Merkel cell tumor of the skin. Ultrastructural and immunohistochemical studies.

A skin tumor of a 66-year-old female was investigated morphologically and immunohistochemically. The tumor was located within the dermis and comprised of rounded cells with scanty cytoplasm, which proliferated forming a small nest or trabecular arrangement. Electron microscopic observation indicated the presence of dense-core granules within the tumor cell cytoplasm suggesting that the tumor was derived from Merkel cells. Occasionally clusters or bundles of the intermediate filaments were found in the perinuclear cytoplasm of the tumor cells. Each tumor cell was connected with desmosomes. Immunohistochemical staining with anti-keratin antiserum showed positive reaction at the perinuclear cytoplasm of the tumor cells indicating that the cluster of the microfilaments presumably contains keratin. Conversely S-100 protein was negative in the tumor cells. The results obtained strongly suggest that the tumor or Merkel cell was considered to be derived from the epidermal immature cells rather than from the neural crest.