7.2 Acute vs Chronic Inflammation & Tissue Repair

7.2 Acute vs Chronic Inflammation & Tissue Repair

Independent study guide by OpenExamPrep.

Core Examination Pearl: Board examinations heavily test the sequential steps and molecular receptors of the leukocyte adhesion cascade (selectins vs. integrins vs. PECAM-1), the biochemical deficiencies in phagocytosis (LAD-1, CGD, Chédiak-Higashi), the arachidonic acid pathway and pharmacologic blockade points, the polarization of M1 vs. M2 macrophages, and the temporal collagen transition (Type III replaced by Type I) and tensile strength recovery milestones during wound healing.

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1. Acute Inflammation: Vascular & Cellular Dynamics

Acute inflammation is the immediate, non-specific response of vascularized tissue to noxious stimuli (infections, trauma, physical/chemical agents, tissue necrosis). It delivers host defense elements—leukocytes and plasma proteins—to extravascular sites of injury.

The Cardinal Signs of Inflammation

• Rubor (Redness) & Calor (Heat): Caused by arteriolar vasodilation and increased local blood flow (hyperemia) mediated primarily by histamine, nitric oxide (NO), and prostaglandins (PGI2, PGE2).
• Tumor (Swelling): Caused by increased microvascular permeability (primarily at the postcapillary venules) allowing protein-rich fluid (exudate) to accumulate within the extravascular interstitial tissue.
• Dolor (Pain): Elicited by direct mechanical tissue stretching combined with chemical stimulation of nociceptors by bradykinin and prostaglandin E2 (PGE2).
• Functio Laesa (Loss of Function): Added by Rudolf Virchow to Celsus's original four signs, reflecting physical immobilization and mechanical compromise from pain and severe edema.

Vascular Events: Permeability & Stasis

1. Transient Vasoconstriction: Lasts only seconds, mediated neurogenically by reflex sympathetic firing and endothelin.
2. Arteriolar Vasodilation: Prompts engorgement of downstream capillary beds (hyperemia).
3. Increased Vascular Permeability: The hallmark of acute inflammation. Endothelial cells in postcapillary venules contract, widening interendothelial gaps (0.5–1.0 μm). Immediate-transient response is mediated by histamine, bradykinin, and cysteinyl leukotrienes (LTC4, LTD4, LTE4).
4. Stasis & Margination: Loss of fluid into tissues concentrates erythrocytes in microvessels, increasing blood viscosity and slowing velocity (stasis). As laminar blood flow decelerates, heavier leukocytes fall out of the central axial stream and push toward the endothelial periphery (margination).

+-----------------------------------------------------------------------------------------+
|                        THE LEUKOCYTE EXTRAVASATION CASCADE                              |
+-----------------------+-----------------------------+-----------------------------------+
| Phase                 | Endothelial / ECM Molecule  | Leukocyte Counter-Receptor        |
+-----------------------+-----------------------------+-----------------------------------+
| 1. Margination &      | • P-selectin (stored in     | • Sialyl-Lewis X modified         |
|    Rolling            |   Weibel-Palade bodies)     |   glycoproteins (PSGL-1)          |
|                       | • E-selectin (induced by    | • L-selectin (shed from leukocyte)|
|                       |   TNF, IL-1)                |                                   |
+-----------------------+-----------------------------+-----------------------------------+
| 2. Tight Adhesion &   | • ICAM-1 (intercellular     | • LFA-1 (CD11a/CD18 integrin)     |
|    Activation         |   adhesion molecule-1)      | • Mac-1 (CD11b/CD18 integrin)     |
|                       | • VCAM-1 (vascular CAM-1)   | • VLA-4 (alpha-4/beta-1 integrin) |
+-----------------------+-----------------------------+-----------------------------------+
| 3. Diapedesis         | • PECAM-1 (CD31) at         | • PECAM-1 (CD31) on leukocyte     |
|    (Transmigration)   |   endothelial junctions     |   pseudopod                       |
+-----------------------+-----------------------------+-----------------------------------+
| 4. Chemotaxis         | • Extracellular matrix      | • G-protein coupled receptors for:|
|                       |   gradient: LTB4, C5a,      |   LTB4, C5a, IL-8, bacterial      |
|                       |   IL-8, formyl peptides     |   N-formylmethionine (fMLP)       |
+-----------------------+-----------------------------+-----------------------------------+

