Macroscopic & Microscopic Structure of Bone

Macroscopic Bone Structure

  • Compact Bone: Dense outer layer, smooth and solid.
  • Spongy Bone: Internal to compact bone, consists of trabeculae (needle-like or flat pieces).
  • Short, Irregular, and Flat Bones: Thin plates of periosteum-covered compact bone and endosteum-covered spongy bone (diploe) housing bone marrow.
  • Long Bones:
    • Diaphysis: Tubular, with a bone collar surrounding a medullary cavity filled with yellow bone marrow in adults.
    • Epiphyses: Ends of bone, spongy bone covered by compact bone and articular cartilage.
    • Epiphyseal Line: Remnant of epiphyseal plate, where longitudinal bone growth occurred.
    • Periosteum: External surface, location of osteogenic cells.
    • Endosteum: Internal surface, location of osteogenic cells.
    • Nutrient Foramen: Opening for nutrient artery and vein to serve bone marrow and spongy bone.
  • Articular Cartilage: Hyaline cartilage (2-4 mm thick) for smooth joint movement; limited healing capacity.
  • Red Bone Marrow: Haematopoietic tissue in spongy bone of long bones and diploe of flat bones; restricted to proximal epiphyses of humerus and femur in adults.
  • Yellow Bone Marrow: Replaces red marrow in diaphysis of adults; forms cartilage, fat, and bone; can convert back to red marrow.
  • Bone Markings: Projections for tendon/ligament attachment, surfaces forming joints, depressions/openings for blood vessels.

Microscopic Bone Structure

  • Bone Cells:
    • Osteoprogenitor Cells: Bone stem cells.
    • Osteoblasts: Secrete and synthesize bone matrix.
    • Osteocytes: Maintain bone matrix.
    • Bone Lining Cells: Monitor and maintain bone matrix.
    • Osteoclasts: Bone resorption and remodeling.
  • Osteon (Haversian System): Structural unit of compact bone.
    • Lamellae: Concentric tubes of bone matrix.
    • Central Canal: Contains blood vessels and nerves.
    • Perforating (Volkmann's) Canals: Connect periosteum to central canals and medullary cavity.
    • Lacunae: Small holes housing osteocytes.
    • Canaliculi: Hair-like channels connecting lacunae and central canal.
    • Circumferential Lamellae: Beneath periosteum, around bone circumference.
    • Interstitial Lamellae: Between intact osteons.
  • Spongy Bone: Lacks osteons, has trabeculae aligned along stress lines with irregular lamellae and osteocytes connected by canaliculi.

Chemical Composition of Bone

  • Organic Components:
    • Cells (osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts, bone lining cells).
    • Osteoid: 1/3 of bone matrix (ground substance and collagen fibers), contributes to flexibility and strength.
  • Inorganic Components:
    • 65% of bone mass.
    • Hydroxyapatites (calcium phosphates): For hardness and compression resistance.
  • Osteoid Composition:
    • 50% of bone volume, 35-40% of bone mass.
    • Type I collagen (90%).
    • Ground substance (chondroitin sulfate and osteocalcin).
  • Bone Strength: Half as strong as steel in compression, as strong as steel in tension.