Learning Objective: Identify the structure, function, and clinical relevance of the major collagen types (I–IV), including disorders associated with collagen defects.
Overview of Collagen
| Feature | Description |
|---|---|
| Definition | Most abundant protein in the human body |
| Function | Organizes and strengthens the extracellular matrix |
| Synthesis | Undergoes extensive post-translational modification (hydroxylation, glycosylation, triple-helix formation) |
| Major Types | I, II, III, and IV — “SCAB” mnemonic |
Major Collagen Types
| Type | Location | Mnemonic / Function | Associated Disorders |
|---|---|---|---|
| Type I | Bone, skin, tendon, dentin, fascia, cornea, late wound repair | “bONE, tENDON” | ↓ in Osteogenesis imperfecta type I |
| Type II | Cartilage (hyaline), vitreous body, nucleus pulposus | “cartwOLAGE” | Joint degeneration disorders |
| Type III | Reticulin: skin, blood vessels, uterus, fetal tissue, early wound repair | “threE D → Ehlers-Danlos (vascular type)” | Deficient in Ehlers-Danlos syndrome (vascular type) |
| Type IV | Basement membrane (glomerulus, cochlea, lens) | “Under the floor (basement membrane)” | Defective in Alport syndrome, targeted in Goodpasture syndrome |
Collagen in Wound Healing
- Myofibroblasts secrete collagen during the proliferative phase.
- Collagen type transitions from type III (early) to type I (late) during wound repair.
- Wound contraction is mediated by myofibroblasts.
Key Points for USMLE Step 1
- Type I – bone, skin, tendon → ↓ in osteogenesis imperfecta
- Type II – cartilage, vitreous → structural support
- Type III – vessels, skin → deficient in Ehlers-Danlos (vascular type)
- Type IV – basement membrane → Alport & Goodpasture
- “SCAB” → Skin/Bone (I), Cartilage (II), Arteries (III), Basement membrane (IV)
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