The 'melanocyte-keratin' mystery revisited: Neither normal human epidermal nor hair follicle melanocytes express keratin 16 or keratin 6 in situ

Background Keratin family proteins are generally accepted as being restricted to epithelial cells. However, several studies have challenged this paradigm by reporting, for example, that melanoma cells can express keratins and that normal human epidermal melanocytes, which derive from the neural crest, express keratin 16 (K16) in situ. Objectives We wished to confirm or refute that K16 and/or its intermediate filament partner, keratin 6 (K6), are expressed in normal human epidermal and/or hair follicle melanocytes in situ. Methods Cryosections of normal human scalp skin were subjected to highly sensitive double immunohistochemistry with specific antibodies against K16 or K6 and against the melanocyte-specific marker NKI/beteb (gp100). Immunoreactivity (IR) was visualized by conventional light microscopy and confocal fluorescence microscopy. Results Despite the use of different, high-sensitivity immunostaining methods, stringent positive and negative controls, and monospecific, well-characterized antikeratin antibodies, we could detect neither K16 nor K6 IR within intraepidermal or intrafollicular pigment cells of normal human scalp skin. Instead, NKI/beteb+ cells were found to be intimately embedded in foci of K16+ and/or K6+ keratinocytes, which might create the illusion of keratin expression by these cells. Conclusions Human epidermal or hair follicle melanocytes do not express K16 and/or K6 while residing in their natural habitat.