RESUMO
A fibroadenoma was diagnosed in the mammary gland of a lamb by immunohistochemical method and measurement of oestradiol hormone. The tumor was characterized by an encapsulated firm mass with a creamy-white cut surface. Histologically, it consisted of variably-sized sinus ducts, covered by a single or multiple layers of proliferated epithelial cells, and embedded in a loose connective tissue. Immunohistochemical results revealed that more than half of the epithelial tumor cells were labeled only for estrogen, but not for progesterone. Moreover, the high level of plasma ER concentration in contrast to the normal PR values was consistent with immunohistochemical findings. Both results suggested that reproductive hormones, exclusively estrogen, influenced the developmental stage of the mammary glands and are responsible for neoplastic changes in this case. This study indicated for the first time the novel evidence of estrogen-induced fibroadenoma in a 2-month-old lamb mammary gland tissue