Testicular efferent ductule cyst of a dog.

A 9-year-old male Shetland Sheepdog had a small mass in the left testis. Grossly, the round to oval cyst was present at the upper pole of the testicular parenchyma near the head of the epididymis. Histologically, the cyst was lined by a single layer of nonciliated and ciliated epithelial cells. Immunohistochemically, the epithelial cells of the cyst showed expression of the low- and high-molecular-weight cytokeratins, vimentin, and desmin similar to that of normal efferent ductules in the dog. The testicular cystic dysplasia was thought to originate from the efferent ductules.