Kat1-antigen - a reliable immunological marker for identifying osteoclast precursors of rats: detection of subpopulations among precursors and initiation of osteoclastogenesis

Previously we found a unique cell surface antigen [Kat1-antigen (Kat1-AE)] expressed on rat osteoclasts, In the present study, we focused on the expre ssion of this antigen in preosteoclasts, mononuclear precursors of osteocla sts. Immunohistochemical and immunoelectron microscopic observations of the Kat1-Ag expressed in vivo showed the antigen to be present on mononuclear cells having the morphological features of preosteoclasts, The relationship between Kat1-Ag expression and calcitonin receptor (CTR) expression was ex amined in detail by a double-detection technique for CTR and Kat1-Ag by use of autoradiography and immunocytochemistry, respectively. In a culture sys tem for forming mononuclear preosteoclast-like cells, almost 100% of the mo nonuclear cells expressing Kat1-Ag also expressed CTR, demonstrating that K at1-Ag is a reliable immunological marker for identifying preosteoclasts. I nterestingly, a significant number of the CTR-positive mononuclear cells di d not express the Kat1-Ag, Detection of these cells expressing CTR but not Kat1-AE strongly suggests the presence of subpopulations in preosteoclasts. We also obtained evidence suggesting that expression of the Kat1-Ag is ini tiated during the postmitotic stage of the osteoclast progenitors.