Merkel Cell: Morphology, Developmental Origin, Function

Overview

Merkel nerve endings are mechanoreceptors in the vertebrate skin. They were named after the person who firstdescribed them – F. S. Merkel (1875). They consist of large, pale cells with lobulated nuclei forming synapse-likecontacts with enlarged terminal endings of myelinated nerve fibres. Inside the cell are intermediate filaments formedof simple cytokeratins and osmophilic granules containing variety of neuropeptides. In mammals, they can be foundin the basal layer of the skin and in those parts of the mucosa, which is derived from ectoderm. In contrast, in birdsthese cells are located in the dermis. The largest accumulation of Merkel nerve endings was found in whiskers ofmost mammals apart from man. There has been a controversy concerning the origin of Merkel cells. Results fromchick/quail chimeras and most recently also from double transgenic mice have shown that Merkel cells are derivedfrom the neural crest.Merkel cell play a role in the mechano-transduction process. In response to mechanical stimulicalcium ions enter Merkel cell and trigger the release of neurotransmitter, probably glutamate. Thus, Merkel cellappears to be essential for characteristic slowly adapting response of these receptors during maintained mechanicalstimuli. Cells in the skin of a similar appearance as Merkel cells are probably part of the diffuse neuroendocrinesystem and they do not function as mechanoreceptors. These cells, rather than those in mechanoreceptors, are mostlikely the origin of the highly malignant skin cancer called Merkel cell carcinoma.

Key words:
Merkel cell, Merkel nerve endings, mechanoreceptor, neuroendocrine cell, sinus hair, Merkel cellcarcinoma.