Bones - Anatomy & Physiology

BACK TO MUSCULOSKELETAL ANATOMY AND PHYSIOLOGY

Bone comprises the structure of the skeletal system and provide lever arms for locomotion. Bone also plays important roles in maintaining mineral homeostasis as well as providing the environment for hematopoesis in marrow.

Development of Bone and Cartilage

• Osteogenesis
  • Intramembranous Ossification
    • Forms the flat bones of skull and mandible
    • No cartilaginous precursor: mesenchyme forms bone directly
      • Mesenchyme condenses, differentiates to pre-osteoblasts, then osteoblasts
      • Osteoblasts synthesize osteoid (collagen and proteoglycans)
      • Mineralization and Bone Spicule formation (spicules produce spongy bone)
      • Appositional Growth: laid down on the exterior (vs interior) surface of developing bone
    • Increasing association with blood vessels
    • Bone Marrow formed by mesenchyme between bone and blood vessels
  • Endochondral Ossification
    • Responsible for embryonic bone formation as well as growth in length
    • Via cartilaginous precursor:
      • Mesenchyme condenses to form bone outline
      • Core cells differentiate to chondroycytes and begin secreting cartilage matrix
      • Peripheral condensation forms perichondrium
      • Interstitial (length) and Appositional (width) growth
      • Central cells in developing diaphysis mature and hypertrophy
      • Matrix surrounding most mature chondrocytes calcifies
      • Perichondrium differentiates to osteoblasts
    • Capillary invasion to central core, forming trabecular bone
• Bone Growth and Remodeling
  • Short bones: endochondral ossification continues in diaphysis until only cartilage rim remains
  • Long bones: secondary center of ossification develops in epiphyses
    • Epiphyseal Growth Plate: transverse disc of cartilate between the two ossification centers, allows longitudinal growth to continue to maturity, then closes
  • Primary bone has not yet been remodelled
    • Contains less mineral and more randomly arranged collagen fibers, trabecular organization
  • Remodelling occurs by Haversian canals
    • Osteoclasts dig out canal, followed by blood vessel invasion
    • Concentric lamellae laid down
    • Phased resorption happens concurrently

Types of Bone

• Long Bone
  • Found in the limbs and act as levers for locomotion
  • An elongated diaphysis (shaft) and two epiphyses (ends), each of which encases a center for ossification

• Short Bone
  • Found in places of articulation, such as the carpus and tarsus
  • All dimensions are relatively equal, generally signifying one center of ossification

• Flat Bone
  • Found in the skull, pelvic girdle, and scapula
  • Expand in two directions, with a broad surface for attachment of large muscle masses and protection of underlying structures

Composition of Bone

Bone is comprised of:

• Matrix:
  • Organic component: "Collagen I", which resists tension
  • Bone-specific proteins: Osteonectin, Osteopontin, Osteocalcin
  • Inorganic component: Calcium, Phosphorus, Bicarbonate, Citrate, Potassium, Magnesium, Sodium, which provide rigidity

• Cells:
  • Osteoblasts: single layer of cuboidal cells which synthesize bone extracellular matrix (ECM)
  • Osteocytes: osteoblasts embedded in their own matrix; reside within lacunae and are interconnected via channels forming canaliculi
    • Canaliculi create connections to form a huge neural-like junctional organization
  • Osteoclasts: giant (multinucleate monocytes) cells which act to resorb bone ECM

All mammalian bone is organized as lamellated sheets:

• Cortical (compact) bone
  • Sheath covers external surface of long bone, thicker in shaft and thins over epiphyses
  • Comprised of thin lamallae in a series of concentric tubes arranged around small central canals (collectively known as an osteone)

• Cancellous (spongy, or trabecular) bone
  • Forms the hematopoeitic center of epiphyses
  • Stacks of parallel or concentrically stacked sheets arranged as rods, plates, and arches

• Medullary Cavity and Cancellous Interstitium: bone marrow storage and production
  • Red Marrow: richly vascularized, gelatinous tissue with hematopoeitic properties found abundantly in young animals
  • Yellow Marrow: fat infiltration converts red marrow to yellow, causing hematopoeitic properties to dwindle