Tissue Repair and Cell Proliferation

• Repair is the replacement of damaged tissue by living tissue.

Types of Repair

• Regeneration replaces destroyed tissue with the same type of cells; an example is bone fracture repair.
• Repair by fibrosis replaces destroyed tissue with granulation tissue, that matures into fibrosis.

Cell Proliferation Capacity

• Classified by proliferative capacity.

Labile Cells

• These cells have a good, continuous proliferation ability for replacing aging cells.
• Examples of labile cells include stratified squamous epithelium of skin and columnar epithelium of the GIT (gastrointestinal tract).

Stable Cells

• Stable cells do not proliferate under normal conditions.
• When needed stable cells can proliferate under certain conditions.
• Examples are parenchymatous cells (liver, pancreas, renal tubules) and mesenchymal cells (fibroblasts, chondroblasts, osteoblasts).

Permanent Cells

• These cells cannot proliferate.
• Examples include muscle and nerve cells.

Granulation Tissue

• Described as red, granular, soft, painless, and prone to bleeding.
• Granulation tissue fills gaps in healing wounds.
• Collagen surrounds fibroblasts, is composed of newly formed blood vessels, scattered macrophages, and some inflammatory cells.
• Scar formation occurs after excessive collagen deposition and capillary obliteration.

Characteristics of Granulation Tissue

• Light red to dark pink from new capillary loops.
• Soft to the touch.
• Moist and granular in appearance because of punctate hemorrhages.
• Pulsatile on palpation.
• Normally painless if healthy.

Wound Healing by Primary Intention

• Occurs in clean surgical wounds with minimal tissue destruction, minimal bleeding, and no bacterial infection.
• Also involves good apposition of wound edges.

Steps of Healing by Primary Intention

• Initial hemorrhage fills the wound cavity with a blood clot.
• Inflammation occurs at 24 hours: PNLs (polymorphonuclear leukocytes) infiltrate the blood clot.
• Epithelial changes occur at 48 hours when epithelial cells from wound edges proliferate
• Early granulation tissue forms on the 3rd day when macrophages replace PNLs.
• Macrophages stimulate ingrowth of fibroblasts and angioblasts, initiating collagen type III formation, and epidermal cells bridge and seal off the defect.
• Fully developed granulation tissue appears between 4–6 days, marked by neovascularization, and the tissue fills the wound gap.
• Scar formation takes place in the 2nd week as inflammation subsides, fibroblastic proliferation continues, and collagen deposition continues.
• Remodeling of the scar happens by the end of the 1st month.

Healing by Secondary Intention

• Occurs in gaping wounds characterized by marked tissue destruction, bleeding, infection, and poor apposition of wound edges.
• Has the same steps as healing by primary intention.
• Has a heightened inflammatory response and abundant granulation tissue formation.
• Epithelization takes longer based on the wound gap.
• Results in excessive fibrosis, wound contraction, and a higher risk of infection and complications.

Factors Affecting Wound Healing: Local

• These factors include the size and site of the wound, presence of infection or foreign bodies, and blood supply to the area.

Factors Affecting Wound Healing: Systemic

• Systemic factors include age, nutritional disorders, metabolic disorders, chronic debilitating diseases (such as diabetes), and drug use (steroids and cytotoxic drugs).

Complications of Wound Healing

• Infection
• Sinus formation: a blind duct lined by epithelium between the wound and external surface.
• Fistula formation: a duct lined by epithelium between the wound and a hollow organ.
• Ulcer: Loss of surface epithelium continuity.
• Keloid: Excessive granulation tissue and scar formation.
• Incisional hernias and cicatrization where scars on flexures interfere with movement.
• Implantation (epidermal) cysts.
• Possible malignant transformation into squamous cell carcinoma.