Phenotypic variation of thymic epithelial cells and partial spontaneous regression in thymoma of a cow

Thymoma was recognized in a 12-year-old Holstein dairy cow presenting with no significant clinical signs. The animal was conventionally slaughtered at an abattoir and, on postmortem examination, a spherical tumor mass was found in the anterior region of the mediastinum. Histologically, the tumor was fibrously encapsulated and exhibiting varying-sized cystoid cavities containing serous fluid and necrotic and hemorrhagic foci. No architectural features showing distinct characteristics to a sufficient degree to allow identification of the thymus or other organs could be seen. The tumor showed diffuse solid sheet-like or nodular patterns of proliferation and consisted of a dual cell population: one was neoplastic epithelial cells that were immunohistochemically positive for cytokeratin, and the other was T lymphocytes that exhibited positive immunohistochemical staining with CD3. This finding enabled a diagnosis of mixed thymoma to be made. Neoplastic epithelial cells represented a wide spectrum of phenotypic expression, including (1) spindle shaped cells with a chiasmus-like, interlacing fascicular, or storiform growth pattern; (2) small, round to oval cells with scant cytoplasm showing indistinct boundary of cells; (3) large cells arranged in a solid sheet-like pattern; (4) vacuolated cells similar to signet ring cells; (5) clear cells that had defined cell membranes; and (6) occasional formation of micronodules composed of neoplastic epithelial cells. In addition, some areas of the tumor exhibited features of partial spontaneous regression characterized by collapse or disintegration of tumor tissues in close association with fibrocollagenous connective tissue deposition. Even if the morphology of thymic epithelial cells in thymoma is known to display variable appearance, it is likely to be unusual that, in the current bovine case, such thymic epithelial cells exhibited a wider spectrum of phenotypic expression within a single tumor. This salient histomorphological heterogeneity shown by thymic epithelial cells might have presumably been associated with innate characteristics specific to pluripotent thymic epithelial cells during thymoma tumorigenesis.