between adjacent collagen fibrils contributing to the tensile strength of collagen Elastic Fibres Provide elasticity to tissues and ability to recoil after tension

12. Glycoproteins – Fibronectin and Laminin These matrix proteins bind to cell adhesion molecules mediating adhesiveness between cells and extracellular matrix Fbronectin A large molecule made up of two glycoprotein chains held together by disulfide bonds Synthesised by fibroblasts and also derived from plasma Binds to many molecules such as collagen, fibrin, proteoglycans and CAM. Finding to collagen and proteoglycans stabilizes the healing tissue Chemoattractant for fibroblasts and macrophages; stimulates fibroblasts to secrete more fibronectin

13. Laminin Most abundant glycoprotein in BM Binds to both ECM and CAM Involved in adhesion of epithelial cells to BM Proteoglycans A mixture of polysaccharide chains, each of two disaccharide units, one of which is always an amino sugar Sulphated glycosaminoglycans in tissues are covalently linked to proteins to form proteoglycans (approx. 95% carbohydate and 5% protein) e.g. chondroitin sulphate, dermatan sulphate, heparan sulphate, heparin sulphate and keratan sulhate Hyaluronidase is the only glycosaminoglycan lacking a sulphate side group

14. Association between the type of collagen present and the type of glycosaminoglycan in tissues. Tissues composed of Type I collagen contain scanty proteoglycans which is almost entirely dermatan sulphate Tissues rich in Type II collagen contain very high levels of chondoitin sulphate Type III collagen is associated with heparan sulphate Sulphated glycosaminoglycans regulate connective tissue structure and permeability Act as modulators of cell growth and differentiation HA Binds water, forming a viscous gel giving CT ability to resist compression forces, helps to provide resilience and lubrication to many connective tissue, especially articular cartilage

15. Inhibits cell-to-cell adhesion and facilitates cell motility Binds CD44 on leucocytes retaining them in tissues at site of inflammation Once tissue is healed, proteoglycans contribute to the organisation and stability of collagen and create an electrical charge that gives basement membrane the property of functioning as molecular “sieves”.

16. WOUND HEALING In the healing of the wound of the skin there is epidermal regeneration and repair of the dermis and subcutaneous tissue This response is therefore considered the archetype of healing. Healing in other organs differs merely by degree rather than of kind

17. Two types of skin wound healing are considered:   Healing of a clean, incised wound with very little loss of tissue, minimal amount of acute inflammatory exudates and necrotic debris and close apposition of the wound edges (surgical wound) – primary union or healing by first intention Healing of a wound with sufficient tissue loss to produce a large tissue defect so that the wound edges are not apposed. Generally, there is extensive inflammatory exudates and necrotic debris that must be removed before healing can occur – secondary union or healing by second intention Thus whether a wound heals by first or second intention is determined not by the healing process but by the type of the wound

18. Healing by First Intention The incision causes death of a small number of epidermal and connective tissue cells without or with minimal damage to adnexal structures The narrow incisional space is rapidly filled with a fibrin-rich exudate as a result of the increased vascular permeability from the inflammatory response initiated by the injury and/or clotted blood containing fibrin due to haemorrhage from severed blood vessels Dehydration of the exudate or blood clot on the surface, results in the formation of scab, which covers the wound

19. First 24 hours : Neutrophils appear at the margins of the incision and invade and release enzymes that digest the clot heralding the stage of demolition The basal cells of the marginal epidermis, in response to stimulatory factors, undergo mitosis and tongues of epithelial cells from the wound edges both migrate and grow along the cut margins depositing basement membrane components as they move The cells only migrate over viable tissue and so they cleave a path between dead and living collagen fibres secreting collagenase and plasminogen activators thus displacing the scab towards the surface

20. Within 48 hours the cells fuse in the midline so that a continuous, though thin, epithelial layer covers the surface of the wound 72 hours : By this time neutrophils have largely been replaced by macrophages Fibrin strands coated by plasma fibronectins chemotactic for macrophages and fibroblasts act as “scaffolding” to facilitate the influx of these cells to the site of injury The macrophages remove the cellular debris along the wound margins and secrete growth factors Fibroblasts briefly undergo intense mitoses begin to secrete type I but mainly type III collagen, elastic fibres , ground substance ( glycosaminoglycans , proteoglycans, glycoproteins) and fibronectins