The Leukocyte Extravasation Cascade

• 1. Rolling: Low-affinity, transient binding of leukocytes to endothelial cells mediated by selectins. Leukocytes slow down and roll along the endothelial surface.
  • P-selectin: Stored pre-formed in endothelial Weibel-Palade bodies and platelet α-granules. Rapidly redistributed to the luminal surface within minutes upon exposure to histamine or thrombin.
  • E-selectin: Synthesized de novo by endothelial cells following stimulation by pro-inflammatory cytokines TNF-alpha and IL-1 (peaks at 4–6 hours).
  • L-selectin: Expressed constitutively on leukocyte microvilli; binds CD34 and GlyCAM-1 on endothelium.
  • Counter-Receptors: Leukocytes express carbohydrate ligands decorated with Sialyl-Lewis X (e.g., PSGL-1).
• 2. Tight (Firm) Adhesion: Leukocytes stop rolling and adhere firmly to the endothelial wall. Mediated by integrins on the leukocyte surface interacting with immunoglobulin superfamily ligands on the activated endothelium.
  • Endothelial ligands: ICAM-1 (Intercellular Adhesion Molecule-1) and VCAM-1 (Vascular Cell Adhesion Molecule-1), upregulated by TNF-alpha and IL-1.
  • Leukocyte integrins: β2-integrins LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18), as well as β1-integrin VLA-4 (CD49d/CD29). Chemokines displayed on the endothelial lumen activate inside-out signaling, converting integrins from a bent (low-affinity) to an extended (high-affinity) conformation.
• 3. Diapedesis (Transmigration): Leukocytes squeeze through widened interendothelial junctions into the extravascular connective tissue. Driven primarily by homotypic binding of PECAM-1 (Platelet Endothelial Cell Adhesion Molecule-1 / CD31) expressed on both the leukocyte and the endothelial junction. Leukocytes secrete collagenases/elastases to pierce the vascular basement membrane.
• 4. Chemotaxis: Locomotion along a chemical concentration gradient toward the site of tissue injury. Leukocytes extend filopodia and actin pseudopods in response to chemoattractants binding G-protein coupled receptors (GPCRs):
  • Exogenous Chemoattractants: Bacterial products, particularly peptides containing N-formylmethionine (fMLP).
  • Endogenous Chemoattractants: Complement fragment C5a, leukotriene LTB4, chemokine IL-8 (CXCL8), and platelet-activating factor (PAF).

