Pathology

Fracture healing occurs in overlapping phases rather than a single event. The fracture haematoma establishes the inflammatory niche; granulation tissue and a soft callus stabilise the gap; hard callus bridges it with woven bone; and remodelling converts that temporary scaffold into organised lamellar bone. Knowing what should happen in each phase helps emergency clinicians recognise normal recovery, detect infection or non-union early and avoid interventions that disrupt biology. Phase map Phase Approximate timing* Dominant events Mechanical result Haematoma and inflammation Hours–about 1 week Clot, cytokines, neutrophils, macrophages, angiogenic signals Very unstable; pain and swelling Soft callus About 1–3 weeks Fibrous tissue and cartilage bridge the gap Flexibility falls, but loading remains limited Hard callus About 3–8 weeks or longer Cartilage is replaced by mineralised woven bone Progressive bridging and stability Remodelling Months–years Woven bone becomes lamellar; excess callus is shaped Strength and architecture improve *Timelines vary by bone, age, fracture pattern, soft-tissue injury, fixation and comorbidity; use serial progress rather than a calendar alone. Learning outcomes After this lesson, the learner should be able to describe the cellular and vascular events of each phase, connect them with clinical and radiographic findings, explain direct versus callus-mediated healing, identify factors that interrupt a phase, and use phase-based reasoning in open fractures, delayed union and non-union. 1. Why phases overlap Inflammatory mediators remain active while soft callus forms; angiogenesis continues as cartilage is converted to bone; and remodelling begins before a fracture has regained full strength. The phase model is therefore a map of dominant activity, not a set of watertight compartments. Severe infection, instability or ischaemia can return a wound to a prolonged inflammatory state and prevent progression. 2. Phase 1—Fracture haematoma and inflammation 2.1 The first minutes and hours Fracture tears periosteal, endosteal and soft-tissue vessels. Blood collects between and around the fragments, forming a fibrin-rich haematoma. Platelets degranulate and create a provisional matrix containing platelet-derived growth factor, transforming growth factor-beta and other signals. The clot provides a temporary scaffold for incoming cells; removing it unnecessarily or stripping the surrounding soft tissue can impair repair. 2.2 Innate immune response Neutrophils arrive early and clear bacteria and debris. Monocytes differentiate into macrophages that remove necrotic tissue and coordinate the transition from inflammation to repair. Cytokines such as interleukin-1, interleukin-6 and tumour necrosis factor-alpha recruit progenitor cells and induce angiogenic and osteogenic pathways. Controlled inflammation is essential. Excessive or prolonged inflammation—especially from infection, retained foreign material or repeated tissue trauma—causes osteolysis and inhibits osteoblast function. 2.3 Vascular events Hypoxia around the fracture activates hypoxia-inducible pathways and vascular endothelial growth factor. New vessels begin to grow from periosteum and surrounding muscle. A small degree of hypoxia favours cartilage formation in the flexible central callus, while later revascularisation is necessary for mineralisation. Complete loss of perfusion, however, produces necrotic fragments and a biologically inactive gap. 2.4 What the clinician sees Pain, local tenderness, bruising, swelling and guarding are expected. Distal pulses and neurological function should be documented; they are not explained away as “part of healing.” Increasing pain out of proportion, pain with passive stretch, tense compartments, fever, crepitus or rapidly spreading erythema are red flags. Plain radiographs show the fracture but usually cannot show the haematoma. CT or MRI is used selectively for complex anatomy, occult fracture or soft-tissue questions. 2.5 How this phase is protected Control haemorrhage, cover open wounds, provide analgesia, align and splint the limb, preserve soft tissue and avoid repeated unnecessary manipulation. In open fractures, early antibiotics and operative debridement according to local protocol reduce the microbial burden before it can convert the inflammatory niche into destructive infection. 3. Phase 2—Granulation tissue and soft callus 3.1 Cellular composition Mesenchymal progenitor cells migrate into the haematoma. Fibroblasts lay down collagen; endothelial cells create capillary loops; and chondrocytes form cartilage where oxygen tension and mechanical stability favour a cartilaginous matrix. This mixture is granulation tissue and soft callus. It bridges the fragments but is not yet strong enough for unrestricted loading. 3.2 Endochondral and intramembranous pathways begin Endochondral ossification: cartilage is produced in relatively hypoxic, more mobile areas and will later be replaced by bone. Intramembranous ossification: osteoblasts form woven bone directly in well-vascularised periosteal regions. These pathways occur side by side. The balance reflects strain, oxygen, blood supply and the fixation construct. 3.3 Mechanical meaning Soft callus reduces gross motion and strain at the fracture. Controlled relative movement can stimulate callus, but excessive shear prevents maturation and leaves fibrocartilage or fibrous tissue between the fragments. A new increase in pain or deformity during this phase suggests loss of fixation, cast failure, premature loading or infection. 3.4 Clinical and radiographic findings Finding Expected progression Concerning pattern Pain Less at rest and with gentle movement Increasing pain, night pain or pain after a period of improvement Swelling Gradually decreases New tense swelling, erythema, fever or drainage Stability Less abnormal motion on protected examination Persistent gross mobility or new angulation Radiograph Early periosteal reaction or faint callus may appear Widening gap, implant migration or no serial progression 4. Phase 3—Hard callus and woven bone 4.1 Cartilage maturation Chondrocytes enlarge and the cartilaginous matrix becomes mineralised. The cells undergo apoptosis, leaving a scaffold invaded by capillaries, osteoclasts and osteoblast precursors. This transition is vulnerable to persistent hypoxia, infection and instability. 4.2 Woven bone formation Osteoblasts deposit osteoid on the mineralised cartilage scaffold and directly along vascularised periosteal surfaces. Woven bone has randomly arranged collagen and is less strong than mature lamellar bone, but it rapidly bridges the fracture. As the external and internal callus connect, abnormal movement and tenderness decline. 4.3 Clinical and imaging progression Weight-bearing or functional use becomes more comfortable within the limits of the prescribed plan. Callus becomes visible and increasingly mineralised on serial radiographs; the fracture line gradually fades. Bridging should be judged across multiple cortices and views, not from a single projection. Apparent radiographic callus with persistent pain or motion warrants assessment for infection, a hypertrophic non-union or an unstable construct. 4.4 Direct healing can bypass a large callus

Fracture healing is a coordinated repair response that restores bone continuity and, when possible, the original strength and shape. It requires a viable blood supply, inflammatory signalling, mesenchymal stem cells, cartilage and bone formation, mechanical stability and later remodelling. Emergency clinicians influence the biology from the first minutes by controlling haemorrhage, protecting soft tissue, reducing gross contamination, restoring alignment and preventing avoidable ischaemia. At a glance Secondary (indirect) healing: the common response to relatively flexible fixation; proceeds through haematoma, inflammation, soft callus, hard callus and remodelling. Primary (direct) healing: occurs with anatomic reduction and very rigid, low-strain fixation; osteonal remodelling crosses the fracture without a large external callus. Union is biological and mechanical: viable cells and vessels cannot compensate for excessive motion, while perfect stability cannot rescue devascularised or infected bone. Major threats: open injury, vascular compromise, infection, tissue interposition, excessive gap or motion, smoking, diabetes, malnutrition and some medicines. Learning outcomes After studying this page, the learner should be able to define fracture healing, describe the cells and signals involved, distinguish primary from secondary healing, outline the phases and expected clinical/radiographic findings, identify factors that accelerate or delay union, recognise delayed union, non-union and malunion, and apply emergency principles to open and complicated fractures. 