A three-dimensional distribution of osteocyte processes revealed by the combination of confocal laser scanning microscopy and differential interference contrast microscopy.

Osteocytes are the most numerous cells in bone, embedded within the mineralized bone matrix. Their slender cytoplasmic processes form a complex intercellular network. In addition, these processes are thought to be important structures in the response to mechanical stress. This study provides an extensive analysis of the three-dimensional structure of the osteocyte and its processes in 16-day-old embryonic chick calvariae, based on nondestructive subsurface histotomography using both confocal laser scanning (CLS) microscopy and differential interference contrast (DIC) microscopy. OB7.3, a chicken osteocyte-specific monoclonal antibody, and Texas Red-X-conjugated phalloidin were used to confirm the osteocyte phenotype and to identify whole cells in the calvariae, respectively. Serial CLS images revealed morphological changes in bone cells up to 20 microm in depth. Osteocytes had widely spread their processes into the osteoblast layer, and we found for the first time that some of these processes had elongated to the vascular-facing surface of the osteoblast layer. Furthermore, stereotype images reconstructed from CLS images could show the three-dimensional distribution of these processes. Using the stereopair image, we could evaluate the frequency of processes between osteocytes and osteoblasts. Complementation of DIC microscopy revealed canaliculi and lacunae with high contrast. The distributional pattern of canaliculi generally coincided with that of the osteocyte processes. We consider that the combination method of CLS microscopy and DIC microscopy using a laser scanning microscope is a very useful new technical approach for investigating osteocytes in bone.