High-Yield Board Genetic Defects in Leukocyte Function

1. Leukocyte Adhesion Deficiency Type 1 (LAD-1):
   • Genetics: Autosomal recessive mutation in the ITGB2 gene encoding the CD18 β2-integrin subunit (shared by LFA-1 and Mac-1).
   • Pathogenesis: Defective leukocyte tight adhesion to endothelium and impaired phagocytic binding to C3b opsonized targets.
   • Clinical Triad: 1. Delayed separation of the umbilical cord (>30 days; normal is 1–2 weeks), 2. Recurrent severe bacterial and fungal infections of skin and mucosal surfaces without pus formation (neutrophils cannot enter tissues), 3. Marked peripheral blood leukocytosis / neutrophilia (>30,000–100,000/μL) because neutrophils cannot adhere to marginal pools and remain trapped in the circulation.
2. Chédiak-Higashi Syndrome:
   • Genetics: Autosomal recessive mutation in the LYST (lysosomal trafficking regulator) gene, causing defective vesicle fusion and microtubule assembly.
   • Pathogenesis: Neutrophils cannot fuse phagosomes with primary lysosomes; impaired degranulation and defective microbial killing. Melanocytes cannot properly distribute melanosomes.
   • Clinical Hallmarks: Recurrent pyogenic infections, partial oculocutaneous albinism, progressive peripheral neuropathy, and pathognomonic giant azurophilic lysosomal granules visible in neutrophils on peripheral blood smear.
3. Chronic Granulomatous Disease (CGD):
   • Genetics: Most commonly X-linked recessive (~70%) mutation in the CYBB gene encoding the gp91phox subunit of the NADPH oxidase enzyme complex.
   • Pathogenesis: Complete inability to generate superoxide radicals (O2•-) during the respiratory burst; neutrophils can ingest bacteria but cannot produce hypochlorite (bleach) to kill them.
   • Microbiology: Patients are susceptible to recurrent infections by catalase-positive organisms (Staphylococcus aureus, Pseudomonas aeruginosa, Serratia marcescens, Burkholderia cepacia, Nocardia, and Aspergillus). These organisms produce catalase that breaks down host/bacterial H2O2, denying the cell any source of peroxide for MPO.
   • Diagnostics: Abnormal Dihydrorhodamine (DHR) flow cytometry (absence of green fluorescence due to lack of oxidative burst) or negative Nitroblue Tetrazolium (NBT) dye reduction test (fails to turn blue).

Temporal Kinetics of Cellular Infiltration

• First 6 to 24 Hours: Neutrophils (Polymorphonuclear Leukocytes - PMNs) predominate. Neutrophils are the most abundant leukocyte in peripheral blood, respond rapidly to chemokines, and bind firmly to selectins and integrins. However, they are short-lived in tissues, undergoing apoptosis within 24–48 hours.
• 24 to 48 Hours & Beyond: Monocytes / Macrophages replace neutrophils. Monocytes survive for weeks to months in tissue, actively phagocytose necrotic debris and apoptotic neutrophils, produce nitric oxide and growth factors, and direct the transition from active inflammation to tissue repair.

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2. Chemical Mediators of Inflammation

Chemical mediators originate either from plasma (synthesized in the liver as inactive precursors activated by proteolytic cascades) or from local cells (stored in secretory granules or synthesized de novo upon stimulation).

+-----------------------------------------------------------------------------------------+
|                         THE ARACHIDONIC ACID PATHWAY CASCADE                            |
+-----------------------------------------------------------------------------------------+
|                                 Membrane Phospholipids                                  |
|                                            |                                            |
|                            [Inhibited by Corticosteroids]                               |
|                                            v (Phospholipase A2)                         |
|                                     Arachidonic Acid                                    |
|                                            |                                            |
|                +---------------------------+---------------------------+                |
|                |                                                       |                |
|                v [Cyclooxygenase (COX-1/COX-2)]                        v [5-Lipoxygenase]|
|      Prostaglandin G2 / H2                                       5-HPETE                |
|                |                                                       |                |
|     +----------+----------+----------+                         +-------+-------+        |
|     |          |          |          |                         |               |        |
|     v          v          v          v                         v               v        |
|   PGI2        TXA2       PGE2       PGD2                     LTB4          LTC4/D4/E4   |
| (Prostacyclin)(Thromboxane)(Pain,   (Vasodilation,         (Neutrophil    (SRS-A: severe|
| Vasodilation, Vasoconstr,   Fever,  Permeability)          Chemotaxis,     Bronchospasm,|
| Inhibit Plt)  Plt Aggreg)   Vasodil)                       Activation)    Permeability) |
+-----------------------------------------------------------------------------------------+

1. The Arachidonic Acid (Eicosanoid) Metabolites

Arachidonic acid (a 20-carbon polyunsaturated fatty acid) is esterified in membrane phospholipids. Upon stimulation, it is cleaved by Phospholipase A2 (PLA2).

