Keratinocyte differentiation in psoriatic scalp: morphology and expression of epithelial keratins.

The morphology of hair follicles was examined in psoriatic scalp biopsies and compared with normal scalp. In scalp psoriasis the lower outer root sheath and hair matrix were not affected by the psoriatic changes, although there was an irregular expansion in the proximal lower outer root sheath. This area has been characterized, by the presence of keratin K19-containing cells, as the putative stem cell region. In addition, marked shrinkage of the sebaceous glands was seen in the psoriatic scalp, as previously reported. A panel of monospecific monoclonal antibodies to individual epithelial keratins was used to analyse scalp specimens immunohistochemically. Keratin expression in scalp was generally unaffected by psoriasis, except for widespread expression of suprabasal keratins K16 and K17 in suprabasal interfollicular psoriatic scalp epidermis. Simple epithelial keratins K8 and K18 were not found in follicular epithelium from either normal or psoriatic scalp, using multiple monospecific antibodies. This study shows that keratin K17 is induced suprabasally during epidermal hyperproliferation, and cannot therefore be considered a hair follicle-specific keratin.