21. Simultaneously, macrophage growth factors and fibroblast fibronectins stimulate angiogenesis and neovascularization of the wound: Solid cords of endothelial cells bud or sprout from surviving capillaries, rapidly become canalized and form a lumen in each endothelial cell; lack basement membrane and are therefore leaky; allow protein-rich fluid to exude into the surrounding tissue Increase the inflammatory oedema ; supplies nutrients to the now very metabolically active macrophages and fibroblasts Proliferating blood vessels and fibroblasts form a special type of tissue called granulation tissue - deriving its name from the pink, granular appearance on the surface of a large wound (the tiny granules are loops of new capillaries)

22. Epithelial cell proliferation continues, causing increase in epidermal thickness By Day 5 : Angiogenesis at its height; collagen is abundant crossing the incision line to bridge the cut edges The epidermis has attained normal thickness; further proliferation of the epithelial cells ceases due to a variety of growth inhibitors Differentiation of surface cells produces mature epidermis Second week : Fibroblasts continue to proliferate and lay down collagen and other ground substances; maximal collagen content reached by about the third week

23. Inflammatory cells and oedema gradually disappear Progressive devascularization begins as vascular channels regress resulting in blanching The process of remodeling occurs during this period; type I collagen (with greater tensile strength) replaces type III collagen (with relatively low tensile strength); the new collagen fibres are oriented according to the new lines of mechanical stress Acquisition of wound strength slowly occurs with remodeling; formation of intra-molecular and intermolecular cross-links within collagen fibrils

24. First month : The wound is now healed and covered by intact epidermis The dermal appendages destroyed in the line of incision do not regenerate and are permanently lost The dermis now contains a connective tissue scar devoid of inflammatory cells and composed predominantly of type I collagen The tensile strength of the healed wound increases with time but never reaches that of the original tissue

25. Healing by Second Intention When there is more extensive loss of tissue producing a wound with widely separated margins, the healing process follows the same pattern as in healing by first intention but there are quantitative differences: The reparative response is slow because of the large tissue defect to be filled Regeneration of parenchymal cells cannot solely accomplish repair because of damage to both parenchymal cells and stromal framework. Dermal papillae and rete ridges are either absent or poorly developed, and skin adnexal structures are absent in the large dermal scars of skin wounds

26. The greater amounts of tissue and cellular debris provoke a more intense inflammatory response More abundant granulation tissue forms to fill the large tissue defect and complete the repair Single most important feature that differentiates healing by first intention from healing by second intention is the phenomenon of wound contraction; more significant in the healing of large wounds: The surface area of a large wound can be reduced by as much as 90% due to wound contraction Attributed to the presence of myofibroblasts, modified fibroblasts containing considerable amounts of contractile proteins actin and myosin

27. FACTORS INFLUENCING WOUND HEALING Factors which may promote or adversely affect wound healing may be divided into local and systemic Local: Blood supply : Actively dividing fibroblasts are confined to regions where tissue oxygen tension is greater than 15 mmHg A good blood supply is essential for fighting infection and bringing nutrients to the metabolically tissue Any factor that impairs arterial blood flow or retards venous drainage will impair wound healing

28. Infection: This is the single most important local cause of delayed wound healing. Infection delays epithelial and fibroblast proliferation and promotes more intense and prolonged inflammatory response as well as production of large amounts of granulation tissue Mechanical factors: Early movement causes mechanical stress of the wound. Granulation tissue is easily disrupted and repeated trauma or movement slows the healing process Foreign bodies: Delay wound healing by exciting macrophage response with foreign body giant cell reaction and/or encouraging infection

29. Size and type of wound: Large wounds heal slower than small ones Location: Wounds in richly vascularised areas heal faster than those in poorly vascularised areas Systemic : Nutrition: Protein deficiency affects acquisition of wound tensile strength through impairment of collagen and intercellular matrix synthesis. Vitamin C deficiency particularly retards wound healing by causing impaired synthesis of normal collagen. The under-hydroxylated collagen is poorly transported out of fibroblasts and is very susceptible to degradation. Zinc deficiency has adverse effects on wound healing as the metal acts as cofactor of several enzymes

30. Systemic disease: Diabetes and Cushing disease are associated with delayed wound healing. Chronic debilitating diseases such as malignancies also delay wound healing Hormones: Glucocorticoids have anti-inflammatory effects, have adverse effects on epithelial regeneration, the prliferation of fibroblasts and the synthesis of extracellular matrix

31. CONTROL OF HEALING Wound healing is controlled by signals derived from polypeptide growth factors, cytokines and growth inhibitors. Other signals are derived from the extracellular matrix Growth Factors: Polypeptides, which attach to cell membrane receptors on various cells and either promote DNA replication ( progression factors ) or “prepare” the cell for DNA replication ( competence factors ) Platelet-derived Growth Factor : A cationic glycoprotein stored in the  granules of platelets, released on platelet activation; also be produced by a variety of cells including activated macrophages, endothelial cells and smooth muscle cells

