Expression and localization of tight junction-associated proteins in human hair follicles

Tight junctions (TJ) are barrier-forming intercellular junctions selectively sealing cells and controlling the paracellular pathway. They have been well-characterized in simple epithelia and endothelia but have only recently been described in stratified epithelia such as epidermis, oesophagus and oral mucosa. Various epithelial layers which are partly in morphogenic continuity with the epidermis and develop therefrom during early fetal life, build the human hair follicle. The barrier function of these epithelial layers seems to be important for the universal continuity of the barrier represented by the skin. We show the presence of the TJ proteins ZO-1, occludin, and various claudins in the hair follicle and demonstrate their impressive heterogeneous distribution pattern within a given stratum as well as within its different epithelial layers. Coexpression of the various TJ proteins, arguing for typical TJ structures, can be observed especially in cell layers facing the hair shaft and the stratum corneum, and in addition at the border between the outer and inner root sheaths. Usually they are found in close proximity to desmosomal and adherens junction proteins. The morphological and biological importance of these findings and the possible roles of TJ in hair follicles, e.g. in follicular penetration, are discussed.