The keratins of the human beard hair medulla: the riddle in the middle.

We have investigated the expression of 52 of the 54 keratins in beard hair medulla. We found that not only 12 hair keratins but, unexpectedly, also 12 epithelial keratins are potentially expressed in medulla cells. The latter comprise keratins also present in outer- and inner-root sheaths and in the companion layer. Keratins K5, K14, K17, K25, K27, K28, and K75 define a "pre-medulla," composed of cells apposed to the upper dermal papilla. Besides K6, K16, K7, K19, and K80, all pre-medullary epithelial keratins continue to be expressed in the medulla proper, along with the 12 hair keratins. Besides this unique feature of cellular keratin co-expression, the keratin pattern itself is highly variable in individual medulla cells. Remarkably, both epithelial and hair keratins behave highly promiscuously with regard to heterodimer- and IF formation, which also includes keratin chain interactions in IF bundles. We also identified cortex cells within the medullary column. These exhibit all the properties of genuine cortex cells, including a particular type of keratin heterogeneity of their compact IF bundles. In both keratin expression profile and keratin number, medulla cells are distinct from all other cells of the hair follicle or from any other epithelium.