When hair grows, either it is pigmented, or it is white. The greyish
appearance of hair is only in fact a kind of optical illusion, produced by
the mixture of coloured hair with white hair. The French expression
"pepper and salt hair" gives a good indication of what this means. It is
therefore obvious that the hair appears increasingly grey as the
percentage of white hairs increases. Furthermore, since hair grows from
the root, it can be coloured and yet have a white base.
As can be clearly seen when hair continues to grow after having
been coloured.
The hair whitening is called canities. It is a very complex phenomenon,
easily explained in the first instance - for as soon as a hair is no
longer pigmented at its conception, it grows white. This is all very
well, but why is it no longer pigmented? It was long believed to be the
natural consequence of a halt in the production of melanin by the
melanocytes. Until researchers made the amazing discovery than although
the papilla producing a white hair contained melanocytes incapable of
producing coloured pigments, there were others
that were working perfectly, but were no longer able to transmit
their melanin to the keratinocytes. For the moment, the reasons for
this interruption of communication between melanocytes and
keratinocytes remains obscure.
It was subsequently discovered that melanocytes were not only to be found
at the bottom of the dermal papilla, but also in a reservoir situated
higher in the external epithelial sheath. These melanocytes are dormant
and produce no pigments. Some of these are recruited by the hair follicle
to repopulate its lower part when it begins to regenerate itself at the
end of the telogen phase. Once they have been selected, these melanocytes
are reactivated and the production of melanin begins once more. But this
reservoir is still to be found in white hair follicles. Which leads us to
think that canities may be the result of a "recruitment deficit" in this
reservoir. Once again, the processes involved remain a mystery.
However, there is every reason to believe that if we can understand these
various mechanisms, it should be possible to prevent hair whitening if we
so wish!
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