The Merkel cell: recent findings and unresolved problems.

Recently acquired knowledge about the Merkel cell is reviewed, and unresolved problems on the development and function of the cell are discussed. Concerning its developmental origin, the epidermal origin hypothesis has become prevalent since the discovery of cytokeratin polypeptides in it. While the significance of this cell is still unclear, the following functions are presumed: 1) the release of neurotransmitters or neuromodulators to the SAI mechanosensory nerve terminals; 2) the attraction and trophic action on peripheral nerve fibers; 3) the stimulation of the proliferation of keratinocytes leading to the three-dimensional development of the epidermal anlagen; 4) the maintenance of the normal differentiation of keratinocytes; and 5) the release of bioactive substances to subepidermal structures. These multiple functions imply a possibility that one Merkel cell sequentially performs all these different roles during its life span. However, it is also possible that Merkel cells consist of several functionally different subpopulations.