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.