Merkel cells--review on developmental, functional and clinical aspects

The objective of this review is to introduce Merkel cells, to provide a basic overview on the theoretical background of function, development and clinical importance of Merkel cells. Merkel cells (MCs) are post-mitotic neuroendocrine cutaneous cells primarily localized in the epidermal basal layer of vertebrates and concentrated in touch-sensitive areas in glabrous, hairy skin and in some mucosa. There is a great site variation in the density of MCs. In routine light microscopy human MCs can hardly be identified. Cytokeratine 20 is a reliable marker with highest degree of specifity. MCs can be also distinguished by electron microscopy. The origin of human MCs has been controversial. Some investigators believe that it is a neural crest derivate, whereas others have proposed that it is a differentiation product of the fetal epidermal keratinocytes. Most studies focus on neuroendocrine functions and their possible malignant transformation into Merkel cell carcinomas (MCC). MCC is an uncommon and often aggressive malignancy and found mainly in elderly patients. It occurs most frequently in the head and neck region. MCC may be difficult to diagnose, it appears as a firm, painless lump. Diagnosis is based on typical histology representation on haematoxylin-eosin stained slides together with the results of immunohistochemistry. Histologically, MCC has been classified into three distinct subtypes: trabecular, intermediate and small cell type. Radical surgery is the recomended procedure for the treatment of primary MCC. Oncological treatment is generally reserved for stage III. (distant metastases) cases of MCC (Tab. 1, Fig. 13, Ref. 58). Full Text in free PDF www.bmj.sk.