EXPRESSION OF THE INTERMEDIATE FILAMENT KERATIN GENE, K15, IN THE BASAL-CELL LAYERS OF EPITHELIA AND THE HAIR FOLLICLE

The intermediate filament keratin, K15, is present in variable abundan ce in stratified epithelia. In this study we have isolated and charact erized the sheep K15 gene, focusing on its expression in the follicles of sheep and mice. We show that K15 is expressed throughout the hair cycle in the basal layer of the outer root sheath that envelops the fo llicle. Strikingly, however, in large medullated wool follicles, a sma ll group of basal outer root sheath cells located in the region though t to contain hair follicle stem cells are K15-negative. In the follicl e bulb K15 is expressed in cells situated next to the dermal papilla b ut not in the inner bulb cells. Elsewhere, K15 is expressed at a low, variable level in the basal layer of the epidermis and sebaceous gland , often in a punctate pattern. In the esophagus of the sheep K15 expre ssion is restricted to the basal layer, in contrast to human esophagus where it is expressed throughout the epithelium. Transgenic mouse lin es established with a 15-kb sheep K15 gene construct exhibited faithfu l expression and showed no phenotypic consequences of K15 overexpressi on. An investigation of transgene expression showed that K15 is contin uously expressed in outer root sheath cells during the hair cycle. Giv en its expression in the mitotically active basal cell layers of diver se epithelia and the follicle, K15 expression appears to signal an ear ly stage in the pathway of keratinocyte differentiation that precedes the decision of a cell to become epidermal or hair-like. (C) 1998 Acad emic Press.