USPIO or SPIO was injected intraperitoneally (500 µmol Fe/kg) into 8-week-old female C57BL/6 mice (Japan SLC, Inc, Tokyo, Japan), each weighing approximately 30 g. Groups of three mice each were treated with USPIO or SPIO and sacrificed at time points of 30 minutes and 1, 3, 12, 24, and 48 hours afterward; two mice were the controls. Organs to be studied were fixed in 10% paraformaldehyde and examined histologically.