The neural dependency of Merkel cell development in the rat: the touch domes and foot pads contrasted

We have used the quinacrine labeling technique and electron microscopy to study the development of the Merkel cell population in the skin of the rat and how this is affected by denervation produced at birth and at various times thereafter. An unexpected difference was found between the Merkel cells of glabrous and hairy skin. In the paw pads of rats aged 1 day or older the Merkel cells differentiated normally and survived quantitatively in the absence of their nerves. In the touch domes however, denervation at 1-4 days prevented the differentiation of the normal Merkel cell population and led to the disappearance of all or most of the Merkel cells that were already present. The Merkel cells in touch domes of the lower leg were affected by denervation like those of the back skin, differing strikingly from the Merkel cells of the footpads, even though the hairy skin of the leg and the glabrous skin of the foot are innervated by the same anatomical nerve. In adult rats, axons regenerating to denervated paws reinnervated epidermal Merkel cells of the pads and restored essentially normal mechanosensitivity to them; thus the Merkel cells of mammalian glabrous skin, like their counterparts in the wholly glabrous skin of lower vertebrates (S. A. Scott, E. Cooper, and J. Diamond, 1981, Proc. R. Soc. London B211, 455-470; K. M. Mearow and J. Diamond, 1988, Neuroscience 26, 695-708), can act as targets for ingrowing nerves. However, even though the differentiation of Merkel cells in hairy skin is nerve dependent, they probably have in common with the Merkel cells of glabrous skin the role of acting as final targets for nerves during development and regeneration.