Overview
- Tissue repair following injury proceeds through overlapping phases of inflammation, cellular proliferation, and extracellular matrix (ECM) remodelling.
- Outcome depends on the nature, extent, and location of injury, and on the regenerative capacity of the affected tissue.
- Where regeneration is insufficient, repair by scar formation (fibrosis) occurs.
- When fibrosis develops within an inflammatory exudate (e.g. pleural or peritoneal space), the process is termed organisation.
- Understanding the mechanisms and pathological deviations of wound healing is essential for recognising clinical complications and interpreting histological findings in surgical pathology specimens.
Phases of Wound Healing
All wound healing proceeds through three interconnected phases:
| Phase | Timing | Key Events |
|---|---|---|
| Inflammation | Hours to days | Coagulation, clot formation, neutrophil then macrophage recruitment, phagocytosis of debris |
| Proliferative (granulation) | Days to weeks | Angiogenesis, fibroblast migration and proliferation, collagen synthesis, re-epithelialisation |
| Remodelling (maturation) | Weeks to months | Collagen cross-linking and increased fibre size, vascular regression, increased tensile strength |
Primary vs Secondary Intention Healing
Healing by Primary Intention (First Intention)
- Applies to wounds with minimal tissue loss and well-apposed edges (e.g. a clean surgical incision approximated with sutures)
- Focal disruption of epithelial basement membrane continuity; relatively few epithelial and connective tissue cells lost
- Within 24-48 hours: basal epidermal cells from both wound edges migrate and proliferate to form a thin but continuous epithelial layer
- Day 3: neutrophils largely replaced by macrophages; granulation tissue begins to invade the incision space; macrophages clear debris and promote angiogenesis and ECM deposition
- Day 5: neovascularisation peaks; granulation tissue fills the incisional space; new vessels are leaky, producing oedematous granulation tissue
- Second week: continued collagen accumulation and fibroblast proliferation; leukocyte infiltrate, oedema, and vascularity substantially diminish
- End of first month: scar composed of acellular connective tissue devoid of inflammatory cells, covered by intact epidermis
- Result: minimal granulation tissue, little contraction, small scar
Healing by Secondary Intention (Second Intention)
- Applies to wounds with significant tissue loss where margins cannot be apposed (e.g. large ulcers, burns, avulsion injuries)
| Feature | Primary Intention | Secondary Intention |
|---|---|---|
| Tissue defect | Minimal | Large |
| Fibrin clot | Small | Large |
| Inflammatory response | Mild | Intense (more necrotic debris/exudate to clear) |
| Granulation tissue | Minimal | Abundant |
| Wound contraction | Absent/minimal | Prominent and essential |
| Scar size | Small, fine | Large, broad |
| Re-epithelialisation | Rapid, minimal migration | Extensive migration required |
- Wound contraction is a hallmark of secondary intention: myofibroblasts (modified fibroblasts expressing $\alpha$-smooth muscle actin with contractile properties) form a network and contract the wound; large skin defects may be reduced to 5-10% of their original size within 6 weeks
- A provisional matrix of fibrin, plasma fibronectin, and type III collagen forms first, then in approximately 2 weeks is replaced by a matrix of predominantly type I collagen
- Granulation tissue scaffold is ultimately converted into a pale, avascular scar; dermal appendages within the wound are permanently lost
- Exaggerated contraction leads to contracture, causing permanent functional deformity (especially after circumferential burns around joints or tubular structures such as gut, the latter causing stenosis/stricture)
Granulation Tissue: Components and Histology
Granulation tissue is the hallmark of repair and must be clearly distinguished from a granuloma (a discrete aggregate of epithelioid histiocytes, a feature of specific chronic inflammatory disorders, with minimal vascularity).
Histological Components
| Component | Role |
|---|---|
| Thin-walled capillary loops (neovascularisation) | Oxygen and nutrient delivery; vessels are leaky, contributing to oedema of new granulation tissue |
| Fibroblasts | Proliferate; synthesise collagen and ECM components |
| Myofibroblasts | Wound contraction; express $\alpha$-smooth muscle actin |
| Alternatively activated (M2) macrophages | Clear debris; secrete TGF-$\beta$, VEGF, and other fibrogenic/angiogenic cytokines |
| Loose ECM (proteoglycans, fibronectin) | Scaffold for cell migration |
| Variable inflammatory infiltrate | Mast cells, lymphocytes, predominantly mononuclear cells in established granulation tissue |
Key Molecular Mediators
- TGF-$\beta$: the most important pro-fibrogenic cytokine; produced by most cells in granulation tissue, especially M2 macrophages; stimulates fibroblast migration and proliferation, increases synthesis of collagen and fibronectin, inhibits MMPs (reducing ECM degradation), and serves as an anti-inflammatory cytokine (inhibits lymphocyte proliferation and leukocyte activity); regulated primarily at the post-transcriptional level (activation of latent TGF-$\beta$) rather than by gene transcription
- VEGF: drives angiogenesis
- PDGF: fibroblast and smooth muscle cell migration and proliferation
- ECM balance: determined by the ratio of metalloproteinases (MMPs, collagenases, gelatinases) to their inhibitors (TIMPs)
- As the scar matures, fibroblasts assume a more synthetic phenotype, ECM deposition increases, and progressive vascular regression transforms vascularised granulation tissue into a pale, largely avascular scar.
