Osteoclast demise in the rat: physiological versus degenerative cell death

Time-lapse video microscopy was used to assess the temporal morphological events undergone by rat osteoclasts after exposure to a variety of agents that promote cell death. Direct observations revealed that there were two morphologically distinguishable forms of cell death, which resembled apoptosis and necrosis, respectively. Marked changes in morphology became apparent after 2-4 h exposure to a variety of agents, including cyclosporine A, tamoxifen, corticosterone and dexamethasone. The cells began to shrink rapidly and within 25 min appeared as small round spheres. At this time, the cell membranes underwent violent distortive boiling, or zeiosis, accompanied by the formation of small membrane buds. The cells maintained a spherical configuration and the membrane appeared to remain intact for several hours. These observations are consistent with the process of physiological cell death or apoptosis. In some cases, post-apoptotic changes or secondary necrosis could be seen, including membrane blebbing and degeneration. In contrast, when cells were exposed to hydrogen peroxide or sodium azide, there was a marked deterioration of the cell membrane after 1-4h. This included the formation of spikes and/or blebbing with the release of intracellular debris, resulting in an overall spattered appearance. This type of appearance is characteristic of degenerative cell death or necrosis. The significance of the mode of osteoclast death is discussed.