Biology Chapter on Tissue Repair

Repair Definition

• Repair is the replacement of damaged tissue with a new, healthy one.

Intended Learning Objectives

• Recall the definition of repair.
• List cell types based on their division capabilities.
• Describe different repair types with examples.
• List phases, types, complications, and factors affecting wound healing.

Cell Types Based on Division Power

• Labile cells: Continuously divide and renew themselves. Examples: skin epithelium, mucosal lining of the gastrointestinal tract (GIT), and hematopoietic cells (blood cells).
• Quiescent (Stable) cells: Divide only when needed. Examples: liver, kidney, pancreas, smooth muscle, and fibroblasts.
• Permanent cells: Non-dividing cells. When injured, healing occurs through fibrous or glial tissue (in the central nervous system, or CNS). Examples: nerve cells and skeletal and cardiac muscle cells.

Types of Repair

• Regeneration: Replacement of damaged cells with new, identical cells. This happens through proliferation of adjacent healthy cells. Occurs with minor damage to labile and stable cells. Examples: mild liver injuries and bone fractures.
• Fibrosis or Gliosis: Replacement of damaged tissue with fibrous tissue or glial tissue (in the CNS). This occurs during the healing process of permanent cells or stable cells with significant damage. Examples: myocardial infarction healed by fibrosis; cerebral infarction healed by gliosis.

Wound Healing Phases

• Hemostasis: Immediately after injury, blood vessels bleed into the wound defect, forming a blood clot that temporarily seals the wound. This involves vasoconstriction, platelet aggregation and adherence to damaged endothelium, and stimulation of the coagulation system (forming fibrin). The fibrin network forms a hemostatic plug that stops bleeding.
• Inflammation: Mediated by polymorphs (PMNs) and macrophages. During the first 6-8 hours, PMNs eliminate any pathogens and liquefy necrotic debris. Monocytes (macrophages) also enter the wound to engulf the remaining debris.
• Granulation Tissue Formation and Re-epithelialization: In 5-7 days, fibroblasts migrate into the wound. Angiogenesis (formation of new capillaries) happens from healthy blood vessels at the wound edges. These capillaries fill the wound defect. Newly formed capillaries joined with fibroblasts form granulation tissue. Re-epithelialization is the proliferation and migration of healthy epidermal cells from the wound edges inwards.
• Fibrous Tissue Formation and Remodeling: After 3 weeks, fibroblasts deposit collagen fibers (types I and III). Macrophages secrete collagenase enzymes that degrade excess collagen in a controlled manner (remodeling).
  • Wound Contraction: Myofibroblasts (modified fibroblasts, resembling smooth muscle cells) contract to strengthen the healing wound.

Types of Wound Healing

• Primary union (first intention): Healing of clean, non-gaping wounds (e.g., stitched surgical incisions). The resulting scar is small, thin, regular, and flat.
• Secondary union (second intention): Healing of wounds with extensive tissue loss (e.g., large wounds, infected wounds, abscesses, ulcers). The resulting scar is large, thick, irregular, and elevated.

Factors Affecting Repair

• Local factors: Type of damaged cells (labile, stable, or permanent), severity of damage, presence of foreign bodies, necrotic tissue, infection, irradiation, and blood supply.
• General factors: Age of patient, nutritional status, diseases (diabetes, malignancy, anemia), and drugs (corticosteroids, chemotherapy).

Complications of Wound Healing

• Ulcers: Discontinuity of the surface epithelium (skin or mucous membrane).
• Sinus: Blind end track connecting an abscess cavity to the outside. Examples: pilonidal sinus.
• Fistula: Tract connecting two epithelial surfaces. Examples: perianal fistula.
• Weak atrophic scars: Can lead to hernias.
• Hypertrophic scar: Large scar that does not extend beyond the original wound boundaries.
• Keloid formation: Large scar that grows beyond the original wound boundaries and infiltrates surrounding tissue.
• Wound contracture: Exaggerated wound contraction, potentially causing functional disabilities.

Explore the fundamentals of tissue repair, including the definition, cell types related to their division capabilities, and various repair methods. This quiz will cover the phases, types, complications, and factors that impact wound healing. Test your understanding and application of these vital biological concepts.