Porth's Essentials of Pathophysiology, 4e

1070

Musculoskeletal Function

U N I T 1 2

Bone remodeling constitutes a process of skeletal maintenance once skeletal growth is complete. It takes place in the (1) osteons of mature bone and consists of a cycle of (2) bone resorption by osteoclasts, (3) followed by bone formation by osteoblasts. Bone remodeling is (4) controlled by cytokines and growth factors that interact with a paracrine system consisting of the RANK ligand (RANKL), the RANK receptor, and osteoprotegerin. U N D E R S T A N D I N G Bone Remodeling

Bone Remodeling Cycle. Mature bone is made up of osteons or units of concentric lamellae (bone layers) and the haversian canal they surround. Bone remodeling consists of a sequence of bone resorption within an osteon by osteoclasts, fol- lowed by new bone formation by osteoblasts. In the adult, the length of one sequence (i.e., bone resorption and formation) is approximately 4 months. Ideally, the replaced bone should equal the resorbed bone. If it does not, there is net loss of bone. In the elderly, for example, bone resorption and formation no lon- ger are perfectly coupled, and bone mass is lost. 1 Bone Resorption. The osteo- clasts, which are bone-resorbing cells derived from monocyte/macrophage precursors, are the cells involved in the initiation of bone remodeling. The sequence of bone resorption and bone formation is activated by many stimuli, including the action of parathyroid hormone and cal- citonin. It begins with osteoclastic resorption of existing bone, during which the organic (protein matrix) and inorganic (mineral) components are removed, creating a tunnel-like space in the osteon. Soluble factors released during resorption aid in the recruitment of osteoblasts to the site, thereby linking bone resorption to bone formation. 2

Concentric lamella

Vessel of haversian canal

Osteocytes

Osteon

Cutting cone

Osteoclasts