Tissue Repair and Regeneration

• Repair refers to the replacement of damaged tissue with living tissue

Types of Repair

• Regeneration involves replacing destroyed tissue with the same type of cells; bone fracture repair exemplifies this
• Repair by fibrosis replaces destroyed tissue with granulation tissue which then matures into fibrosis

Cell Proliferation Capacity

• Cells are classified based on their proliferative capacity
• Labile cells have a high capacity for continuous proliferation to replace aging cells; examples include stratified squamous epithelium of skin and columnar epithelium of the GIT
• Stable cells typically do not proliferate but will when needed, such as parenchymal cells like liver, pancreas, and renal tubules, also mesenchymal cells like fibroblasts, chondroblasts and osteoblasts
• Permanent cells cannot proliferate; muscle and nerve cells are examples

Granulation Tissue

• A red, granular, soft, and painless tissue that bleeds easily, formed in the gaps of healing wounds
• Consists of fibroblasts surrounded by collagen, newly formed blood vessels, scattered macrophages, and some inflammatory cells
• Excess collagen deposition and obliteration of the capillaries leads to scar formation

Characteristics of Granulation Tissue

• Light red or dark pink due to perfusion with new capillary loops
• Soft to the touch
• Moist and granular in appearance, with punctate hemorrhages
• Pulsatile on palpation
• Painless when healthy

Wound Healing by Primary Intention

• Occurs in clean surgical wounds with minimal tissue destruction, minimal bleeding, no bacterial infection, and good apposition of wound edges
• The initial step involves the wound cavity filling with a blood clot
• Inflammation occurs within 24 hours as PNLs infiltrate the blood clot
• Epithelial cells from wound edges proliferate within 48 hours, leading to epithelial changes
• Early granulation tissue develops by the third day, with macrophages replacing PNLs
• Macrophages stimulate the ingrowth of fibroblasts and angioblasts, beginning the formation of collagen type III, while epidermal cells bridge and seal off the defect
• Fully developed granulation tissue occurs around days 4-6, with neovascularization filling the wound gap
• Scar formation occurs in the second week as inflammation subsides, with fibroblastic proliferation and collagen deposition continuing
• Remodeling of the scar occurs by the end of the first month

Healing by Secondary Intention

• Occurs in gaping wounds characterized by marked tissue destruction, bleeding, infection, and poor apposition of wound edges
• The healing process follows the same steps as primary intention, but with, a greater inflammatory response, and abundant granulation tissue formation
• Epithelization takes longer, excessive fibrosis and wound contraction occur, increasing liability to infection and complications

Factors Affecting Wound Healing (Local)

• Size and site of the wound
• Presence of infection and foreign bodies
• Blood supply to the area: poor blood supply delays repair

Factors Affecting Wound Healing (Systemic)

• Age
• Nutritional disorders like anemia and malnutrition delay repair
• Metabolic disorders
• Chronic debilitating diseases and diabetes increase susceptibility to infection
• Drugs like steroids and cytotoxic drugs delay repair

Complications of Wound Healing

• Infection
• Sinus: A blind duct lined by epithelium between the wound and external surface
• Fistula: A duct lined by epithelium between the wound and a hollow organ
• Ulcer: Loss of continuity of surface epithelium
• Keloid: Excessive granulation tissue and scar formation
• Incisional hernia
• Cicatrization: Scars on flexures interfere with movement
• Implantation (epidermal) cyst
• Malignant transformation: Squamous cell carcinoma