# Lecture 03: Chronic Inflammation and Tissue Repair ## Unit 2.11: Pathology --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the characteristics and causes of chronic inflammation 2. Explain the cells involved in chronic inflammation 3. Describe granulomatous inflammation 4. Explain the mechanisms of tissue repair and regeneration 5. Describe wound healing processes 6. Explain the factors affecting wound healing --- ## Lecture Outline ### I. Chronic Inflammation Overview Chronic inflammation represents a prolonged inflammatory response lasting weeks to months or even years, characterized by the simultaneous occurrence of tissue destruction and repair processes. Unlike acute inflammation, which is dominated by vascular changes and neutrophil infiltration, chronic inflammation features infiltration by mononuclear cells including macrophages, lymphocytes, and plasma cells. The tissue damage in chronic inflammation results from ongoing inflammation rather than resolution, with persistent inflammatory stimuli preventing the normal resolution pathways from terminating the response. Fibrosis and angiogenesis occur concurrently with inflammation, reflecting the body's attempt to repair tissue even as destruction continues. The causes of chronic inflammation fall into several categories, each preventing the normal resolution of inflammatory responses. Persistent infections with organisms resistant to killing by phagocytes, including Mycobacterium tuberculosis, certain viruses such as hepatitis B and C, and fungi such as Histoplasma, trigger sustained immune responses. Autoimmune diseases, including rheumatoid arthritis, systemic lupus erythematosus, and inflammatory bowel disease, result from immune responses directed against self-antigens that cannot be eliminated. Prolonged exposure to toxic agents, whether exogenous substances such as silica and asbestos or endogenous products such as oxidized lipids in atherosclerosis, maintain chronic inflammatory states. The distinction between acute and chronic inflammation extends beyond simple duration to fundamental differences in pathogenesis and cellular participants. Acute inflammation develops rapidly over minutes to hours, while chronic inflammation may develop insidiously over weeks to months without a preceding acute phase. The predominant cells in acute inflammation are neutrophils, which are short-lived and die within 24-48 hours, whereas chronic inflammation features long-lived macrophages and lymphocytes that can persist for extended periods. Tissue destruction in acute inflammation is generally limited and followed by complete resolution, whereas chronic inflammation often produces progressive tissue damage leading to scarring and functional impairment. The clinical significance of chronic inflammation extends across a broad spectrum of human diseases, contributing substantially to morbidity and mortality. Tuberculosis remains one of the leading infectious causes of death worldwide, driven by chronic granulomatous inflammation. Rheumatoid arthritis causes progressive joint destruction and disability through persistent synovial inflammation. Atherosclerosis, now recognized as a chronic inflammatory disease of arterial walls, underlies the majority of cardiovascular deaths. Cirrhosis develops from chronic hepatic inflammation regardless of the underlying cause, whether viral, alcoholic, or autoimmune. Understanding chronic inflammation mechanisms has become central to developing targeted therapies for many common diseases. Panel A: Side-by-side comparison of acute versus chronic inflammation showing timeline of weeks to years, predominant cell types with macrophages and lymphocytes, and concurrent tissue destruction with fibrosis. Panel B: Diagram illustrating the major causes of chronic inflammation including persistent infections (TB, viral hepatitis), autoimmune diseases (RA, SLE), and prolonged toxin exposure (silica, oxidized lipids). Panel C: Histologic images comparing acute inflammation with neutrophil predominance to chronic inflammation with mononuclear cell infiltrate. Panel D: Clinical examples showing chronic inflammatory diseases affecting different organs including joint synovium in RA, liver in cirrhosis, and arterial wall in atherosclerosis. --- ### II. Cells of Chronic Inflammation The macrophage is the central cellular player in chronic inflammation, derived from circulating blood monocytes that emigrate into tissues and undergo activation. Tissue macrophages have an extended lifespan of months to years and can be activated through two distinct pathways that produce functionally different phenotypes. Classical or M1 activation, induced by interferon-gamma and microbial products, produces macrophages specialized for killing microorganisms through production of reactive oxygen species and nitric oxide. Alternative or M2 activation, induced by IL-4 and IL-13, generates macrophages oriented toward tissue repair, producing growth factors and