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BI6.1-3 | Extracellular Matrix — Part 1
CLINICAL SCENARIO
A 5-year-old child is brought to the district hospital with painful swollen legs and difficulty walking. His mother says he has been irritable and reluctant to eat for the past 2 months. The family is from a low-income household and the child's diet consists mainly of rice, rotis, and lentils. On examination: perifollicular haemorrhages (small bleeding spots around hair follicles) on the limbs, swollen gums that bleed on touch, and a large haematoma on the right thigh.
A surgical postgraduate who sees the child says: "The problem is in his collagen — it cannot be cross-linked properly without a specific vitamin."
What is the vitamin? What is the molecular defect? And why does it specifically affect collagen?
WHY THIS MATTERS
The extracellular matrix (ECM) is the scaffold on which every tissue and organ is built. Without it, cells cannot organise, blood vessels cannot maintain integrity, joints break down, and wounds do not heal. Understanding the ECM explains:
- Why scurvy (vitamin C deficiency) causes bleeding gums and poor wound healing
- Why Ehlers-Danlos syndrome patients have hyperflexible joints
- Why the collagen in bone is different from the collagen in skin
- Why hyaluronic acid injections are used for osteoarthritis
- Why matrix metalloproteinases (MMPs) are targets in cancer therapy
This is not just basic biochemistry — it is the molecular basis of surgery, orthopaedics, dermatology, and oncology.
RECALL
Recall from your Anatomy and Cell Biology:
- Connective tissue consists of cells embedded in an extracellular matrix (fibroblasts, chondrocytes, osteoblasts)
- Collagen is the most abundant protein in the human body (30% of total protein)
- Vitamin C (ascorbic acid) is a water-soluble vitamin
- From Biochemistry (protein structure): the triple helix conformation of collagen, the Gly-X-Y repeating sequence
Build on these foundations in this module.
Components of the ECM — The Big Picture
The extracellular matrix (ECM) is not just filler between cells — it is an active, dynamic scaffold that regulates cell behaviour, migration, proliferation, and differentiation.
Figure: Components of the ECM — The Big Picture
The ECM has three main component classes:
1. Fibrous structural proteins — provide tensile strength and elasticity
- Collagens (strength, structure)
- Elastin (recoil — found in skin, arteries, lung)
- Fibronectin (adhesion between cells and matrix)
- Laminin (basement membrane organisation)
Figure: The ECM has three main component classes:
2. Glycosaminoglycans (GAGs) and Proteoglycans — form hydrated gels, resist compression
- Hyaluronic acid, chondroitin sulphate, heparan sulphate, keratan sulphate
3. Adhesive glycoproteins — link cells to matrix
- Fibronectin, laminin, entactin, tenascin
Two architectural zones:
- Interstitial matrix — surrounds connective tissue cells (fibroblasts, smooth muscle cells)
- Basement membrane — thin specialised ECM underlying all epithelial cells, endothelial cells; contains type IV collagen, laminin, perlecan
Collagen — Structure and Types
Clinically Important Collagen Types
| Type | Structure | Distribution | Clinical Relevance |
|---|---|---|---|
| Type I | Fibrillar ([alpha1(I)]2 alpha2(I)) | Bone, skin, tendon, cornea, dentine | Most abundant collagen; mutated in Osteogenesis Imperfecta |
| Type II | Fibrillar ([alpha1(II)]3) | Cartilage, vitreous humour | Arthritis; chondrodysplasias |
| Type III | Fibrillar ([alpha1(III)]3) | Skin, blood vessels, gut, uterus | Defective in vascular Ehlers-Danlos syndrome (Type IV EDS — life-threatening) |
| Type IV | Network-forming | All basement membranes | Mutated in Alport syndrome (X-linked nephritis, deafness) |
| Type VII | Anchoring fibrils | Dermal-epidermal junction | Defective in dystrophic epidermolysis bullosa |
Collagen is the most abundant protein in the human body, accounting for ~30% of total body protein. It provides tensile strength to tissues.
