Mammary fibroadenoma in a young Holstein cow.

A mammary mass approximately 60 cm in diameter, which developed in the left udder of a 17-month-old Holstein cow, was removed surgically. The mass was well demarcated from the surrounding tissues and its cut surface was lobular in appearance. Microscopically, the mass consisted of a proliferation of well differentiated ducts and mucinous connective tissue. Immunohistochemically, the proliferative stroma contained two distinct cell types; vimentin-positive fibroblasts and spindle cells positive for vimentin and actin at the periphery of the ducts. The biological features of the present mammary tumor are thought to be generally compatible with those of mammary fibroadenoma reported in humans and dogs.