Identification of Metallothionein- and parathyroid hormone-related peptide (PTHrP)-positive cells in salivary gland tumours.

Ductal basal cells and myoepithelial cells (MEC) of normal salivary gland share metallothionein (MT)-positivity, while PTHrP positivity is restricted to ductal basal cells. We studied 21 benign and 4 malignant tumours in which MEC are thought to play a role using immuno-histochemical methods for detecting the presence of MT and PTHrP positive cells. In benign tumours, a shared positivity for MT and PTHrP is found in the inner layer of tubulo-ductal and trabecular structures, in part of the cells in the myxoid and chondroid matrices of pleomorphic adenoma, and in the basal epithelial lining of Warthin's tumours. In myoepithelioma almost all tumour cells demonstrate MT reactivity and a restricted positivity for PTHrP. MT-positive cells in oncocytoma were demonstrated in the periphery of some oncocytic islets, while PTHrP positivity was restricted to a few oncocytic cells. In malignant tumours, positivity for MT is found in the periphery of epithelial clusters of mucoepidermoid carcinomas, while PTHrP-positive cells are seen in cyst-like structures and scattered cells in solid arrangements of squamous cells. Although the biologic significance of the presence of MT in neoplastic cells is not yet clearly understood, MT may be necessary for the growth and differentiation in actively growing cells. The variability of MT expression in salivary gland tumours could be a reflection of the morphological heterogeneity and correlate with the degree of differentiation and maturation of the tumour cells. The observations suggest that MT may be considered an oncodevelopmental product.