Figure: Collagen — Structure and Types
Structural feature: Every collagen chain has a repeating (Gly-X-Y)ₙ sequence, where:
- Glycine must be at every third position (smallest amino acid — allows tight winding of the helix)
- X is often proline (rigidity)
- Y is often hydroxyproline or hydroxylysine (cross-linking, stability)
Three chains wind together to form the right-handed triple helix — the hallmark collagen structure.
Clinically important collagen types:
| Type | Distribution | Clinical Relevance |
|---|---|---|
| Type I | Bone, skin, tendon, cornea, dentine | Most abundant; OI mutations |
| Type II | Cartilage, vitreous humour | Arthritis, chondrodysplasia |
| Type III | Skin, blood vessels, gut, uterus | Ehlers-Danlos type IV (vascular) |
| Type IV | Basement membranes | Alport syndrome (X-linked nephritis) |
| Type VII | Anchoring fibrils at dermo-epidermal junction | Epidermolysis bullosa |
Memory hook: "BSTCO" for Type I — Bone, Skin, Tendon, Cornea, organs
Figure: Clinically important collagen types:
Collagen Synthesis — Step by Step
Collagen synthesis is a multi-step process with several post-translational modifications — each step is a potential disease target.
Figure: Collagen Synthesis — Step by Step
Step 1 — Gene transcription and translation: Pro-α chains are synthesised on ribosomes of rough ER. They contain signal peptides + propeptide extensions (N- and C-terminal) that prevent premature fibre assembly.
Step 2 — Hydroxylation (in ER lumen):
- Prolyl hydroxylase converts proline → hydroxyproline (requires: Fe²⁺, O₂, α-ketoglutarate, and Vitamin C as cofactor)
- Lysyl hydroxylase converts lysine → hydroxylysine (same cofactors)
- Vitamin C (ascorbic acid) keeps iron in the Fe²⁺ (reduced) state, reactivating the hydroxylase. Without Vitamin C → hydroxylases inactive → underhydroxylated collagen → unstable triple helix → SCURVY
Step 3 — Glycosylation: Galactose or glucose attached to hydroxylysine residues in the ER.
Step 4 — Triple helix assembly (procollagen): Three pro-α chains wind together to form the procollagen triple helix, held together by disulfide bonds at the C-terminal propeptide.
Step 5 — Secretion of procollagen into extracellular space.
Step 6 — Propeptide cleavage: N- and C-terminal propeptides are removed by procollagen N-proteinase and procollagen C-proteinase → yields tropocollagen.
- Failure here → Dermatosparaxis (Ehlers-Danlos type VIIC) — skin fragility
Step 7 — Cross-linking (fibril assembly):
- Lysyl oxidase (requires Cu²⁺) oxidises lysine/hydroxylysine → allysine
- Allysine residues react covalently → pyridinoline cross-links → stable collagen fibrils
- Copper deficiency → defective cross-linking → weak connective tissue (Menkes disease)
CLINICAL PEARL
Scurvy — the Vitamin C-collagen connection: Vitamin C (ascorbic acid) is required to keep the Fe²⁺ in prolyl hydroxylase active. Without it, proline and lysine are not hydroxylated → collagen triple helix is unstable and degraded → classic features:
- Perifollicular haemorrhages — capillary fragility (basement membrane and vessel wall collagen weak)
- Bleeding gums, gingivitis — gingival collagen breakdown
- Impaired wound healing — new collagen cannot be properly cross-linked
- Corkscrew hairs — follicular collagen abnormality
- Bone pain — periosteal haematomas (type I collagen in bone)
Scurvy is rare in India today but still seen in infants on exclusively milk diets, elderly living alone with poor diet, and institutionalised populations. Treatment: ascorbic acid 300–1000 mg/day. Response is dramatic — gum bleeding stops within 1–2 weeks.