• Corticosteroid Blockade: Systemic and local corticosteroids (e.g., dexamethasone, triamcinolone, prednisone) induce the synthesis of lipocortin (annexin A1), which directly inhibits Phospholipase A2, shutting down both the cyclooxygenase and lipoxygenase pathways.
• Cyclooxygenase (COX-1 & COX-2) Pathway:
  • Prostacyclin (PGI2): Produced by vascular endothelium. Causes potent vasodilation and inhibits platelet aggregation.
  • Thromboxane A2 (TXA2): Produced by platelets via thromboxane synthase. Causes potent vasoconstriction and promotes platelet aggregation.
  • Prostaglandin E2 (PGE2): Produced by macrophages and endothelial cells. Mediates fever (crosses blood-brain barrier to trigger hypothalamic thermoregulation), sensitizes sensory nerve endings to pain (hyperalgesia), and causes vasodilation.
  • Pharmacologic Inhibition: Aspirin irreversibly acetylates both COX-1 and COX-2; non-steroidal anti-inflammatory drugs (NSAIDs such as ibuprofen, naproxen, indomethacin) cause reversible competitive inhibition of COX-1 and COX-2; Celecoxib selectively inhibits COX-2.
• 5-Lipoxygenase (5-LOX) Pathway:
  • Leukotriene B4 (LTB4): Potent chemoattractant and activator of neutrophils; stimulates neutrophil degranulation and ROS release ('Neutrophils B-line to LTB4').
  • Cysteinyl Leukotrienes (LTC4, LTD4, LTE4): Known collectively as the Slow-Reacting Substance of Anaphylaxis (SRS-A). Cause intense smooth muscle contraction (severe bronchospasm in asthma), intense vasoconstriction, and markedly increased vascular permeability (up to 1,000 times more potent than histamine).

2. Cytokines & Chemokines

• Tumor Necrosis Factor-alpha (TNF-alpha) & Interleukin-1 (IL-1): Master pro-inflammatory cytokines produced primarily by activated macrophages and dendritic cells.
  • Endothelial Activation: Upregulate E-selectin, P-selectin, ICAM-1, and VCAM-1; induce production of chemokines (IL-8) and procoagulant tissue factor.
  • Systemic Acute-Phase Response: Travel via circulation to hypothalamus to induce fever; stimulate hepatic production of acute-phase reactants (CRP, fibrinogen, serum amyloid A).
  • Sustained Exposure: Prolonged high levels drive systemic vasodilation, myocardial depression, and septic shock.
• Interleukin-6 (IL-6): Synthesized by macrophages and T cells. Acts on hepatocytes to stimulate the primary synthesis of C-Reactive Protein (CRP) and fibrinogen (which coats red cells, accelerating erythrocyte sedimentation rate [ESR]).
• Interleukin-8 (IL-8 / CXCL8): The primary chemokine recruiting neutrophils to the site of infection ('Clean up on aisle 8').

3. Vasoactive Amines & Kinins

• Histamine: Stored pre-formed in granules of mast cells, basophils, and platelets. Released in response to physical trauma, cold/heat, IgE-antigen binding to FcεRI, and anaphylatoxins (C3a, C5a). Binds H1 receptors on microvascular endothelium, causing rapid arteriolar vasodilation and opening interendothelial junctions in postcapillary venules.
• Bradykinin: A nonapeptide cleaved from high-molecular-weight kininogen (HMWK) by the plasma serine protease kallikrein (activated by Factor XII / Hageman factor). Causes arteriolar vasodilation, increased vascular permeability, extravascular smooth muscle contraction, and intense pain (dolor) by activating sensory pain fibers.

4. The Complement System

• C3a & C5a (Anaphylatoxins): Trigger mast cell degranulation, causing histamine release, vasodilation, and increased permeability.
• C5a: Potent chemoattractant for neutrophils, monocytes, eosinophils, and basophils; stimulates oxidative burst.
• C3b: Major opsonin; coats bacterial surfaces to facilitate phagocytic recognition via macrophage CR1 receptors.
• C5b-9 (Membrane Attack Complex - MAC): Assembles a lytic transmembrane pore causing osmotic cell lysis (critical defense against Neisseria species).

