Evidence for polymorphism of Merkel cells in the adult human oral mucosa.

We recently reported that Merkel cells in the normal palatine mucosa of adult rodents are highly polymorphic. In order to ascertain whether or not this polymorphism is also evident in the human oral mucosa, palatine mucosae from cadavers without oral diseases and perilesional palatine mucosae of patients with pleomorphic adenoma were examined by immunohistochemistry using an antibody against cytokeratin 20. Findings showed that Merkel cells in the human normal palatine mucosa were polymorphic, and a number of irregular-shaped Merkel cells (dendritic Merkel cells) with apparent cytoplasmic projections were present among typical oval to round Merkel cells. The mucosa usually contained a small number of oval to round Merkel cells residing in ectopic places such as prickle and granular cell layers. On the other hand, the slightly inflamed perilesional palatine mucosa contained an increased incidence of dendritic Merkel cells. Ectopic Merkel cells were rare in the perilesional palatine mucosa. Characteristics of dendritic Merkel cells were examined using specimens from perilesional palatine mucosae by means of immunohistochemistry and electron microscopy. It was shown that every dendritic Merkel cell and most roundish Merkel cells in the perilesional mucosa lacked innervation. Electron microscopy suggested that dendritic Merkel cells release secretory granules from the tip of the cytoplasmic process and the basal cytoplasm towards the lamina propria mucosae, in a manner resembling the case of similar cells in rodents.