TRIPARTITE DIFFERENTIATION (SQUAMOUS, GLANDULAR, AND MELANOCYTIC) OF A PRIMARY CUTANEOUS NEUROENDOCRINE CARCINOMA - AN IMMUNOCYTOCHEMICAL AND ULTRASTRUCTURAL-STUDY

We report a case of primary cutaneous neuroendocrine carcinoma (PCNEC) with squamous, glandular, and melanocytic differentiation and associa ted Bowen disease. The paranuclear globular positivity of low-molecula r-weight cytokeratins agrees with the ultrastructural observations of paranuclear fibrous bodies in the small neuroendocrine cells, while th e diffuse cytoplasmic positivity cor-responds to the sparse intermedia te filaments in large cells with squamous differentiation. ''Transitio nal forms'' are characterized by both diffuse and globular cytoplasmic positivity for cytokeratins and by the ultrastructural evidence of ne uroendocrine and squamous features. Therefore the ultrastructural demo nstration of intracytoplasmic tonofibrils and tonofilaments, intercell ular glandular lumina, lined by well-formed microvilli, and immature p remelanosomes in the neurosecretory cells supports the proposed tripar tite differentiation of neuroendocrine cells of this case of PCNEC.