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.