๐Ÿฆด Chapter 746 ยท Bone Structure, Growth, and Hormonal Regulation

Nelson Textbook of Pediatrics 22nd Edition | Bone formation: endochondral ossification (cartilage model โ†’ bone) vs intramembranous ossification (direct mesenchymal โ†’ bone). Vitamin D metabolism: 7-dehydrocholesterol โ†’ skin (UV) โ†’ vitamin D3 โ†’ liver (25-hydroxylase) โ†’ 25(OH)D โ†’ kidney (1ฮฑ-hydroxylase) โ†’ 1,25(OH)2D (calcitriol). Calcium homeostasis: PTH, calcitriol, calcitonin. Phosphate regulation: FGF23 (bone-derived) inhibits renal phosphate reabsorption. Rickets: defective mineralization at growth plate; osteomalacia: defective mineralization of trabecular bone.

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๐Ÿ“‹ 30 Clinical Scenarios โ€” Bone Structure, Growth, & Hormonal Regulation

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๐Ÿฉบ Clinical Recognition: Bone & Mineral Disorders

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๐Ÿ“‹ Stepwise Approach to Bone & Mineral Disorders

๐Ÿ”‘ Key Principles โ€” Nelson Chapter 746
โ€ข Bone formation: Endochondral ossification (majority of skeleton โ€” long bones, vertebrae) vs intramembranous (flat bones โ€” skull, clavicle).
โ€ข Vitamin D metabolism: UV skin โ†’ cholecalciferol (D3). Liver 25-hydroxylase (CYP2R1) โ†’ 25(OH)D (storage form). Kidney 1ฮฑ-hydroxylase (CYP27B1) โ†’ 1,25(OH)2D (active form). 24-hydroxylase (CYP24A1) inactivates vitamin D.
โ€ข Calcitriol (1,25(OH)2D) actions: Increases intestinal calcium and phosphate absorption, bone resorption (with PTH), renal calcium reabsorption, suppresses PTH.
โ€ข PTH: Increases serum calcium โ†’ bone resorption, renal calcium reabsorption, renal phosphate excretion, increases 1,25(OH)2D production.
โ€ข FGF23 (fibroblast growth factor 23): Phosphatonin โ€” decreases serum phosphate by inhibiting renal phosphate reabsorption (downregulates NaPi-2a/c) and suppresses 1,25(OH)2D production (inhibits CYP27B1, stimulates CYP24A1).
โ€ข Rickets vs Osteomalacia: Rickets occurs in growing children (growth plate abnormality). Osteomalacia occurs at any age (defective trabecular bone mineralization).

    โšก Reflex Prompts โ€” Clinical Decisions

    ๐Ÿ“– Summary: Bone Structure, Growth & Hormonal Regulation (Nelson 746)