- Tensile strength results from excess collagen synthesis over degradation and cross-linking of collagen fibres during the first 2 months.
- Carefully sutured wounds reach approximately 70% of normal skin strength due to suture placement; wound strength reaches approximately 70-80% of normal by 3 months and does not substantially improve beyond that point.
Abnormal Scarring: Keloid vs Hypertrophic Scar
Both represent excessive collagen deposition following dermal injury.
| Feature | Hypertrophic Scar | Keloid |
|---|---|---|
| Definition | Raised scar confined within original wound boundaries | Scar tissue extending beyond original wound margins |
| Regression | Regresses spontaneously over months | Does not regress; may continue to enlarge |
| Typical cause | Thermal or deep dermal traumatic injury | Surgery, burn, or even minor wound |
| Genetic predisposition | Less pronounced | Strong individual predisposition; more prevalent in individuals of African descent |
| Recurrence after excision | Lower | High; excision alone insufficient |
| Histology | Abundant myofibroblasts; organised collagen | Thick, glassy, haphazardly arranged ("keloidal") collagen bundles in dermis |
| Location predilection | Any deep dermal wound | Ear lobes, sternum, shoulders, upper back |
Related Abnormalities
- Exuberant granulation tissue ("proud flesh"): excessive granulation tissue protruding above skin surface, blocking re-epithelialisation; requires removal by cautery or surgical excision
- Desmoid tumour (aggressive fibromatosis): rare exuberant proliferation of fibroblasts and connective tissue elements following incisional or traumatic injury; recurs after excision; lies at the interface between benign and low-grade malignant neoplasia
- Contracture: pathological exaggeration of normal wound contraction; particularly prone to develop on palms, soles, and anterior thorax; commonly seen after serious burns; may compromise joint movement
Organ Fibrosis: Mechanisms and Grading Schemes
General Pathogenesis
- Fibrosis in parenchymal organs shares fundamental mechanisms with dermal scar formation.
- Persistent or repeated injury leads to chronic inflammation, sustained TGF-$\beta$ pathway activation, and progressive ECM deposition replacing functional parenchyma, often with substantial organ dysfunction or failure.
Key fibrogenic effector cells vary by organ:
| Organ | Key Fibrogenic Effector Cell |
|---|---|
| Liver | Hepatic stellate cell (Ito cell), activated by chronic injury |
| Kidney | Interstitial fibroblast / pericyte |
| Lung | Fibroblast / myofibroblast |
Important fibrotic disorders include: liver cirrhosis, systemic sclerosis (scleroderma), idiopathic pulmonary fibrosis (IPF), pneumoconioses, drug- and radiation-induced pulmonary fibrosis, end-stage kidney disease, and constrictive pericarditis.
Hepatic Fibrosis Grading
Multiple validated staging systems exist. Grading refers to degree of necroinflammatory activity; staging refers to degree of fibrosis, both must be reported on liver biopsy.
METAVIR (primarily chronic viral hepatitis):
| Stage | Description |
|---|---|
| F0 | No fibrosis |
| F1 | Portal fibrosis without septa |
| F2 | Portal fibrosis with few septa |
| F3 | Numerous septa without cirrhosis (bridging fibrosis) |
| F4 | Cirrhosis |
Ishak (0-6; finer granularity at bridging/cirrhosis transition):
| Score | Description |
|---|---|
| 0 | No fibrosis |
| 1-2 | Fibrous expansion of some/most portal areas |
| 3-4 | Bridging fibrosis (portal-portal or portal-central) |
| 5 | Marked bridging with occasional nodules |
| 6 | Cirrhosis |
- Specific systems also exist for NAFLD/NASH (NASH CRN/Brunt system) and primary biliary cholangitis.
- Across systems, advanced fibrosis is defined as bridging fibrosis (stage 3) progressing to cirrhosis (stage 4/6), characterised by distortion of normal architecture with regenerative nodule formation, decreased hepatocellular mass, altered blood flow, activation of hepatic stellate cells, and increased collagen/ECM deposition.
- Decompensated cirrhosis is defined by ascites, hepatic encephalopathy, or variceal bleeding.
