Neuroendocrine (Merkel cell) carcinoma of the skin. A histologic and ultrastructural study of two cases.

The clinical, histologic, and ultrastructural features of two cases of a primary cutaneous neuroendocrine neoplasm probably arising from Merkel cells are presented and compared with previously reported examples. This recently described tumor arises in the dermis of elderly individuals, is often locally aggressive, may metastasize to regional lymph nodes, and eventually may cause death. Microscopically, sheets of primitive cells fill and expand the dermis. Ultrastructural study of our cases revealed morphologic similarities to normal cutaneous Merkel cells, including peripherally situated dense-core neurosecretory-like granules, zonula adherens-type intercellular junctions, prominent Golgi apparatus, and varying numbers of mitochondria and rough endoplasmic reticulum. This unusual tumor is readily confused with cutaneous lymphoma and various undifferentiated primary or metastatic neoplasms by conventional light microscopy. The definitive diagnosis can be made only by electron-microscopic examination.