Hair raising experiments

Who isn’t worried about hair? Age and gender  take a  united stand against this common enemy.   And what challenges!    Either it is too oily or too dull;  too thin or  too bushy;  too curly or too straight to be styled;  greying too fast or too dark to take up any color!  Every third commercial in the TV suggests solutions with desirable outcomes. Pick your choice.

Besides being the crowing glory, body hair as an integral part of skin plays  an important  role in maintaining homeostasis. The hair that sticks out of the skin is actually a string of dead cells. But can there be death without birth?  Indeed hair  too goes through a birth and  growth period before embracing death.

Life cycle of the hair is actually   the  life cycle of  the hair follicle.  No new hair follicles are made postnatally, the lower portion of the follicle goes through cycles of regeneration  to produce new hair to replace the old.  For this  we are born with a storehouse of stem cells  Anatomically this storehouse is  called the “bulge” ( a rather unscientific term, I agree) at the base of the hair follicle.  These stem cells  are pluripotent, means they can   differentiate into adult hair follicles, epidermis or  sebaceous glands.  Because of the pluripotency  and also because they can be easily cultured in the lab,  skin stem cells  have attracted considerable scientific curiosity.  Several laboratories  around the world are  actively involved in  unravelling the mysteries of the skin and hair.   

Anagen and Telogen are the active and resting phases of the hair follicle  cycle.  The dynamic transition from growth state to rest state is the catagen phase.  During the anagen phase the hair follicle sports  vigorous growth and hair forms.  How long the anagen phase prolongs decides how long the hair will be. But this is genetically predetermined. At the end of the growth phase,  the blood and nutrient supply to the follicle is cut off and it  shrinks and shrivels.  The nascent   hair is pushed up to replace the old one. The shrivelled follicle rests before getting rejuvenated for the next cycle.  Every single hair follicle goes through these three stages albeit  not at the same time.  The  green, amber and red signals  during the hair  life cycle is definitely not synchronised. Imagine  shedding all our sclap hair in one go! What a disaster. Likewise hair on the scalp  has a different  cycle time than the one on the eyebrow or for that matter hair on the hands.  There are instances of synchronized seasonal  cycles.  For example it has been reported that in several breeds of  sheep  the hair is in the telogen phase during winter months. When the ambience warms up in spring the follicles move into anagen phase and  get  ready to shed the old hair.  

Sure enough there is a circadian rhythm at work and scientists  had recognized this very early. The biological  Master clock is in the hypothalamus, (suprachiasmatic nuclei to be precise) but then there are a few autonomous ones scattered elsewhere in other tissues, skin being one among them.  The circadian rhythm is implemented through  a very intriguing interplay among   sets of proteins called, CLOCK, BML1, CRYs and PERs.  The dancers frequently change partners,  take quick  forward and backward steps, ( positive and negative feedbacks)   to  regulate the anagen, catagen, telogen phases.      

Janich et al (3)  conducted in vivo and in vitro studies on special breed of rats. They focussed on the  “bulge” stem cells.  The stem cells have two classes of population: one group  in an “Ever Ready to Go” state, while the remaining   lazily dozing off.    What causes this class divide.  isn’t yet clear. But      Janich et al observed that by disrupting circadian rhythm  they could either increase or decrease the population ratio.    However   upsetting the circadian rhythm  proved  detrimental because it  led to premature aging.

1. Epidermal stem cells: Properties, markers, and location

Robert M. Lavker  and Tung-Tien Sun

PNAS December 5, 2000 vol. 97 no. 25 13473-13475

2. Epidermal stem cells of the skin.Blanpain C, Fuchs E.  Ann. Rev. Cell Dev Biol  2006;22:339-73.

 

3. The circadian molecular clock creates epidermal stem cell heterogeneity.

Jannich et al Nature Vol. 480, 209-214, 2011.

4. Clock genes, hair growth and aging

Mikhail Geyfman and Bogi Andersen AGING, Vol 2, No 3 , pp 122-128, 2010