There are three layers to each strand of hair. Picture three concentric cylinders, with the innermost one being the medulla. The medulla layer is a space that is variable, sometimes not present and whose function is debatable. The middle layer is the cortex, which is the primary source of mechanical strength. The cortex layer also contains melanin, which determines the fiber color. The shape of the cortex also corresponds with the shape of the hair follicle. Circular cortex cross-sections correspond to straight hair and oval cross-sections curly. Finally, the superficial or outer layer is called the cuticle. This layer contains a hydrophobic (water-repelling) lipid layer.
The hair follicle also contains the papilla, matrix, root sheath, and bulge. There are associated structures as well, including sebaceous glands, apocrine glands, the arrector pili muscle, and mechanoreceptors that can respond to touch.
There are four phases of the hair cycle: anagen, catagen, telogen, and exogen. The growth phase is anagen. Up to 90% of the hair follicles are in this phase. The next phase is catagen, the involuting or regressing phase. 1 to 2% of follicles are in this phase. Telogen is the resting phase. 10 to 14% of follicles are in this phase. Finally, exogen is the shedding phase. During this phase, one of several hairs within a single follicle is shed.
The total length of the hair follicle depends on the area of the body where the follicle is located. The cycle for hair in the eyebrow is approximately 4 months versus 3 to 4 years on the scalp.
There are between 250,000 and 500,000 hair follicles on the scalp and as many as 5 million on the whole body.
Reference: Santos Z, Avci P, Hamblin MR. Drug discovery for alopecia: gone today, hair tomorrow. Expert Opin Drug Discov. 2015 March ; 10(3): 269–292