Biology: Bone Structure and Function

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Bone Structure and Function

• Bone provides internal support, attachment for muscles and tendons, and encloses blood-forming elements of the bone marrow and vital organs of the cranial and thoracic cavities.
• Bone is a connective tissue characterized by a mineralized extracellular matrix, producing a hard tissue capable of providing support and protection.
• Bone matrix contains mainly type I collagen, along with other matrix proteins and traces of type IV, III, XI, and XIII collagens.
• Four main groups of non-collagenous proteins found in the bone matrix:
  • Proteoglycans (contribute to compressive strength, bind growth factors, and inhibit mineralization)
  • Multiadhesive glycoproteins (e.g. osteonectin, osteopontin, sialoprotein I and II)
  • Bone-specific, vitamin K-dependent proteins (e.g. osteocalcin, protein S, matrix Gla-protein)
  • Growth factors and cytokines (e.g. insulin-like growth factors, tumor necrosis factor, transforming growth factor, platelet-derived growth factors, bone morphogenic proteins, interleukins)

Bone Cells

• Osteoprogenitor cells: derived from mesenchymal stem cells, give rise to osteoblasts, and are found on external and internal surfaces of bones.
• Osteoblasts:
  • Synthesize organic components of bone matrix
  • Polarized cells, secreting matrix components at the bone surface
  • Can become osteocytes or bone-lining cells
• Osteocytes:
  • Enclosed in lacunae, connected by canaliculi
  • Involved in maintaining bone matrix
  • Can become quiescent, formative, or resorptive osteocytes
• Osteoclasts:
  • Large, motile cells with multiple nuclei
  • Resorb bone matrix, lying in enzymatically etched depressions (resorption bays)

Bone Matrix

• Inorganic material (50% of dry weight):
  • Hydroxyapatite
  • Bicarbonate
  • Citrate
  • Magnesium
  • Potassium
  • Sodium
  • Amorphous calcium phosphate
• Organic matter:
  • Type I collagen
  • Ground substance (proteoglycan aggregates, multiadhesive glycoproteins)
  • Calcium-binding glycoproteins (e.g. osteocalcin)
  • Phosphatases

Periosteum and Endosteum

• Periosteum:
  • Dense fibrous outer layer
  • Osteoprogenitor cells found in the innermost cellular layer
  • Provides nutrition, repair, and growth of bone
• Endosteum:
  • Thin, single layer of connective tissue
  • Covers small spicules or trabeculae of bone
  • Lines internal marrow cavities

Types of Bone

• Compact bone:
  • Dense, with no cavities
  • Covers external surfaces of bone
• Cancellous (spongy) bone:
  • Areas with numerous interconnecting cavities
  • Found in the ends of long bones, and in the interior of short bones

Osteogenesis

• Intramembranous ossification:
  • Osteoblasts differentiate directly from mesenchyme
  • Primary bone tissue appears first
• Endochondral ossification:
  • Matrix of preexisting hyaline cartilage is eroded and replaced by osteoblasts
  • Both processes produce primary bone tissue

Histophysiology of Bone

• Vitamin D deficiency:
  • Rickets in children
  • Osteomalacia in adults
  • Impaired calcium absorption and bone calcification
• Vitamin C deficiency:
  • Impaired collagen production and maintenance
  • Delayed healing of fractures
• Vitamin A deficiency:
  • Impaired skeletal growth and bone remodeling
  • Increased bone resorption

Hormones Influencing Bone Growth and Maintenance

• Parathyroid hormone (PTH):
  • Regulates bone resorption and calcium release
  • Acts on the kidney to increase calcium reabsorption
  • Balances calcium levels in the blood
• Calcitonin:
  • Inhibits bone resorption
  • Lowers calcium levels in the blood
  • Produced in response to hypercalcemia
• Growth hormone:
  • Essential for normal bone growth
  • Excess or deficiency leads to abnormalities
• Sex hormones (gonads):
  • Affect skeletal maturation and closure of epiphyseal ossification centers
  • Influence bone growth and development

Bone Repair

• Fracture repair:
  • Hemorrhage and clotting
  • Formation of granulation tissue (procallus)
  • Transformation to cartilage (temporary callus)
  • Remodeling of bone

Joints

• Diarthroses:
  • Joints that permit free bone movement
  • Have a synovial cavity lined by a synovial membrane
  • Contain synovial fluid
• Synarthroses:
  • Joints that have very limited or no movement
  • Unite bones using bone, cartilage, or fibrous tissue
• Components of a diarthrosis:
  • A capsule continuous with a covering ligament
  • A synovial or joint cavity lined by a synovial membrane
  • Synovial fluid