Detection of apoptosis of bone cells in vitro.

Studies during the last decade demonstrated that apoptosis is as important as mitosis for the growth and maintenance of the skeleton and provided information on the significance and molecular regulation of apoptosis of bone cells. It is now known that: (1) all osteoclasts die by apoptosis after completing a bone resorption cycle; (2) the majority of osteoblasts also die, whereas the remainder become lining cells or osteocytes; and (3) osteocytes, although long-living cells, also can die prematurely. Furthermore, mounting evidence indicates that systemic hormones, local growth factors, cytokines, and pharmacological agents, as well as mechanical forces regulate the rate of bone cell apoptosis. This chapter summarizes the methods developed in the last few years to examine apoptosis of cultured bone cells and identify the signaling pathways and molecules involved in apoptosis regulation by diverse skeletal stimuli.