IL-1 mediates TNF-induced osteoclastogenesis in vivo. WT and IL-1RI–deficient mice were subcutaneously injected daily for 5 days at the base of the skull with vehicle or 0.75 or 1.5 μg TNF. The animals were sacrificed on day 6 and TRAP-stained histological sections of calvariae histomorphometrically analyzed for osteoclast number. (A) Representative histological sections. (B) Histomorphometric analysis of osteoclast number (*P < 0.05, **P < 0.01 vs. WT).