The hair follicle grows in cycles. The Anagen (growing phase) lasts on average 3 years in humans (varies between 2 years and up to 7 years). Between 85% to 90% of the scalp hairs are in Anagen at any given time. Then there is a brief transition phase called Catagen (transition phase) that occurs when hair follicles shift from the growing phase to the Telogen (resting phase). Catagen lasts between 2 to 3 weeks in humans and 2% of scalp hairs are in Catagen at any given time. Telogen lasts on average 3 months in humans. 10% to 13% of scalp hairs are in Telogen at any given time. Those are the three main phases of the hair cycle. The relative duration of these phases in particular Anagen varies with the individual’s nutritional status, age, hormonal function and other physiologic and pathologic factors.

Humans have between ten and 20 hair cycles in a life time from adulthood. Those people with only 10 hair cycles and a short Anagen phase of say only 2 years, will lose most of their hair after 20 years into adulthood. Those people who have 20 hair cycles and a long Anagen phase of say 7 years, will almost never lose their hair during their life time. Most people are somewhere in-between these two extremes and there is a natural curve (Gauss) distribution in the general population. Humans have an asynchronous or mosaic type pattern of hair cycling. Neighbouring follicles are at different stages of the hair cycle. In Rodents for example, the hair cycle occurs in a wave pattern. If Humans had a similar pattern to Rodents, then we would lose all our scalp hair every 3 years and it would last for 3 months at a time. Based on 100,000 hairs on the scalp and when 10% are in Telogen then we lose approximately 100 hairs daily as part of cycle shedding.

Normally this cycle of hair production and inactivity will continue for the duration of the individual’s life but other factors can influence and inhibit hair production and in some cases lead to physical destruction of the hair follicle.

Factors may include adverse reactions to drugs and cosmetics, or as a result of scarring, tumours, radiation, the genetic profile of the person and normal ageing, effects from hormones and the immune system.

During the Anagen phase in humans, hair shafts grow at a rate of 0.35mm to 0.45mm per day, or 1 cm to 1.2cm per month.

During the Catagen phase the hair follicle deletes the old hair shaft factory and initiates formation of a new hair shaft factory in a process of forming a new hair follicle. Possible factors that may play a role in signalling the start of the Catagen phase may include, environmental factors such as trauma, chemicals, experimentally administered endogenous hormones like ACTH (adrenocorticotropic hormone) and 17 Beta estradiol can induce Catagen. Growth Factors such as Fibroblast Growth Factor 5 may also trigger Catagen.

The Telogen phase lasts 3 months on average in humans. During this time the hair matures into a “club” hair. The club hair later sheds with washing and brushing. Usually when you just run your hand through your hair and you get hairs, those are Telogen hairs, not Anagen hairs. Any Anagen hairs seen like that is abnormal. The way to differentiate between Telogen hairs and Anagen hairs is to do a Hair Pull Test. Grasp about 20 to 50 hairs between your thumb and forefinger at the base of the hair shaft and then pull away from the scalp while allowing your fingers to slip upwards (distally) along the hair shafts. In a normal scalp, you will only pull Telogen hairs out, and no more than 4 or 5 Telogen hairs, mostly less. That is normal. Anything more than 5 Telogen hairs and any Anagen hairs that are pulled out that way is abnormal. The difference between Telogen hairs and Anagen hairs can easily be seen under magnification when analizing the root or bulb area of the hair shaft. Telogen hairs will not have a bulb, they simply break off or come loose easily.

At the end of Telogen and the beginning of a new Anagen phase, the Dermal Papillae enlarges and a projection grows down into the deep dermis and subcutaneous tissue and a new Anagen hair follicle starts.

Sources:
– ISHRS Basics Course Lectures by Dr. Nusbaum; Dr Washenik; Dr B Farjo
– Hair Transplantation, Edition 5, Unger, Shapiro
– Various Hair Transplant Forum International Articles (2010 to 2015)

Read more about hair transplant surgery.

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