[New perspectives in hair research: in search of the "biological clock" of the hair cycle].

The hair follicle is not only a clinically and commercially important tissue, but can also serve as a fascinating model for studying epithelial-mesenchymal-neuroectodermal interactions. Recently, this has led to a renaissance of hair research, which this speculative review attempts to outline. The unanswered key questions in hair biology are defined before we focus on the crucial search for the "biological clock" that governs the hair cycle. Specifically, we delineate the clinical importance of understanding the mechanisms of catagen induction and propose an immunological mechanism of catagen induction. New research trends are discussed against the background of their potential significance for improving the still frustrating management of hair diseases.