Bone Resorption The unique ability of self-renewal in bone tissue

Osteoclasts Multinucleated bone cells (many nuclei). Function: to remove damage or old bone tissue by creating an acidic environment between bone surface and cell. Leaves behind an empty space within bone tissue known as a resorption pit.

Osteoblasts Mononucleated bone cells (single nucleus). Function: to deposit a fresh bone matrix in the resorption pit left behind by the osteoclasts. Responsible for the deposition of new bone.

Osteocytes Osteoblasts which become stuck in the fresh bone matrix they have recently laid down. Communication between osteocytes is vital for bone tissue health. Disruptions within this communication network signals for bone to be remodeled or replaced. Lacuna- Space in which individual osteocytes reside within bone tissue. Canaliculus- Cellular protrusion which connect individual osteocytes with one another. Forms a communication network within bone tissue.

Osteoclasts Osteoblasts Osteocytes Osteoblast activity must equal osteoclast activity Summary of Bone Cells

Remodeling Why does bone need to be remodeled? – It has become hypermineralized (old) or had been damage. How does bone know it needs to under go remodeling? – Disruption in osteocyte communicate initiates the process of bone remodeling. Steps of Bone Remodeling 1.Osteoclasts resorb bone. 2.Osteoblasts deposit fresh bone matrix 3.Osteoblasts become embedded within this matrix, transitioning into osteocytes. 4.Communication has been re-established.

Blood Vessel Lining cells Osteocytes Osteoblasts Osteoclasts Microcrack Osteocyte apoptosis Local factors Cement line New bone Animation: Bajaj D. Ph.D., adapted from Bone Remodeling/Repair Bone

Osteoporosis Osteoporosis is an age related skeletal disease. Osteoporosis means “porous bone”. About 10 million Americans are diagnosed with Osteoporosis, of which 80% are women.

Courtesy of Michael J. Klein, M.D. Fritton & Schaffler, 2008 “Bone Quality” HealthyOsteoporotic

Pharmaceutical Countermeasures of Osteoporosis How can drugs alter this process of bone remodeling? – Bisphosphonates, which inhibits bone resorption (osteoclast activity). – Parathyroid Hormone, which stimulates bone formation (osteoblast activity).

Inhibition of Bone Resorption How do Bisphosphonates Function? Ruffled Border Osteoclast Apoptosis Absence of Ruffled Border Animation by Bajaj, D. Ph.D.

To think about: Why is bone resorption vital to bone health? What can happen if bone cell activity is not balanced? How can drugs treat this imbalance in bone cell activity which is observed in Osteoporosis?