Ultrastructure of monomorphic adenoma (ductal type) of the minor salivary glands.

A monomorphic adenoma (ductal type) of the palatal minor salivary glands is reported. Ultrastructurally, the predominant tumor cells contained zymogenlike dense secretory granules. There was a continuum from tumor cells with little or no secretory granules to ones with increasing numbers of granules. Except for that, the tumor cells shared many of their fine structural features. Histogenetically, the tumor originated from neoplastic proliferations of "undifferentiated" stem cells corresponding to "end-bud" stage of salivary glands embryogenesis. Under neoplastic influences, such cells underwent further cytodifferentiation and functional maturation along the secretory cell lines. In mature salivary glands, such pluripotential cells reside as "reserve" cells in the intercalated ducts.