[Dr. Praetoria]

Why does the hair on the head or face seem to grow longer than elsewhere on the body?  Also, do monkeys need haircuts?  []
Doc

"Getting older is no problem. You just have to live long enough."
                                        ---Groucho Marx

[chris]

Good question.

Hair is produced by hair follicles, which are specialised structures that contain stem cells and are embedded in the dermis. They are produced during embryogenesis (development) and by 22 weeks a developing foetus has five million of them, one million on the head in total, and 100,000 on the scalp.

The developing follicles inherit a different genetic programme according to the part of the body on which they form. This genetic programme governs the three phases of a follicle's life, which are 1) its anagen phase, when it actively grows a hair 2) the catagen phase, when growth stops and the hair falls out, and 3) the telogen phase, when the follicle rests.

By genetically varying the relative durations of these phases, the body can control the rate of growth and the length a hair can reach. So head and beard hairs have very long anagen phases (lasting years), which enables the hair to grow very long and make nice pony-tails or, as seems to be the case with Chinese magicians, an incredibly long beard.

Conversely, eyebrows and eyelashes have very short anagen phases lasting only a few weeks, which is fortunate because otherwise we'd be blinking out from beneath a row of three-foot long eyelashes!

Chris

[JimBob]

Chris,

By stem cells do you mean the same type of cells obtained from embryos?

[chris]

The stem cells in hair follicles are a cluster of cells with the capacity to divide many times to produce hair. Unlike embryonic stem cells, which have the potential to turn into a much broader range of cell types, these follicle stem cells are specialised for this purpose.

However, it may be feasible to un-specialise (in other words "de-differentiate" them) so that they may be turned into other tissues, if the right chemical signals are applied.

Here's a reference to the paper published in the Journal of Clinical Investigation in which the follicle stem cells are described:

http://www.jci.org/cgi/content/full/116/1/249

Chris