The Origins of Hair: By week 22, a developing fetus has all of its hair follicles formed. At this stage of life there are about 5 million hair follicles on the body. There are a total of one million on the head, with one hundred thousand of those follicles residing on the scalp. This is the largest number of hair follicles a human will ever have, since we do not generate new hair follicles anytime during the course of our lives. Most people will notice that the density of scalp hair is reduced as they grow from childhood to adulthood. Part of the reason is that our scalps expand as we grow.
Hair Follicles: Hair has two distinct structures - the follicle itself, which resides in the skin, and the shaft, which is what is visible above the scalp. The hair follicle is a tunnel-like segment of the epidermis that extends down into the dermis. The structure contains several layers that all have separate functions. At the base of the follicle is the papilla, which contains capillaries, or tiny blood vessels that nourish the cells. The living part of the hair is the very bottom part surrounding the papilla, called the bulb. The cells of the hair bulb divide every 23 to 72 hours which is faster than any other cell in the body. Two hair 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. The sebaceous gland is vital because it produces sebum, which conditions the hair and skin. After puberty our body produces more sebum but as we age we begin to make less sebum. Women have far less sebum production than men do as they age. 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. At any given time, a random number of hairs will be in one of three stages: anagen, catagen, and telogen.
- Anagen is the active phase of the hair. The cells in the root of the hair are dividing rapidly. During this phase the hair grows about 1 cm every 28 days. Scalp hair stays in this active phase of growth for two to six years. People who have difficulty growing their hair beyond a certain length have a short active phase of growth. The hair on the arms, legs, eyelashes, and eyebrows have a very short active growth phase of about 30 to 45 days
-Catagen phase is a transitional stage and about 3% of all hairs are in this phase at any time. This phase lasts for about two to three weeks. 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.
- Telogen is the resting phase and usually accounts for 6% to 8% of all hairs. This phase lasts for about 100 days for hairs on the scalp and longer for hairs on the eyebrow, eyelash, arm, and leg. During this phase, the hair follicle is completely at rest and the club hair is completely formed. About 25 to 100 telogen hairs are shed normally each day.
* All information subject to change. Images may contain models. Individual results are not guaranteed and may vary.