anti-inflammatory mediators while promoting angiogenesis and fibrosis. The activated macrophage in chronic inflammation serves multiple functions that both propagate inflammation and initiate repair. Phagocytosis and killing of microorganisms represents the primary defensive function, utilizing the arsenal of antimicrobial mechanisms including the respiratory burst and lysosomal enzymes. Antigen presentation to T lymphocytes links innate and adaptive immune responses, sustaining the immune reaction to persistent antigens. Secretion of pro-inflammatory cytokines including TNF-alpha, IL-1, and IL-6 amplifies inflammation and produces systemic effects. Production of growth factors and matrix metalloproteinases simultaneously promotes tissue remodeling and fibrosis, reflecting the dual role of macrophages in destruction and repair. Lymphocytes are essential participants in chronic inflammation, providing antigen specificity and immunological memory that sustain responses to persistent antigens. CD4-positive helper T cells coordinate the immune response through cytokine secretion, with Th1 cells producing interferon-gamma that activates macrophages and Th2 cells producing IL-4 and IL-5 that recruit eosinophils and promote antibody production. CD8-positive cytotoxic T cells directly kill infected or transformed cells through perforin-granzyme mechanisms and Fas ligand interactions. B cells and their terminally differentiated progeny, plasma cells, produce antibodies that opsonize pathogens and activate complement but may also contribute to tissue damage in autoimmune conditions. Additional cell types contribute to the chronic inflammatory milieu in specific contexts. Eosinophils are prominent in parasitic infections and allergic conditions, releasing cytotoxic granule contents that can damage parasites but also injure host tissues. Mast cells, typically associated with immediate hypersensitivity reactions, participate in chronic inflammation through cytokine production and interaction with T cells. Fibroblasts, activated by growth factors from macrophages and other cells, proliferate and synthesize extracellular matrix components, ultimately producing the fibrosis that characterizes many chronic inflammatory diseases. Multinucleated giant cells, formed by fusion of macrophages, are characteristic of granulomatous inflammation and certain foreign body reactions. Panel A: Diagram of monocyte-to-macrophage differentiation with classical M1 versus alternative M2 activation pathways, showing inducing cytokines and functional outputs for each phenotype. Panel B: Illustration of activated macrophage functions including phagocytosis, antigen presentation to T cells, cytokine secretion, and growth factor production. Panel C: T lymphocyte subsets in chronic inflammation showing CD4 helper cells (Th1 and Th2), CD8 cytotoxic cells, and their respective cytokine profiles and effector functions. Panel D: Assembly of cells in a chronic inflammatory infiltrate showing macrophages, lymphocytes, plasma cells, and fibroblasts in characteristic tissue arrangement. --- ### III. Granulomatous Inflammation Granulomatous inflammation represents a distinctive pattern of chronic inflammation characterized by the formation of granulomas, organized collections of activated macrophages with an epithelioid appearance. The formation of granulomas serves to wall off agents that are difficult to eliminate, limiting their spread while concentrating immune effector cells at the site of infection or foreign material. The granuloma structure consists of a central collection of epithelioid histiocytes and multinucleated giant cells, surrounded by a collar of lymphocytes, with variable degrees of fibrosis at the periphery. This organized structure differentiates granulomatous inflammation from other patterns of chronic inflammation and carries specific diagnostic implications. The cellular components of granulomas include specialized macrophage populations that differ in morphology and function from typical tissue macrophages. Epithelioid cells are activated macrophages with abundant pink cytoplasm that appears to merge with adjacent cells, giving the aggregate an epithelium-like appearance that gives rise to the name. These cells have enhanced secretory function but reduced phagocytic capacity compared to typical macrophages. Giant cells form through fusion of epithelioid cells and may contain numerous nuclei. Langhans giant cells, characteristic of tuberculosis, have nuclei arranged in a horseshoe pattern at the cell periphery. Foreign body giant cells have nuclei scattered randomly throughout the cytoplasm and form around inert foreign materials. The classification of granulomas as caseating or non-caseating carries important diagnostic significance. Caseating granulomas contain a central zone of necrosis with a cheesy, caseous appearance that represents the death of accumulated macrophages, typically seen in tuberculosis and certain fungal infections. The