CHARACTERISTICS AND PROPERTIES OF OSTEOCYTES IN CULTURE

Although the osteocyte is the most abundant among the highly different iated cells of mature bone (osteocytes, lining cells, osteoblasts, and osteoclasts), its properties and functions are the least known and un derstood. Here we isolated osteocytes from mixed populations of bone c ells 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 coupl ed osteocytes from the population using a magnet. Isolated osteocytes were found to be highly differentiated, postmitotic cells that require d their typical stellate morphology in culture. Osteocyte populations had alkaline phosphatase (ALP) activity somewhat lower than that of th e osteoblast-like cell populations from which they were separated by t he immunodissection procedure. On the single-cell level, the ALP activ ity was highly variable. Parathyroid hormone (PTH) receptors were foun d 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 t he osteoblast-like cells. Osteocytes appeared to be somewhat more sens itive to PTH than osteoblasts. When seeded onto dentin slices, osteocy tes did not corrode the dentin surface to any appraisable degree. We t herefore found no evidence to support the notion that osteocytes play a role in the calcium homeostasis through osteocytic osteolysis. Wheth er osteocytes play an important role in perceiving and transducing hor monal and/or mechanical stimuli remains open for future research.