Merkel Cells Are Postmitotic Cells of Neural Crest Origin

Chick/quail chimeras and double transgenic Wntl -cre/R26R mice, in which neural crest cells are permanently marked by expression of s-galactosidase, are evidence that Merkel cells in Merkel nerve endings of birds and mammals are neural crest derivatives. Like melanoblasts their precursors colonise the skin. Cytokeratin 8- and 18-positive Merkel cells in mouse whisker follicles are postmitotic cells. They are Ki 67-negative and their nuclei exhibit features of differentiated resting cells. In contrast, Merkel cell progenitors are likely to be proliferating cells. Simple cytokeratins are not suitable markers for their identification. Possible progenitors of Merkel cells that contain dense core granules can be identified using electron microscopy in the connective tissue surrounding the bulge region of the whisker follicle. Possible progenitors of Merkel cells in epidermis are s-galactosidase-positive, but cytokeratin-negative. Verification of the Merkel cell precursor nature of these cells requires specific markers that will enable us to distinguish them from precursors of melanoblasts. Thus, the question of the location of proliferative Merkel cell progenitors remains open.