Biology of the hair follicle: the basics.

The mammalian hair follicle represents a unique, highly regenerative neuroectodermal-mesodermal interaction system that contains numerous stem cells. It is the only organ in the mammalian organism that undergoes life-long cycles of rapid growth (anagen), regression (catagen), and resting periods (telogen). These transformations are controlled by changes in the local signaling milieu, based on changes in expression/activity of a constantly growing number of cytokines, hormones, neurotransmitters, and their cognate receptors as well as of transcription factors and enzymes that have become recognized as key mediators of hair follicle cycling. Transplantation experiments have shown that the driving force of cycling, the "hair cycle clock," is located in the hair follicle itself. However, the exact underlying molecular mechanisms that drive this oscillator system remain unclear. These controls of hair follicle cycling are of great clinical interest because hair loss or unwanted hair growth largely reflect undesired changes in hair follicle cycling. To develop therapeutic agents for the management of these hair cycle abnormalities, it is critical to decipher and pharmacologically target the key molecular controls that underlie the enigmatic "hair cycle clock."