An additional phase of the hair cycle called exogen was recently recognised; this involves hair shaft shedding from the telogen follicle [86], an active process, accompanied by the activation of proteolytic processes in the follicular root [87]. Exogen was also recently characterized in human follicles. It was shown that while anagen and telogen hairs are firmly anchored to the follicle, exogen hairs are passively retained within the follicles. In addition, exogen clubs do not retain remnants of the outer root sheath, in contrast to plucked telogen hairs [88].
The new hair formed during the next anagen may resemble its predecessor, like most human scalp hair, or may differ markedly like the brown summer and white winter hairs of Scottish hares [9]. The type of hair produced depends on the regulatory dermal papilla [89,90] although the cell biology and biochemistry of their mechanisms are not fully understood. The duration of hair cycle stages varies in different body areas. Human scalp hair follicles have the longest anagen phase, which can last up to several years; they also display a relatively short catagen phase (1-2 weeks) followed by a telogen phase lasting several months. The majority of scalp hair follicles are in anagen (80-85%), with the rest either in catagen (2%) or telogen (10-15%). The anagen phase of follicles in other body regions is substantially shorter, for example on the arms, legs, and thighs it ranges from 3 to 4 months [26]. It is clear that anagen length generally determines hair length; long scalp hairs are produced by follicles with anagens over 2 years, while short finger hairs only grow for around 2 months [91].