Apoptosis of Bone Cells

This chapter describes the phenomenon of apoptosis of bone cells. Apoptosis is controlled by a variety of physiological and pathological factors and is involved in the control of cell number during development, repair, and maintenance of regenerating epithelial tissues. This chapter begins by describing general features of apoptosis and its regulation. It then focuses on the occurrence and control of apoptosis in chondrocytes, osteoclasts, osteoblasts, and osteocytes. Furthermore, it highlights the role of apoptosis of osteoclasts and osteoblasts during remodeling by the basic multicellular unit (BMU), the role of osteocyte apoptosis in adaptation to mechanical strain, and the significance of bone cell apoptosis in the pathophysiology of bone disease and its treatment. Apoptosis of chondrocytes, osteoclasts, osteoblasts, and osteocytes plays a critical role in the development of the skeleton and in the maintenance of bone mass and strength throughout life. Dysregulation of bone cell apoptosis contributes to joint disease, as well as to the development of osteoporosis that occurs with sex steroid deficiency, glucocorticoid excess, and aging. Apoptosis may also be an important therapeutic target because the beneficial effects of intermittent parathyroid hormone are associated with a reduction in osteoblast and osteocyte apoptosis. Future innovations that facilitate detection of this important event in human bone, as well as a deeper understanding of the factors and signaling pathways that control bone cell survival, should provide a better appreciation of the role of apoptosis in bone physiology, pathophysiology, and treatment.