Effect of human myeloma cells on bone nodules formation in BM cultures. Human BM cells were cocultured in the appropriate medium with (A) HMCLs (RPMI-8226, XG-1, XG-6, OPM2) and (B) freshly purified MM cells of 2 different patients (MM1 and MM2), or with normal BM B lymphocytes. Colony-forming bone nodules (CFU-OBs) were stained with alizarin red after 21 days and counted at the light microscopy as described in “Patients, materials, and methods.” Graphs represent the mean number of CFU-Fs/well ± SD of 6 independent experiments performed in triplicate. (C) CFU-OBs stained with alizarin red (original magnification, × 5) of representative fields. Graphs show the mean ± SD of bone nodules/field. C indicates control; ^*P < .05; ^**P < .01. Images were obtained as described for Figure 1.