Does anyone know WHY hair follicles have a resting phase? I understand that a follicle normally grows it's hair shaft for a few years and then sheds the hair and "rests" for a couple of months before growing a new hair shaft, but why? I know this is sort of an acedemic question, but it is related to HTs because the transplanted follicles are shocked into their resting phase. I'm just curious.

Dear Gary:

Please excuse the typos.

You have asked a great question. Unfortunately nobody has a great answer. The hair follicle is remarkable in that it continually renews itself during the life of a human being.

There are presumably chemical signals that play a role in the cyling of hair. One factor is probably fibroblast growth factor 5. In mice, lack of this factor causes a delay in catagen.

There may be structural factors that induce a "controlled cell death". This cell death is referred to as apoptosis.The structural factors may contribute to hair cycling.

In terms of evolutionary value, it may be that the hair cycling is related to the shedding of hair in many fur bearing animals. There could have been value in accomodating environmental changes. It may be that the hair cyle is a vestige of our more primitive ancestors.It is all speculative at best.

Controlling the hair cycle may however prove to be an important therapeutic device for treating hair loss.

Paul T. Rose, MD

Dr. Rose,

Thanks for the very interesting answer.