Trichoblastoma with abundant plump stromal cells in a dog.

Histopathological and immunohistochemical examinations were made on a cutaneous tumor on the head of an 11-year-old female mixed-breed dog. The tumor was well demarcated and comprised multilobular structures of neoplastic epithelial cells with abundant plump peritumoral stromal cells. The neoplastic cells formed irregular cell cords or trabeculae and were arranged in characteristic palisades at the periphery. Immunohistochemically, neoplastic cells were positive for p63 and the several cytokeratins examined. In contrast, the plump peritumoral stromal cells were positive for vimentin and unevenly for nestin, a neuroepithelial stem cell protein. The stromal cells prominently proliferated in proximity to epithelial neoplastic cells, suggesting a close interaction between these two cell types.