Are Merkel and Grandry cells two varieties of the same cell in birds?

For over a century it has been held that the Grandry corpuscle and the Merkel corpuscle are unique sensory organs in aquatic and nonaquatic birds, respectively. In other words, the Grandry and the Merkel cells are two varieties of the same receptor cell and never coexist together. Contrary to what has been believed, we found unmistakable Merkel cells in addition to Grandry cells in the lingual mucosa of the duck, Anas platyrhynchos var. domestica, an unprecedented observation in an aquatic bird. This study concerns the fine structure of these cells in the tongue of the duck. Although some ultrastructural features are shared by the Grandry and the Merkel cells, i.e., the presence of numerous dense-cored granules and microvillous projections at the cell surface, the morphology of these two types of cells clearly differ, especially in their size. Cells suggestive of stages intermediate between these two cells have never been recognizable. The present study indicates that the Grandry and the Merkel cells are not two varieties of the same cell in birds. It is suggested that the Merkel cell may exist in every vertebrate species, while the Grandry cell is unique to aquatic birds.