Simultaneous Expression of Type IX Collagen and an Inhibin-related Antigen in Proliferative Myoepithelial Cells with Pleomorphic Adenoma of Canine Mammary Glands

To identify an alcian blue-positive component expressed in the early stage of mammary mixed tumor and to pursue the possible involvement of a tumorigenic inducing factor, monoclonal antibodies to type IX collagen were generated and used to investigate the immnnohistochemical kinetics of type IX collagen expression by the myoepithelial cell-derived chondrocyte-Uke cells during the development of chondrometaplasia. We also examined the expression of inhibin-related antigen using antibodies to a synthetic peptide spanning amino acids 1–30 of the inhibin α chain. At the earliest stage of chondrometaplasia, where myoepithelial cells began to proliferate inside the basement membrane, the cells expressed type IX collagen together with an inhibin-related antigen which was immunoreactive with the anti-inhibin peptide antibodies. The expression of the inhibin-related antigen was also demonstrated in normal embryonic chondrocytes and myoblasts, but was much less strong in mature chondrocytes and myotubes, strongly suggesting that the inhibin-related antigen is involved in the development of chondrocytes and myoblasts from undifferentiated mesenchymal cells as well as proliferating myoepithelial cells as a chondro-progenitor cell in the mammary mixed tumor. The pathophysiological significance of type IX collagen expression as a possible cell marker of the progenitor cell in myoepithelial cell-related chondrometaplasia is also discussed.