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3. Chronic & Granulomatous Inflammation

Chronic inflammation is inflammation of prolonged duration (weeks, months, to years) in which active inflammation, tissue destruction, and attempts at repair proceed simultaneously.

Cellular Hallmarks of Chronic Inflammation

Unlike acute inflammation (dominated by neutrophils and vascular fluid leakage), chronic inflammation is histologically characterized by:

1. Infiltration by mononuclear leukocytes: macrophages, lymphocytes (T and B cells), and plasma cells.
2. Tissue Destruction: Driven largely by products of persistently activated macrophages and lymphocytes.
3. Attempts at Healing: Characterized by connective tissue replacement, angiogenesis (granulation tissue), and progressive fibrosis (scarring).

Macrophage Polarization: M1 vs. M2 Pathways

Monocytes extravasate into tissues and differentiate into macrophages, which undergo distinct functional polarization depending on microenvironmental signals:

+-----------------------------------------------------------------------------------------+
|                        MACROPHAGE POLARIZATION: M1 vs. M2                               |
+-----------------------+-----------------------------+-----------------------------------+
| Feature               | M1 (Classically Activated)  | M2 (Alternatively Activated)      |
+-----------------------+-----------------------------+-----------------------------------+
| Inducing Signals      | • Interferon-gamma (IFN-g)  | • Interleukin-4 (IL-4)            |
|                       | • Microbial LPS (endotoxin) | • Interleukin-13 (IL-13)          |
| Primary Effector      | • Reactive Oxygen Species   | • Arginase (produces proline      |
| Molecules             | • Nitric Oxide (iNOS)       |   for collagen synthesis)         |
|                       | • Lysosomal proteases       | • TGF-beta (fibrogenesis)         |
|                       | • IL-1, IL-6, IL-12, TNF    | • VEGF, FGF (angiogenesis)        |
| Biological Function   | Microbicidal host defense,  | Anti-inflammatory resolution,     |
|                       | pro-inflammatory killing,   | tissue repair, matrix deposition, |
|                       | pathological tissue damage  | wound remodeling, fibrosis        |
+-----------------------+-----------------------------+-----------------------------------+

Granulomatous Inflammation

Granulomatous inflammation is a distinctive, specialized pattern of chronic inflammation characterized by microscopic aggregates of epithelioid histiocytes (activated macrophages with abundant pale pink cytoplasm and elongated, vesicular nuclei resembling epithelial cells) surrounded by a collar of lymphocytes and plasma cells.

• Multinucleated Giant Cells: Formed by the cell fusion of multiple epithelioid macrophages.

  • Langhans Giant Cells: Nuclei are arranged in a regular peripheral horseshoe or crescentic pattern along the outer cytoplasmic rim. Classically associated with mycobacterial and fungal granulomas.
  • Foreign Body Giant Cells: Nuclei are haphazardly and irregularly distributed throughout the center and cytoplasm. Characteristic of non-immune foreign body reactions.

• Caseating vs. Non-Caseating Granulomas:

  • Caseating Granulomas: Exhibit a central zone of acellular, necrotic, friable debris with loss of cellular detail. Pathognomonic for Tuberculosis (Mycobacterium tuberculosis, confirmed via acid-fast Ziehl-Neelsen or Fite stain) and systemic deep fungal infections (Histoplasma, Coccidioides).
  • Non-Caseating Granulomas: Lack central necrosis; composed entirely of viable epithelioid histiocytes, giant cells, and lymphocytes. Classic for Sarcoidosis (non-caseating granulomas containing Schaumann bodies and asteroid bodies), Crohn's disease, Berylliosis, and foreign body granulomas.
  • Podiatric Foreign Body Granuloma: Common in the plantar foot following penetrating puncture wounds from sewing needles, retained suture material (nylon, braided silk), wooden splinters, or sea urchin spines. Macrophages surround the foreign material, which often exhibits birefringence under polarized light microscopy.

