An Alizarin red-based assay of mineralization by adherent cells in culture: comparison with cetylpyridinium chloride extraction.

Alizarin red S (ARS) staining has been used for decades to evaluate calcium-rich deposits by cells in culture. It is particularly versatile in that the dye can be extracted from the stained monolayer and assayed. This study describes a sensitive method for the recovery and semiquantification of ARS in a stained monolayer by acetic acid extraction and neutralization with ammonium hydroxide followed by colorimetric detection at 405 nm. This method was three times more sensitive than an older method involving cetylpyridinium chloride (CPC) extraction and resulted in a better signal to noise ratio, especially for weakly stained monolayers. The assay facilitates detailed inspection of mineralization by phase microscopy and semiquantification of the entire monolayer by extraction and quantification. The sensitivity of the assay is improved by the extraction of the calcified mineral at low pH and, since the mineral is already stained in a quantitative manner, there is no requirement for an additional colorimetric quantification step. Furthermore, the linear range is much wider than those of conventional assays for calcium, making dilutions of mineral extracts prior to measurement unnecessary. It has a wide range of potential uses including tumor characterization, mesenchymal stem cell evaluation, and osteogenic compound screening. Although more labor intensive than CPC extraction, the protocol is more sensitive and yields more reliable results for weakly mineralizing samples.