Von Kossa Staining Alone Is Not Sufficient to Confirm that Mineralization In Vitro Represents Bone Formation

Numerous techniques are currently used to characterize biological mineralization in intact tissues and cell cultures; the von Kossa staining method, electron microscopic analysis (EM), X-ray diffraction, and Fourier transform infrared spectroscopy (FTIR) are among the most common. In this study, we utilized three of these methods to compare the mineralization of cultured fetal rat calvarial cells (FRC) and the osteoblast cell lines 2T3 and MC3T3-E1 with the in vivo mineral of rat calvarial bone. The cells were cultured with or without ascorbic acid (100 µg/ml) and β-glycerophosphate (2.5, 5, or 10 mM βGP), and harvested between 16 and 21 days (FRC cells and 2T3 cells) or at 30 days of culture (MC3T3-E1 cells). In the FRC cultures, maximal von Kossa staining was observed with 2.5 and 5 mM βGP in the presence of 100 µg/ml ascorbate. FRC cells also showed some von Kossa staining when cultured with βGP alone. In contrast, maximal von Kossa staining for MC3T3-E1 cells was observed with 10 mM βGP. Only the cultures of MC3T3-E1 cells that received both ascorbate and βGP produced von Kossa positive structures. The 2T3 cultures produced von Kossa positive staining only upon treatment with ascorbic acid and βGP, which was greatly accelerated by bone morphogenic protein-2 (BMP-2). FTIR was performed on the mineral and matrix generated in FRC, MC3T3, and 2T3 cultures, and the results were compared with spectra derived from 16-day-old rat calvaria. The mineral-to-matrix ratios calculated from FTIR spectra for rat calvaria ranged from 2.97 to 7.44. FRC cells made a bonelike, poorly crystalline apatite, and, with increasing βGP, there was a statistically significant (P ≤ 0.02) dose-dependent increase in the mineral-to-matrix ratio (0.56 ± 0.16, 1.00 ± 0.32, and 2.46 ± 0.76, for 2.5, 5, and 10 mM βGP, respectively). The mean carbonate-to-phosphate ratios of the FRC cultures were 0.015, 0.012, and 0.008, in order of increasing βGP concentration, compared with rat calvaria values of 0.009–0.017. The 2T3 cells treated with BMP-2 also made bonelike crystals, similar to those observed in FRC cultures. In contrast, the cultures of von Kossa positive MC3T3-E1 cells did not display a significant amount of mineral (maximum mineral-to-matrix ratio was 0.4). Thus, although the von Kossa stainings of FRC, 2T3, and MC3T3-E1 were very similar, FTIR analysis indicated that calcium phosphate mineral was not present in the MC3T3 cultures. By EM, the mineral in FRC cell cultures and 2T3 cultures was generally associated with collagen, whereas rare or sparse dystrophic mineralization of unknown chemical origin was evident in the MC3T3-E1 cultures. These studies demonstrate that von Kossa staining alone is not appropriate for the identification and quantitation of bonelike mineral and, hence, other techniques such as X-ray diffraction, EM, or FTIR should be utilized to verify the presence and quality of calcium phosphate phases.