Metaphyseal factors promote calcium incorporation in physeal chondrocyte cultures

Our hypothesis is that physiological mineralization within the mammalian growth plate is a consequence of communication between cartilage chondrocytes and cells within metaphyseal bone. To test this hypothesis, chondrocytes were isolated from the proliferative region of the fetal ovine physis and co-cultured with cells or conditioned medium from cells characteristic of those in metaphyseal bone. The mineralization potential of chondrocytes alone and in the presence of other cells or conditioned medium was determined by ⁴⁵calcium incorporation. Co-culture of chondrocytes with a crude cell isolate from metaphyseal bone resulted in a stimulation of ⁴⁵calcium incorporation of 93% above that observed in the individual cell populations alone. Conditioned medium from metaphyseal bone cultures also stimulated ⁴⁵calcium incorporation. This response to conditioned medium was dose-dependent and stable to 90°C. Vascular endothelial cells and conditioned medium from chondrocyte and osteoblast cultures did not stimulate ⁴⁵calcium incorporation by physeal chondrocytes. Thus, cells found in the metaphyseal bone produce a soluble factor, which promote calcium incorporation by physeal chondrocytes. The source of this factor is not chondrocytic, osteoblastic, or endothelial in origin.