Tissue Repair (1)

Wound Healing and Tissue Repair

Objectives of the Lecture

  • Understanding features of wound healing, including:

    • Cells involved

    • Regenerating capacity of cells

    • Mechanisms of healing

    • Complications

    • Factors affecting wound healing

    • Process of wound healing in the skin

Learning Outcomes

At the end of this lecture, students will be able to:

  1. Describe terms regeneration and repair.

  2. Classify cells according to their proliferative capacity.

  3. Explain steps in tissue repair by connective tissue.

  4. Relate tissue repair processes to the wound healing of skin.

  5. Describe factors influencing tissue repair.

  6. Identify abnormalities/complications of wound healing.

Importance of Wound Healing

  • Critical for organism survival to repair damage from toxic insults and inflammation.

  • Inflammatory response aims to eliminate dangers and initiate repair processes.

Healing and Repair

Definitions

  • Healing/Repair: Replacement of dead or damaged cells with new, healthy cells, restoring tissue architecture and function post-injury.

  • Two processes for tissue repair:

    1. Regeneration: Restores normal cells.

    2. Scarring: Deposition of connective tissue (fibrous tissue).

Regeneration

  • Involves proliferation of cells and tissues for complete restoration of lost or damaged tissues.

  • Driven by growth factors and depends on the integrity of the extracellular matrix (ECM) and mature cell development from stem cells.

Repair (Scar Formation)

  • Occurs through deposition of collagen in various organs due to chronic inflammation or after myocardial infarction.

  • If tissues cannot regenerate or support structures are excessively damaged, fibrosis occurs, referred to as organization.

Cell Types Involved in Tissue Repair

Classification by Proliferative Capacity

  • Labile Cells: Continuously dividing (e.g., bone marrow cells, surface epithelium).

  • Stable Cells: Quiescent but can divide in response to injury (e.g., liver, kidney).

  • Permanent Cells: Non-dividing (e.g., neurons, cardiac muscle).

Functions of Cell Types in Repair

  • Remnants of Injured Tissue: Restore normal structure.

  • Vascular Endothelial Cells: Form new blood vessels for nutrient supply.

  • Fibroblasts: Produce fibrous tissue for scar formation.

Granulation Tissue

  • Characterized by:

    • Proliferation of fibroblasts and new blood vessels (angiogenesis).

    • Presence of inflammatory cells, particularly macrophages.

  • Appearance: Pink, soft, and granular; forms beneath scabs of skin wounds.

Process of Wound Healing

Stages of Healing

  1. Haemostasis: Formation of a clot stops bleeding.

  2. Inflammation: Clears debris and signals repair.

  3. Angiogenesis: Formation of new blood vessels.

  4. Fibroblast Proliferation: Fibroblasts lay down collagen and form granulation tissue.

  5. Scar Formation: Eventually leads to matured scar tissue.

  6. Tissue Remodeling: Balance between synthesis and degradation of ECM.

Factors Affecting Tissue Repair

  • Extrinsic Factors:

    • Infection, diabetes, nutritional status, glucocorticoids.

  • Intrinsic Factors:

    • Mechanical factors, foreign bodies, type and extent of injury, location of injury.

Healing of Skin Wounds

Healing by Intention

  • First Intention: Clean cuts where edges align (i.e., surgical incisions).

  • Second Intention: More extensive tissue loss with granulation tissue and fibrosis present.

Complications of Wound Healing

  • Deficient Scar Formation: Wound dehiscence, ulceration (e.g., venous leg ulcers).

  • Excessive Scar Formation: Hypertrophic scars and keloids, which can cause deformities.

  • Contractures: Result in deformities of the wound and surrounding tissue.

Types of Scars

Hypertrophic Scars

  • Excess collagen leading to raised scars that may regress over time.

Keloid Scars

  • Exceed original wound boundaries and do not regress; more common in certain populations (e.g., African Americans).

Conclusion

  • Understanding wound healing and the complex processes involved is critical for ensuring effective tissue repair and managing complications that may arise.