The drug minoxidil takes advantage of the responsiveness of hair follicles to hormones. Testosterone has just the opposite effect on hair follicles of the face. In men it is the onset of puberty and the elevated levels of testosterone that stimulate the follicles of the face to make beard hairs, and beards are exclusively a masculine feature. Extremely old women can grow stubble like a man's when their estrogen dries up, because women do produce small amounts of testosterone. When their source of estrogen is completely gone, there is enough to stimulate their facial hair follicles.
Equivalent phenomena occur in domestic animals.
Any agent, physical or chemical, which affects rapidly dividing cells will affect hair growth, usually by stopping it. Parasites, disease, and poor nutrition all affect hair growth. The condition of an animal's hair coat is usually a very good diagnostic clue to its internal medical problems. Structure of Hairs and Hair Follicles Start with slide 11, the nose of a dog. This slide has a fair number of large hairs on it, and you will be able to identify the principal parts.