Seventy-three mammary
tumors and three mammary tissue specimens were examined to elucidate the expression of
bone morphogenetic protein (BMP)-6 in the myoepithelial cells of canine mammary gland
tumors. Morphologically, the myoepithelial cells were classified into four types: resting and proliferating cells inside the basement membrane, and spindle- and star-shaped cells proliferating in the outer area of the basement membrane. The characteristics of these myoepithelial cells were confirmed by immunohistochemistry using
antibodies raised against
keratin,
cytokeratin 19, alpha-smooth muscle actin, and
vimentin. In simple
adenoma, a small number of resting myoepithelial cells was immunopositive for
BMP-6. In complex
adenomas and benign mixed
tumors, all types of myoepithelial cells, depending in some cases on their specific location within the
tumor, were immunopositive for
BMP-6, but almost all of the tubular epithelial cells were immunonegative. Foci consisting of a proliferation of BMP-6-positive star- and spindle-shaped cells had mucinous stroma with marked hyaline and chondroid changes. In contrast, the foci with BMP-6-negative spindle- and star-shaped cells tended to have mucinous stroma without chondroid change. Several types of mesenchymal cells including chondrocytes, osteoblasts, and fibroblastlike cells in the mixed
tumors, showed an intense immunopositive reaction for the
BMP-6 antibody, and were located close to the ectopic cartilage and bone matrix. No significant immunoreactivity for
BMP-6 was observed in most of the malignant mammary
tumors; only one
malignant mixed tumor was examined. All of these findings indicate that
BMP-6 expression in myoepithelial cells may increase in complex
adenomas and benign mixed
tumors in canine mammary glands, and that
BMP-6 expression is most intense in the vicinity of chondroid matrix in these
tumors.