caseation is believed to result from the combined effects of hypoxia in the granuloma center and the cytotoxic effects of inflammatory mediators. Non-caseating granulomas lack this central necrosis and are characteristic of sarcoidosis, Crohn disease, and foreign body reactions. The distinction requires careful histologic examination, as some tuberculous granulomas may not show caseation, particularly in immunocompromised patients. The pathogenesis of granuloma formation involves complex interactions between macrophages and T lymphocytes mediated by cytokines. The process begins when macrophages encounter an agent they cannot destroy, leading to prolonged activation and cytokine production. Interferon-gamma, produced primarily by Th1 cells, is the critical cytokine that transforms macrophages into epithelioid cells and promotes their fusion into giant cells. TNF-alpha is essential for granuloma maintenance, explaining why anti-TNF therapy can cause reactivation of latent tuberculosis. The granuloma may ultimately resolve, heal with fibrosis and calcification, or persist as a chronic structure that contains but does not eliminate the inciting agent. Panel A: Low-power histologic view of a well-formed granuloma showing central epithelioid cells, surrounding lymphocyte collar, and peripheral fibrosis with labeled components. Panel B: High-power comparison of Langhans giant cells with horseshoe nuclear arrangement versus foreign body giant cells with scattered nuclei. Panel C: Side-by-side comparison of caseating granuloma with central necrosis and non-caseating granuloma with compact structure and no necrosis. Panel D: Diagram of granuloma formation showing T cell-macrophage interaction, IFN-gamma signaling, and the role of TNF in granuloma maintenance. --- ### IV. Causes of Granulomatous Inflammation Mycobacterial infections represent the most clinically significant cause of granulomatous inflammation, with tuberculosis serving as the prototypical granulomatous disease. Mycobacterium tuberculosis survives within macrophages by inhibiting phagolysosome fusion and resisting the microbicidal mechanisms that normally kill ingested bacteria. The host response to this persistent intracellular infection is formation of granulomas that contain but often fail to eliminate the organism. Tuberculous granulomas are typically caseating, with the characteristic cheesy central necrosis. The granulomas may calcify and persist as stable structures for decades, with viable organisms remaining within and capable of reactivation if host immunity wanes. Fungal infections frequently produce granulomatous responses, particularly those caused by endemic dimorphic fungi that can persist within macrophages. Histoplasma capsulatum, endemic to the Ohio and Mississippi River valleys, causes granulomatous lung disease that may closely mimic tuberculosis. Coccidioides immitis, found in the southwestern United States, produces granulomas containing distinctive spherules filled with endospores. Blastomyces dermatitidis causes granulomatous inflammation in the lungs and skin, with organisms visible as broad-based budding yeasts within the granulomas. Identification of the causative fungus by special stains or culture is essential, as the clinical and histologic features may be indistinguishable from tuberculosis. Sarcoidosis represents the paradigm of non-caseating granulomatous disease, characterized by systemic granuloma formation in the absence of an identifiable infectious agent. The lungs, lymph nodes, skin, and eyes are most commonly affected, though granulomas may develop in virtually any organ. Sarcoid granulomas may contain inclusions including Schaumann bodies (laminated calcium concretions) and asteroid bodies (stellate eosinophilic structures), though these are not specific for the diagnosis. Laboratory findings may include elevated serum angiotensin-converting enzyme and hypercalcemia from granuloma production of vitamin D. The etiology remains unknown, though the immunologic phenotype suggests response to an unidentified antigen. Additional causes of granulomatous inflammation include Crohn disease, in which non-caseating granulomas develop in the intestinal wall and associated lymph nodes, foreign body reactions around sutures, splinters, and surgical materials, and certain drug reactions. Cat-scratch disease, caused by Bartonella henselae, produces stellate granulomas with central suppurative necrosis in lymph nodes. Leprosy demonstrates the spectrum of granulomatous response, with tuberculoid leprosy showing well-formed granulomas and few organisms while lepromatous leprosy shows poorly formed granulomas packed with organisms, reflecting the host immune response. The differential diagnosis of granulomatous inflammation requires clinical correlation, special stains for organisms, and often culture or molecular testing. Panel A: Tuberculosis granuloma histology showing caseating necrosis centrally with surrounding epithelioid cells, giant