Physiology of Hair Follicle
In the human body we grow hair on every area with the exception of our palms and the soles of our feet. Of course, some of the hair is so fine that it is virtually invisible. A protein, Keratin, is produced from the outer layer of the skin and a hair is formulated. Normally, the follicle is producing new hair cells consistently and they replace the native ones.
These old hair cells, the hair shafts, are creating the hairs that we can see on our scalp. Each terminal hair is divided into the hair follicle that is under the skin and the hair shaft. Usually a scalp has 100,000 to 150,000 hairs. Generally, blonde people have more hair than brunette people. We lose hair every day and it is completely normal to shed up to 100 hairs per day. This does not mean that an individual has a hair-loss problem. Each hair has a normal hair cycle in which about 90% of normal hair is growing. It is important to understand that each hair may be at a different phase of its individual life-cycle. The life-cycle can be affected by many factors such as age, some medications, severe diseases, stress, etc.
Normally, hair grows about 15 cm per year and, as people age, their speed of hair growth slows.
We distinguish four phases in the life cycle of the hair:
Anagen – active hair growth with a duration of between two and seven years. The longer the hair stays in the anagen phase, the faster and longer it will grow. About 85 to 90% of hairs are in this phase at any one time.
Catagen – a transitional hair growth phase that lasts two to three weeks. About 1 to 2% of hairs at any given time are in this phase. During this stage, due to disintegration, the follicle shrinks to a sixth of its usual length and detaches from the dermal papilla while the new hair starts to form in the bulb.
Telogen – the resting phase which lasts about two to four months. At the end of the resting phase, the hair sheds and a new hair replaces it and the growing cycle starts again with the anagen phase. About 10 to 15 per cent of the head hair is in this phase at any one time.
Exogen – the new hair phase. The old hair sheds and a new one continues to grow. Approximartely 100 hairs per day may fall and this is considered to be a normal shedding.
At some point, the hair follicle will begin to grow again. The hair base will break free from the root and then the hair will be shed. The new hair shaft will begin to emerge within two weeks.
During hair loss, the hair growth cycle is being affected. The most common cause of Male-Pattern Baldness, a hereditary form of hair loss, is where testosterone, with the help of an enzyme called Alpha Reductase, is transformed into DHT. DHT attacks and causes the papilla to shrink and the hair suffocates; subsequently the anagen phase becomes shorter and shorter until the follicles stop producing hair.
Hair loss is the result of the progressive miniaturisation of the follicles that eventually causes the growth cycle to become shorter and the hairs finer until they fall permanently without the emergence of a new hair.