Tumor αvβ3 integrin enhances osteoclast recruitment and osteolysis following intratibial injection of tumor cells. (a) The relative tumor burden was measured by real-time quantitative PCR 14 days following direct intratibial injection of 66cl4pBabe and 66cl4beta3 cells. Representative H&E-stained sections through tibias injected with (b) 66cl4beta3 or (c) 66cl4pBabe cells. Osteoclasts are marked with arrows. B, bone; T, tumor. Scale bar = 50 μm. (d) Serial sections adjacent to those stained by H&E were stained for tartrate-resistant acid phosphatase (TRAP) activity, and the active osteoclasts (TRAP-positive) present per millimeter of bone in tibias injected with either 66cl4beta3 or 66cl4pBabe cells were counted. Results of two independent experiments are shown. n = 4 mice/group per experiment *P < 0.001.