cells, and lymphocytes, with acid-fast stain inset showing mycobacteria. Panel B: Comparison of endemic fungal infections showing Histoplasma yeast forms within macrophages, Coccidioides spherules, and Blastomyces broad-based budding yeasts. Panel C: Sarcoidosis granulomas showing non-caseating structure with Schaumann body and asteroid body inclusions in higher magnification insets. Panel D: Spectrum of granulomatous diseases arranged by etiology including infectious (TB, fungi), immune-mediated (sarcoidosis, Crohn), and foreign body causes. --- ### V. Tissue Repair Overview Tissue repair following injury occurs through two fundamental processes: regeneration, in which damaged tissue is replaced by cells of the same type, and healing by connective tissue deposition (scarring), in which the original tissue is replaced by fibrous tissue. The outcome depends on the regenerative capacity of the injured tissue, the extent of injury, and whether the underlying tissue framework remains intact. When regeneration occurs, function is restored; when scarring occurs, structural integrity is maintained but at the cost of functional impairment. Most injuries heal through a combination of both processes, with the relative contribution of each depending on the specific circumstances. The capacity for regeneration varies dramatically among different cell types, leading to the classification of tissues based on their proliferative potential. Labile cells divide continuously throughout life to replace cells that are constantly being lost, including epithelial cells of the skin, gastrointestinal tract, and respiratory tract, as well as hematopoietic cells. These tissues readily regenerate after injury. Stable cells are normally quiescent but retain the capacity for rapid proliferation when stimulated by appropriate signals, including hepatocytes, renal tubular cells, and endothelial cells. Permanent cells have exited the cell cycle and cannot proliferate, including neurons of the central nervous system and cardiac myocytes, though this traditional classification has been modified by recognition of limited regenerative capacity in some contexts. Tissue regeneration requires not only the presence of proliferating cells but also preservation of the underlying tissue scaffold. The extracellular matrix provides structural support and contains basement membranes and interstitial matrix that guide regenerating cells to their proper locations. When the matrix scaffold is intact, regenerating epithelial and parenchymal cells can repopulate the framework and restore normal tissue architecture. When matrix destruction accompanies cell loss, as occurs with extensive necrosis or destruction of basement membranes, regeneration is impaired and healing by scarring becomes necessary. This explains why superficial injuries that spare the basement membrane heal by regeneration, while deeper injuries that destroy it heal by scarring. Stem cells provide the cellular source for regeneration in many tissues, residing in specialized niches that protect them and regulate their activity. Adult tissue stem cells are generally committed to producing the differentiated cells of their resident tissue, though some plasticity may exist. Intestinal stem cells in the crypt base continuously regenerate the epithelium. Hepatic progenitor cells in the canals of Hering contribute to liver regeneration when hepatocyte proliferation is impaired. Satellite cells beneath the skeletal muscle fiber sarcolemma provide myogenic precursors for muscle regeneration. Understanding the location and regulation of tissue-specific stem cells has become central to regenerative medicine approaches. Panel A: Comparison diagram showing regeneration with restoration of normal tissue structure versus healing by scarring with replacement by fibrous connective tissue. Panel B: Classification of cells by proliferative capacity showing labile cells (skin, GI epithelium), stable cells (hepatocytes, endothelium), and permanent cells (neurons, cardiac myocytes) with their regenerative potential. Panel C: Role of extracellular matrix in regeneration showing intact scaffold guiding epithelial regrowth versus destroyed matrix requiring scar formation. Panel D: Tissue stem cell niches in different organs including intestinal crypts, liver canals of Hering, and muscle satellite cells. --- ### VI. Wound Healing Healing by first intention occurs when wound edges are closely apposed, as in clean surgical incisions with minimal tissue loss. The narrow incisional space fills with blood clot within 24 hours, and neutrophils migrate to the wound margins. Epithelial cells from the wound edges begin proliferating and migrating within 24 to 48 hours, covering the defect with a thin epithelial layer. The underlying dermis is repaired by granulation tissue formation and collagen deposition, producing a thin scar with minimal contraction. The entire process from injury to mature scar may take only two to three