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... How does it [hair] know when to stop growing when it reaches the intended length?
Follicles are specialised collections of cells that produce the keratin filaments which we call hair. The follicles are not continuously active and have three growth phases which vary in length depending upon where on the body surface they are. These three phases are the anagen phase, when the follicle is actively producing a hair, a catagen phase during which growth stops and the hair falls out, and a thelogen phase when the follicle rests before recommencing growth.The relative lengths of these phases can alter depending upon the role of the follicle, and this governs the ultimate length of the resulting hair. So a pubic hair has an anagen phase lasting a few weeks, and eyelash a week or so and a head-hair several years.