Ito cell tumor: immunohistochemical investigations of a rare lesion in the liver of mice.

In 2 lifespan transgeneration experiments using a total of 4,682 CBA/J mice, we observed uncommon lipomatous lesions in the livers of 8 mice independent of the treatment. Macroscopically, the lesions were described as pale white areas (2) or nodules (6) during necropsy. The lesions ranged from 1 to 15 mm in diameter. Microscopically, the lesions consisted of nodular aggregations of round to spindle-shaped cells that partly caused distinct compression of the adjacent hepatic parenchyma. The tumor cells were smaller than hepatocytes and had dark oval nuclei. Many of the more spherical cells contained clear vacuoles of various sizes, which were shown to be lipid droplets by oil red O staining. In addition to Gomori's silver and Masson's trichrome staining, several immunohistochemical stains were used to characterize the origin of the proliferating cells. Tumor cells were labeled by vimentin, actin, desmin, and proliferating cell nuclear antigen. The 2 cell phenotypes showed similar staining characteristics. Increased amounts of laminin and tenascin, 2 extracellular matrix proteins of the liver, were detected within these neoplasms. Summarizing, we suggest that these tumors are of Ito cell origin.