Collagen Fibres are Not Required for Initial Matrix Mineralization by Bone Cells

Passaged primary cultures of young adult rat bone marrow cells were maintained in medium containing combinations of the supplements dexamethasone, ascorbic acid and Na-{3-glycerophosphate. The cultures were examined by both light and scanning electron microscopy (with additional energy dispersive X-ray analysis). In cultures with fully supplemented medium, an extracellular matrix formed at the culture dish surface, beneath developing bone nodules; this matrix was mineralized but collagen-fibre free and corresponded to that which we have reported previously as the equivalent of the cement line in vivo. In selected samples this interfacial, cement line, matrix was analyzed by X-ray photoelectron spectroscopy. No evidence of this matrix was seen in the absence of dexamethasone, although collagen production was evident if ascorbic acid was present. No ex tracellular matrix of any type was visualized in cultures devoid of all three supplements. In the dexamethasone-containing cultures, collagen production was not evident in the absence of ascorbic acid. Addition of {3-glycerophosphate augmented the amount of mineralized matrix. However, matrix was also seen in the absence of this supplement, demonstrating that Na-{3-glycerophosphate was not essential to derive this biologically produced interfacial matrix. The results illustrate that the cement line matrix is the result of matrix production by differentiating osteogenic cells.