Expression of trichohyalin in dermatological disorders: a comparative study with involucrin and filaggrin by immunohistochemical staining

To investigate the function of trichohyalin during terminal differentiation of the skin, immunohistochemical studies were performed on trichohyalin and its related proteins, filaggrin and involucrin, the components of the cornified cell envelope. In skin disorders unrelated to tumours, weak trichohyalin expression was found in a few granular cells or in the horny layer of psoriasis, ichthyosis, keratosis pilaris, porokeratosis, chronic dermatitis and callus. Similar trichohyalin expression was found in epidermal tumours, such as seborrheic keratosis, actinic keratosis, Bowen's disease and well-differentiated squamous cell carcinoma. In follicular tumours, trichohyalin expression was positive in trichoepithelioma, keratotic basal cell epithelioma, proliferating trichilemmal tumour, trichilemmoma, pilomatricoma and keratoacanthoma. From comparative studies with filaggrin and involucrin, trichohyalin expression was co-localized with them in molluscum contagiosum, keratoacanthoma and pilomatricoma. From this study, trichohyalin is revealed to have close functional relationship with other markers of terminal differentiation as a precursor of the cornified cell envelope of the skin.