Merkel cell hyperplasia in hypertrophic varieties of actinic keratoses

Hyperplasia and dysplasia of the epidermal neuroendocrine cells (i.e., Merkel cells, MC) have been suggested in lesional skin of actinic keratosis and chronic radiation dermatitis. Because several histological types of actinic keratosis exist, we investigated the occurrence of such MC hyperplasia according to each of the histological patterns. Using an immunoperoxidase technique and the cytokeratin monoclonal antibody CAM 5.2, we counted 0.02 CAM 5.2 + cells (i.e., MC)/mm epidermis in normal-appearing perilesional skin; 0.05 MC/mm epidermis in nonhypertrophic actinic keratoses and 4.09 MC/mm epidermis in hypertrophic actinic keratoses. This number was as high as 14.40 MC/mm epidermis when hypertrophic actinic keratoses showed club-like epithelial proliferations. In contrast to normal epidermis, where MC were found isolated within the basal cell layer, MC clustered in these club-like epithelial proliferations as they did in the normal parakeratotic zone of the rabbit lip epithelium. The significance of this selective MC hyperplasia is discussed.