Our pervious electron microscopic studies indicated that Merkel cells (MCs) in the gerbil palatine mucosa were polymorphic, possibly reflecting different function. In order to verify and extend this evidence, the shape of and the innervation to MCs in the palatine mucosa of six different species of rodents including the Mongolian gerbil and the rat were examined by immunohistochemistry and transmission electron microscopy. Immunohistochemistry using anti-cytokeratin 20 (CK20) antibody revealed that in the gerbil palatine mucosa, approximately half of MCs were dendritic. Confocal laser scanning microscopy after double labeling with anti-cytokeratin and anti-PGP 9.5 or anti-Na+/K(+)-ATPase beta 1 subunit antibodies indicated that most of the dendritic MCs (DMCs) in these mucosae were free of innervation. Electron microscopy showed that all species of rodents examined contained abundant dendritic MCs as well as roundish (oval to round) MCs (RMCs) with typical innervation. Secretory granules of the RMCs were usually concentrated at the synaptic site, whereas those of the DMCs tended to accumulate in the tips of the cytoplasmic processes and in the cytoplasm facing the basal lamina. Some MCs showed features intermediate between those of the RMC and DMC. These results indicate that MCs in rodent palatine mucosae are consistently polymorphic, and that DMCs may represent a distinctive subset with specific, presumably including endocrine and paracrine, functions different from those of RMCs.