weeks, with excellent cosmetic and functional results when the wound is properly approximated. Healing by second intention occurs when wound edges are separated by significant tissue loss, as in extensive burns, large ulcers, or abscess cavities. The healing process involves the same fundamental mechanisms as first intention healing but occurs on a larger scale and with additional features. Granulation tissue must fill the entire wound defect, requiring extensive angiogenesis and fibroblast proliferation. Wound contraction, mediated by myofibroblasts, progressively reduces the wound size by pulling the edges inward. Epithelialization must cover a larger area, with epithelial cells migrating over the granulation tissue from wound margins. The resulting scar is larger and often cosmetically inferior to first intention healing. The phases of wound healing proceed in an overlapping sequence of inflammation, proliferation, and remodeling. The inflammatory phase, beginning immediately after injury and lasting several days, involves hemostasis through platelet aggregation and fibrin deposition, followed by recruitment of neutrophils and macrophages. The proliferative phase, from approximately day three to week three, features formation of granulation tissue with angiogenesis, fibroblast proliferation, and collagen synthesis, along with epithelialization. The remodeling phase, extending from week three to one year or longer, involves maturation and reorganization of collagen, with gradual increase in wound strength and regression of vascularity. Granulation tissue is the specialized tissue that fills wounds during the proliferative phase, composed of new capillaries, proliferating fibroblasts, and loose connective tissue matrix. The tissue has a distinctive red, granular appearance on gross examination, with the "granular" texture resulting from the numerous new capillary loops. Macrophages in granulation tissue clear debris and secrete growth factors that coordinate repair. Myofibroblasts, specialized fibroblasts expressing alpha-smooth muscle actin, develop from local fibroblasts or circulating precursors and mediate wound contraction through their contractile properties. As repair progresses, granulation tissue matures into scar tissue with dense collagen and decreased vascularity. Panel A: Side-by-side comparison of first intention healing (clean sutured incision) versus second intention healing (open wound with tissue loss) showing differences in epithelialization, granulation tissue, contraction, and final scar size. Panel B: Timeline diagram showing the three overlapping phases of wound healing - inflammatory, proliferative, and remodeling - with key cellular events and timeframes. Panel C: Histologic appearance of granulation tissue showing new capillary loops, fibroblasts, macrophages, and loose matrix, with gross photograph inset showing red granular appearance. Panel D: Myofibroblast function in wound contraction showing alpha-smooth muscle actin expression and reduction of wound size over time. --- ### VII. Growth Factors in Repair Platelet-derived growth factor (PDGF), released from platelet alpha granules during hemostasis, initiates the repair cascade by serving as a potent chemoattractant for neutrophils, macrophages, and fibroblasts. Beyond its chemotactic effects, PDGF stimulates fibroblast proliferation and extracellular matrix synthesis, promoting the formation of granulation tissue. Macrophages become an important source of PDGF as they accumulate in wounds, sustaining the stimulus for fibroblast activity. PDGF also induces myofibroblast differentiation and contraction, contributing to wound closure. The critical importance of PDGF is demonstrated by impaired wound healing in animals with defective PDGF signaling. Vascular endothelial growth factor (VEGF) serves as the master regulator of angiogenesis, stimulating endothelial cell proliferation, migration, and new vessel formation. Hypoxia in the wound center is a potent stimulus for VEGF production through stabilization of hypoxia-inducible factor. Macrophages and keratinocytes are important VEGF sources in healing wounds. New vessel formation provides oxygen and nutrients to the metabolically active granulation tissue and facilitates removal of waste products. The angiopoietin-Tie2 system works in concert with VEGF to stabilize newly formed vessels and promote their maturation. Excessive angiogenesis in pathologic conditions such as tumor growth can be targeted by anti-VEGF therapies. Transforming growth factor-beta (TGF-beta) has pleiotropic effects in wound healing, serving as the most important pro-fibrogenic cytokine. TGF-beta stimulates fibroblast chemotaxis, proliferation, and synthesis of collagen and other matrix components. It promotes formation of granulation tissue and, when signaling is excessive, contributes to pathologic fibrosis and scarring. TGF-beta also suppresses inflammation through inhibition of lymphocyte proliferation and macrophage