The Cellular and Biochemical Aspects of Bone Remodeling

Abstract

Metabolic bone diseases are fundamentally disorders of bone remodeling. Bone remodeling is the highly integrated process of resorption and successive formation of bone tissue that results in the maintenance of skeletal mass with renewal of the mineralized matrix. This is accomplished by focal cell-mediated degradation and regeneration of bone tissue without compromising the overall architecture of the anatomy of bones (Fig. 1). The remodeling sites are known as basic multicellular units (BMUs). The renewal of bone tissue occurs through orchestrated cycles of activity called ActivationResorption-Formation (ARF). The first step is activation of quiescent osteoclasts and precursors that begin to excavate a cavity on a bony surface. After an amount of time, osteoblast precursors are activated and refill the excavation site. Under ideal conditions, the amount of bone fill equals the amount resorbed, with no net change in the volume of bone. Consequently, the molecular composition of the adult skeleton is not static, but it changes as new bone fills each excavation site.