A novel epithelial keratin, hK6irs1, is expressed differentially in all layers of the inner root sheath, including specialized huxley cells (Flügelzellen) of the human hair follicle.

In this study we have characterized a novel human type II keratin, hK6irs1, which is specifically expressed in the inner root sheath of the hair follicle. This keratin represents the ortholog of the recently described mouse inner root sheath keratin mK6irs. The two keratins were highly related and migrated at the same height as keratin 6 in two-dimensional gel electrophoresis. Both RNA in situ hybridization and indirect immunofluorescence studies of human hair follicles demonstrated hK6irs1 expression in the Henle and Huxley layers as well as in the cuticle of the inner root sheath. In all three layers, the expression of hK6irs1 mRNA and protein began simultaneously in adjacent cells of the lowermost bulb above the germinative cell pool. Higher up in the follicle, the detection limits for both hK6irs1 mRNA and protein precisely coincided with the asynchronous onset of abrupt terminal differentiation of the Henle layer, inner root sheath cuticle, and Huxley layer. Mainly above the level of terminal Henle cell differentiation, both indirect immunofluorescence and immunoelectron microscopy revealed the occurrence of distinct Huxley cells that developed pseudopodal hK6irs1-positive extensions passing through the fully keratinized Henle layer. These outwardly protruding foot processes abutted upon cells of the companion layer, with which they were connected by numerous desmosomes. These specialized Huxley cells have previously been termed "Flügelzellen", which means "winged cells", with reference to their characteristic foot processes. We provide evidence that, together with Henle cells, Flügelzellen ensure the maintenance of a continuous desmosomal anchorage of the companion layer along the entire inner root sheath. This tightly connected companion layer/inner root sheath unit provides an optimal molding and guidance of the growing hair shaft.