Distinguishing matrix-associated mineralization from solution-driven calcium phosphate precipitation in osteogenic culture

Mineral accumulation in osteogenic culture is often interpreted as evidence of cell-guided extracellular matrix mineralization. However, measurements could also capture calcium-phosphate precipitation outside of the organized extracellular matrix. High concentrations of β-glycerophosphate (β-GP) could promote nonphysiological mineral deposition. We developed a kinetic model to examine when mineral deposition occurs within the matrix and when precipitation from the medium becomes the dominant mechanism. The model couples β-GP hydrolysis, medium renewal, matrix-associated precursor formation and maturation, and solution-driven precipitation. We examined model behavior using parameter sweeps, global sensitivity analysis, comparison with published mineralization time courses, and equifinality analysis to determine whether similar mineralization amounts could arise from different pathway contributions and parameter combinations. Under baseline conditions (10 mM β-GP, 2-day renewal), the matrix-associated pathway activated first, followed by solution-driven precipitation about 18 h later. Once matrix capacity was approached, additional phosphate mainly increased non-matrix deposition. Parameter sets with similar total mineral gave matrix-associated fractions ranging from 0.17 to 1.00. Thus, the total amount of mineral does not necessarily indicate how much formed within the matrix, highlighting the importance of phosphate supply and medium renewal in interpreting osteogenic culture results.