Determination of the optimal cell density of the inoculated D3 ES cells for osteoclastogenesis on ST2 and OP9 stromal cell lines. The duration of cultures was as outlined in culture system A of Fig 1. The indicated numbers of cells obtained at day 5 were seeded on ST2 or OP9 monolayers grown on 24-well plates. After 6 days TRAP⁺ MNCs were counted. Each column represents means ± SD of triplicate cultures.