Healing of skin wounds is a classic example of how the body restores damaged tissue. Wound repair involves a combination of two processes:
- Regeneration: Replacement of lost cells with the same type of cells.
- Repair: Replacement of damaged tissue with scar tissue (fibrosis).
Skin wounds heal in two main ways:
- Healing by first intention (primary union)
- Healing by second intention (secondary union)
1. Healing by First Intention (Primary Union)
This occurs when:
- The wound is clean and uninfected
- The incision is surgical and neat
- There is minimal tissue loss
- The wound edges are closed with sutures
Sequence of Events in Primary Healing
1) Initial Haemorrhage
Right after injury, blood fills the gap between the wound edges. It forms a clot that closes the wound and protects it from dehydration and infection.
2) Acute Inflammatory Response
Within the first 24 hours, neutrophils appear at the wound. By the 3rd day, macrophages replace neutrophils and begin clearing debris.
3) Epithelial Changes
Basal cells from the edges start dividing and migrate across the wound. A thin epithelial layer forms within 48 hours. By the 5th day, a multilayered new epidermis is formed.
4) Organization (Fibroblast Activity)
By the 3rd day, fibroblasts migrate into the wound. By the 5th day, collagen deposition begins. In 4 weeks, a scar with few cells and vessels forms, covered by new epithelium.
5) Suture Tracks
Each suture itself acts like a mini wound. It undergoes the same sequence—bleeding, inflammation, epithelial proliferation and collagen deposition.
When stitches are removed (around 7 days), small epithelial remnants disappear. Sometimes, stitch abscess or epithelial cysts develop.
Primary intention results in a neat, minimal scar.
2. Healing by Second Intention (Secondary Union)
This occurs when the wound:
- Is open or infected
- Has large tissue loss
- Is not closed with sutures
The steps are similar to primary healing, but the process is slower and the scar is larger because more tissue must be replaced.
Sequence of Events in Secondary Healing
1) Initial Haemorrhage
The wound fills with blood and fibrin, forming a clot that dries on the surface.
2) Inflammatory Phase
Acute inflammation occurs, followed by macrophage activity to remove necrotic material.
3) Epithelial Changes
Epidermal cells migrate from the margins. However, they can only fully cover the surface after granulation tissue begins filling the wound from the base upward.
4) Granulation Tissue Formation
This is the main feature of secondary healing.
Granulation tissue consists of:
- Proliferating fibroblasts
- Newly formed blood vessels (neovascularisation)
It appears red, granular and fragile. As collagen increases and vascularity decreases, the scar becomes pale and firm.
Hair follicles and sweat glands do not regenerate unless their remaining structures survive.
5) Wound Contraction
A key feature of secondary intention. Myofibroblasts in granulation tissue contract the wound, reducing size by 1/3 to 1/4. This helps close large wounds but may deform skin if contraction is excessive.
6) Presence of Infection
Bacterial contamination delays healing due to toxins that cause more necrosis and inflammation. Debridement (removal of dead tissue) is needed to support healing.
Differences Between Primary and Secondary Healing
- Primary healing: minimal tissue loss, quick healing, thin scar
- Secondary healing: large tissue loss, slow healing, wide/ugly scar, contraction present
Factors Influencing Wound Healing
A. Local Factors
- Infection: Most important factor delaying healing.
- Poor blood supply: Slows repair (e.g., leg ulcers heal slowly).
- Foreign bodies: Sutures, dirt or fragments delay healing.
- Movement: Disrupts forming tissue.
- Radiation: Delays granulation tissue formation.
- Ultraviolet light: Speeds up healing.
- Type, size and location: Determines healing pattern.
B. Systemic Factors
- Age: Healing is faster in young people.
- Nutrition: Protein, vitamin C and zinc deficiency slow healing.
- Systemic infection: Slows overall recovery.
- Glucocorticoids: Delay healing due to anti-inflammatory effects.
- Diabetes: Poor circulation and infections delay healing.
- Blood disorders: Neutropenia or bleeding disorders affect wound repair.
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