Bone Structure and Function

Bone Composition & Cells - Microscopic Marvels

• Composition (Matrix):
  • Organic (~35%):
    • Collagen Type I (90% of organic): Tensile strength.
    • Ground Substance: Proteoglycans, Glycoproteins (e.g., osteonectin, osteocalcin).
  • Inorganic (~65%):
    • Hydroxyapatite crystals: $Ca_{10}(PO_4)_6(OH)_2$. Compressive strength.
• Cells (The Bone Workforce): 📌 Blasts Build, Clasts Carve.
  • Osteoblasts: Form bone. Synthesize osteoid (unmineralized matrix). Marker: Alkaline Phosphatase (ALP).
  • Osteocytes: Mature osteoblasts in lacunae. Mechanosensors, orchestrate remodeling.
  • Osteoclasts: Resorb bone. Multinucleated. Found in Howship's lacunae. Marker: TRAP.
  • Bone Lining Cells: Quiescent osteoblasts on bone surface.

⭐ Osteocalcin, secreted by osteoblasts, is a key non-collagenous protein; serum levels reflect bone formation rate (Vitamin K-dependent).

Bone Architecture - Skeleton's Scaffolding

• Macroscopic:
  • Cortical (Compact): Dense outer (80% mass). Resists bending/torsion. Unit: Osteon.
  • Cancellous (Spongy): Inner porous (20% mass). Trabeculae on stress lines. Metabolically active.
• Microscopic:
  • Woven Bone: Immature, disorganized collagen. Rapid formation (fetus, fracture, tumors).
  • Lamellar Bone: Mature, organized collagen layers. Stronger. Forms osteons, trabeculae.
• Key Lamellar Features:
  • Osteon (Haversian System): Central canal (vessels/nerves), lamellae, lacunae (osteocytes), canaliculi.
  • Volkmann's Canals: Connect Haversian canals.
• Linings:
  • Periosteum: Outer fibrous (Sharpey's fibers), inner osteogenic.
  • Endosteum: Lines internal surfaces; osteogenic.

⭐ Wolff's Law: Bone adapts to loads; trabeculae align with stress.

Bone Formation & Remodeling - Dynamic Development

• Bone Formation (Ossification):
  • Intramembranous: Direct formation from mesenchymal condensations. Osteoblasts deposit osteoid. E.g., skull, clavicle.
  • Endochondral: Cartilage scaffold replaced by bone. Primary (diaphysis) & secondary (epiphysis) ossification centers. E.g., long bones.
    • Epiphyseal plate zones: Resting, Proliferation, Hypertrophy, Calcification, Ossification (📌 RPHCO: "Real People Have Career Options").
• Bone Remodeling: Lifelong resorption (osteoclasts) & formation (osteoblasts) by Basic Multicellular Unit (BMU).
  • Phases: Activation → Resorption → Reversal → Formation → Quiescence.
  • Wolff's Law: Bone adapts to load. ↑ stress → ↑ density.

    ⭐ Wolff's Law: Mechanical stress dictates bone architecture; crucial for fracture healing and osteoporosis understanding.

  • Hormonal control: PTH, Calcitonin, Vit D.

Bone Mechanics & Blood Supply - Force & Flow

• Bone Mechanics
  • Wolff's Law: Bone remodels to adapt to mechanical stress. "Form follows function."
  • Stress-Strain Curve: Describes bone's response to load.
    • Elastic deformation: Recovers.
    • Plastic deformation: Permanent.
    • Ultimate failure point: Fracture.
  • Anisotropy: Strength varies with direction of force.
  • Viscoelasticity: Response depends on loading rate (stiffer with faster load).
• Blood Supply (Primarily centrifugal flow: marrow outwards)
  • Nutrient Artery: Main diaphyseal supply (inner 2/3 cortex).
  • Periosteal Arteries: Outer 1/3 cortex; vital in fractures, growth.
  • Metaphyseal & Epiphyseal Arteries: From periarticular vascular plexus.

  ⭐ Femoral head AVN post #NOF (Neck of Femur fracture) is high-risk due to precarious supply mainly from medial circumflex femoral artery.

High‑Yield Points - ⚡ Biggest Takeaways

• Bone is a dynamic, specialized connective tissue; Type I collagen provides tensile strength, while hydroxyapatite crystals confer compressive strength.
• Osteoblasts are responsible for bone formation (ossification & calcification); Osteoclasts are responsible for bone resorption.
• Woven bone (immature) is characterized by random collagen fiber orientation, found in fetal development and fracture calluses; Lamellar bone (mature) is organized and stronger.
• The Haversian system (osteon) is the fundamental structural unit of compact (cortical) bone.
• Wolff's Law states that bone adapts its structure to the mechanical loads it experiences.
• The periosteum is crucial for blood supply, appositional growth, and fracture healing, especially in children.
• Vitamin D is essential for calcium and phosphate absorption, vital for bone mineralization and health.