The Hair Clock
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The Hair Clock

The Hair Clock,

Or (to put it another way) 

Why is hair growth cyclical?

Andrea Marliani, M.D.



For medical hair specialists, and also for hair transplantation surgeons, it is very important to understand the physiology of  hair because this can suggest simple therapies that may really improve the outcome of a transplant.

Hair growth is cyclical so hair and body hair do not grow indefinitely. Therefore, the hair of a 50 year-old person will not reach 6 or 7 meters in length. Hair cuts are not part of natural processes.

How long the anagen phase lasts is determined by the amount of energy available. If the anagen phase and protein synthesis are to be maintained, the follicle needs ATP energy. This energy is released by the metabolism of glucose by way of Glycolysis and by the Pentose Phosphate Shunt. It continues to be produced during the Krebs Cycle. ATP is produced during Glycolysis, while NADPH is produced in the Pentose Phosphate Shunt.

The metabolism of glucose is activated (is turned both on and off) by the Adenil cyclase enzyme. When this enzyme is withheld, Glycolysis stops, as do the Pentose Phosphate Shunt and the Krebs Cycle. The interruption of glucose metabolism turns off the supply of energy and ends the anagen phase.

The hair cycle is controlled by sex steroids. Not by the hormones circulating in the blood but by hormones that are produced within the follicle itself. Dihydrotestosterone inhibits the Adenil cyclase enzyme, while Estrone increases it. A follicle in the anagen phase makes a “physiological attempt” to reach the catagen stage and then the telogen stage.

For the follicle to be able to move on to the catagen stage, 5 alpha reduction is required. The 5 alpha Reductase enzyme changes testerone into Dihydrotesterone. 5 alpha reduction uses the NADPH produced in the Pentose Phosphate Shunt. It is, therefore, dependent on NADPH. Glycolysis is stopped at the end of the anagen phase, as is the Pentose Phosphate Shunt. And NADPH is no longer produced. There is no 5 alpha reduction, and all metabolic activity is geared to aromatization. There is an abundant production of Estrone at the close of the anagen phase, and this activates the Adenil cyclase enzyme. Glycolysis begins again and the cycle is set once more in motion.

Why is the anagen phase longer in women than in men? The reason is that, in males, the most natural metabolic process is for testosterone to be turned into Dihydrotestosterone. In women, the most natural development is one leading to the production of Estrone. The result is that the anagen phase lasts 3 years in males and 6 years in females. Men have short anagen phases and rapid hair cycles. In women, anagen is long and the cycle is slow.

A shorter anagen phase results in a quicker cycle. This does not mean there will be involution of the follicle, nor miniaturization of the hair. A shorter anagen cycle is in no way synonymous of baldness. But that’s for another story…




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