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Merkel cell hyperplasia in chronic radiation‐damaged skin: its possible relationship to fibroepithelioma of Pinkus

Moderate hyperplasia of Merkel cells (MC) in chronic sun‐damaged skin and hypertrophic actinic keratoses is well known. In the present study we investigated the number of MC in 24 samples of chronic radiation dermatitis and 19 cases of fibroepithelioma of Pinkus (FP), which is known to arise preferably in radiation‐damaged skin. Using antibodies against the low molecular weight cytokeratins 8, 18, and 20 and chromogranin A to visualize MC, we found hyperplasia of MC in chronic radiation dermatitis. Additionally, in all FPs we could detect many MC, especially in areas with a pronounced fenestrated pattern. Recently, regulative functions of MC on the growth of follicular epithelium under various conditions were discussed. Thus, MC hyperplasia suggests a causal role also in the development of FP. In this context, hyperplasia of MC in chronic radiation dermatitis could explain the frequent occurrence of FP due to radiation exposure. As we recently found MC also in trichoblastomas but not in basal‐cell carcinomas, the MC in FP may indicate its relationship to the benign trichoblastoma rather than to the basal‐cell carcinoma. It is possible that regulative influences of the MC are important for the clinically rather benign course of FP.