Abstract
Spontaneous hemangiosarcoma is generally uncommon in rats. Furthermore, there are only a few case reports in young rats. This report describes a spontaneous hemangiosarcoma in a young 7-week-old rat. At necropsy, no remarkable changes were noted in any organs including the liver. Histopathologically, multifocal small tumors were located mainly in the perilobular region of the liver. The tumors comprised polygonal to short spindle-shaped cells that showed cellular atypia and local infiltration. In the foci, there were blood-filled spaces lined by spindle-shaped cells. Reticular fibers, which were loosely connected together and formed an irregular network within the foci, were noticed with Azan and silver stain. Immunohistochemistry revealed that tumor cells expressed vascular endothelial cell markers: von willebrand factor and CD34, respectively. These features indicate that the tumor originated from vascular endothelial cells. Although the primary lesion was uncertain, the foci were all small and multicentrically located mainly in the perilobular region, indicating that the liver lesion likely formed by hematogenic metastasis. Taken together, we diagnosed this case as a metastatic hemangiosarcoma.