How Hair Cycles; Why Human Hair Is Unique
Most hair-bearing mammals shed in waves which cycles are often seasonally driven. In rats and mice for example, hair is shed in a cephalocaudal pattern beginning at the tip of the nose and progressing rearward toward the tail. In humans, each individual hair cycles individually -- this distinction is known as a mosaic pattern. Scalp hairs typically grow in the anagen portion of the cycle for two to five years then transition to a telogen resting phase where the follicle sheds its hair and does not produce a new hair for 90 days.
When scalp hair follicles undergo pattern hair loss the cycle is modified so that the anagen growing cycle is shortened and the telogen resting cycles are more frequent. Ultimately, susceptible follicles may basically end up in permanent telogen. Anatomically, the follicle structure remains intact but profoundly miniaturized.
Surgical hair transplantation is based on an intriguing physiological precept which came to light in the 1940's. Here, it was found that hair bearing tissue transplanted into an anatomical region which had been denuded of hair continued to retain its donor identity. In other words, it did not matter where the healthy hair bearing transplant tissue was placed, it continued to produce healthy vigorous hair. This paradigm, which became far more refined as years and decades passed, was used to replace grafts of hair producing skin tissue into zones where hair loss had occurred.
One of the most significant challenges facing hair loss research scientists today is the fact that no animal model for human pattern hair loss exists. By way of contrast, many human diseases are faithfully replicated in animal models -- providing an invaluable insight into mechanisms and potential treatments. Those of us studying human pattern hair loss have been compelled to rely on cell-based models and other inferential tools. It is unquestionable that this caveat has affected the rate at which scientific progress has occurred -- or failed to occur -- in this field.