1. What is a fracture and what does healing mean? A fracture is a disruption of the structural continuity of bone. It may be traumatic, stress-related, pathological or iatrogenic, and it may be closed or open. Healing means restoration of a load-bearing bridge across the fracture, with progressive recovery of stiffness, strength and function. Radiographic bridging is important but is not identical to biological union: a patient may have pain-free clinical union before complete remodelling, or an apparently bridged film with persistent weakness in a high-demand bone. Bone heals differently from skin. It can regenerate with tissue that is structurally similar to the original bone, but only if the local cells, blood vessels, matrix and mechanical environment remain suitable. A fracture also injures periosteum, endosteum, marrow, muscle and vessels, so the repair response is a tissue-level process rather than simply “new bone filling a crack.” 2. The local biology of a fracture 2.1 Blood supply Periosteal vessels supply the outer cortex and are especially important in children and in callus formation. Nutrient and endosteal vessels supply marrow and the inner cortex; disruption can produce cortical necrosis and delayed union. Soft-tissue stripping, high-energy injury, repeated surgery, infection and smoking can reduce perfusion even when the main artery remains patent. 2.2 Cells Cell Main role Clinical relevance Platelets and inflammatory cells Initiate clotting and release cytokines and growth factors Early inflammation is necessary; uncontrolled infection causes destructive inflammation Mesenchymal stem/progenitor cells Differentiate toward fibroblasts, chondrocytes and osteoblasts Provide the cellular pool for soft and hard callus Fibroblasts Produce collagen-rich fibrous tissue Form the early flexible bridge Chondrocytes Produce cartilage in low-oxygen, relatively unstable areas Build the soft callus for endochondral ossification Osteoblasts Synthesise osteoid and mineralise new bone Produce woven bone directly or after cartilage replacement Osteoclasts Resorb damaged or excess bone Shape the callus and participate in remodelling Endothelial cells/pericytes Generate new microvasculature Angiogenesis is essential for oxygen delivery and osteogenesis 2.3 Signalling pathways Platelet-derived growth factor, transforming growth factor-beta, fibroblast growth factors, vascular endothelial growth factor and bone morphogenetic proteins (BMPs) coordinate inflammation, angiogenesis and osteogenic differentiation. Wnt signalling supports osteoblast activity and bone formation. Hypoxia-inducible pathways help recruit vessels, but prolonged severe hypoxia, acidosis or infection shifts the environment toward necrosis and fibrous non-union. These pathways interact with parathyroid hormone, vitamin D, calcium-phosphate metabolism and local mechanical signals. 3. Phases of secondary (indirect) fracture healing These phases overlap; they are a useful framework rather than rigid calendar boxes. The duration varies with bone, fracture pattern, age, soft-tissue injury and fixation. Phase 1: Haematoma and inflammation (hours to approximately 1 week) Ruptured vessels produce a fracture haematoma and fibrin scaffold. The initial clot is not merely debris; it concentrates cells and growth factors. Platelets, neutrophils and macrophages remove damaged material and release mediators that recruit progenitor cells and stimulate angiogenesis. Necrotic bone at the fracture ends is gradually demarcated. Excessive tissue destruction, an open wound or a large devascularised fragment enlarges this zone and worsens prognosis. Clinically, pain, swelling, bruising and loss of function are prominent. Fever or severe systemic toxicity is not a normal consequence and should prompt assessment for infection or another injury. Phase 2: Granulation tissue and soft callus (approximately 1–3 weeks) Fibroblasts and chondrocytes bridge the gap with collagen and cartilage. The soft callus stabilises the fracture enough for later bone formation but has limited load-bearing strength. New capillaries grow into the repair tissue. Excessive motion maintains fibrous tissue and prevents progression; moderate controlled strain can stimulate callus in secondary healing. Phase 3: Hard callus and woven bone (approximately 3–8 weeks, often longer) As vascularity improves, cartilage is replaced by woven bone through endochondral ossification. Osteoblasts also lay down woven bone directly in well-oxygenated areas. The external callus becomes mineralised and bridges the fracture; pain and abnormal movement diminish. Radiographs show increasing callus and fading fracture lines, but the woven bone is not yet organised like mature lamellar bone. Phase 4: Remodelling (months to years) Osteoclasts remove excess callus and osteoblasts replace it with stronger lamellar bone. The medullary canal is re-established and trabeculae align along functional stress (Wolff’s law). Children remodel angulation and some deformity more effectively than adults, particularly near a growing physis; rotational deformity and large displacement remodel poorly at any age. Remodelling may continue long after the patient is clinically functional. 4. Primary (direct) fracture healing Primary healing requires near-anatomic reduction, minimal fracture gap and very rigid fixation that reduces interfragmentary strain. There may be little or no visible external callus. Cutting cones of osteoclasts cross the fracture from existing Haversian systems, followed by osteoblasts that create new osteons and restore the cortex. Contact healing occurs when the gap is extremely small; gap healing fills a small space with woven bone that later remodels. If fixation

Healing by Second Intention (Secondary Union) Large tissue defect • open wound • abundant granulation • wound contraction • slower epithelialisation • larger scar Healing by second intention occurs when wound edges cannot be closely approximated because tissue is missing, contaminated, infected, necrotic or under excessive tension. The defect fills from the base with granulation tissue, new extracellular matrix and blood vessels. Myofibroblasts contract the wound and keratinocytes gradually cover the surface. Secondary healing is slower and more vulnerable to infection, dehydration, contracture and excessive scarring, but it is often the safest strategy for contaminated or infected wounds because it allows drainage and surveillance. Learning outcomes Define secondary intention and identify when it is appropriate. Explain the roles of granulation tissue, angiogenesis, fibroblasts, myofibroblasts and epithelial migration. Describe why contraction and scar are greater than in primary union. Recognise complications and principles of open-wound management. Distinguish secondary from primary and delayed primary closure. 1. Indications for secondary healing Situation Why the wound remains open Large tissue loss Edges cannot be brought together without excessive tension. Contamination or infection Closure would trap bacteria, pus or necrotic debris. Necrotic tissue Viability must be assessed and devitalised tissue removed. Abscess cavity Drainage is necessary while the cavity fills from its base. Pressure ulcer Underlying pressure, ischaemia and tissue loss require staged healing. Burn or avulsion Skin/subcutaneous tissue is missing or unsuitable for immediate closure. Risky anatomic site Closure could compromise drainage, perfusion or function. 2. Sequence of secondary healing 2.1 Haemostasis and early inflammation Fibrin forms a provisional matrix, but the clot is larger than in a small incision. Neutrophils and macrophages must clear more debris and bacteria. Persistent contamination prolongs inflammation and increases protease-mediated tissue destruction. 2.2 Granulation tissue and angiogenesis Fibroblasts migrate from the wound edges and underlying tissue. Hypoxia stimulates VEGF and capillary growth. The wound base becomes red, moist and granular. Granulation tissue fills depth and creates a scaffold for the epidermis. 2.3 Epithelial migration Keratinocytes migrate from the edges and skin appendages. The larger the defect, the longer the surface remains fragile and vulnerable to desiccation, shear and infection. 2.4 Contraction Myofibroblasts attach to collagen and contract the matrix, pulling edges inward. Contraction can close a substantial portion of the wound but can distort nearby joints, eyelids, mouth, fingers and neck. 2.5 Remodelling Collagen III is replaced by collagen I, vessels regress and the scar becomes paler. The final scar is larger than in primary healing and may remain functionally limiting. 3. Why the scar is larger The wound contains more dead tissue and matrix to replace. Granulation tissue and angiogenesis are extensive. Fibroblasts deposit more collagen and extracellular matrix. Contraction produces a broad, irregular scar. Inflammation persists longer, stimulating TGF-β and fibrosis. 