biology of hair growth
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How Hair Grows

If there is one simple truth we learn as the years pass by, probably the simplest is that nothing in life is as simple as it seems!

So, it should come as no surprise that the single strand of hair which you found clinging to your brush this morning is really a very complex part of your body.

Hair begins appearing on the human body while it is still in the womb. By the time the developing fetus is 22 weeks old, there are already 5 million follicles on the body. Those are all the follicles that you will ever have no matter how long you live. No new follicles will ever be added.

The Anatomy of a Hair

Hair is made up of a follicle, which is essentially a root 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. A papilla lies at the base of each follicle. Capillaries are connected to the papilla and they supply blood to the cells surrounding the bottom part of the hair strand called the bulb.

There are two sheaths, inner and outer, surrounding the follicle itself. These are intended and 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 sebaceous, or oil 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 in moments of fright or awe, for example.

The hair shaft itself is comprised of three layers of dead protein cells called keratin. The innermost layer, which is known as the medulla, is not always present in every hair shaft. The second layer, known as the cortex, is what 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. The cuticle is comprised of a series of overlapping cells. The hair's luster and sheen will come from the cuticle.

How Does Hair Grow?

Scalp hair normally 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 is a transitional phase that lasts 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 outer portion of the hair root sheath shrinks and attaches itself to the root.

  • Telogen Phase
    This phase, sometimes referred to as the "resting period" is experienced by approximately 15% of your hair. Telogen will last for around 100 days for scalp hair and significantly longer for other body hair.

  • Anagen Phase
    This phase is marked by a period of activity where hair cells divide and new hair growth occurs. This phase can last for as long as two to six years. Shorter growth periods are found in people who have trouble growing long hair.

Hair Growth Disorders

There are two common hair growth disorders that affect humans. Hirsutism is a condition which mainly affects females. It is a condition where excessive thick and dark hair growth is experienced in areas typically reserved for males. These areas will include the face, chest and the areolae, the 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. The good news is that both of these conditions are treatable.



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Biology of Hair Growth and Hair Removal Techniques
Page Updated 5?42 PM Friday, March 5, 2021