Morphologic evidence of the histogenesis of epithelial thymoma in a cow.

Light and electron microscopic studies were performed on an epithelial thymoma obtained from a cow. Tumor masses were found in the cervical and dorsal mediastinal portions, in the hilus and parenchyma of the lung, on the surface of the kidney, and in a uterine horn. The masses were encapsulated and divided into distinct lobules or nodules by prominent trabeculae. Examination of the tumor cells by light microscopy showed squamous epithelial characteristics in both the eosinophilic granules and the periodic acid-Schiff-positive granules. Electron microscopic observations revealed that the tumor cells had intercellular microvilli, which appeared to be the interdigitations between the cells, tonofilaments, and desmosomes. A few of the tumor cells contained round vesicles which resembled the hypertrophic type of epithelial cells of a normal thymus. These findings indicated that the tumor originated in the thymus' epithelial cells.