4. Normal appearance and monitoring Finding Usually expected Concerning Wound bed Moist pink/red granulation tissue. Black eschar, grey slough, pallor or exposed tendon/bone. Drainage Small amount of clear or blood-tinged fluid. Increasing purulence, foul odour or heavy bleeding. Pain Gradually improves. New severe pain or pain out of proportion. Edges Slow inward contraction and epithelial growth. Expanding defect, undermining or rapidly spreading erythema. Systemic state Stable, afebrile and improving function. Fever, confusion, hypotension, oliguria or tachypnoea. 5. Open-wound management principles Assess perfusion and depth: pulses, capillary refill, tendon/nerve function and exposed structures. Control contamination: irrigation, appropriate debridement, foreign-body removal and tetanus prevention. Manage infection: culture deep tissue when indicated, drain collections and use antimicrobials for clinically significant infection. Protect granulation: use a moist, non-adherent dressing and avoid traumatic dressing changes. Offload pressure and shear: reposition, use support surfaces and immobilise when necessary. Optimise systemic factors: glucose, nutrition, oxygenation, anaemia, smoking and vascular supply. Consider advanced closure: negative-pressure therapy, skin graft, flap or delayed primary closure when granulation and source control permit. 6. Complications Complication Mechanism Clinical consequence Chronic ulcer Persistent pressure/ischaemia, infection, neuropathy or inflammatory disease. Long-term drainage, pain, osteomyelitis and cancer risk. Contracture Excess myofibroblast contraction across a functional structure. Reduced movement, deformity and disability. Hypertrophic scar/keloid Excess collagen and persistent TGF-β signalling. Raised, itchy or painful scar. Exuberant granulation Granulation tissue rises above the wound edge. Epithelial migration is blocked. Infection/abscess Open defect and retained necrotic tissue provide microbial access. Cellulitis, sepsis or delayed closure. Bleeding and desiccation Fragile capillaries and inadequate protection. Anaemia, pain and loss of viable tissue. 7. When secondary wounds need urgent escalation Pain out of proportion, rapidly spreading redness, bullae, crepitus or skin anaesthesia. Exposed vessel, tendon, bone, joint or implanted material. Absent pulses, cold limb, worsening numbness or motor loss. Fever, hypotension, confusion, oliguria or rising lactate. Failure to improve despite offloading, cleaning and infection control. 8. Secondary versus primary and tertiary intention Feature Primary Secondary Tertiary Edges Closely apposed immediately. Open and fill naturally. Open initially, closed later. Granulation Minimal. Abundant. Moderate after control. Contraction Limited. Marked. Variable. Scar Small. Large, often irregular. Intermediate. Best use Clean, low-tension wound. Defect, infection or contamination. Contaminated wound suitable for later closure. 9. Applied cases Case 1: Pressure ulcer Pressure, shear and ischaemia produce tissue loss. Relief of pressure, staging, debridement, moisture-balanced dressings, nutrition and infection assessment are essential. A persistently non-healing ulcer needs vascular and systemic evaluation. Case 2: Drained abscess The cavity is left open so pus can drain and the base can granulate. Packing or negative-pressure therapy is selected according to size and local protocol; unnecessary traumatic packing can injure new tissue. Case 3: Burn contracture Deep burns heal by contraction and scar. Early specialist referral, physiotherapy, splinting and later grafting may preserve movement and prevent deformity. 10. Quick self-test What is secondary intention?Answer: Healing of an open, tissue-deficient, contaminated or infected wound by granulation, contraction and epithelialisation. Why is contraction greater?Answer: Myofibroblasts must reduce a large wound surface area. What does healthy granulation look like?Answer: Moist, pink-red, granular and vascular tissue. Name three reasons to leave a wound open.Answer: Infection, contamination, necrosis, abscess, large tissue loss or uncertain viability. What findings suggest necrotising infection?Answer: Severe pain out of proportion, rapid spread, bullae, crepitus, skin

Healing by First Intention (Primary Union) Closely apposed wound edges • clean incision • minimal tissue loss • rapid epithelialisation • small scar Healing by first intention, also called primary union, occurs when a clean wound has closely approximated edges, minimal tissue loss, good perfusion and controlled contamination. A sutured surgical incision is the classic example. The wound still passes through haemostasis, inflammation, proliferation and remodelling, but the gap is small and the scar is limited. Learning outcomes Define primary intention and list the conditions required for it. Describe the microscopic and clinical timeline after primary closure. Explain why tension, infection, haematoma and ischaemia cause dehiscence. Compare primary union with secondary and delayed primary closure. Apply principles to laceration assessment and emergency wound care. 1. Requirements for primary union Requirement Why it matters Clean wound Low bacterial burden prevents prolonged neutrophilic inflammation and infection. Viable edges Perfused tissue provides oxygen, immune function and cells for epithelialisation. Close apposition Minimises the gap that must be filled with granulation tissue and collagen. Minimal tissue loss Reduces matrix deposition, contraction and scar size. Haemostasis Prevents haematoma, dead space and pressure on the edges. Low tension Prevents ischaemia, edge separation and widened scar. Stable protection Limits shear, contamination and repeated mechanical injury. 2. Timeline after primary closure Time What occurs Clinical expectation Minutes–hours Platelet plug, fibrin clot and inflammatory mediator release. Small amount of blood-stained ooze; mild swelling. First 24 hours Neutrophils enter; basal keratinocytes begin migration. Edges may be tender and mildly erythematous. 24–48 hours Macrophages clear debris; epithelium bridges the narrow gap. Edges should remain aligned without spreading redness. Days 3–5 Fibroblasts and capillaries form limited granulation tissue; collagen III accumulates. Swelling settles; wound gains early strength. Weeks Collagen I replaces collagen III; vascularity decreases. Scar becomes flatter and paler. Months Matrix remodelling and fibre alignment continue. Strength improves but remains below normal tissue. 3. Cellular and molecular events Platelets: provide fibrin and release PDGF, TGF-β, VEGF and EGF. Neutrophils: remove microbes and small amounts of necrotic debris. Macrophages: clear apoptotic cells and shift from inflammatory to repair signalling. Keratinocytes: migrate across the wound and restore the epidermal barrier. Fibroblasts: deposit fibronectin, collagen III and then collagen I. Endothelial cells: form capillary loops in limited granulation tissue. Myofibroblasts: provide modest contraction because the gap is small. 4. Closure methods Method Use Key precautions Sutures Precise edge alignment and layered closure. Avoid strangulation and excessive tension; remove at appropriate time/site. Staples Rapid closure of selected skin and scalp wounds. Check alignment, bleeding and infection risk. Adhesive strips Low-tension, superficial wounds. Edges must be dry and stable. Tissue adhesive Small, clean, superficial wounds with minimal tension. Do not use in contaminated, wet, mucosal or high-tension sites unless appropriate. 5. Benefits of primary union Rapid epithelial barrier restoration. Less granulation tissue and collagen than secondary healing. Reduced wound contraction and contracture. Smaller, more cosmetically acceptable scar. Shorter open-wound exposure to contamination. 6. Complications and why they occur Complication Mechanism Clues Infection Bacteria multiply in a closed or contaminated space. Increasing pain, warmth, pus, fever, spreading erythema. Haematoma/seroma Bleeding or fluid collects between edges and separates tissue. Fluctuant swelling, bruising, drainage or pressure. Dehiscence Infection, ischaemia, excessive tension, coughing or weak collagen. Gaping wound, serous discharge or visible deeper tissue. Hypertrophic scar Excess collagen remains within original wound boundary. Raised, itchy scar that may regress. Keloid Collagen extends beyond original boundary. Firm scar spreading into normal skin. Stitch abscess/foreign-body reaction Retained suture or material maintains inflammation. Persistent local nodule, drainage or sinus. 7. Factors that jeopardise primary healing Crush injury, devitalised edges or missed foreign bodies. Delayed presentation with bacterial contamination. Diabetes, peripheral vascular disease, smoking or anaemia. Malnutrition, vitamin C/zinc deficiency or immunosuppression. High wound tension, movement, pressure or repeated trauma. Systemic corticosteroids or medications that impair collagen. 