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.

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