activation, helping to transition from the inflammatory to the proliferative phase. The balance between TGF-beta effects on matrix production and inflammation helps determine whether healing proceeds normally or results in excessive scarring. Epidermal growth factor (EGF) and related molecules including transforming growth factor-alpha stimulate epithelial cell proliferation and migration, promoting re-epithelialization of wounds. Keratinocyte growth factor (KGF or FGF7) specifically stimulates keratinocyte proliferation. Fibroblast growth factors (FGFs) have diverse effects including stimulation of angiogenesis and fibroblast proliferation. Hepatocyte growth factor (HGF) promotes epithelial cell proliferation and motility in multiple tissues. The coordinated action of multiple growth factors, each with specific cellular targets and effects, orchestrates the complex process of tissue repair. Growth factor signaling pathways represent therapeutic targets for promoting wound healing in difficult wounds and preventing excessive fibrosis. Panel A: Temporal sequence showing growth factor release during wound healing, starting with platelet PDGF at injury, followed by macrophage cytokines, VEGF-driven angiogenesis, and TGF-beta-mediated matrix deposition. Panel B: VEGF signaling pathway showing hypoxia stabilization of HIF, VEGF production, and endothelial cell responses including proliferation, migration, and tube formation. Panel C: TGF-beta effects on fibroblasts showing chemotaxis, proliferation, collagen synthesis, and the dual role in normal healing versus pathologic fibrosis. Panel D: Epithelial growth factors including EGF, TGF-alpha, and KGF showing their sources and effects on keratinocyte proliferation and migration during re-epithelialization. --- ### VIII. Scar Formation and Remodeling The development of scar strength follows a predictable time course that has important clinical implications. At one week after injury, the wound has approximately 10% of its final strength, held together primarily by fibrin and granulation tissue. By three months, scar strength reaches 70-80% of normal unwounded tissue, with continued slow increase in strength for up to one year. Full normal tissue strength is never achieved, with scars reaching a maximum of approximately 80% of unwounded tissue strength. The increasing strength results from progressive collagen cross-linking by lysyl oxidase, which creates covalent bonds between collagen molecules. The remodeling phase involves dramatic changes in the composition and organization of the scar matrix. Early granulation tissue contains predominantly type III collagen, which is gradually replaced by type I collagen that provides greater tensile strength. Collagen fibers become progressively organized along lines of stress, aligning perpendicular to the wound edges in skin. Proteolytic enzymes including matrix metalloproteinases (MMPs) degrade excess matrix, while tissue inhibitors of metalloproteinases (TIMPs) regulate this degradation. The balance between matrix synthesis and degradation determines whether the scar resolves normally or progresses to excessive fibrosis. Vascularity decreases as the scar matures, changing the appearance from red granulation tissue to pale mature scar. Pathologic scarring represents dysregulation of the normal wound healing process, resulting in either inadequate or excessive scar formation. Hypertrophic scars remain confined to the original wound boundaries but are raised and thickened, containing excessive collagen in nodular arrangements. Keloids extend beyond the original wound margins into surrounding normal skin and do not regress spontaneously, representing a more severe form of abnormal scarring that tends to recur after excision. Both conditions are more common in individuals with darker skin pigmentation and in areas of high skin tension. Wound contracture, while a normal component of second intention healing, becomes pathologic when it causes functional limitation, particularly when it occurs across joints or in the face. Fibrosis represents the excessive accumulation of collagen and other matrix components that replaces normal parenchyma and impairs organ function. While localized to wounds in normal healing, fibrosis becomes a pathologic process when it occurs throughout an organ in response to chronic injury or inflammation. Hepatic cirrhosis, pulmonary fibrosis, and cardiac fibrosis exemplify the devastating consequences of organ-level fibrosis. The molecular mechanisms parallel those of wound healing, with TGF-beta playing a central role in driving fibroblast activation and matrix production. Unlike wound healing, which normally resolves, pathologic fibrosis is often progressive and irreversible, making prevention and early treatment critical. Panel A: Graph showing wound tensile strength over time from zero at injury to approximately 80% of normal by one year, with molecular events (collagen synthesis, cross-linking) mapped