8. Emergency wound assessment Control bleeding and assess airway/breathing/circulation in major trauma. Inspect depth: tendon, nerve, vessel, joint, bone and foreign-body involvement. Assess perfusion, sensation, motor function and distal pulses before and after anaesthesia/closure. Irrigate and remove contamination; debride non-viable tissue when indicated. Assess tetanus status and infection risk. Close primarily only when the wound is suitable, viable and adequately cleaned. Arrange follow-up for infection, dehiscence, suture removal and function. 9. Primary versus other intentions Feature Primary intention Secondary intention Tertiary intention Edges Closely apposed. Open, tissue-deficient or widely separated. Left open initially, closed later. Granulation Minimal. Abundant. Moderate after contamination resolves. Contraction Limited. Marked. Variable. Scar Small. Larger. Intermediate. Typical use Clean surgical incision. Pressure ulcer/large contaminated defect. Contaminated wound needing delayed closure. 10. Applied cases Case 1: Clean forearm laceration After neurovascular and tendon assessment, irrigation, haemostasis and local anaesthesia, a clean low-tension wound may be approximated primarily. Document function before closure and provide infection/tetanus advice. Case 2: Wound with haematoma A tense swelling under a fresh incision separates the edges and provides a medium for bacteria. Reassess bleeding, drainage and perfusion; do not simply add more sutures over an unresolved collection. Case 3: Wound dehiscence Separation with visible fascia, bowel or deep tissue is an emergency. Cover with sterile saline-moistened dressings, avoid pushing exposed tissue back, treat shock and obtain urgent surgical review. 11. Quick self-test What is primary union?Answer: Healing of a clean, closely apposed wound with minimal tissue loss and a small scar. Why is the scar smaller than in secondary healing?Answer: The gap is narrow, so less granulation tissue, collagen and contraction are required. Name three causes of dehiscence.Answer: Infection, haematoma, ischaemia, excessive tension, coughing, malnutrition or weak collagen. What must be assessed before closing a traumatic laceration?Answer: Depth, contamination, tissue viability, foreign bodies, tendon/nerve/vessel/joint injury and tetanus risk. Why is a haematoma dangerous?Answer: It separates edges, compromises perfusion and supports bacterial growth. 12. Take-home summary Primary intention occurs when viable, clean wound edges are closely apposed with minimal tissue loss and tension. Healing still includes haemostasis, inflammation, epithelialisation, fibroblast activity and remodelling. Good irrigation, haemostasis, perfusion, alignment, low tension and follow-up reduce complications. Increasing pain, spreading erythema, pus, haematoma,

Phases of Wound Healing Haemostasis • inflammation • proliferation • remodelling • cell and mediator timeline • normal versus delayed healing Wound healing is an overlapping sequence rather than four isolated boxes. Haemostasis begins within seconds, inflammation develops over hours, proliferation becomes prominent over days, and remodelling continues for weeks to months. Platelets, neutrophils, macrophages, endothelial cells, fibroblasts, keratinocytes and myofibroblasts communicate through cytokines, growth factors and extracellular matrix. A wound heals normally only when bleeding is controlled, the tissue is perfused, infection is contained, the edges are mechanically stable and the patient has adequate oxygen, protein and micronutrients. Learning outcomes Describe the timing, dominant cells, mediators and matrix changes in each phase. Explain how haemostasis creates a provisional scaffold for inflammation and repair. Describe the neutrophil-to-macrophage transition and why resolution is essential. Explain re-epithelialisation, angiogenesis, fibroblast migration, collagen deposition and contraction. Describe collagen remodelling and the time course of tensile strength. Recognise normal healing versus infection, dehiscence, excessive scar and chronic non-healing. Apply the timeline to wound review and emergency decision-making. 1. Master timeline Phase Approximate timing Dominant cells/signals Main events Haemostasis Seconds to hours. Platelets, coagulation factors, fibrin. Vasoconstriction, platelet plug, fibrin clot and provisional matrix. Inflammation Hours to days. Neutrophils first; macrophages later. Debris/microbial clearance, cytokines, transition to repair. Proliferation Day 2 to several weeks. Keratinocytes, fibroblasts, endothelial cells, myofibroblasts. Re-epithelialisation, granulation, angiogenesis, collagen III and contraction. Remodelling/maturation Weeks to months or longer. Fibroblasts, MMPs/TIMPs and collagen I. Matrix turnover, fibre alignment, scar contraction and increasing strength. Phases overlap: macrophages remain active during proliferation, and remodelling begins before the wound is fully closed. 2. Phase 1 — Haemostasis 2.1 Immediate vascular response Injury exposes collagen and tissue factor. Brief vasoconstriction limits blood loss; endothelial contraction and local reflexes reduce flow. Excessive vasospasm or arterial injury can instead worsen tissue ischaemia. 2.2 Platelet adhesion and activation Von Willebrand factor binds exposed collagen and platelet glycoprotein Ib. Platelets change shape and release ADP, thromboxane A2, serotonin and calcium. Activated glycoprotein IIb/IIIa binds fibrinogen and cross-links platelets. Platelet-derived growth factor (PDGF), TGF-β, VEGF and EGF signal to fibroblasts, endothelial cells and keratinocytes. 2.3 Coagulation and provisional matrix Tissue factor activates thrombin, which converts fibrinogen to fibrin. The fibrin-fibronectin clot traps red cells and provides a temporary scaffold. It also concentrates growth factors and creates a surface across which leukocytes and fibroblasts can migrate. 2.4 Clinical relevance Anticoagulants, platelet disorders and severe liver disease can prolong bleeding. Haematoma separates tissue planes, increases infection risk and may require drainage. Ischaemia, hypothermia and acidosis impair coagulation in major trauma. 3. Phase 2 — Inflammation 3.1 Early neutrophilic phase Neutrophils arrive within hours in response to IL-8/CXCL8, C5a, LTB4 and bacterial peptides. They phagocytose microbes and necrotic debris and release ROS, proteases and antimicrobial granules. Excessive neutrophil activity damages viable matrix and delays closure. 3.2 Macrophage transition Monocytes enter after the first day and differentiate into macrophages. Early inflammatory macrophages release TNF, IL-1, IL-6 and chemokines. As the trigger is cleared, macrophages perform efferocytosis, release IL-10 and TGF-β, and stimulate fibroblasts, endothelial cells and keratinocytes. 3.3 Resolution checkpoint Healing cannot progress if the wound remains infected, necrotic or contaminated. Neutrophils must undergo apoptosis, oedema must drain and macrophages must switch from inflammatory clearance to repair. Retained sutures, foreign bodies, biofilm and poor perfusion prolong this phase. 3.4 Clinical appearance Mild warmth, redness, swelling and tenderness are expected early. Increasing pain, expanding erythema, purulent discharge, foul odour, fever, crepitus, skin anaesthesia or systemic toxicity suggests infection or necrotising injury rather than normal inflammation. 4. Phase 3 — Proliferation 4.1 Re-epithelialisation Keratinocytes at the wound margin loosen desmosomes, migrate across the provisional matrix and proliferate behind the leading edge. EGF, TGF-α, keratinocyte growth factor and adequate moisture support migration. The epidermal barrier is initially fragile and easily disrupted by friction or desiccation. 4.2 Angiogenesis Hypoxia stabilises HIF-1α and increases VEGF. Endothelial cells degrade their basement membrane, migrate through the matrix, proliferate and form capillary loops. New vessels deliver oxygen and nutrients but make granulation tissue red and prone to bleeding. 4.3 Fibroblast recruitment PDGF, FGF and TGF-β recruit fibroblasts into the wound. Fibroblasts produce fibronectin, proteoglycans, collagen III and later collagen I. They remodel the provisional fibrin matrix and connect the wound edges. 4.4 Granulation tissue Granulation tissue contains capillaries, fibroblasts, macrophages, loose extracellular matrix and type III collagen. Healthy granulation is moist, pink-red and finely granular. Pale, necrotic or malodorous tissue suggests poor perfusion or infection. 4.5 Myofibroblasts and contraction Myofibroblasts express contractile actin and attach to collagen. Their contraction reduces wound area, especially in secondary healing. Excessive contraction across joints, eyelids, mouth or the neck creates functional contractures. 5. Phase 4 — Remodelling and maturation 5.1 Collagen transition Early collagen III is relatively weak and loosely organised. Fibroblasts and matrix metalloproteinases replace it with stronger collagen I. Cross-linking and alignment along lines of tension improve tensile strength. 5.2 MMPs and TIMPs Matrix metalloproteinases degrade damaged collagen and basement membrane. Tissue inhibitors of metalloproteinases limit their activity. An imbalance causes chronic ulceration, excessive matrix breakdown or pathological fibrosis. 5.3 Tensile strength Approximate stage Strength and appearance First days Strength depends mainly on sutures, fibrin and wound support; collagen is weak. 1–3 weeks Rapid collagen deposition and cross-linking; epithelial surface closes but deep tissue remains vulnerable. 1–3 months Collagen I alignment and remodelling increase strength; scar becomes less vascular. Long term Strength plateaus below original tissue, commonly around 70–80%. 6. How phases interact Interaction Why it matters Platelet clot → inflammation Fibrin and platelet growth factors recruit leukocytes and repair cells. Macrophage resolution → proliferation Pro-repair macrophages release VEGF, TGF-β and PDGF. Hypoxia → angiogenesis Moderate hypoxia induces VEGF; severe persistent hypoxia prevents repair. Inflammation → remodelling Proteases clear damaged matrix, but excess protease causes ulceration. Mechanical tension → collagen alignment Appropriate tension strengthens scar; excessive tension causes dehiscence or hypertrophy. 7. Normal versus abnormal timeline Expected finding Concerning deviation Possible cause Mild early redness, warmth and tenderness. Rapidly expanding redness, severe pain or systemic

