In search of the "hair cycle clock": a guided tour

The hair follicle, a unique characteristic of mammals, represents a stem cell-rich, prototypic neuroectodermal-mesodermal interaction system. This factory for pigmented epithelial fibers is unique in that it is the only organ in the mammalian body which, for its entire lifetime, undergoes cyclic transformations from stages of rapid growth (anagen) to apoptosis-driven regression (catagen) and back to anagen, via an interspersed period of relative quiescence (telogen). While it is undisputed that the biological "clock" that drives hair follicle cycling resides in the hair follicle itself, the molecular nature of the underlying oscillator system remains to be clarified. To meet this challenge is of profound general interest, since numerous key problems of modern biology can be studied exemplarily in this versatile model system. It is also clinically important, since the vast majority of patients with hair growth disorders suffers from an undesired alteration of hair follicle cycling. Here, we sketch basic background information and key concepts that one needs to keep in mind when exploring the enigmatic "hair cycle clock"(HCC), and summarize competing models of the HCC. We invite the reader on a very subjective guided tour, which focuses on our own trials, errors, and findings toward the distant goal of unravelling one of the most fascinating mysteries of biology: Why does the hair follicle cycle at all? How does it do it? What are the key players in the underlying molecular controls? Attempting to offer at least some meaningful answers, we share our prejudices and perspectives, and define crucial open questions.