Hair Fol­li­cles

Hair has two dis­tinct struc­tures — first, the fol­li­cle itself, which resides in the skin, and sec­ond, the shaft, which is what is vis­i­ble above the scalp.

Two sheaths, an inner and outer sheath, sur­round the fol­li­cle. These struc­tures pro­tect and form the grow­ing hair shaft. The inner sheath fol­lows the hair shaft and ends below the open­ing of a seba­ceous (oil) gland, and some­times an apoc­rine (scent) gland. The outer sheath con­tin­ues all the way up to the gland. A mus­cle called an erec­tor pili mus­cle attaches below the gland to a fibrous layer around the outer sheath. When this mus­cle con­tracts, it causes the hair to stand up which also causes the seba­ceous gland to secrete oil.

Hair Shafts

The hair shaft is made of a hard pro­tein called ker­atin and is made in three lay­ers. This pro­tein is actu­ally dead, so the hair that you see is not a liv­ing struc­ture. The inner layer is the medulla. The sec­ond layer is the cor­tex and the outer layer is the cuti­cle. The cor­tex makes up the major­ity of the hair shaft. The cuti­cle is a tightly formed struc­ture made of shingle-like over­lap­ping scales. It is both the cor­tex and the medulla that holds the hair’s pig­ment, giv­ing it its color.

Hair Growth Cycle

Hair on the scalp grows about .3 to .4 mm/day or about 6 inches per year. Unlike other mam­mals, human hair growth and shed­ding is ran­dom and not sea­sonal or cycli­cal. At any given time, a ran­dom num­ber of hairs will be in one of three stages of growth and shed­ding: ana­gen, cata­gen, and telogen.