32. Is a powerful chemoattractant for fibroblasts, smooth muscle cells and monocytes and neutrophils directing these cells to the site of tissue damage Is a competence factor, which stimulates migration and proliferation of fibroblasts, smooth muscle cells and monocytes Transforming Growth Factor  : TGF-  is also found in the  granules of platelets, endothelial cells, monocytes and lymphocytes It is a growth inhibitor for epithelial cells In low concentrations it is an indirect mitogen for fibroblasts and smooth muscle by inducing synthesis and release of PDGF. In high concentrations it is growth inhibitory

33. Also stimulates wound contraction Involved in the formation and conservation of the extracellular matrix through combined stimulation of fibroblasts chemotaxis and collagen synthesis, while inhibiting secretion of proteases (matrix metalloproteinase [MMP] enzymes) that degrade collagen, and increasing the secretion of protease inhibitors (tissue inhibitors on matrix metalloproteinases [TIMPs]) Insulin-like growth factor 1 : IGF-1 stimulates collagen deposition by fibroblasts and reduce cellular expression of MMP Vascular Endothelial Growth Factor : VEGF induces the proliferation of endothelial cells and tube formation characteristic of capillaries

34. Epidermal Growth Factor and Transforming Growth Factor  : These two have the same effects; they are progression factors and enhance epidermal proliferation and stimulate fibroblasts to proliferate Cytokines : These are small molecules that signal between cells, inducing growth, differentiation, chemotaxis, activation, enhanced cytotoxicity and/or regulation of immunity Monokines : Secreted by monocytes and macrophages Have many different local and systemic activities that are critical to immune defense and inflammation

35. In response to appropriate stimulus, macrophages secrete IL-1, IL-6, IL-8, IL-12 and TNF  These molecules are called pro-inflammatory cytokines IL-1, TNF  and IL-6 activate lymphocytes, increase body temperature (decreasing pathogen replication and increasing specific immune responses), mobilize and activate neutrophils for phagocytosis, activate vascular endothelium (in preparation for neutrophil chemotaxis) and induce the release of acute phase proteins and thus complement activation and opsonization IL-1 also induces systemic production of IL-6 while TNF  also activates macrophages and induces their production of NO

36. TNF  is also produced by T cells IL-8 increases access for, and chemotaxis of, neutrophils. It also activates binding by integrins, which facilitate neutrophil binding to endothelial cells and migration into tissues IL-12, also produced by B cells, activates NK cells, which then produce IFN  , a cytokine important in inducing differentiation of Th cells to Th 1 cells and protects surrounding cells from infection by viruses released from virus-infected cells

37. COMPLICATIONS Deficient granulation tissue formation Wound dehiscence Wound rupture is common after abdominal surgery (burst abdomen); due to increased abdominal pressure Wound breakdown or ulceration during healing is due to inadequate vascularization Incisional hernia Excessive granulation tissue formation Also called exuberant granulation or proud flesh; prevents re- epitheliazation ; must be removed by excision or cautery to allow restoration of epithelial continuity Hypertrophic scar Accumulation of excessive collagen gives rise to a raised scar

38. HYPERTROPHIC SCAR

39. HYPERTROPHIC SCAR

40. Keloid Inherited tendency to produce excessive amounts of collagen; particularly prevalent among blacks There is excessive fibroblast proliferation and collagen production Scar tissue grows beyond the boundaries of the original wound and does not regress Forms a dermal mass and raised scar Desmoids Exuberant proliferation of fibroblasts and other connective tissue elements May recur after excision

41. KELOID

42. KELOID

43. Contracture An exaggeration of wound contraction Results in deformities of scar and surrounding tissues; can compromise movement of joints

44. HEALING OF FRACTURE Fracture repair is a healing process by regeneration and remodeling and with potential for a return of optimal function in many cases Three phases: Inflammatory – lasting about 1 week Reparative – lasting 6-12 weeks Remodeling – lasting several months to years At the moment of fracture, blood vessels through the haversian system are torn at the fracture site Local haemorrhage and formation of a haematoma that fills the fracture gap and surrounds it Forms the scaffolding for granulation tissue

45. The inflammatory process which follows immediately after the injury brings in recruited leukocytes . By the end of week 1 Phagocytic cells have removed most of the haema-toma, and neovascularization and initial fibrosis are occurring Degranulated platelets and activated leukocytes have release PDGF, TGF- , FGF, and cytokines, which activate osteoprogenitor cells in the periosteum, medullary cavity, and surrounding soft tissues and stimulate osteoclastic and osteoblastic activity

