Phases of Wound Healing

Introduction

Wound healing is a remarkably orchestrated biological process that restores tissue integrity following injury. This ancient survival mechanism involves the precise coordination of multiple cell types, molecular signals, and matrix components in a temporal sequence that, when successful, closes wounds and restores function.

Understanding wound healing is essential for dermatologists because disruptions in this process underlie chronic wounds, keloids, hypertrophic scars, and poor surgical outcomes. Moreover, many skin diseases and therapeutic interventions affect wound healing, making this knowledge clinically actionable.

The wound healing cascade is classically divided into four overlapping phases: hemostasis, inflammation, proliferation, and remodeling. Each phase builds upon the previous one, with complex feedback mechanisms ensuring orderly progression.

Overview of Wound Healing Phases

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Phase Overview

Phase	Timing	Key Cells	Key Events	Key Mediators
Hemostasis	0-1 hours	Platelets	Clot formation, vasoconstriction	Thrombin, fibrin, TXA₂
Inflammation	1-7 days	Neutrophils, macrophages	Debridement, pathogen defense	IL-1, TNF-α, IL-6, TGF-β
Proliferation	4-21 days	Keratinocytes, fibroblasts, endothelial cells	Re-epithelialization, granulation tissue, angiogenesis	VEGF, PDGF, FGF, EGF
Remodeling	21 days-2 years	Fibroblasts, myofibroblasts	Collagen maturation, scar formation	TGF-β, MMPs, TIMPs

Phase I: Hemostasis (Minutes to Hours)

Immediate Response to Injury

The moment skin is injured, a cascade of events is triggered to stop bleeding and create a provisional matrix for subsequent healing.

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Platelet-Derived Growth Factors

Platelet alpha granules release critical growth factors that orchestrate subsequent healing phases.

Growth Factor	Source	Target Cells	Function
PDGF	Platelets, macrophages	Fibroblasts, SMCs	Chemotaxis, proliferation
TGF-β	Platelets, macrophages	Multiple	Matrix synthesis, fibrosis
EGF	Platelets, macrophages	Keratinocytes	Re-epithelialization
VEGF	Platelets	Endothelial cells	Angiogenesis
FGF	Platelets	Multiple	Proliferation, angiogenesis

Provisional Matrix

The fibrin clot serves as a provisional matrix—a temporary scaffold that:

Provides structural support
Traps growth factors
Creates a chemotactic gradient for migrating cells
Protects the wound from pathogens

Phase II: Inflammation (Days 1-7)

Purpose of Inflammation

The inflammatory phase serves critical functions:

Debridement: Removal of dead tissue and debris
Pathogen defense: Bacterial killing
Signal amplification: Recruitment of more cells
Transition signaling: Preparing for proliferative phase

Cellular Choreography

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Neutrophil Functions

Function	Mechanism	Products
Phagocytosis	Engulfment of bacteria/debris	—
Oxidative burst	NADPH oxidase	Superoxide, H₂O₂, HOCl
Degranulation	Enzyme release	Elastase, cathepsins, MMPs
NETs formation	DNA extrusion	Neutrophil extracellular traps
Cytokine release	Signaling	IL-1β, IL-6, TNF-α

Macrophage Polarization

Macrophages are the "master regulators" of wound healing, transitioning from pro-inflammatory (M1) to pro-healing (M2) phenotypes.

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Feature	M1 Macrophage	M2 Macrophage
Timing	Days 1-3	Days 3-7+
Inducers	IFN-γ, LPS, TNF-α	IL-4, IL-10, IL-13, TGF-β
Function	Pro-inflammatory	Pro-healing
Products	iNOS, ROS, IL-1, TNF	Arginase, TGF-β, VEGF
Metabolism	Glycolytic	Oxidative phosphorylation
Wound role	Debridement	Resolution, repair

Resolution of Inflammation

The transition from inflammation to proliferation requires active resolution:

Neutrophil apoptosis: Programmed death (not necrosis)
Efferocytosis: Macrophage engulfment of apoptotic neutrophils
Pro-resolving mediators: Lipoxins, resolvins, protectins
M1→M2 switch: Anti-inflammatory reprogramming

Clinical Pearl: Impaired M1→M2 transition is a hallmark of chronic wounds (diabetic ulcers), leading to persistent inflammation and failed healing.

Phase III: Proliferation (Days 4-21)

Overview of Proliferative Events

The proliferation phase involves three concurrent processes:

Re-epithelialization: Closure of epithelial gap
Granulation tissue formation: New connective tissue
Angiogenesis: New blood vessel formation

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Re-epithelialization

Keratinocytes at the wound edge undergo a phenotypic switch to migratory cells.

Event	Mechanism	Key Molecules
Activation	Phenotypic change	EGF, KGF, HGF
Migration	Crawling/leapfrogging	Integrins (α5β1, αvβ6), MMPs
Proliferation	Mitosis behind leading edge	EGF, IGF-1
Differentiation	Stratification resumes	Calcium gradient
Contact inhibition	Stop signal	Cadherin reconnection

Migration Pattern: Epithelial Tongue

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Granulation Tissue

Granulation tissue is the provisional connective tissue that fills the wound—so named for its granular, red appearance clinically.

