Re: Re: Can hair manufacture more melanin due to sun exposure?

Mayra,

What you report is what we experience here with patients who spend months
(notice the plural) in more tropical climates and return to the Northeast 
USA.  You are not imagining the deepening of hair color darkness in 
response to lengthened light stimulus.  It is quite real, especially if 
you are of olive or darker skin complexion.  

Perhaps a refresher note on melanocyte and hair pigmentation would be 
appropriate.  For brevity, we will leave out graying of hair.

Melanocytes do not color hair as it is originally formed.   Fresh hair, 
under the skin has no color, it picks up color as it lengthens.  The hair 
is going to grow no matter if the hair is colored or not.  The hair shaft 
develops in the follicle, surrounded by melanocytes.  The  melanocyte 
pigments enter the hair shaft cortex by diffusion through the cuticle and 
keratin.   So the important fact is that the melanocytes act independently 
of the growth of hair.  Melanocytes simply behave as melanocytes and the 
pigment is “picked up” adventitiously by the growing hair.

There are two types of pigment that gives hair its color, eumelanin
(brown/black), and pheomelanin(yellow/red). A low concentration of 
eumelanin in the hair and low concentration of pheomelanin will result in 
ash blond color, more eumelanin, “dirty blond” or light brown color.   
Higher eumelanin will result in deep black pigmented hair. Pheomelanin in 
low concentrations causes a yellow tone, increasing to strawberry red to  
higher concentrations causeing increasing red color effects.  Pheo- and eu-
 melanins usually occur paired and create the many varied effects we see, 
such as auburn hair.

Pheomelanin breaks down more slowly than eumelanin when oxidized, which 
accounts for darker hair turning red to orange to yellow during the 
bleaching process.  That is also the reason why mummies all have red hair.
Color producing melanocytes are not sensitive to light stimulus, per se.  
However, keratinocytes are very sensitive to light.  When exposed to 
sunlight, keratinocytes release melanocyte-stimulating hormone (MSH), 
which binds to the melanocyte activating receptor, Mc1r on the 
melanocyte.   Thus the melanocytes are activated and produce pigment.  
Melanocytes increase their production of  pheo- and eu- melanins, as 
programmed by the genetic make up of the individual.

For your first Question:        If one changes geography to a climate of 
longer day time, assuming they spend a long enough time there to notice,  
ie:  greater than 6 months, will experience increased hair follicle 
melanocyte production of the respective melanins.  That will be noticed by 
pronounced hair color change.

Whether that is a deepening or an apparent bleaching of color is dependent 
on the melanins produced by melanocytes that are responding to MSH.  
MSH, light time and your genetics are the cumulation of your hair color 
deepening while in Brazil.  Enjoy!

Thank you for this very interesting question!

On your second question: 
     I have asked a number of my collegues.  We 
are a bit stymied by your second question and are most curious.  We 
collectively can not think of a rationale or reference to this 
phenomenon.  Sorry.  Perhaps you should re-post it to the moderator in an 
effort to locate an adequate answer.  

Thank you very much!
Peter

Ref: http://fo
cus.hms.harvard.edu/2006/101306/dermatology.shtml http://www.ishrs.org/articles/hair-color.htm http://www.ncbi.nlm.n
ih.gov/entrez/query.fcgi?
cmd=Retrieve&db=PubMed&list_uids=12894999&dopt=Abstract http://www.pubmedcentral.nih.gov/articlerender.fcgi?
artid=1201498