Absolute proliferation of OPC1 cells maintained in six types of tissue culture medium over three two-week periods . At day 5, the OPC1 cell counts in the wells containing vitamin D metabolites ± osteogenic factors (BM+) were statistically lower than the negative control (BM−). OPC1 treated with BM+ and 1,25OH₂D₃ experienced the largest statistically antiproliferative effect. By day 10, OPC1 treated with BM+ and active 1,25OH₂D₃ remained around 50% of the BM− cellular confluency. The effect of vitamin D₃ was similar to 1,25OH₂D₃ in both groups of media. Means ± SEM are shown with linear trendlines in the same color scheme for each experimental group.