Characteristics and properties of osteocytes in culture.

Although the osteocyte is the most abundant among the highly differentiated cells of mature bone (osteocytes, lining cells, osteoblasts, and osteoclasts), its properties and functions are the least known and understood. Here we isolated osteocytes from mixed populations of bone cells liberated from fetal chick calvariae by alternate treatments with collagenase and EDTA. The osteocytes were removed from the bone cell populations by binding them via an osteocyte-specific antibody (MAb OB 7.3) to magnetic beads and removing the beads together with the coupled osteocytes from the population using a magnet. Isolated osteocytes were found to be highly differentiated, postmitotic cells that required their typical stellate morphology in culture. Osteocyte populations had alkaline phosphatase (ALP) activity somewhat lower than that of the osteoblast-like cell populations from which they were separated by the immunodissection procedure. On the single-cell level, the ALP activity was highly variable. Parathyroid hormone (PTH) receptors were found to be present on osteocytes as well as on osteoblast-like cells, but not on fibroblast-like cells of the outer periosteum. In response to PTH, osteocytes increased their intracellular levels of cAMP, as did the osteoblast-like cells. Osteocytes appeared to be somewhat more sensitive to PTH than osteoblasts. When seeded onto dentin slices, osteocytes did not corrode the dentin surface to any appraisable degree. We therefore found no evidence to support the notion that osteocytes play a role in the calcium homeostasis through osteocytic osteolysis. Whether osteocytes play an important role in perceiving and transducing hormonal and/or mechanical stimuli remains open for future research.