Mammalian hair follicles represent one of the most rapidly proliferating tissue systems in a living organism.  In humans, scalp hair typically grows at a rate of 1/2" per month, representing a remarkably robust rate of cell division.  Equally interesting, is the fact that hair goes through several distinct cycles, loosely analogous to the cycles deciduous tree leaves go through as the seasons change.  Like the leaves on a tree, hair follicles go through a growing stage (anagen) a shedding stage (catagen) and a resting stage (telogen).

On the other hand, each individual human scalp hair is set to its own clock.  This is known as a mosaic growth pattern and it explains why humans do not shed hair seasonally as do many other mammals.   Strikingly, it is to to the rapid rate of cell turnover that cancer chemotherapy often causes hair loss in treated patients.  These drugs attack rapidly dividing cells throughout the body because usually such rapid cell division is a hallmark characteristic of neoplastic (i.e. cancerous) activity.  In the past decade, however, new drugs (e.g. CD2 inhibitors) have been created that can offer patients substantial protection from chemotherapy-induced hair loss.