Up to now, bicomponent
keratohyalin has only been described for rat epithelium and human intraepidermal sweat ducts and fetal nail organ cells. In normal human interductal epidermis, the
keratohyalin appears homogeneous, osmiophilic and stellate in shape. Under pathological conditions, bicomponent
keratohyalin has been observed in different
palmoplantar keratoses and has therefore been thought to be associated with abnormal
keratosis. We studied the keratinization process in normal human plantar epidermis, in which
keratohyalin was found to exhibit several morphological differences as compared to that seen in non-ridged skin. The most striking feature was seen in upper granular cells, where the
keratohyalin granules consisted of two components of differing electron density. The electron-dense component formed the main part of the composite granule and was found in the cytoplasm of lower and upper granular cells. The less-electron-dense component was attached to the main component and appeared in the cytoplasm of upper granular cells, forming the convex contact zone. No intranuclear osmiophilic inclusions were present. The respective electron densities of the two
keratohyalin components of ridged skin were obviously different to that of the bicomponent
keratohyalin granules seen in the epidermal sweat-duct cells of the same specimen. These findings indicate the presence of at least two different types of
keratohyalin proteins in normal human ridged skin. They can be distinguished at the electron-microscope level and differ from the
keratohyalin of human non-ridged skin as well as from bicomponent
keratohyalin granules derived from human epidermal sweat-duct cells or from rat epithelium.