Review Article: Merkel Cell Carcinoma

Abstract

In 1875, FRIEDRICH Sigmund Merkel (1845-1919) described a unique epidermal nondendritic, nonkeratinocyte cell, which he called a tactile cell (Tastzelle). This cell, now bearing his name, was thought to be a primary touch receptor.1 Merkel cells are now generally believed to be primary neural cells, found as single cells within the basal layer of the epidermis or grouped together as a component of the tactile hair disc of Pinkus in the hairbearing skin of mammals, functioning as slowly adapting type I mechanoreceptors.2,3 Their origin from the neural crest is supported by recent interspecies embryonic transplantation models in birds,4 but other authors invoke an epidermal origin from keratinocytes.5,6 In 1972, Toker7 described five cases of a trabecular cell carcinoma of the skin, which was initially thought to be derived from sweat glands. However, in 1978 Tang and Toker8 found densecore granules typical of Merkel cells and other neuroendocrine cells on electron microscopy in these trabecular tumors, suggesting an origin from the Merkel cell. Also called (neuro)endocrine cancer of the skin or small-cell carcinoma of the skin, the name Merkel cell carcinoma (MCC) was suggested by De Wolf-Peeters et al9 in 1980. Approximately 2,000 cases have thus far been reported, and this disease has not previously been the subject of a comprehensive review in the oncology literature.