Original StudyDermal and Intraepidermal Merkel Cell Carcinoma With Squamous Cell Carcinoma: A Report of a Rare Case With Special Reference to the Touch DomeNarisawa, Yutaka MD, PhD; Inoue, Takuya MD, PhD; Nagase, Kotaro MD, PhDAuthor Information Division of Dermatology, Department of Internal Medicine, Faculty of Medicine, Saga University, Saga, Japan. Correspondence: Yutaka Narisawa, MD, PhD, Division of Dermatology, Department of Internal Medicine, Faculty of Medicine, Saga University, 5-1-1 Nabeshima, Saga, Saga, Japan 8498501 (e-mail: [email protected]). The authors declare no conflicts of interest. The American Journal of Dermatopathology: January 2021 - Volume 43 - Issue 1 - p 15-20 doi: 10.1097/DAD.0000000000001618 Buy Metrics Abstract In skin containing hair follicles, specialized epithelial structures known as “touch domes (TDs)” are located where the Merkel cells are clustered. We explored the histogenetic relationship between intraepidermal and dermal Merkel cell carcinomas (MCCs) and investigated which transformed progenitor cells can develop into intraepidermal MCC. We encountered an association between an extremely rare case of dermal and intraepidermal MCC with squamous cell carcinoma, which was examined using standard immunohistochemical methods with various epithelial, neuroendocrine, and TD markers including several immunohistochemical markers. Differential expression levels of CK20 and CD56 were found between intraepidermal and dermal MCCs, indicating molecularly distinct MCC populations. CK15 and CK17, expressed in TDs, were partially expressed in the intraepidermal neuroendocrine component at the tumor periphery in intraepidermal MCC with squamous cell carcinoma. These differences may suggest that the origin of dermal and intraepidermal MCCs is different under pathological conditions. We hypothesize that intraepidermal MCC is derived from tissue-specific stem cells localized within TDs. Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.