Neuroendocrine carcinoma of the skin: an immunohistochemical study of tumor markers and neuroendocrine products

Fifteen neuroendocrine carcinomas of the skin (Merkel cell tumors) were stained within the constraints of tissue availability by the Grimelius method and immunohistochemically for keratin, neuron‐specific enolase (NSE), carcinoembryonic antigen (CEA), S‐100, common leukocyte antigen (CLA), met‐enkephalin, bombesin, calcitonin, ACTH, gastrin, and somatostatin. Focal argyrophilia was present in 5 of 12 tumors. All tumors tested demonstrated immunoreactivity for NSE and 5 tumors were positive for keratin. One tumors appeared to demonstrate focal ACTH‐like immunoreactivity, but otherwise no immunoreactivity for the above mentioned polypeptide hormones was noted in 11 completely studied tumors. One tumor contained histologically obvious areas of squamous differentiation in addition to areas of Merkel cell tumor. In various tumors, keratin immunoreactivity was present either in areas of histologically obvious squamous differentiation, in randomly scattered single cells not histologically identifiable as squamous, or in a paranuelear dot‐like distribution. Immunoreactivity for CEA, S‐100, and CLA was not present in any tumors. The lack of met‐enkephalin and the presence of squamous differention in these tumors indicates multidirectional differentiation in a fashion not phenotypically typical of Merkel cells.