BONE REMODELING

HOMEOSTATIC PROCESS OF BONE REMODELING

Key Functions

• Regulates calcium blood levels
• Repairs worn-out bone
• Responds to bone stress

Actions

• Osteoblasts form bone from calcium in blood and that osteoclasts break down bone and push calcium into blood.
• LOW blood (plasma) calcium levels stimulate osteoclast activity and inhibit osteoblast activity.
• HIGH blood (plasma) calcium levels inhibit osteoclast activity and stimulate osteoblast activity.
• Reabsorbed bone releases calcium into blood and PTH (parathyroid hormone) is a key physiologic mediator for bone homeostasis.

Clinical Correlation: In Osteoporosis, bone resorption exceeds deposition.

THE BIOLOGICAL PROCESS OF BONE REMODELING

OSTEOBLAST GENERATION

Osteoblasts are the primary mediators of bone formation.

Osteoprogenitor cells

• Spindle-shaped osteoblast precursors.
• They are funneled into ossification centers for linear bone growth.
• They line both the periosteum and the endosteum for appositional bone growth.

Osteoblasts

• Lie along bone matrix. Bone matrix comprises an inorganic component: hydroxyapatite and an organic component: osteoid.
• Osteoblasts are critical to bone formation, they:
  -Secrete osteoid (the organic (unmineralized) portion of bone – ie, the type 1 collagen fibers and ground substance).
  -Mineralize hydroxyapatite (the hydroxylated calcium and phosphate component of bone) via osteocalcin and osteonectin
  -Mediate osteoclastogenesis (the formation of osteoclasts) via M-CSF (macrophage colony stimulating factor) and RANKL with inhibition by osteoprotogerin.

• Stimulation of osteoprogenitor cell differentiation:
  -Members of the (bone morphogenetic protein) BMP family
  -Transforming Growth Factor Beta