Calcification of differentiating skeletal mesenchyme in vitro.

Embryonic limb-bud mesenchyme was induced to calcify in culture by the addition of 3 mM inorganic phosphate to the medium. Phosphate enhanced calcification of the matrix produced by mesenchymal or fibroblast-like cells, whereas no calcification was evident in areas where cartilage had developed. However, calcification was induced throughout the cell layer by altering the cartilage matrix properties with certain enzymes or by changing the phenotypic expression of the cells with vitamin A.