Component	Source	Function
Fibroblasts	Dermis, bone marrow	Matrix production
Capillaries	Angiogenesis	Oxygen/nutrient delivery
Collagen III	Fibroblasts	Provisional matrix
Fibronectin	Fibroblasts, plasma	Cell migration scaffold
Proteoglycans	Fibroblasts	Hydration, signaling
Inflammatory cells	Circulation	Ongoing signaling

Angiogenesis

New blood vessel formation is essential to supply the metabolically active healing tissue.

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Key Growth Factors in Proliferation

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Phase IV: Remodeling (Day 21 to 2 Years)

Overview

The remodeling (maturation) phase is the longest phase, during which the initial wound matrix is reorganized to approach (but never reach) normal tissue.

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Collagen Remodeling

Feature	Early (Type III)	Late (Type I)
Timing	Days 3-21	Weeks 3 to years
Fiber diameter	Small	Large, thick bundles
Organization	Random	Parallel to stress lines
Cross-linking	Minimal	Extensive (lysyl oxidase)
Tensile strength	15-20% normal	70-80% normal (max)

MMP/TIMP Balance

The remodeling phase requires a delicate balance between matrix metalloproteinases (degradation) and tissue inhibitors of metalloproteinases (preservation).

Factor	Function	Imbalance Effect
MMPs	Collagen degradation	↑ = Chronic wound
TIMPs	MMP inhibition	↓ = Excessive degradation
TGF-β	↑ Collagen synthesis, ↓ MMPs	↑ = Excessive scarring

Scar Formation

Normal wound healing results in a scar—tissue that differs from the original:

Feature	Normal Skin	Scar Tissue
Collagen organization	Basket-weave pattern	Parallel bundles
Tensile strength	100%	70-80% maximum
Appendages	Present (hair, glands)	Absent
Elasticity	Normal	Reduced
Color	Normal	Initially red, then pale

Wound Healing Types

Primary vs. Secondary Intention

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Wound Contraction

In secondary intention healing, wound contraction significantly reduces wound size:

Mechanism	Mediator	Contribution
Myofibroblast contraction	α-SMA	Primary mechanism
Fibroblast traction	Collagen gel compaction	Early contribution
Rate	~0.6-0.75 mm/day	Variable by site
Maximum	40-80% size reduction	Depends on wound shape

Factors Affecting Wound Healing

Local Factors

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Systemic Factors

Factor	Effect on Healing	Mechanism
Diabetes	↓↓ Impaired	Hyperglycemia, neuropathy, vasculopathy
Advanced age	↓ Slower	Decreased cell function, thin skin
Malnutrition	↓↓ Impaired	Protein, vitamin C, zinc deficiency
Immunosuppression	↓ Impaired	Reduced inflammatory phase
Corticosteroids	↓ Impaired	↓ Inflammation, ↓ collagen synthesis
Smoking	↓ Impaired	Vasoconstriction, ↓ oxygen
Obesity	↓ Impaired	↓ Perfusion, ↑ tension
Peripheral vascular disease	↓↓ Impaired	↓ Perfusion, ↓ oxygen

Chronic Wound Environment

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Summary: Wound Healing at a Glance

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Key Clinical Pearls

Topic	Pearl
Hemostasis	Platelet α-granules release growth factors that drive subsequent healing
Inflammation	M1→M2 macrophage transition is critical; failure = chronic wound
Proliferation	Granulation tissue is red and granular; pale tissue suggests ischemia
Re-epithelialization	Keratinocytes migrate over provisional matrix, not exposed dermis
Remodeling	Scar never exceeds 80% normal tensile strength
Moist healing	Moist wound environment promotes faster healing than desiccation
Diabetes	Hyperglycemia impairs neutrophil, macrophage, and fibroblast function
Biofilm	Present in 60-90% of chronic wounds; requires debridement

Cross-References

Volume 03, Chapter 5: Skin Aging - Impaired healing in elderly
Volume 05: Immunology - Inflammatory mediators
Volume 33: Therapeutics - Wound care products
Volume 35: Wound Healing - Clinical management
Volume 36: Procedural Dermatology - Surgical wounds

References

Gurtner GC, et al. Wound repair and regeneration. Nature 2008;453:314-321.
Eming SA, Martin P, Tomic-Canic M. Wound repair and regeneration: mechanisms, signaling, and translation. Sci Transl Med 2014;6:265sr6.
Sindrilaru A, Scharffetter-Kochanek K. Disclosure of the culprits: macrophages—versatile regulators of wound healing. Adv Wound Care 2013;2:357-368.
Werner S, Grose R. Regulation of wound healing by growth factors and cytokines. Physiol Rev 2003;83:835-870.
Diegelmann RF, Evans MC. Wound healing: an overview of acute, fibrotic and delayed healing. Front Biosci 2004;9:283-289.