Repair and healing of damaged tissues begin soon after the onset of inflammation. This process restores tissue integrity and function through a series of overlapping stages involving cells, cytokines, and the extracellular matrix (ECM).
Phases of Tissue Repair
| Phase | Main Events | Key Cells / Components | Purpose |
|---|---|---|---|
| 1. Hemostasis | Vasoconstriction → platelet aggregation → clot formation | Platelets, coagulation factors | Prevent blood loss and form a temporary matrix |
| 2. Inflammation | Removal of necrotic tissue and microbes | Neutrophils, macrophages, lymphocytes, and mast cells | Clean the wound site and release growth factors |
| 3. Regeneration | Replacement of lost cells by proliferation | Stem cells, differentiated parenchymal cells | Restore original tissue structure and function (if possible) |
| 4. Fibrosis (Scar Formation) | Fibroblast activation and angiogenesis (granulation tissue formation) → collagen deposition (type III) | Macrophages, fibroblasts, and endothelial cells | Fill defect with connective tissue when regeneration is incomplete |
| 5. Remodeling (Maturation) | Conversion of type III → type I collagen, tissue contraction | Fibroblasts, macrophages, matrix metalloproteinases (MMPs) | Strengthen and reorganize scar tissue |
The Extracellular Matrix (ECM)
The ECM provides the structural framework essential for tissue repair, cell migration, and signaling.
It exists in two main forms:
| Type | Location | Main Components |
|---|---|---|
| Interstitial Matrix | Between connective tissue cells | Fibrillar collagens (types I, III, V), elastin, fibronectin, proteoglycans |
| Basement Membrane | Beneath epithelial cells and around vessels | Type IV collagen, laminin, entactin, heparan sulfate |
Main Components of ECM
| Component | Function / Example |
|---|---|
| Collagens & Elastins | Provide tensile strength and elasticity |
| Gels (Proteoglycans & Hyaluronan) | Provide resilience, lubrication, and compressive resistance |
| Glycoproteins & Cell Adhesion Molecules (CAMs) | Mediate cell attachment, migration, and signaling (e.g., fibronectin, laminin, integrins) |
Key Points to Remember
- Repair involves both regeneration and scar formation (fibrosis).
- Inflammation and repair overlap; macrophages are central to both.
- Granulation tissue is the hallmark of early repair, rich in new capillaries and fibroblasts.
- Collagen type III → type I conversion gives strength to the mature scar.
- Basement membrane integrity is critical for organized regeneration.
Learning Objective
By the end of this topic, students should be able to:
- Describe the sequential phases of tissue repair and healing.
- Identify the major cellular and molecular components involved.
- Explain the role of ECM in tissue repair and remodeling.