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4. Cutaneous Wound Healing & Tissue Repair

Tissue repair following injury proceeds through two competing pathways: regeneration (re-population of damaged tissue by identical parenchymal cells; possible in labile and stable tissues with intact extracellular matrix scaffolds) and repair by connective tissue replacement / scar formation (fibrous tissue deposition; occurs when supporting stroma is damaged or in permanent non-dividing tissues).

+-----------------------------------------------------------------------------------------+
|                        THE FOUR PHASES OF CUTANEOUS WOUND HEALING                       |
+-----------------------+-----------------------------+-----------------------------------+
| Phase                 | Temporal Window             | Dominant Cells & Molecular Events |
+-----------------------+-----------------------------+-----------------------------------+
| 1. Hemostasis         | Immediate (0 to hours)      | • Platelet plug & fibrin clot     |
|                       |                             | • Release of PDGF, TGF-b, PF4     |
+-----------------------+-----------------------------+-----------------------------------+
| 2. Inflammatory Phase | Day 1 to Day 3              | • Neutrophils (peak at 24 h)      |
|                       |                             | • Macrophages (peak at 48–72 h)   |
|                       |                             | • Phagocytosis & debridement      |
+-----------------------+-----------------------------+-----------------------------------+
| 3. Proliferative Phase| Day 3 to Week 3             | • Angiogenesis (VEGF)             |
|                       |                             | • Granulation tissue (Type III col)|
|                       |                             | • Epithelialization (keratinocyte)|
|                       |                             | • Wound contraction (myofibroblast|
+-----------------------+-----------------------------+-----------------------------------+
| 4. Remodeling Phase   | Week 3 to 12+ Months        | • Type III replaced by Type I col |
|                       |                             | • Zinc-dependent MMP collagenases |
|                       |                             | • Tensile strength reaches 70–80% |
+-----------------------+-----------------------------+-----------------------------------+

The Four Chronological Phases of Wound Healing

1. Hemostasis (Immediate to Hours)

• Disrupted blood vessels trigger reflex vasoconstriction, platelet adherence to exposed subendothelial von Willebrand factor (vWF) and collagen, and activation of the extrinsic coagulation cascade.
• The resulting fibrin clot stops hemorrhage and establishes a provisional extracellular matrix scaffold.
• Degranulating platelets release critical mitogenic cytokines into the wound bed: Platelet-Derived Growth Factor (PDGF) and Transforming Growth Factor-β (TGF-β), which act as potent chemoattractants for circulating neutrophils, monocytes, and fibroblasts.

2. Inflammatory Phase (Day 1 to 3)

• Neutrophils enter within hours, undergoing the extravasation cascade to phagocytose contaminating bacteria and necrotic cell debris.
• At 24–48 hours, circulating monocytes extravasate and differentiate into tissue macrophages. Macrophages act as the master orchestrators of repair: they debride the wound, phagocytose apoptotic neutrophils, and transition from M1 to M2 phenotype, releasing growth factors (VEGF, FGF-2, PDGF, TGF-β, and EGF) that initiate the proliferative phase.

3. Proliferative Phase (Day 3 to Week 3)

• Angiogenesis: Driven by Vascular Endothelial Growth Factor (VEGF) and basic Fibroblast Growth Factor (bFGF), endothelial cells bud from pre-existing venules, creating dense, leaky capillary networks.
• Granulation Tissue Formation: The hallmark histological feature of the proliferative phase. Characterized grossly as soft, pink, fleshy, granular tissue that bleeds easily on contact. Microscopically consists of proliferating new capillary loops embedded in an edematous extracellular matrix rich in hyaluronic acid and newly synthesized Type III collagen.
• Fibroplasia: Fibroblasts migrate into the wound bed in response to PDGF and TGF-β, synthesizing amorphous ground substance and immature Type III collagen.
• Re-Epithelialization: Driven by Epidermal Growth Factor (EGF) and Keratinocyte Growth Factor (KGF), epidermal keratinocytes at the wound margins detach, migrate across the provisional wound matrix, and proliferate until contact inhibition halts migration upon monolayer confluence.
• Wound Contraction: Mediated by myofibroblasts (differentiated fibroblasts containing intracellular α-smooth muscle actin bundles). Myofibroblasts attach to extracellular fibronectin fibrils and contract, pulling the opposing edges of the wound toward the center. Wound contraction is especially prominent in large defects healing by secondary intention.