Wound Healing Haemostasis • inflammation • proliferation • remodelling • healing by first, second and third intention • complications Wound healing is the coordinated restoration of tissue after injury. It depends on haemostasis, controlled inflammation, epithelial migration, angiogenesis, fibroblast activity, collagen deposition, matrix remodelling and contraction. Healing may restore normal architecture (regeneration) or replace damaged tissue with scar. The same processes that close a wound can create excessive scar, contracture or chronic non-healing ulcers. Emergency and clinical care therefore aims to control bleeding and infection, preserve perfusion, align tissue, protect the wound and correct systemic factors that impair repair. Learning outcomes Describe the phases of wound healing and their major cells, mediators and matrix changes. Explain healing by primary, secondary and tertiary intention. Distinguish regeneration from scar formation and understand wound contraction. Recognise normal versus delayed or excessive healing. Identify local and systemic factors that impair repair. Apply wound-healing principles to emergency wound assessment and aftercare. 1. Regeneration versus repair Process Outcome Determinants Regeneration Replacement by cells of the same type with near-normal function. Intact extracellular matrix, viable stem/progenitor cells and limited tissue loss. Repair by scar Collagen and matrix replace lost tissue; strength improves but function may not fully return. Large defect, damaged matrix, non-regenerating tissue or persistent inflammation. Mixed healing Some areas regenerate while others scar. Most real wounds, infarcts and chronic ulcers. 2. Phase 1: Haemostasis Haemostasis begins immediately after injury. Vasoconstriction: limits blood loss. Platelet adhesion: von Willebrand factor bridges platelets to exposed collagen. Platelet activation: ADP, thromboxane A2 and serotonin recruit more platelets. Coagulation: thrombin converts fibrinogen to fibrin, stabilising the plug. Provisional matrix: fibrin, fibronectin and platelet proteins provide a scaffold for migrating cells. Platelets release PDGF, TGF-β, VEGF and EGF, linking clotting to inflammation and repair. 3. Phase 2: Inflammation Neutrophils arrive early to remove bacteria and debris. Macrophages then clear apoptotic neutrophils and necrotic material and coordinate the transition to repair. Histamine and prostaglandins increase flow and permeability. Neutrophil ROS/proteases kill organisms but can injure tissue. Macrophage IL-1/TNF recruit cells; later IL-10/TGF-β support resolution. Persistent contamination or necrosis prolongs inflammation and delays closure. 4. Phase 3: Proliferation 4.1 Re-epithelialisation Basal keratinocytes loosen attachments, migrate across the wound and proliferate behind the leading edge. Growth factors from platelets, macrophages and keratinocytes coordinate the process. Moist, protected wound surfaces support epithelial migration. 4.2 Angiogenesis Hypoxia stabilises HIF-1α, which increases VEGF. Endothelial cells degrade basement membrane, migrate, proliferate and form capillary loops. New vessels make granulation tissue red and bleed easily. 4.3 Fibroblast migration and collagen deposition PDGF, TGF-β and FGF recruit fibroblasts. Early collagen is mainly type III; fibronectin and proteoglycans provide a provisional matrix. Myofibroblasts express alpha-smooth-muscle actin and contract the wound. 4.4 Granulation tissue Granulation tissue contains new capillaries, fibroblasts, macrophages, loose extracellular matrix and type III collagen. It fills defects and supports epithelium; healthy granulation is moist, red and granular. 5. Phase 4: Remodelling and maturation Over weeks to months, collagen III is replaced by stronger collagen I. Matrix metalloproteinases degrade old matrix while tissue inhibitors restrain them. Collagen fibres align along lines of tension and cross-link. Time Typical event Days Fibrin matrix, neutrophils and early macrophages. 1–3 weeks Granulation tissue, angiogenesis, fibroblasts and type III collagen. Weeks–months Collagen I maturation, reduced vascularity and increasing tensile strength. Months+ Scar becomes paler and flatter; strength plateaus below uninjured tissue. Scar strength rises rapidly early but generally reaches only about 70–80% of original tissue strength; it never regains the exact normal architecture. 6. Healing by primary intention Primary intention occurs when wound edges are closely apposed, as in a clean sutured incision. Small clot and limited necrosis. Minimal inflammation and granulation tissue. Rapid epithelial bridging. Thin scar with little contraction. Lower infection risk if contamination is controlled. Accurate alignment, gentle tissue handling, haemostasis and early closure favour primary healing. 7. Healing by secondary intention Secondary intention occurs when there is a large tissue defect, tissue necrosis, contamination or wound edges cannot be approximated. More inflammation and granulation tissue. Greater collagen deposition and wound contraction. Slower epithelialisation and a larger scar. Higher risk of infection, hypertrophic scar and contracture. Examples include pressure ulcers, large abscess cavities and wounds left open after debridement. 8. Healing by tertiary intention (delayed primary closure) Tertiary intention leaves a contaminated or infected wound open initially for drainage, observation and debridement. Once bacterial burden and oedema decrease, the wound is closed. It combines the safety of delayed source control with the cosmetic and functional advantages of primary closure. 9. Wound contraction Myofibroblasts attach to collagen and pull wound edges together. Contraction reduces the surface area but can distort joints, eyelids, mouth, fingers or the neck. Excessive contraction after burns causes contractures and functional limitation. 10. Factors that affect healing Factor Effect on healing Clinical action Infection Prolongs inflammation, destroys matrix and increases dehiscence. Clean, culture when indicated, drain/debride and use appropriate antimicrobials. Ischaemia/hypoxia Impairs collagen synthesis, angiogenesis and leukocyte killing. Restore perfusion, stop smoking and optimise oxygenation. Foreign bodies Maintain inflammation and sinus formation. Remove when safe; image retained material. Diabetes Microvascular disease, neuropathy, glycation and immune dysfunction. Glucose control, foot care and vascular assessment. Malnutrition Protein, vitamin C, zinc and iron deficiency impair matrix and immunity. Nutrition assessment and protein/micronutrient replacement. Glucocorticoids Suppress inflammation, fibroblasts and collagen. Balance indication; do not stop essential steroids abruptly. Mechanical stress Shear, tension and pressure disrupt fragile repair. Immobilise, offload, support and close appropriately. Ageing Reduced angiogenesis, stem-cell reserve and collagen response. Prevent pressure injury, optimise nutrition and mobilise safely. 11. Complications Complication Mechanism/feature Wound dehiscence Separation from infection, poor collagen, tension or malnutrition; emergency if bowel protrudes. Incisional hernia Weak scar permits abdominal contents to bulge. Hypertrophic scar Excess collagen remains within wound boundaries and may regress. Keloid Excess collagen extends beyond wound margins; recurrence is common. Contracture Excessive contraction limits movement, especially after burns. Chronic ulcer Persistent pressure, ischaemia, neuropathy, infection or systemic disease prevents closure. Exuberant granulation “Proud flesh” rises above the wound edge and blocks epithelial migration. 12.

