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The Hair Cycle,

The Hair Cycle, Or (to put it another way) Why is hair growth cyclical?

Hair growth is cyclical so that hair, and body hair, will not
grow indefinitely. So that the hair of a 50 year-old person will
not be 6 or 7 metres long. 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 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 easy metabolic process is for testosterone
to be turned into Dihydrotestosterone. In women, the most easy
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…

References:

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