What Was Covered (All 5 Topics + Interregulation)

1. Collagen (Part 1)

• Structure: Triple helix, Gly-X-Y repeats, hydroxyproline/hydroxylysine, 25+ types with tissue-specific distribution
• Biosynthesis (step-by-step): Pre-pro-alpha chains → RER hydroxylation (Vit C/Fe2+) → glycosylation → triple helix (procollagen) → secretion → propeptide cleavage (tropocollagen) → fibril assembly → cross-linking (lysyl oxidase, Cu2+)
• Collagenopathies: Scurvy, EDS, OI, Lathyrism, Menkes

2. ECM - Collagen and Proteoglycans (Part 2)

• Proteoglycan = core protein + GAG chains (bottle-brush); aggregates via hyaluronic acid backbone
• GAG types: hyaluronic acid, chondroitin sulfate, keratan sulfate, heparan sulfate, heparin, dermatan sulfate - each with unique composition, location, and function
• ECM functions: structural support, molecular sieving, water retention (osmotic), growth factor binding

3. Bone and Cartilage Composition (Part 3)

• Bone: 65% hydroxyapatite (Ca10(PO4)6(OH)2) + 35% organic (90% type I collagen + osteocalcin, osteopontin, alkaline phosphatase)
• Cartilage: Type II collagen + aggrecan + hyaluronic acid + 65-80% water; avascular, reliant on diffusion

4. Vitamin D Metabolism (Part 4)

• Skin (UV) → Cholecalciferol → Liver (25-hydroxylase, unregulated) → Calcidiol (storage form) → Kidney (1-alpha-hydroxylase, regulated) → Calcitriol (active)
• Acts via nuclear VDR → calbindin induction (intestine), renal Ca2+ reabsorption, bone mobilization

5. Hormonal Interregulation of Ca2+ and PO4 (Part 5)

• The integrated PTH-Calcitriol-Calcitonin-FGF23 axis with a detailed flow of signals, target organs, and feedback loops shown in the tables and diagrams