Inflammatory Cells in Chronic Inflammation Macrophages • T and B lymphocytes • plasma cells • eosinophils • mast cells • neutrophils • fibrosis and tissue damage Chronic inflammation is defined microscopically by a persistent mononuclear infiltrate accompanied by tissue destruction and repair. The inflammatory cells do not act independently: macrophages present antigen and direct repair, T cells activate macrophages, B cells make antibodies, plasma cells sustain humoral responses, eosinophils injure tissue in allergy and parasites, and mast cells rapidly amplify vascular and fibrotic signals. Learning outcomes Identify the major inflammatory cells in chronic inflammation and their microscopic appearance. Explain how each cell is recruited, activated and removed. Compare M1-like and M2-like macrophage functions. Describe T-cell subsets, B cells/plasma cells, eosinophils, mast cells and persistent neutrophils. Link cells to granulomas, fibrosis, allergy, autoimmunity, infection and cancer. Interpret cell-rich pathology and select useful diagnostic tests. 1. Overview of the cellular network Cell Main role Potential harm Macrophage Phagocytosis, antigen presentation, cytokines, repair and fibrosis. ROS/proteases, persistent cytokines and excessive fibrosis. T lymphocyte Antigen-specific activation and macrophage/leukocyte control. Autoimmunity, cytotoxic injury and chronic cytokine loops. B cell/plasma cell Antibody production and immune memory. Immune complexes, autoantibodies and tissue deposition. Eosinophil Helminth defence and type 2 inflammation. Granule-protein injury in asthma and allergy. Mast cell Rapid histamine, lipid mediator and cytokine release. Anaphylaxis, bronchospasm, oedema and fibrosis. Neutrophil Acute killing and debris removal. Persistent neutrophilic injury in chronic infection/COPD. 2. Macrophages 2.1 Origin and recruitment Macrophages arise from resident tissue populations and recruited monocytes. CCL2, CSF-1, complement and microbial signals guide recruitment. They survive longer than neutrophils and can repeatedly ingest material. 2.2 M1-like inflammatory activation Microbial products, IFN-γ and TNF induce an inflammatory phenotype. These macrophages produce IL-1, TNF, IL-6, IL-12, ROS, nitric oxide and proteases. They kill organisms and activate T cells but can damage bystander tissue. 2.3 M2-like repair activation IL-4, IL-13, immune complexes and repair signals produce a matrix-remodelling phenotype. M2-like cells release TGF-β, VEGF and growth factors, promote angiogenesis and activate fibroblasts. Excessive activation causes fibrosis and tumour-supporting stroma. 2.4 Macrophage morphology Abundant eosinophilic or foamy cytoplasm. Round or oval nuclei; lipid-laden macrophages appear as foam cells. Epithelioid macrophages have abundant pale cytoplasm and indistinct borders in granulomas. Multinucleated giant cells form by macrophage fusion. 3. T lymphocytes Subset Key cytokines/function Clinical association Th1 IFN-γ activates macrophages; IL-2 supports cellular immunity. Tuberculosis, intracellular infection, granulomas. Th2 IL-4, IL-5, IL-13; IgE, eosinophils, mucus and fibrosis. Asthma, helminths and allergy. Th17 IL-17 and IL-22 recruit neutrophils and strengthen mucosal barriers. Fungal/bacterial defence, psoriasis and autoimmunity. Regulatory T cells IL-10 and TGF-β suppress excessive immune activation. Tolerance, resolution and tumour immune escape. CD8 cytotoxic cells Perforin/granzyme and death receptors kill target cells. Viral hepatitis, graft rejection and tumour surveillance. 4. B cells and plasma cells B cells present antigen, form memory and differentiate into plasma cells. Plasma cells have eccentric “clock-face” nuclei and basophilic cytoplasm due to abundant rough ER. They produce antibodies in chronic infection, autoimmune disease and mucosal inflammation. Autoantibodies can directly injure cells or activate receptors. Immune complexes activate complement and neutrophils. Tertiary lymphoid structures may form in chronically inflamed organs. Monoclonal plasma-cell populations suggest neoplasia and require immunophenotyping. 5. Eosinophils Eosinophils have bilobed nuclei and bright eosinophilic granules containing major basic protein, eosinophil cationic protein, peroxidase and neurotoxin. IL-5 promotes their production and survival; eotaxins recruit them. Protective: kill helminths too large for phagocytosis. Harmful: granule proteins injure bronchial epithelium, nerves and endothelium. Clinical: asthma, allergic rhinitis, drug reactions, parasitic disease and eosinophilic gastrointestinal disorders. 6. Mast cells Mast cells reside near vessels, nerves and epithelial barriers. IgE cross-linking triggers rapid degranulation; complement, physical injury and neuropeptides can also activate them. Mediator Effect Histamine Permeability, itching, vasodilation and bronchospasm. Tryptase/chymase Matrix remodelling and proteolysis. Leukotrienes/prostaglandins Bronchoconstriction, mucus, pain and sustained permeability. TNF/chemokines Leukocyte recruitment and chronic amplification. 7. Neutrophils in chronic disease Neutrophils are not exclusive to acute inflammation. They persist when organisms, smoke, crystals, necrotic tissue or cytokines continually recruit them. Chronic neutrophilic conditions include bronchiectasis, COPD, chronic osteomyelitis, inflammatory bowel flares and non-healing wounds. 8. Cells in granulomatous inflammation Epithelioid macrophages form the central cellular aggregate. Th1 cells produce IFN-γ and maintain macrophage activation. Multinucleated giant cells surround indigestible organisms or foreign material. Peripheral lymphocytes and fibroblasts form a rim and may create a fibrotic capsule. Caseous necrosis suggests some infections but is not diagnostic without microbiology. 9. Cell interactions and fibrosis Macrophage–T-cell feedback sustains inflammation. Macrophage TGF-β and platelet-derived growth factor activate fibroblasts; IL-13 promotes collagen and mucus; endothelial VEGF drives angiogenesis. Matrix metalloproteinases and their inhibitors determine whether tissue is degraded, remodelled or replaced by scar. 10. Diagnostic clues Finding Likely cell/process Useful test Foamy macrophages Lipid, cholesterol or storage material. Oil Red O, lipid profile, storage-disease testing. Granulomas Epithelioid macrophages, giant cells and T cells. AFB/fungal stains, cultures, polarised light. Eosinophil-rich infiltrate Allergy, helminths or drug reaction. FBC differential, parasite tests, medication review. Plasma-cell infiltrate Chronic antigen or autoimmune response. Immunoglobulins, electrophoresis, clonality studies. Mast-cell increase Allergic or mast-cell disease. Tryptase, CD117 and specialist evaluation. 11. Clinical applications Tuberculosis: Th1–macrophage granulomas contain infection but can destroy lung tissue. Asthma: Th2 cytokines, mast cells and eosinophils cause bronchospasm, mucus and airway remodelling. Rheumatoid arthritis: macrophages, T cells and plasma cells sustain synovitis and pannus destruction. Inflammatory bowel disease: lymphocytes, macrophages, neutrophils and plasma cells interact with barrier/microbial signals. Chronic hepatitis: lymphocyte and macrophage injury causes fibrosis, cirrhosis and cancer risk. 12. Emergency relevance Chronic disease lowers organ reserve; acute infection or trauma can cause rapid decompensation. Eosinophilic or mast-cell inflammation can produce life-threatening bronchospasm or anaphylaxis. Immunosuppressed patients may have muted cell counts and still develop sepsis. Before escalating immunosuppression, exclude occult infection and obtain tissue/microbiology when appropriate. 13. Quick self-test Which cell coordinates chronic inflammation and fibrosis?Answer: The macrophage. Which T-cell subset drives macrophage activation in tuberculosis?Answer: Th1 cells through IFN-γ. Which subset is associated with allergy and eosinophils?Answer: Th2 cells. What granules injure tissue in eosinophilic inflammation?Answer: Major basic protein, eosinophil

