Ultrastructural immunolocalization of a cartilage-specific proteoglycan, aggrecan, in salivary pleomorphic adenomas

A pleomorphic adenoma (PA) is the most common epithelial tumor in the salivary glands, but it frequently shows a mesenchyme-like histology, including the presence of myxoid and chondroid areas. Cartilage-specific matrix proteins are deposited in PA. Aggrecan is a major component of cartilage-specific proteoglycans. The present study examined the ultrastructure of the stromal areas in ten salivary PA specimens and investigated the distribution of aggrecan by immunoelectron microscopy. Aggrecan was deposited in the myxoid and chondroid stroma of PA. Ultrastructural observations revealed many proteoglycan cores and fibrils in the myxoid stroma and some spindle-shaped neoplastic myoepithelial cells with vacuoles and actin filaments in the myxoid areas. By immunoelectron microscopy, positivity for aggrecan was observed in the vacuoles of neoplastic myoepithelial cells, which coexisted with the viscous materials, and it was also frequently seen in electron-dense crystals in the myxoid stroma. These findings suggest that neoplastic myoepithelial cells produce aggrecan and release it from vacuoles, and aggrecan is then deposited in the myxoid stroma. Aggrecan deposition is therefore considered to play an important role in the formation of the mesenchyme-like stroma, especially the myxoid stroma.