Abstract
Angiosarcomas arising in ocular tissues of four aging horses are described. Tumors were locally invasive and eventually metastasized via the mandibular and cervical lymph nodes. Pathologically, the tumors contained well-differentiated regions, in which vascular channels were lined by pleomorphic endothelial cells, as well as poorly-differentiated regions, in which vascular channels were either rudimentary or absent. Red blood cells were scarce in vascular structures formed by the tumors. Factor VIII related antigen (VIII:RAg), a blood vascular endothelial marker, was demonstrated by immunohistochemistry in tumor cells in both the well-differentiated and poorly-differentiated regions. The results suggest that ocular tumors arose in blood vascular endothelium and represented true hemangiosarcomas although a lymphatic endothelial origin could not be excluded. Immunohistochemical demonstration of VIII:RAg is a valuable adjunct to conventional stains in the diagnosis of equine vascular neoplasia.