Chronic Inflammation Persistent injury • macrophages and lymphocytes • tissue destruction • repair and fibrosis • granulomas • systemic disease Chronic inflammation is prolonged inflammation in which active inflammation, tissue destruction and repair occur at the same time. It may follow unresolved acute inflammation or begin insidiously with persistent infection, autoimmunity, foreign material, toxin exposure or metabolic stress. The response can continue for months or years and remodel an organ permanently. Unlike acute inflammation, chronic inflammation is not defined only by duration. It is recognised by the combination of mononuclear inflammatory cells, ongoing tissue injury, angiogenesis and fibrosis. Learning outcomes Define chronic inflammation and distinguish it from acute and acute-on-chronic inflammation. List major causes and explain why the stimulus persists. Describe macrophage, lymphocyte, plasma-cell, eosinophil and mast-cell roles. Explain M1/M2 macrophage activation, cytokine loops and fibrosis. Recognise granulomatous inflammation and important causes. Relate chronic inflammation to systemic disease, cancer, organ failure and emergency flares. Outline diagnostic and management principles. 1. Hallmarks Hallmark What is seen? Mononuclear infiltrate Macrophages, lymphocytes and plasma cells; variable eosinophils and mast cells. Ongoing tissue destruction Necrosis, epithelial injury, parenchymal loss or vascular damage from the cause and inflammatory cells. Repair and fibrosis Angiogenesis, fibroblast activation, collagen deposition and remodelling. Persistent mediator network TNF, IL-1, IFN-γ, IL-17, TGF-β, chemokines and growth factors. 2. Causes Cause Examples Why it persists Persistent infections Tuberculosis, fungi, parasites, chronic viral hepatitis. Organism resists eradication or hides intracellularly. Autoimmunity Rheumatoid arthritis, SLE, inflammatory bowel disease. Self-antigen and memory immune responses continue. Prolonged toxin exposure Silica, tobacco smoke, asbestos, drugs. Continued exposure or retained particles. Foreign bodies Sutures, splinters, implants, crystals. Material cannot be degraded or removed. Metabolic stress Obesity, atherosclerosis, urate and cholesterol crystals. Lipotoxicity, inflammasomes and repeated tissue stress. Unresolved acute inflammation Chronic abscess, obstructed organ, persistent necrotic tissue. Source control is incomplete. 3. Macrophages: the central chronic cell Monocytes enter tissue and differentiate into macrophages under the influence of chemokines and growth factors. Resident macrophages also expand locally. They ingest microbes and debris, present antigen and direct repair or fibrosis. 3.1 Classical (M1-like) activation IFN-γ, microbial products and TNF produce an inflammatory phenotype. M1-like macrophages release ROS, NO, IL-1, TNF, IL-6, IL-12 and proteases. They are effective at microbial killing but can destroy host tissue. 3.2 Alternative (M2-like) activation IL-4, IL-13 and other signals produce a repair/fibrotic phenotype. M2-like macrophages release TGF-β, VEGF, growth factors and matrix-remodelling enzymes. They support angiogenesis and wound healing but can cause excessive fibrosis and tumour support. 3.3 Macrophage self-amplification Macrophages release chemokines that recruit monocytes and cytokines that activate lymphocytes. Lymphocytes release IFN-γ, IL-17 and other signals that further activate macrophages. This positive feedback loop sustains chronic inflammation after the initial trigger becomes difficult to detect. 4. Lymphocytes and other cells Cell Function in chronic inflammation T lymphocytes CD4 subsets activate macrophages or recruit neutrophils; CD8 cells kill target cells. B cells/plasma cells Produce antibodies; organise tertiary lymphoid structures; may drive immune complexes. Eosinophils Helminth defence and allergy; granule proteins injure tissue. Mast cells Release histamine, proteases and cytokines; participate in allergy and fibrosis. Neutrophils Persist in chronic infection, smoking-related disease and acute-on-chronic flares. Fibroblasts/endothelium Produce matrix, angiogenesis and remodelling signals. 5. Granulomatous inflammation Granulomas are organised collections of epithelioid macrophages, often surrounded by lymphocytes and giant cells. They form when a stimulus is difficult to eradicate or when T-cell responses strongly activate macrophages. Type Examples Diagnostic approach Caseating infectious Tuberculosis, histoplasmosis and other deep fungi. AFB/fungal stains, culture, PCR and exposure history. Non-caseating immune Sarcoidosis, Crohn disease, beryllium. Exclude infection and foreign body; assess organ pattern. Foreign-body Suture, talc, splinter, keratin or crystals. Polarised light and search for material. Granulomas can cause fibrosis, obstruction, vascular compromise and organ dysfunction. A biopsy pattern is not a complete diagnosis without microbiology and clinical correlation. 6. Tissue destruction and repair Macrophage proteases and ROS destroy extracellular matrix and parenchymal cells. TNF and IL-1 activate endothelium and sustain leukocyte recruitment. VEGF promotes angiogenesis, while TGF-β activates fibroblasts and collagen deposition. Matrix metalloproteinases degrade tissue; tissue inhibitors control remodelling. Repeated injury and repair produce architectural distortion, strictures and loss of specialised function. 7. Fibrosis and organ consequences Organ/condition Chronic injury and consequence Liver Chronic hepatitis/steatohepatitis → bridging fibrosis, cirrhosis, portal hypertension. Lung Interstitial inflammation → collagen deposition, restrictive disease and hypoxia. Kidney Interstitial inflammation → nephron loss, fibrosis and chronic kidney disease. Heart/vessels Atherosclerotic inflammation → plaque rupture, infarction and stroke. Gut Crohn disease/ulcerative colitis → strictures, fistulae, bleeding and malabsorption. Joints Rheumatoid synovitis → pannus, cartilage destruction and deformity. 8. Systemic effects Persistent IL-6 and TNF cause anaemia of inflammation, thrombocytosis, fatigue and cachexia. Chronic inflammation increases cardiovascular risk through endothelial activation and plaque instability. Immune dysregulation produces fever, night sweats, weight loss and recurrent flares. Chronic infection can produce amyloidosis through sustained serum amyloid A. Inflammatory cytokines and oxidative stress promote DNA damage and cancer development. 9. Chronic inflammation and cancer Persistent inflammatory signalling promotes angiogenesis, cell proliferation, DNA damage, matrix remodelling and immune evasion. Examples include H. pylori-associated gastric cancer, chronic hepatitis and hepatocellular carcinoma, inflammatory bowel disease and colorectal cancer, and chronic pancreatitis with pancreatic cancer risk. 10. Diagnosis History: duration, infection exposure, autoimmune symptoms, occupation, smoking, drugs and foreign material. Examination: organ-specific signs, lymphadenopathy, hepatosplenomegaly, joint changes, fibrosis and nutritional status. Laboratory: FBC, CRP/ESR, renal/liver function, autoantibodies, cultures, viral tests and targeted metabolic markers. Imaging: ultrasound, CT, MRI, endoscopy, echocardiography or pulmonary function tests for remodelling and fibrosis. Biopsy: identifies cell type, granuloma, fibrosis, organisms, vasculitis and neoplasia. 11. Management principles Eradicate persistent infection with appropriate antimicrobials and source control. Remove foreign bodies and stop toxin exposure, including smoking. Use immunomodulation for proven autoimmune disease after excluding infection. Control metabolic drivers such as obesity, diabetes, dyslipidaemia and hyperuricaemia. Prevent and treat fibrosis complications; monitor organ function and cancer risk. Rehabilitate with nutrition, exercise, vaccination and functional support. 12. Emergency flares Sepsis on chronic liver, lung or kidney disease can cause rapid multi-organ failure. Acute asthma or COPD exacerbation adds neutrophilic/eosinophilic inflammation

