Osteoclast development in marrow cultured in calvaria-conditioned media

The precise signals responsible for recruitment and differentiation of osteoclasts (OCs) from their mononuclear precursors are poorly understood. Marrow mononuclear cells, a reputed source of OC precursors, fuse in culture, forming multinucleated cells. These cells, although similar to OCs, differ from osteoclasts in cell-surface morphology and are not recognized by an OC-specific monoclonal antibody. We have used the expression of an osteoclast-specific membrane epitope designated by monoclonal antibody 121F to delineate OCs from marrow-derived giant cells (MAGC). In this report we describe a series of experiments designed to better define the role of the bone environment in the osteoclast differentiation process. Periosteum-free calvariae from hatchling chicks or their conditioned media were combined with adherent Day 1 cultured marrow cells. The time course of OC marker expression was monitored by ELISA and the requirement for live bone and PTH was investigated. Freshly isolated marrow, MAGC, and calvariae were devoid of OC expression. Antigen expression developed in cultured MAGC after 4 days of coplating with either live bone or live bone-conditioned media. The presence of PTH in the cocultures or conditioned media from PTH-treated calvariae did not significantly alter the level of expression. These data indicate that live bone is, in part, responsible for the production of osteoclasts from mononuclear precursors.