The aim of this study was to investigate the expression of tumour endothelial marker 8 (TEM8) in canine mammary gland tumours (MGTs) by immunohistochemistry and to evaluate the association between tumour cell TEM8 expression and tumour histological features, histological grades and expression of luminal and basal/myoepithelial cell markers. TEM8 expression was detected in >60 % of neoplastic epithelial cells in all simple adenomas (n = 25), simple carcinomas (n = 43) and invasive micropapillary carcinomas (n = 5) studied. Six of the 18 solid carcinomas studied showed TEM8 expression in >60% of carcinoma cells present in solid structures and in 12 of the 18 solid carcinomas, <30% of the luminal structure-forming carcinoma cells showed TEM8 expression. TEM8 expression in the neoplastic cells was not associated with histological malignancy in canine MGTs. TEM8+ tumour cells frequently showed the luminal-like phenotype cytokeratin (CK)19+/p63-/α-smooth muscle actin (SMA)-, while most TEM8- tumour cells exhibited the basal-like phenotype CK19-/p63+/αSMA-. These findings indicate that TEM8 may be involved in maintaining the characteristics of luminal cells in canine MGTs and that TEM8 would be useful in identifying the type of neoplastic epithelial cell in MGTs.
