The Biology of Hair Growth

An old saying regarding hair goes; “A hair on the head is worth two on the brush!”. So it might come as no shock that the strand of hair that you located on your hairbrush this morning is an amazingly complex component of your body.

Generally speaking, hair starts appearing on the human body while it is still in the womb. By the time a forming fetus attains 5 month old, there are already five million follicles on the body. Interestingly, that is all of the follicles that will ever develop regardless of how long we actually live. No additional hair will ever be added.

The Anatomy of Hair

Hair consists of a follicle, which is embedded in the skin, and the shaft of hair which appears on our body. The follicle itself consists of multiple layers with each segment having a specific purpose. The papilla lies at the bottom of the follicle. Capillaries are connected to the papilla and they furnish blood to the cells which encompass the bottom portion of the hair strand called the bulb.

Encompassing the follicle are two sheaths, inner and outer, which are designed to both safeguard the hair shaft from harm, and to help it grow out in the appropriate direction. The inner sheath runs adjacent to the hair shaft and ends at the oil (sebaceous) gland. The outer sheath runs to the gland and ends at the erector pili muscle. Somewhat amuzingly, this is the muscle that causes our hair to “stand on end” when it contracts.

Now let’s take a look at the hair shaft. It is comprised of three layers of dead protein cells known as keratin. The innermost layer, known as the medulla, is not always existing in every hair shaft. The second layer, known as the cortex, provides the bulk of the hair shaft. The hair color is primarily decided by the pigmentation contained in this layer. The outermost layer is called the cuticle. It is comprised of a series of overlapping cells. It is important to note that the hair’s luster and sheen comes from the cuticle.

How Hair Grows

Have you ever thought about the groth rate of your hair? Hair on your head generally grows at an standard rate of six inches per year, or just .3-.4 mm per day. There are three distinct stages of hair growth.

Catagen Phase
This transitional stage runs around two to three weeks. At any given time, roughly 3% of your hair is in this phase. There is no hair growth during this period while the out portion of the hair root sheath shrinks and attaches itself to the root.

Telogen Phase
This phase, known as the “resting period” is experienced by approximately 15% of your hair. In this situation, telogen lasts for around 100 days for scalp hair and dramatically longer for other body hair.

Anagen Phase
This phase defines a interval of activity where the hair cells are dividing and new hair development occurs. It is this stage that can last for as long as 2-6 years. Shorter growth periods are indicative of people who have trouble growing their hair long.

Hair Growth Disorders

It is important to note that there are two hair growth disorders that inflict humans. Hirsutism is a condition affecting mostly females. It is defined as a state where excess thick and pitch-dark hair growth is incurred in locations typically reserved for males. These areas comprise the face, chest and the areolae or area encompassing the nipple.

Hypertrichosis is a state which causes excess hair growth in areas of the body where visible hair is not generally present. Both of these conditions are treatable, but not curable.