Apoptotic bone cells may be engulfed by osteoclasts during alveolar bone resorption in young rats

The alveolar bone is a suitable in vivo physiological model for the study o f apoptosis and interactions of bone cells because it undergoes continuous, rapid and intense resorption/remodelling, during a long period of time, to accommodate the growing tooth germs. The intensity of alveolar bone resorp tion greatly enhances the chances of observing images of the extremely rapi d events of apoptosis of bone cells and also of images of interactions betw een osteoclasts and osteocytes/osteoblasts/bone lining cells. To find such images, we have therefore examined the alveolar bone of young rats using li ght microscopy, the TUNEL method for apoptosis, and electron microscopy. Fr agments of alveolar bone from young rats were fixed in Bouin and formaldehy de for morphology and for the TUNEL method. Glutaraldehyde-formaldehyde fix ed specimens were processed for transmission electron microscopy. Results s howed TUNEL positive round/ovoid structures on the bone surface and inside osteocytic lacunae. These structures - also stained by hematoxylin - were t herefore interpreted, respectively, as osteoblasts/lining cells and osteocy tes undergoing apoptosis. Osteoclasts also exhibited TUNEL positive apoptot ic bodies inside large vacuoles; the nuclei of osteoclasts, however, were a lways TUNEL negative. Ultrathin sections revealed typical apoptotic images - round/ovoid bodies with dense crescent-like chromatin - on the bone surfa ce, corresponding therefore to apoptotic osteoblasts/lining cells. Osteocyt es also showed images compatible with apoptosis. Large osteoclast vacuoles often contained fragmented cellular material. Our results provide further s upport for the idea that osteoclasts internalize dying bone cells; we were however, unable to find images of osteoclasts in apoptosis. (C) 2001 Harcou rt Publishers Ltd.