Strontium promotes osteoblasts' activity in vitro. (A) The mineralization of bone nodules was detected by alizarin red S staining in primary osteoblast cultures that had been incubated for 21 days in osteogenic media alone (i) or osteogenic media containing 1 mmol/L Sr (ii) or 10 ng/mL PTH (iii). The mRNA expression levels of runx2 (B) and RANKL (C) were determined by real-time RT-PCR in primary murine osteoblasts. Cells plated in 6-well plates were treated in complete media with 1 mmol/L Sr or 10 ng/mL PTH and harvested after 2, 4, 8, 12, and 24 hours. The results are reported as fold change in gene expression relative to untreated controls after being normalized to GAPDH (ΔΔC_T method), and columns represent mean value (± SEM) of 3 different experiments.