Hair Growth Cycle in Subcutaneous Implants of Skin

The critical stages of hair development and the associated skin changes in the mouse have been well described (1, 2, 3). The term anagen has been applied to the period of active proliferation of the pilo-sebaceous unit which in the mouse lasts for about 17 days. The mouse is born with the hair entering its first cycle. Hair growth ceases at the age of 17 days, passes through the involutional phase of “catagen” during the 17th-19th day, and enters the resting phase known as the “telogen”. The latter phase persists for approximately 10 days and at the end of this time a new cycle of anagen begins which in turn is again terminated at approximately the 45th day of life. The phase of telogen occurring at this time, however, and those thereafter, persist for varying and prolonged periods, even though the length of anagen remains the same.