A case report of a metastatic yolk sac carcinoma in the pulmonary artery of a young female Sprague-Dawley rat

Yolk sac carcinoma is an extremely rare tumor in rats and is usually found in the genital system of aged animals. We encountered a yolk sac carcinoma in the pulmonary artery of an 18-week-old female Sprague-Dawley rat. In a repeated dosing toxicity study (once weekly for 4 weeks, intraperitoneal), this rat was unexpectedly found dead on the 55th day after the final administration of the test article. At necropsy, grayish white nodules were found on the lung surface. Histopathologically, tumor emboli were observed in the trunk and branch of the pulmonary artery. Tumor cells with slightly basophilic vacuolated cytoplasm and large vesicular nuclei formed nests or clusters and were embedded in a homogenous eosinophilic and periodic acid-Schiff reaction positive matrix. The tumor cells and matrix were immunoreactive for laminin. The embolic tumor resembled yolk sac carcinoma showing a parietal pattern in rodents. Although the primary site was unknown, the tumor was considered to be a metastatic yolk sac carcinoma.