6.02 - Merkel Cells

Abstract

Merkel nerve endings are found in the skin of all vertebrates, functioning as mechanoreceptors. They consist of large cells (Merkel cells) with lobulated nuclei and dense-core granules in the cytoplasm facing the associated afferent nerve terminal. In mammals, Merkel cell nerve endings are located in the basal layer of the epidermis both in hairy and in glabrous skin appearing often in clusters forming specific structures like touch domes or special arrangements around sinus hairs. Recent developmental studies have shown that the Merkel cells are not modified epithelial cells but originate from the neural crest. Functionally, mammalian Merkel cell nerve endings are slowly adapting mechanoreceptors. New experimental evidence strongly suggests a synaptic link between Merkel cell and nerve terminal using glutamate as neurotransmitter. The role of the Merkel cell appears to be particularly important for the slowly adapting responses of these receptors to maintained stimuli. Occasionally, Merkel cells lacking contact with nerve terminals are found and thought to be part of a diffuse endocrine APUD (amino precursor uptake and decarboxylation) system. However, so far there is no evidence for exocytosis of substances from Merkel cells. A highly malignant type of skin cancer called Merkel cell carcinoma may be derived from cells originating from the neural crest.