Life Cycle of Hair

Life Cycle of Hair

Many of us probably have noticed the fact that some areas in our scalp have different hair growth rates. For example, some people who shaved off their hair in order to boost its growth observe that some regions are thinner with a slower hair growth rate. The reason for this is that our hair has a dynamic growth structure (mosaic cycle), in which hair fibers are found in different phases of their life cycle.

Even though there is diversity between races (Asians and Africans tend to have a lower hair density compared to Caucasians), a normal individual has around 100,000 – 125,000 terminal hair fibers.

Follicular Unit term has emerged in 1984, when a pathologist named John T. Headington analyzed a punch biopsy cut in a transverse fashion instead of a longitudinal mode.

What is a FOLLICULAR UNIT?

A follicular unit consists of 1, 2 or 3 (rarely 4 or 5) hair fibers, which are surrounded by a collagen band. It also includes the region with the related sebaceous gland and arrector pili muscles that are attached to each fiber. Most of the scalp hair is made up of these follicular units, which usually have 2-3 hair fibers.

Hair Life Cycle

As mentioned above, hair has a dynamic growth structure that includes continuous elongation, thinning and fall out phases. Each hair fiber is independent from each other and might be found in a particular phase of its life cycle.

Anagen Phase

The hair follicles in Anagen phase are thick, healthy and in continuous elongation. Approximately 84 – 90 % of all hair follicles are found in Anagen phase, which typically lasts for 3 years.

Catagen Phase

Catagen phase is a transition phase; usually lasting about 3 weeks. Programmed cell death (apoptosis) is initiated within this phase. The dermal papillae shrinks and the hair follicle is elevated within the skin. The hair begins to lose its pigment and gets thinner (club hair). Roughly 2% of our hair is found in catagen phase.

Telogen Phase

Telogen phase lasts an average of 3 months. In telogen phase, thin and depigmented club hair falls out. In the majority of the cases, a new hair follicle is produced by the hair stem cells, which are induced by a yet unknown signal. Sometimes, the hair follicles go into a latent state called the Lag phase and the follicles remain dormant for 2 – 5 weeks. Approximately 10 – 15 % of our scalp hair is in telogen phase.

10 % of the hair follicles of an individual, who has a total of 100,000 hair follicles, are theoretically found in telogen phase. Given this, the loss of an average of 100 hair fibers per day – taking into account that a telogen phase that lasts about 90 to 100 days – should be considered as normal.

The Arrector pili muscle is attached to the hair follicle at a region called the Bulge Region. In laboratory studies, it is shown that if the cells from this region (follicular stem cells) are isolated from mice follicles and engrafted to another region on the animal, they have the ability to produce new hair follicles. Such studies would enhance the creation of new strategies for hair transplantation operations within the next decades.