An unusual
tumor with a controversial name as well as histogenesis, the
neuroendocrine carcinoma of the skin (also known as "
Merkel cell carcinoma," "trabecular
carcinoma of the skin") has previously been extensively studied by immunohistochemical methods at the light-microscopic level. Ultrastructural descriptions of this
tumor have also been extensive, although immunocytochemical study of this
neoplasm at the electron-microscopic level has been limited. In this report, we have used postembedding
protein A-
gold immunocytochemistry on thin sections from
tumor embedded in
Lowicryl K4M to investigate the expression and ultrastructural localization of a panel of commercially available, diagnostically useful
antibodies.
Antibodies associated with epithelial derivation included anti-
keratin monoclonal antibody AE1/AE3, polyclonal anti-
keratin, and monoclonal anti-
cytokeratin cocktail (MAK-6), as well as a
monoclonal antibody against
epithelial membrane antigen (EMA).
Antibodies associated with neuroendocrine derivation included monoclonal anti-
chromogranin A and monoclonal
anti-synaptophysin. Although staining with a polyclonal antibody directed against
neuron-specific enolase (NSE) was equivocal, there was no labeling with a monoclonal anti-neurofilament antibody. The finding of positive
keratin labeling of filaments arranged in paranuclear aggregates correlates well with the previously described immunohistochemical staining pattern at the light-microscopic level. Moreover, the presence of cytoplasmic
synaptophysin and
chromogranin positivity over dense-core granules exemplifies the neuroendocrine differentiation present in this fascinating
tumor of the skin.