MINERALIZATION OF BONE-LIKE EXTRACELLULAR-MATRIX IN THE ABSENCE OF FUNCTIONAL OSTEOBLASTS

When grown in medium containing ascorbic acid and beta-glycerol phosph ate, mouse MC3T3-E1 cells express an osteoblast phenotype and produce a highly mineralized extracellular matrix. The purpose of this study w as to independently examine the role of the collagenous matrix and fun ctional osteoblasts on the mineralization process. Cultures with and w ithout an extensive collagenous matrix were prepared by growing MC3T3- E1 cells in the presence and absence of ascorbic acid. Matrix-rich cul tures mineralized at much lower calcium phosphate ion products than no nmatrix cultures. At higher ion products, spontaneous precipitation in the medium and cell layers of nonmatrix cultures were observed. In co ntrast, mineral in matrix-rich cultures was still exclusively associat ed with collagen fibrils and not with ectopic sites in the cell layer or medium. To examine the effect of cell viability on matrix mineraliz ation, cells were grown 8 or 16 days in the presence of ascorbic acid, then killed and incubated in a mineralizing medium. Significant miner alization was not observed in the collagenous matrix of 8-day killed c ultures or age-matched controls, At 16 days mineral was associated wit h collagen fibrils at specific foci in the matrix of both viable and k illed cultures, This observation is consistent with the concept that c ollagenous matrices must undergo a maturation process before they can support mineral induction and growth. It further shows that osteoblast -like cells are not required for mineralization of mature matrices, bu t are required for matrix maturation.