1. ¹German Cancer Research Center, Department of Cell Biology, University of Heidelberg, Heidelberg, Germany
2. ²Section of Normal and Neoplastic Epidermal Differentiation, University of Heidelberg, Heidelberg, Germany
3. ³Institute of Pathology, University of Heidelberg, Heidelberg, Germany

Correspondence: Dr Lutz Langbein, German Cancer Research Center, Department of Cell Biology, A010, Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany. E-mail: langbein@dkfz.de

Received 4 May 2006; Revised 12 June 2006; Accepted 14 June 2006; Published online 27 July 2006.

Abstract

The recent elucidation of the human type I keratin gene domain allowed the completion of the so far only partially characterized subcluster of type I keratin genes, KRT25–KRT28 (formerly KRT25A–KRT25D), representing the counterparts of the type II inner root sheath (IRS) keratin genes, KRT71–KRT74 (encoding proteins K71–K74, formerly K6irs1–K6irs4). Here, we describe the expression patterns of the type I IRS keratin proteins K25–K28 (formerly K25irs1–K25irs4) and their mRNAs. We found that K25 (K25irs1), K27 (K25irs3), and K28 (K25irs4) occur in the Henle layer, the Huxley layer, and in the IRS cuticle. Their expression extends from the bulb region up to the points of terminal differentiation of the three layers. In contrast, K26 (K25irs2) is restricted to the upper IRS cuticle. Apart from the three IRS layers, K25 (K25irs1), K27 (K25irs3), and K28 (K25irs4) are also present in the hair medulla. Based on previous, although controversial claims of the occurrence in the IRS of various "classical" epithelial keratins, we undertook a systematic study using antibodies against the presently described human epithelial and hair keratins and show that the type I keratins K25–K28 (K25irs1–K25irs4) and the type II keratins K71–K74 (K6irs1–K6irs4) represent the IRS keratins of the human hair follicle.