Understanding hair loss cycles and growth cycles are helpful when trying to understand hair loss and can serve as a useful background for anyone interested in hair restoration.
Human Hair loss and Growth Cycle
Hair growth can be divided into three distinct phases:
Anagen (active growth)
Catagen (active loss)
To understand these phases, one must first understand that scalp hair consists of 100,000 to 150,000 follicles, which is a small fraction compared to the approximate 5 million follicles covering the rest of the body. In the nonbalding scalp, 90% of hairs remain in the active growth, or anagen phase, which extends over a period of 3 years.
The next phase is the catagen phase, which usually lasts about 1 to 3 weeks. During this time, the hair separates from the dermal root while remaining in place only by a thin strand of connective tissue.
Telogen, or the resting phase, comes next as the basal attachment becomes even more dissipated, leading to the hair shaft eventually falling out. Nonbalding hair on average remains in the telogen phase about 10% of the time, and the phase lasts around 3 to 4 months. Thinning of hair begins to occur when the rate of hair loss exceeds that of hair growth.
Male Pattern Baldness
In addition to actual hair loss, male pattern baldness is characterized by the conversion of terminal (thick and healthy) hair into vellus (thin and fine), also referred to as “baby” hair. Hair loss has one main controlling factor, which is derived from a single, autosomal gene that is regulated by freely flowing levels of androgens (male hormones that consist of testosterone and dihydrotestosterone).
The arrival of puberty for men causes a surge in androgens, which drives the adaptation from the terminal to vellus hair and starts the beginning of hair loss. The ongoing rate of hair loss and change to vellus hair is dependent principally upon genetic forces.
Varying degrees of both male hormones (testosterone and dihydrotestosterone) are found in men and women. Testosterone is necessary for pubic, beard, and axillary (underarm) hair growth, but does not affect scalp hair growth or loss. On the contrary, the presence of dihydrotestosterone leads to hair loss in hairs that are destined for hair loss, without having an effect on other non-susceptible hairs.
Usually, in men, susceptible hairs cover the front and top of the head (crown), but the sides and back of the head usually consist of non-susceptible hairs, which are resistant to hair loss caused by the presence of dihydrotestosterone. This means that there is still a large quantity of hair in the back and sides of the head on even the baldest of individuals.
These areas of the head can serve as donor hair for transplantation to areas of the scalp that are bald or balding, such as the crown. The areas used to gather donor hair for a transplant are genetically programmed to remain forever, which means the donor areas will never undergo hair loss.
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