The Biology Behind Hair Growth

Nothing in life is as simple as it seems, so it should come as no surprise that the strand of hair that you found on your brush this morning is an amazingly complex part of your body. Hair begins appearing on the human body while it is still in the womb. By the time a developing fetus reaches 22 weeks old, there are already 5 million follicles on the body. Interestingly enough, that is all of the follicles that will ever develop regardless of how long we live. None will ever be added.
Hair Anatomy
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 layer having a specific function. The papilla lies at the base of the follicle. Capillaries are connected to the papilla and they supply blood to the cells which surround the bottom part of the hair strand called the bulb.
Surrounding the follicle are two sheaths, inner and outer, which are designed to both protect the hair shaft from damage, and to help it grow out in the proper direction. The inner sheath runs next 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. This is the muscle that causes our hair to "stand on end" when it contracts.
The hair shaft is comprised of three layers of dead protein cells called keratin. The innermost layer, known as the medulla, is not always present in every hair shaft. The second layer, known as the cortex, provides the bulk of the hair shaft. The hair color is mostly determined by the pigmentation contained in this layer. The outermost layer is called the cuticle. It is comprised of a series of overlapping cells. The hair's luster and sheen comes from the cuticle.
How Hair Grows
Scalp hair grows at an average rate of six inches per year, or only .3-.4 mm per day. There are three distinct stages of hair growth.
- Catagen Phase
This transitional phase runs about two to three weeks. At any given time approximately 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. Telogen lasts for around 100 days for scalp hair and significantly longer for other body hair.
- Anagen Phase
This phase defines a period of activity where the hair cells are dividing and new hair growth occurs. This phase can last for as long as two to six years. Shorter growth periods are indicative of people who have trouble growing their hair long.
Hair Growth Disorders
There are two hair growth disorders that affect humans. Hirsutism is a condition affecting mostly females. It is defined as a condition where excessive thick and dark hair growth is experienced in areas typically reserved for males. These areas include the face, chest and the areolae or area surrounding the nipple. Hypertrichosis is a condition which causes excessive hair growth in areas of the body where visible hair is not normally present. Both of these conditions are treatable.