Current understanding of Merkel cells, touch reception and the skin

Abstract

Merkel nerve endings are found in human skin, as well as the skin of all vertebrates functioning as mechanoreceptors. Merkel cells are located in the basal layer of the epidermis both in hairy and in glabrous skin, and are often arranged in clusters. In hairy skin, Merkel cells are typically associated with large guard hairs and specialized sensory complexes known as touch domes. Merkel cells have lobulated nuclei and dense-core granules in the cytoplasm facing the associated afferent nerve terminal. Developmental studies have shown that avian Merkel cells are derived from the neural crest. In mammals, studies on transgenic mice have resulted in contradictory findings. Thus, the question of the developmental origin of mammalian Merkel cells remains to be resolved. Functionally, mammalian Merkel cell nerve endings are slowly adapting mechanoreceptors. In response to maintained mechanical stimulation they keep firing action potentials with characteristic irregular patterns. New experimental evidence strongly suggests a synaptic link between Merkel cell and nerve terminal using glutamate as the neurotransmitter. Occasionally, Merkel cells lacking contact with nerve terminals are found and thought to be part of a diffuse endocrine system. However, so far there is no evidence for exocytosis of substances from Merkel cells. It is still unclear whether the highly malignant type of skin cancer called Merkel cell carcinoma is actually derived from the same precursor cells as those functioning as mechanoreceptors. © 2010 Expert Reviews Ltd.