Healing Processes and Cell Types

Healing Definition

• Healing is the body's replacement of damaged tissue with living tissue.

Types of Healing Processes

• Regeneration: Replacement of lost tissue with similar tissue types.
• Repair (Healing by Scarring): Replacement of lost tissue by granulation tissue, eventually forming a scar.

Wound Healing Process Steps

• Tissue Damage → Inflammation → Removal of Dead Tissue & Injury Agent → Replacement:
  • Replacement occurs by either specialized tissues (regeneration) or fibrous tissue (scarring).
  • This leads to healing.

Cell Types Based on Proliferative Capacity

• Labile Cells: Continuously turn over through stem cell division. Found in the surface epithelium of the gastrointestinal tract, urinary tract, and skin.
• Stable Cells: Have a lower replication rate than labile cells but can rapidly divide in response to injury. Examples include liver cells, endocrine glands, and renal tubular epithelium.
• Permanent Cells: Non-dividing cells. Replacement not possible if lost. Examples include adult neurons and lens cells.

Regeneration Process

• Proliferation: Division of surviving cells to replace lost tissue.
• Migration: Movement of surviving cells into the vacant space.

Factors Affecting Tissue Regeneration

• Proliferative Ability: The tissue's capacity for cell division.
• Damage to Stromal Framework: The structural support of the tissue.
• Type and Severity of Damage: The nature and extent of the injury.

Repair (Healing by Scarring) Details

• Repair involves the orderly replacement of lost tissue with a scar.

Wound Contraction

• Wound contraction is a mechanical process that reduces the size of a wound.
• Wounds contract by 70-80% of their original size.
• This process speeds up healing by decreasing the amount of damaged tissue needing replacement.
• Preventing contraction leads to slow healing & a large scar.

Types of Wound Healing

• Healing by First Intention (Primary Union):
  • Occurs when a wound is clean, closed with sutures, and edges are approximate.
  • The incision site is immediately filled, sealed and the area heals relatively quickly.
  • A scab is often formed.
• Healing by Second Intention (Secondary Union):
  • Occurs with extensive tissue loss/damage.
  • Large defects are created. Examples include infarctions, ulcers, abscesses, and severe surface wounds.
  • Healing is more drawn out.

Local Factors Affecting Wound Healing

• Wound Type, Size, and Location: Clean surgical incisions heal faster than wounds caused by blunt trauma.
• Vascular Supply: Adequate blood flow is crucial for delivering oxygen and nutrients to the healing area.
• Infection: Infections significantly delay or prevent healing.
• Ionizing Radiation: Radiation harms tissue and slows down the healing process.

Systemic Factors Affecting Wound Healing

• Circulatory Status: Poor circulation reduces the supply of nutrients and oxygen to the injured area.
• Metabolic Status: Poorly controlled diabetes impairs blood circulation and sensation and delays healing. Diseases and conditions can disrupt the wound healing process.
• Nutritional Deficiencies: Poor nutrition reduces healing speed and impairs wound repair processes.
• Hormones (Corticosteroids): Corticosteroids can hinder wound healing. Other hormones such as thyroid, androgens, estrogens, and growth hormone influence wound healing as well.

Complications of Wound Healing

• Infection: Bacteria entering the wound site can cause delays, or stop healing completely.
• Deficient Scar Formation: Results in conditions such as wound dehiscence (wound gap), incisions hernias, and ulcerations.
• Excessive Scar Formation: Results in raised scars (keloids) which can recur periodically. Hypertrophic scars are similar to keloids, but do not recur after surgical excision.

Bone Regeneration (Stages in Fracture Healing)

• Stage 1: Hematoma formation
• Stage 2: Inflammation
• Stage 3: Demolition (removal of dead tissue)
• Stage 4: Formation of granulation tissue
• Stage 5: Woven bone and cartilage formation
• Stage 6: Formation of lamellar bone
• Stage 7: Remodeling