The life cycle of a hair is divided into three phases. The actively growing (Anagen) phase, the transitional (Catagen) phase, and the resting (Telogen) phase.
Hair cycle
During the anagen phase, protein and keratin are continuously made. It is during this phase that the hair shaft is manufactured and pushed upward to its natural length. Note the large healthy bulb at the hair base. A hair’s anagen, or growth phase, lasts from 3 to 5 years, and represents what is occurring to about 90% of the hair on your head at any given time.
In the catagen, or transitional phase, there are chemical and structural changes in the hair follicle. The hair stops growing, and remains in this phase for only two to three weeks before moving into the next phase.
Finally, hair enters the telogen phase where it basically just sits on your head for about 3 months. Then, it falls out only to be replaced by the next budding hair in the anagen phase which begins to grow from the same hair follicle. These replacement hairs get finer and thinner as a person ages. In most settings of baldness, the hair follicle simply shuts down and refuses to put out more hair to replace the ones that have fallen out.
A person normally sheds up to 100 hairs per day. Hair growth occurs at about a half inch per month, faster when it’s hot (summer) and slower when it’s cold (winter). This rate slows down with age, and shuts off in more and more hair follicles as time marches on. Things that influence hair growth include not only hormones, but nutrition, vitimins, emotional states, and many unknown factors.
Hair growth starts before we’re even born. We don’t really know what triggers it, or what keeps it going. Our first hair, Lanugo (Latin for fine wool), begins to grow approximately 3-6 months after conception. It is usually shed before, or soon after birth and is replaced with the coarser hair we all know. The pattern of our hair is believed to be related to the formation of hair follicles as our skin is stretched over the developing fetus. There are many different patterns and possibilities of hair growth, though some are much more common than others.
After birth, different hormones affect the hair follicle allowing growth or causing hair follicles to move into the resting (telogen) phase. A hormone called dihydrotestosterone (DHT), a product made from testosterone, acts on the hair follicle causing growth to slow and ultimately stop. DHT only works on certain hair follicles that have the genetic predisposition to be shut off. Usually, these are on the front and top of our heads.
An interesting fact is that castrated males do not make testosterone, and therefore can’t make DHT because they do not have testicles, they do not have male pattern balding unless they are given injections of testosterone. If a castrated man receives testosterone injections, they will immediately start again to have the potential of losing hair in the classic horseshoe fashion. Men receiving a vasectomy to prevent pregnancy in their mate (cutting the sperm tubes attached to the testicles) has no effect on testosterone levels and are therefore still producing DHT.
