An in vitro model for mineralization of human osteoblast-like cells on implant materials

In in vitro mineralizing cell–implant system was developed to study osteoblast attachment, secretion of extracellular (ECM) matrix proteins and mineralization. Saos-2 cells were plated on Tivanium (Tiv, Ti-6Al-4V), Zimaloy (Zim, Co–Cr–Mo) and glass disks. The cells were cultured in α-MEM medium with 10% fetal bovine serum and 50 μg ml-1 ascorbic acid. The cultures were analyzed for calcification and for mRNA expression for ECM proteins after 1, 2, 4 and 6 weeks. Calcium content was significantly higher in cells on Tiv, less on Zim and least on glass disks. With the addition of 3 mimage β-glycerophosphate (β-GP), the cell layer was more calcified on Zim than on Tiv and all substrates had three times more calcium than cultures without β-GP. All subsequent experiments were performed without β-GP. Phalloidin immunofluorescence microscopy of the actin-based cytoskeleton at 2 weeks demonstrated nodules composed of multilayered, cobblestone-appearing osteoblasts overlying calcified matrix which was stained with calcein. On Tiv, calcified nodules were connected in a trabecular-like pattern while on Zim, calcification was dispersed throughout the cell layer. Northern blots for alkaline phosphatase, bone sialoprotein, osteocalcin and α1(I) procollagen mRNAs were performed at different time points. The amount and pattern of calcification as well as the expression of ECM-mRNAs differed on each implant material. The results indicate that Tiv stimulates the production of more ECM proteins and mineralized matrix than Zim or glass in this osteoblast-like cell/implant culture.