The Biology of Hair
Hair is not as simple as it appears. It is a complicated and also the only body structure that is completely renewable without scarring. The major part of the body surface has hair. Hair helps transmit sensory information. This article explains the biology of hair.
By 22nd week a developing fetus gets all its hair follicles. During this period 5 million follicles are present all over the body. The head has one million of those, and 100,000 are on the scalp. It must be noted that follicles are never added during life and this is the period (fetus) when the largest number of follicles any one will ever have. As we age the density of the hair follicles on the skin decreases.
Hair has two separate structures: follicle and the shaft. The structure that we see is shaft while the follicle is in the skin.
Hair Follicle: The hair follicle is a complex mini-organ. It is composed of multiple mesenchymal and epithelial cell layers. All of these comprise of more than 20 different cell populations. Along with the sebaceous gland and the arrrector pili muscle, the hair follicle becomes the part of the so-called “pilosebaceous unit”. However, different hair follicle types may contain these components in varying proportions. The inner sheath follows the hair shaft and ends below the opening of a sebaceous 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. The sebaceous gland produces sebum which is a natural conditioner. The production of sebum increases after puberty. The sebum production decreases in women throughout their lives. The production also decreases in men, but not as much as in women.
Shaft: The hair shaft is made up of 3 layered dead, hard protein called keratin. The inner layer is medulla and may not be present. The second layer is the cortex and the outer layer is called cuticle. The majority of the hair shaft is made of cortex. The cuticle is formed by tightly packed scales in an overlapping structure similar to roof shingles. There are pigment cells that are distributed throughout the cortex and medulla giving the hair it's characteristic color.
Hair on the scalp grows about 1/2 inch per month or about 6 inches per year. Hair growth and loss in humans is random and not seasonal or cyclic. At any given time, a random number of hairs will be in various stages of growth and shedding. There are three stages of hair growth: catagen, telogen, and anagen.
Catagen: The catagen phase lasts for about 2-3 weeks and is a transitional stage. 3% of all hairs are in this phase at any time. During this phase growth stops and the outer root sheath shrinks and attaches to the root of the hair. This is the formation of what is known as a club hair. The catagen is the transitional phase.
Telogen: Telogen is the resting phase and accounts for 10-15% of all hair. Telogen phase lasts for about 100 days for hairs on the scalp and longer for hairs on the eyebrow, eyelash, arms and legs. During this phase the hair follicle is at rest and the club hair is completely formed. A solid, hard, dry and white material at the root can be seen if one pulls out a hair in this phase. Normally, 25-100 telogen hairs are shed each day. Tellogen is the resting phase.
Anagen: Anagen is the active or growth phase of the hair. A new hair is formed and pushes the club hair up the follicle and eventually out. Approximately 85% of all hairs are in the growing phase at any one time. The Anagen phase or growth phase can vary from two to six years. Hair grows approximately 10cm per year and any individual hair is unlikely to grow more than one meter long. The hair on the arms, legs, eyelashes, and eyebrows have a very short active growth phase of about 30-45 days explaining why they are so much shorter than scalp hair.
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