46. From week 2, osteoclasts clear away necrotic bone and osteoprogenitor cells deposit woven bone in the subperiosteal region and within the medullary cavity Activated mesenchymal cells in the surrounding soft tissue differentiate into chondroblasts and lay down fibro and hyaline cartilage. In uncomplicated fractures, by the end of the 2 nd or 3 rd week the fracture site is immobilised by fibrocartilagenous tissue called soft callus but not yet strong enough for weight bearing The newly formed cartilage undergoes endochodral ossification and connects to the newly formed woven bone resulting in formation of bony callus

47. Mineralisation of the callus leads to increased stiffness and strength so that controlled weight bearing can be tolerated. During remodeling, the callus which is laid in excess is resorbed and the bone remodels in response to the mechanical stresses placed on it. If the bone ends are not perfectly aligned, the volume of callus is greatest in the concave portion of the fracture site. With maturation and transmission of weight-bearing forces, the portions that are not physically stressed are resorbed and the callus is reduced in size until the shape and outline of the bone is established

48. The medullary cavity is restored After remodelling is completed, it may be impossible to demonstrate the site of the healed fracture. Overall time for bone healing varies depending on: The bone involved The fracture site Fracture type Treatment –external versus internal immobilization, need for bone grafting or use of graft substitute Degree of soft tissue injury Treatment complications Other factors as for wound healing

49. Complications Sequence of events in the healing of a fracture can be hindered or even blocked. Several factors can delay or arrest the process: Movement Excessive movement prevents normal constituents of callus from forming so that callus is composed of fibrous tissue and cartilage – results in fibrous union Continued movement results in cystic degeneration of central portion of fibrous callus ending in formation of a false joint – pseudoarthrosis Movement of lesser degree results in excessive callus and prevents or slows down bone union as the callus takes a long time to be resorbed Interposed soft tissue between broken ends delays healing and may cause non-union

50. Misalignment Displaced fractures heal slowly and result in some deformity (e.g. shortening) leading to increased risk of osteoarthritis in adjacent joints Comminution Comminuted fractures frequently result in deformity and predispose to development of osreoarthritis Infection At the fracture site is a serious obstacle to healing and there is the additional risk of chronic osteomyelitis More likely in comminuted and compound fracture (skin over fracture is broken)

51. Pre-existing bone disease Abnormal bone is likely to fracture after trivial force – pathological fracture Abnormality may due to primary bone disorder or secondary involvement of bone by some other condition e.g. metastatic cancer Most pathological fractures heal satisfactorily but some require treatment of underlying cause first Vitamin deficiencies C and D can impair bone healing Calcium and phosphate deficiencies Systemic infection Diabetes mellitus

52. Vascular insufficiency Malnutrition

53. HEALING OF TENDONS AND LIGAMENTS Tendons and ligaments Are dense fibrous connective tissue Composed of 78% water, 20% collagen (type I) and 2% proteoglycans Can sustain high unidirectional tensile loads, transfer forces, provide strong flexible, and help the tissue respond to normal loads while resisting excessive mechanical or shearing forces and deformation Have viscoelastic properties so can undergo deformation under tensile or compressive forces and return to original state after removal of force

54. Tendons attach muscles to their bony origins and insertions, whereas ligaments attach bone to bone providing support to joints Rupture may require surgical repair with apposition to restore function Following injury, inflammatory process ensues and debris are cleared from the area by macrophages. Migration and proliferation of fibroblasts and capillary buds from surrounding connective tissue results in granulation tissue formation About 2 weeks into healing process, collagen fibrils are oriented and rearranged into thick bundles to provide greater strength

55. During this period immobilisation relieves stress and prevent re-rupturing; collagen is deposited randomly When healing has progressed sufficiently for adequate tissue integrity to be achieved, mobility results in remodelling Collagen realigns to the lines of stress provided by motion to its usual parallel arrangement Takes 40 to 50 weeks for tendons and ligaments to regain normal strength The scar tissue formed is weaker, has increased amount of minor collagens (III, V, VI), increased proteoglycans and decrease cross-links

56. HEALING OF CARTILAGE Types of cartilage include: articular or hyaline cartilage at ends of bones, fibrocartilage in menisci, annulus fibrosis, and at insertions of tendons and ligaments into bone, and elastic cartilage found in ligamentu flavum, ear and epiglottis Hyaline cartilage Composed of chondrocytes, type II collagen and proteoglycans Is avascular and aneural Does not regenerate after adolescence Heals by fibrosis or not at all Fibrous scarring predisposes to osteoarthritis