to strength increases. Panel B: Histologic comparison showing immature scar with type III collagen and high vascularity versus mature scar with organized type I collagen and decreased vessels. Panel C: Clinical photographs comparing normal scar, hypertrophic scar (raised but within wound boundaries), and keloid (extending beyond wound margins). Panel D: Progression of organ fibrosis showing normal liver, early fibrosis with septa, and established cirrhosis with nodular regeneration and extensive scarring. --- ### IX. Factors Affecting Wound Healing Local factors at the wound site have direct effects on the healing process and are often amenable to intervention. Infection represents the most important local impediment to healing, with bacterial contamination prolonging inflammation, damaging regenerating tissues, and competing for oxygen and nutrients. Wound debridement and appropriate antimicrobial therapy address this common problem. Blood supply determines the delivery of oxygen, nutrients, and inflammatory cells to the wound; ischemic tissues heal poorly, and restoration of perfusion through revascularization may be necessary for healing. Mechanical factors including tension, pressure, and movement can disrupt healing, requiring appropriate wound closure techniques and immobilization when necessary. Systemic factors influencing wound healing include nutritional status, metabolic conditions, and medication effects. Adequate protein intake is essential for collagen synthesis, and severe malnutrition significantly impairs wound healing. Vitamin C deficiency causes scurvy, characterized by defective collagen synthesis due to failure of proline and lysine hydroxylation. Zinc serves as a cofactor for numerous enzymes involved in wound healing and epithelialization. Glucocorticoids impair wound healing through multiple mechanisms including suppression of inflammation, inhibition of collagen synthesis, and impairment of angiogenesis, necessitating careful wound care in patients on chronic corticosteroid therapy. Diabetes mellitus impairs wound healing through multiple interconnected mechanisms, making diabetic foot ulcers a major clinical problem. Peripheral neuropathy leads to unrecognized trauma and pressure injuries. Peripheral vascular disease reduces blood supply to healing wounds. Leukocyte function is impaired, increasing susceptibility to infection. Advanced glycation end products alter collagen structure and impair cellular function. Hyperglycemia itself causes multiple cellular defects that impair the proliferative and remodeling phases of healing. Optimal glycemic control, pressure offloading, vascular assessment and intervention, and meticulous wound care are all essential components of managing diabetic wounds. Age affects wound healing through multiple mechanisms, including decreased inflammatory response, reduced cellular proliferation, diminished collagen synthesis, and impaired angiogenesis. Elderly patients may have comorbidities and medication effects that compound age-related healing impairment. Nutritional deficiencies are more common in older patients and may contribute to poor healing. Despite these factors, healthy elderly individuals generally heal adequately, and chronologic age alone is not a contraindication to surgery. Malignancy and its treatment may impair wound healing through nutritional depletion, immunosuppression, and direct effects of chemotherapy and radiation on proliferating cells. Panel A: Local factors affecting wound healing including infection with bacterial burden, ischemia with reduced oxygen tension, foreign bodies, and mechanical forces, each with intervention strategies. Panel B: Nutritional factors showing protein requirements for collagen synthesis, vitamin C role in hydroxylation, and zinc cofactor functions in wound healing enzymes. Panel C: Pathophysiology of diabetic wound healing impairment showing neuropathy, vascular disease, infection susceptibility, and AGE effects converging to prevent healing. Panel D: Age-related changes in wound healing showing decreased inflammation, reduced cellular proliferation, and delayed collagen remodeling compared to young tissue. --- ### X. Clinical Aspects of Wound Healing Wound complications represent significant causes of morbidity following surgical procedures and traumatic injuries. Dehiscence, the partial or complete separation of wound layers, typically occurs between postoperative days 5 and 14 when wound strength is lowest. Risk factors include obesity, malnutrition, infection, diabetes, increased intra-abdominal pressure, and technical factors in wound closure. Superficial dehiscence may be managed conservatively, while fascial dehiscence with evisceration is a surgical emergency. Incisional hernia, protrusion of abdominal contents through a healed wound, represents a late complication of impaired fascial healing. Wound infection manifests with the classic signs of inflammation and may progress to abscess formation, requiring