Electron microscopy of developing calvaria reveals images that suggest that osteoclasts engulf and destroy osteocytes during bone resorption.

It is generally accepted that osteoclasts are responsible for the breakdown and removal of bone matrix constituents. However, very little is known about the fate of osteocytes during bone resorption. In the present study we have examined sites of bone destruction in calvaria of young rats aged 4-9 days in the hope of obtaining information on the fate of osteocytes. Decalcified glutaraldehyde-formaldehyde-fixed specimens were prepared for ultrathin section electron microscopy. When sequentially arranged, the images obtained suggest that osteoclasts engulf and destroy osteocytes during bone degradation. We propose that the following sequence of events takes place when a lacuna is opened up by an osteoclast: (1) When the osteoclast comes in contact with an osteocyte, the villi of the ruffled border become flat and broad. (2) Long osteoclastic extensions surround the osteocyte. (3) The osteocyte is subsequently internalized with apparent degradation.