Soluble matrix from osteoblastic cells induces mineralization by dental pulp cells

Dental pulp cells have a capacity to differentiate into mineralization-inducing cells. To clarify the molecular mechanism, we established an in vitro mineralization-inducing system by rat clonal dental pulp cell line, RPC-C2A, and tried to purify a mineralizationinducing factor in conditioned medium (CM) from preosteoblastic MC3T3-E1 cells. The active factor was impermeable to an ultrafiltrating membrane, and sedimented by ultracentrifugation. The sedimented factor was found as a needle-like structure about 1.3 μm in average length as observed by transmission electron microscopy. The factor contained type I collagen, suggesting not a matrix vesicle, but a soluble matrix. The mineralizationinducing activity was also detected in CM from primary culture of rat calvaria (RC) cells. These results suggested that the soluble matrices from osteoblastic cells serve, at least in part, as differentiation-inducing agents.