Bone Resorption

Bone resorption is the physiological process by which existing bone tissue is broken down and removed. It is carried out primarily by osteoclasts, large multinucleated cells derived from the monocyte-macrophage lineage. Bone resorption is a normal part of skeletal growth, repair, mineral homeostasis, and lifelong bone remodeling rather than inherently a sign of bone disease .^1,2

To resorb bone, an osteoclast attaches tightly to the bone surface and creates a sealed compartment beneath itself. The cell pumps hydrogen ions into this space, lowering the pH and dissolving hydroxyapatite, the mineral component of bone. Proteolytic enzymes, particularly cathepsin K, then degrade the exposed organic matrix, which consists largely of type I collagen. Breakdown products are subsequently removed from the resorption site .¹

Bone resorption contributes to mineral homeostasis because bone serves as the body's major reservoir of calcium and phosphate. Hormonal and local signals regulate osteoclast formation and activity. Signaling through RANKL promotes osteoclast differentiation, while parathyroid hormone can increase bone resorption indirectly by altering signals produced by osteoblast-lineage cells and osteocytes .^1,2

Bone resorption is distinct from bone formation. Resorption removes existing mineralized tissue, whereas osteoblasts form new bone matrix and promote its mineralization. In normal adult bone remodeling, these processes are closely coupled so that old or damaged bone is replaced. If resorption persistently exceeds formation, net bone loss can occur .^1,2