Chemical Mediators of Inflammation Sources • receptors • vascular effects • leukocyte recruitment • fever and pain • complement • resolution • therapeutic targets Chemical mediators are short-lived signals that initiate, amplify, coordinate and terminate inflammation. They arise from plasma proteins or activated cells and act locally or systemically. Some are preformed and act within seconds; others are synthesised after stimulation and persist longer. The final response depends on the balance between pro-inflammatory and pro-resolution mediators. Learning outcomes Classify mediators as cell-derived or plasma-derived and preformed or newly synthesised. Explain the actions of histamine, serotonin, eicosanoids, platelet-activating factor, cytokines, chemokines, complement, kinins, nitric oxide, ROS and lysosomal enzymes. Link mediators to redness, swelling, pain, fever, bronchospasm, leukocyte recruitment and shock. Describe inactivation and checks that prevent uncontrolled inflammation. Map common emergency treatments to mediator pathways without confusing mechanism with diagnosis. 1. Classification Category Examples Typical timing Cell-derived, preformed Histamine, serotonin, lysosomal enzymes. Seconds to minutes. Cell-derived, newly synthesised Prostaglandins, leukotrienes, PAF, cytokines, chemokines, NO. Minutes to hours. Plasma-derived Complement, kinins, coagulation/fibrinolytic products. Rapidly activated by cascades. Pro-resolution Lipoxins, resolvins, protectins, maresins, IL-10, TGF-β. Later termination and repair. 2. Histamine Histamine is stored mainly in mast-cell granules and also in basophils and platelets. It is released by IgE cross-linking, complement C3a/C5a, neuropeptides, physical injury and some drugs. Receptor/effect Inflammatory result H1 on endothelium Venular contraction, permeability, itching and bronchoconstriction. H1 on smooth muscle/nerves Urticaria, pain and bronchospasm. H2 Vasodilation and gastric acid secretion; modulates immune cells. Histamine is rapidly degraded by methyltransferase and diamine oxidase. Antihistamines block H1 effects but do not reverse the hypotension or airway compromise of anaphylaxis; adrenaline remains first-line. 3. Serotonin Serotonin is stored in platelets and enterochromaffin cells. Platelet activation, tissue injury and some neuroendocrine tumours release it. It can increase vascular tone or permeability and contribute to pain, nausea and bronchial effects. Its role is generally less dominant than histamine in acute inflammation. 4. Eicosanoids Membrane phospholipids are converted by phospholipase A2 to arachidonic acid. Cyclo-oxygenase (COX-1/COX-2) produces prostaglandins and thromboxane; 5-lipoxygenase produces leukotrienes. Aspirin and NSAIDs inhibit COX; glucocorticoids reduce phospholipase A2 activity and COX-2 expression. Mediator Main source/action Clinical relevance PGE2 Vasodilation, pain sensitisation and fever through hypothalamic set-point change. Fever, pain, dysmenorrhoea; reduced by NSAIDs. PGI2 (prostacyclin) Endothelial vasodilation and inhibition of platelet aggregation. Balances thromboxane; vascular protection. TXA2 Platelet aggregation and vasoconstriction. Thrombosis and platelet haemostasis. LTB4 Potent neutrophil chemotaxis and activation. Airway inflammation and leukocyte recruitment. LTC4, LTD4, LTE4 Bronchoconstriction, increased permeability and mucus secretion. Asthma and anaphylaxis. Lipoxins Inhibit neutrophil recruitment and promote resolution. Endogenous anti-inflammatory pathway. 5. Platelet-activating factor (PAF) PAF is produced by platelets, leukocytes, endothelium and mast cells. It increases platelet aggregation, leukocyte adhesion, chemotaxis, degranulation, bronchoconstriction and vascular permeability. At high concentrations it causes vasodilation, hypotension and shock. It amplifies histamine, eicosanoid and cytokine responses. 6. Cytokines Cytokine Main source Major effects TNF Macrophages, T cells, mast cells. Endothelial activation, fever, leukocyte recruitment, cachexia and shock. IL-1 Macrophages, endothelium. Fever, endothelial adhesion molecules and acute-phase response. IL-6 Macrophages, T cells, endothelium. CRP/fibrinogen production, fever and B-cell support. IL-8/CXCL8 Macrophages, endothelium. Neutrophil chemotaxis and activation. IL-12 Dendritic cells, macrophages. NK/T-cell IFN-γ response against intracellular organisms. IFN-γ Th1 cells, NK cells. Macrophage activation and granulomatous immunity. IL-17 Th17 cells. Neutrophil recruitment and mucosal defence. IL-10 Macrophages, T regulatory cells. Suppresses inflammatory cytokines and supports resolution. 7. Chemokines Chemokines are small chemotactic cytokines. Endothelial and stromal chemokines create gradients that direct leukocytes to tissue. Different chemokines select different cells. CXCL8/IL-8: neutrophils. CCL2/MCP-1: monocytes. CCL11/eotaxin: eosinophils. CXCL13: B-cell and follicular organisation. CCL19/CCL21: lymphocyte and dendritic-cell trafficking. 8. Complement mediators Component Function C3a, C5a Anaphylatoxins; mast-cell degranulation and vascular permeability. C5a Powerful leukocyte chemoattractant and activator; increases adhesion and ROS. C3b/iC3b Opsonisation for phagocytosis. C5b–9 Membrane attack complex; lysis of susceptible microbes and cells. Complement is regulated by soluble and membrane inhibitors. Deficiency predisposes to infection or immune disease; excessive activation contributes to sepsis, vasculitis and reperfusion injury. 9. Kinins and coagulation-linked mediators Bradykinin Bradykinin is generated from high-molecular-weight kininogen by kallikrein. It causes pain, vasodilation, increased permeability and bronchial smooth-muscle contraction. ACE degrades bradykinin; ACE inhibitors can produce cough and, rarely, life-threatening angioedema. Coagulation and fibrinolysis Thrombin activates platelets, endothelium and protease-activated receptors. Fibrin contains infection but can organise into scar. Plasmin degrades fibrin and can activate complement and inflammatory mediators. In sepsis, dysregulated coagulation creates microthrombi and bleeding simultaneously. 10. Nitric oxide Endothelial NOS produces NO for physiologic vasodilation; inducible NOS in macrophages and other cells produces larger amounts during inflammation. NO kills microbes and relaxes smooth muscle, but excess NO contributes to vasoplegia and septic shock. Reactive nitrogen species can combine with superoxide to form peroxynitrite. 11. Reactive oxygen species and lysosomal enzymes Neutrophils and macrophages use ROS and lysosomal enzymes to kill organisms. Extracellular release damages host tissue. ROS: superoxide, hydrogen peroxide, hydroxyl radical and hypochlorous acid. Enzymes: elastase, collagenase, cathepsins, phospholipases and matrix metalloproteinases. Protection: catalase, superoxide dismutase, glutathione, alpha-1-antitrypsin and tissue inhibitors of metalloproteinases. 12. Neuropeptides and alarmins Substance P, CGRP and other neuropeptides increase vasodilation, permeability and pain. ATP, HMGB1, mitochondrial DNA and heat-shock proteins act as alarmins/DAMPs. Their release links nerve activation, tissue damage and innate immunity. 13. Pro-resolution mediators Mediator Resolution function Lipoxins Stop neutrophil recruitment and promote efferocytosis. Resolvins/protectins/maresins Reduce leukocyte infiltration, enhance debris clearance and restore barrier function. IL-10 Suppresses macrophage and dendritic-cell inflammatory cytokines. TGF-β Limits inflammation and stimulates repair/fibrosis. Adenosine Dampens leukocyte activation and promotes vascular regulation. 14. Mediator inactivation and checks Short half-lives and rapid enzymatic degradation. Antioxidants and protease inhibitors neutralise damaging products. Complement regulators prevent runaway cascade activation. Endothelial barrier repair limits ongoing leak. Neutrophil apoptosis and macrophage efferocytosis terminate cellular injury. Lymphatic drainage removes fluid, proteins and cells. 15. Drug–mediator connections Drug/class Mediator pathway affected Important clinical point H1 antihistamines Block histamine H1 effects. Relieve itch/urticaria; not a substitute for adrenaline in anaphylaxis. NSAIDs Inhibit COX and prostaglandin synthesis. Analgesia/antipyresis; GI, renal and cardiovascular risks. Corticosteroids Suppress cytokines, phospholipase A2 and COX-2