Bone Structure and Composition

Jul 17, 2024

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Bone Composition and Structure

Bone Matrix

  • Composed primarily of an extracellular calcified material
  • Also known as the collagen matrix

Cell Types in Bones

  1. Osteocytes
    • Located in cavities (lacunae) between bone matrix layers
    • Assist with bone nutrition
  2. Osteoblasts
    • Mainly found on bone surfaces and within the bone matrix
    • Responsible for synthesizing and mediating mineralization of bone matrix
  3. Osteoclasts
    • Large multinucleated cells
    • Remove calcified bone matrix, facilitating bone turnover and remodeling

Bone Morphology

  • Compact (Cortical) Bone: Dense bone closer to the surface
  • Cancellous (Trabecular) Bone: Found deeper in bones with interconnecting cavities

Microscopic Organization of Bone

  • Lamellar Bone
    • Bone matrix arranged in sheets
    • 80% found in compact bone, 20% in trabecular bone
  • Woven Bone
    • Collagen fibers arranged randomly
    • Found in developing/growing bones and healed fractures

Comparison of Long Bone and Flat Bone

  • Long Bone
    • Compact bone along the outer edge
  • Flat Bone
    • Compact bone on both outer flat surfaces

Trabecular Bone

  • Structure
    • Small beam-like structures (spicules)
    • Interconnecting spaces containing bone marrow
  • Examples
    • Vertebrae (mostly trabecular with a thin compact layer)
    • Osteoclasts within bone marrow; distinguishable by multiple nuclei

Compact Bone

  • Haversian System (Osteons)
    • Structural units of compact bone
    • Consist of concentric lamellae (layers) around a central Haversian canal
    • Haversian canal contains blood vessels and nerves
    • Haversian lamellae: Layers of mineralized bone matrix radiating from the canal
    • Cement line: Thin basophilic line marking the boundary of each osteon
    • Lacunae: White spaces containing remnants of osteocytes
    • Osteoblasts: Found lining the inner surface of the Haversian canal; large spindle-shaped or cuboidal cells with basophilic cytoplasm

Recap

  • Two main types of bone morphology: compact (cortical) and cancellous (trabecular)
  • Microscopically: lamellar bone arranged in sheets, woven bone with random collagen fiber arrangement
  • Three main bone cell types: osteocytes, osteoblasts, and osteoclasts