4. Remodeling / Maturation Phase (Week 3 to 12+ Months)

• Collagen Transition (Type III to Type I): The immature, slender, mechanically weak Type III collagen deposited during the proliferative phase is systematically degraded by zinc-dependent Matrix Metalloproteinases (MMPs / collagenases) produced by fibroblasts and macrophages.
• Fibroblasts simultaneously synthesize robust, thick, highly cross-linked bundles of Type I collagen (the dominant collagen of mature dermis, bone, and tendon).
• Recovery of Tensile Strength:
  • At 1 week (at time of surgical suture removal): Tensile strength is approximately 10% of unwounded skin.
  • At 3 weeks: Tensile strength reaches approximately 20%.
  • At 3 months: Tensile strength plateaus at approximately 70% to 80% of original intact tissue strength.
  • A healed scar never recovers 100% of the tensile strength of native, unwounded skin!

Primary Intention vs. Secondary Intention

• Healing by Primary Intention (First Intention): Occurs in clean, uninfected surgical incisions where the wound edges are neatly approximated with sutures, staples, or adhesive tape. Minimal tissue loss occurs, requiring minimal granulation tissue. Re-epithelialization is complete within 24–48 hours, yielding a narrow, fine, linear scar with minimal wound contraction.
• Healing by Secondary Intention: Occurs in extensive, gaping wounds with separated edges, tissue loss, avascularity, or active bacterial contamination (e.g., debrided diabetic plantar ulcers, severe crush injuries, open fasciotomy sites). The wound cannot be approximated and must heal from the base up. Characterized by extensive necrotic debridement, exuberant amounts of granulation tissue, and marked wound contraction mediated by myofibroblasts, resulting in substantial scarring and tissue distortion.

Aberrant Wound Healing: Hypertrophic Scar vs. Keloid

+-----------------------------------------------------------------------------------------+
|                        HYPERTROPHIC SCAR vs. KELOID DIFFERENTIATION                     |
+-----------------------+-----------------------------+-----------------------------------+
| Clinical Feature      | Hypertrophic Scar           | Keloid                            |
+-----------------------+-----------------------------+-----------------------------------+
| Boundary Extension    | Remains strictly **CONFINED**| **EXTENDS BEYOND** the original   |
|                       | within the original borders | wound borders into normal tissue  |
| Collagen Organization | Parallel, organized bundles | Thick, haphazard, disorganized    |
|                       | of Type III / Type I collagn| hyalinized 'keloid collagen'      |
| Collagen Synthesis    | Increased collagen synthesis| Massively increased (up to 20x);  |
|                       | balanced by degradation     | dysregulated TGF-b signaling      |
| Onset & Progression   | Develops rapidly (weeks);   | Appears late (months to years);   |
|                       | often regresses over time   | progressive, permanent enlargement|
| Genetic Predilection  | No specific ethnic bias;    | Marked predilection in dark skin  |
|                       | common across tension lines | (African, Asian, Hispanic descent)|
| Anatomic Sites        | Flexor surfaces, joints     | Earlobes, sternum, deltoid, feet  |
| Recurrence Post-Op    | **Low recurrence rate**     | **Extremely high recurrence rate**|
| Excision              | following surgical revision | (often recurs larger if excised)  |
+-----------------------+-----------------------------+-----------------------------------+