Abstract
A testicular yolk sac carcinoma (YSCA) was diagnosed in a 28-day-old male Japanese black calf. Macroscopically, the abdominal cavity was filled with reddish gelatinous masses of various sizes. There were no testes in the scrotum or pelvic cavity. Histologically, the masses consisted of the tissues showing a variety of patterns; loose reticular network, pseudopapillary arrangement, festoon, solid nest, and labyrinthine pattern. Tumor cells were round to oval, with single central or polar nuclei with sharply defined nuclear borders and deeply basophilic chromatin. Tumor cells sometimes had glycogen granules, periodic acid–Schiff (PAS)-positive inclusions, or sudanophilic droplets in their cytoplasm. Hyaline matrices were found in extracellular areas and were intensely PAS positive. Immunohistochemically, most of the tumor cells and hyaline matrices had positive reactivity to anti-alpha-fetoprotein and anti-placental-alkaline-phosphatase antibodies. Positive reaction to anti-laminin antibody appeared only in hyaline matrices. This is the first case of a tumor in a domestic animal with histologic and immunohistochemical features analogous to those of human YSCA, endodermal sinus type.