drainage and antibiotic therapy. Chronic wounds are defined as wounds that fail to heal in the expected timeframe, typically three months, and represent a major healthcare burden particularly in aging populations. Venous leg ulcers result from venous insufficiency and chronic venous hypertension, characteristically appearing on the medial lower leg with surrounding hemosiderin deposition and lipodermatosclerosis. Arterial ulcers result from peripheral arterial disease and typically appear distally on the feet and toes, with features of ischemia including absent pulses and pallor on elevation. Pressure ulcers develop from sustained pressure over bony prominences in immobilized patients, progressing from superficial skin changes to deep tissue destruction. Each type requires specific interventions addressing the underlying cause. The management of chronic wounds involves addressing both local and systemic factors that impair healing. Debridement removes necrotic tissue and bacterial biofilm, converting a chronic wound to an acute wound that can progress through normal healing phases. Offloading pressure, whether through specialty beds, positioning, or therapeutic footwear, is essential for pressure ulcer and diabetic foot ulcer management. Compression therapy is the cornerstone of venous ulcer treatment, reducing venous hypertension and edema. Revascularization through endovascular or surgical techniques may be necessary for arterial ulcers. Advanced wound care products including growth factors, bioengineered skin substitutes, and negative pressure wound therapy may accelerate healing in recalcitrant wounds. Negative pressure wound therapy (NPWT), commonly known as vacuum-assisted closure (VAC), has become an important tool for managing complex wounds. The application of controlled negative pressure to a wound promotes granulation tissue formation, reduces edema, removes wound exudate, and may enhance bacterial clearance. NPWT is particularly useful for open wounds with significant tissue loss, exposed bone or hardware, and contaminated wounds that are not candidates for primary closure. Skin grafting may be necessary to cover large defects that cannot close by secondary intention. Split-thickness skin grafts, containing epidermis and partial dermis, are used to cover granulating wounds, while full-thickness grafts are reserved for areas requiring better cosmetic results and durability. Panel A: Wound dehiscence showing superficial versus fascial separation with evisceration, timeline of occurrence, and risk factors. Panel B: Classification and characteristics of chronic wounds showing venous ulcer (medial leg, hemosiderin staining), arterial ulcer (distal, punched out), and pressure ulcer (over bony prominence) with their specific features. Panel C: Treatment modalities for chronic wounds including debridement technique, compression bandaging for venous disease, and offloading for diabetic foot ulcers. Panel D: Negative pressure wound therapy setup showing foam dressing, adhesive seal, and vacuum unit with mechanism of action promoting granulation tissue. --- ## Summary - Chronic inflammation is characterized by prolonged duration, mononuclear cell infiltration, and concurrent destruction and repair - Macrophages exist in M1 (pro-inflammatory) and M2 (pro-repair) activation states and are central to chronic inflammation - Granulomas are organized collections of epithelioid macrophages; caseating granulomas suggest TB while non-caseating suggest sarcoidosis - Cell proliferative capacity determines regenerative potential: labile, stable, and permanent cell classifications - Wound healing occurs by first intention (clean, apposed edges) or second intention (open, requiring granulation tissue fill) - Healing phases include inflammation, proliferation, and remodeling spanning weeks to months - Key growth factors include PDGF, TGF-beta, VEGF, and EGF with specific roles in repair - Scar strength never reaches 100% of normal tissue, plateauing at approximately 80% - Factors impairing healing include infection, ischemia, diabetes, malnutrition, and corticosteroids - Pathologic scarring includes hypertrophic scars, keloids, and organ fibrosis --- ## Key Terms | Term | Definition | |------|------------| | Granuloma | Organized collection of activated macrophages with epithelioid appearance | | Epithelioid cell | Activated macrophage with abundant pink cytoplasm found in granulomas | | Caseating necrosis | Cheesy central necrosis in granulomas characteristic of tuberculosis | | Regeneration | Replacement of damaged tissue with cells of the same type | | Granulation tissue | New vascular and fibroblastic tissue filling wounds during healing | | Myofibroblast | Contractile fibroblast expressing smooth muscle actin in wound healing | | Keloid | Scar tissue extending beyond original wound margins | | Fibrosis | Excessive collagen deposition replacing normal parenchyma | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*