Pulmonary Fibrosis Grading
- Pulmonary fibrosis (IPF, pneumoconioses, drug-induced, radiation-induced) is assessed histologically on surgical lung biopsy, with HRCT correlation.
- Histological pattern carries independent prognostic weight.
The usual interstitial pneumonia (UIP) pattern (recognised by ATS/ERS/JRS/ALAT guidelines and WHO classification) is most important:
- Temporal and spatial heterogeneity
- Subpleural, basal-predominant distribution
- Honeycombing with or without peripheral traction bronchiectasis
- Fibroblastic foci at the advancing edge of fibrosis (marker of active disease progression; high density predicts more rapid functional decline and shorter survival in IPF)
- Other histological patterns: NSIP, DIP, RB-ILD, organising pneumonia, each with distinct prognostic implications.
- Semi-quantitative biopsy scoring assesses proportion of affected parenchyma, honeycombing, and fibroblastic focus density.
Renal Fibrosis Grading
Interstitial fibrosis and tubular atrophy (IFTA) represent the final common pathway of chronic kidney injury and are the strongest histological predictors of progression to end-stage renal disease.
Banff Classification (transplant biopsies) scores interstitial fibrosis (ci) and tubular atrophy (ct) separately on a 0-3 scale:
| Score | Interstitial Fibrosis (ci) | Tubular Atrophy (ct) |
|---|---|---|
| 0 | ≤5% of cortex | ≤5% of tubules |
| 1 | 6-25% | 6-25% |
| 2 | 26-50% | 26-50% |
| 3 | >50% | >50% |
- For native renal biopsies, IFTA is typically reported as percentage of cortical area: mild (<25%), moderate (25-50%), or severe (>50%).
- IFTA >25% is associated with significantly accelerated progression to end-stage renal failure.
Factors Modifying Wound Healing
| Factor | Effect |
|---|---|
| Infection | Most important cause of delayed healing; prolongs inflammation, increases local tissue injury |
| Diabetes mellitus | Impaired angiogenesis, neuropathy, immune dysfunction, poor collagen synthesis |
| Malnutrition (protein or vitamin C deficiency) | Inhibits collagen synthesis, retards healing |
| Glucocorticoids | Anti-inflammatory; inhibit TGF-$\beta$ production, reduce fibrosis, weaken scar |
| Ischaemia/vascular insufficiency | Reduces oxygen and nutrient delivery |
| Obesity | Increases risk of wound dehiscence |
| Wound dehiscence | Internal or external wound reopening; precipitated by obesity, malnutrition, infection, vomiting, coughing |
Diagnostic Pitfalls
- Granulation tissue vs granuloma: granulation tissue contains capillary loops, fibroblasts, myofibroblasts, and loose ECM in an oedematous stroma; a granuloma is a discrete aggregate of epithelioid histiocytes ± Langhans giant cells with minimal vascularity. These must not be confused, particularly on superficial biopsies
- Keloid vs hypertrophic scar: the key discriminator is whether collagen deposition extends beyond wound margins (keloid) or remains confined (hypertrophic scar); microscopically, keloid collagen is characteristically thick, glassy, and haphazardly arranged
- Fibrosis vs normal fibrous stroma: Masson trichrome and Sirius red stains are essential for delineating collagen deposition in organ biopsies
- Staging vs grading in liver biopsies: grading = degree of necroinflammatory activity; staging = degree of fibrosis; both are complementary and must be reported
- Fibroblastic foci in UIP: must not be mistaken for organising pneumonia; location at the leading edge of established fibrosis and the UIP architectural pattern are discriminating features
- Wound contraction vs contracture: contraction is a normal physiological process reducing wound size; contracture is a pathological exaggeration causing permanent functional deformity
- Organisation vs resolution: in cavitary spaces, small exudates may resolve by proteolytic digestion and resorption (restoring normal architecture); larger accumulations undergo organisation, granulation tissue grows into the exudate and a fibrous scar forms
Clinical Correlates and Prognosis
- Wound tensile strength reaches a plateau of 70-80% of normal by 3 months and does not substantially improve beyond this point
- Keloids carry a high recurrence rate after excision alone; combination therapy (excision + intralesional corticosteroids, radiotherapy, or pressure garments) is required
- Hepatic fibrosis stage is the strongest predictor of liver-related morbidity and mortality in chronic liver disease; stage F3-F4 (bridging fibrosis/cirrhosis) confers significantly increased risk of hepatic decompensation, portal hypertension, and hepatocellular carcinoma
- IFTA >25% on renal biopsy is associated with accelerated progression to end-stage renal failure
- In IPF with UIP pattern, high fibroblastic focus density predicts more rapid decline in lung function and shorter survival
- Anti-fibrotic therapies (targeting TGF-$\beta$, MMP/TIMP balance) represent an active therapeutic frontier across all organ systems