part of society. People suffering from hair loss are generally perceived as older people who are physically and socially less attractive (Goh & Zippin, 2009).
Hair loss has been for many years a common problem for dermatologists consultation. For both men and women, it is extremely important that hair is healthy, full of plenitude, movement and freshness, as it directly influences our mood and character and it can define our professional, personal and even sentimental success.
Each hair follicle is constantly subjected to cycles consisting of three stages: a stage of rapid growth and hair shaft formation (anagen), followed by a regression stage based on apoptosis (catagen) and a resting period of the hair follicle (telogen).
During the initial stages of hair regeneration (end of the telogen phase/beginning of the anagen phase), the hair follicle stem cells are quiescent (Rompolas et al., 2012).
The activation of the anagen phase starts with a signal from the dermal papilla towards the secondary germ stem cells, which activate and proliferate in order to initiate hair growth. The bulge stem cells are the next ones to activate and proliferate. These cells are responsible for extending the outer root sheath (it separates the hair follicle from the dermis) and maintaining the matrix that supports hair growth (Myung et al., 2013). These stem cells differentiate into matrix cells.
Then, matrix cells quickly proliferate to produce the hair shaft. Hair pigmentation is due to melanocytes inserted between these cells (Paus & Cotsarelis, 1999).
Finally, the stem cells are silenced and return to a state of quiescence. In addition, during the transition from the anagen phase to the catagen phase, proliferating cells in the matrix are induced into a coordinated apoptosis (Plikus, 2012).
Approximately 85-90% of the follicles are in the anagen phase (Rittié et al., 2009), which usually lasts from 2 to 8 years (Paus & Cotsarelis, 1999). The duration of the anagen phase determines hair length, which varies between individuals and declines with age.
It is the end of the hair fiber production phase, when the follicle is subjected to a controlled process of regression. Most follicular keratinocytes undergo apoptosis. Cell growth and pigmentation stop, the bulb is separated from the dermal papilla and follicular shortening occurs (Paus & Cotsarelis, 1999).
It is the shortest phase of the cycle and it only lasts two to three weeks. Therefore, at any given time only between 1% and 2% of the follicles are in the catagen phase (Restrepo, 2010).
At the end of this phase, the follicle retracts into the surface of the scalp (it does not protrude from the dermis) and its size is significantly reduced.
The telogen phase typically lasts from 2 to 4 months, before the follicles reenter the anagen phase and the cycle starts again. Therefore, it determines when new hair is originated.
During this phase, the hair shaft matures into fully keratinized hair that detaches from the follicle (usually due to hair combing or washing). Most people lose 50 to 150 hairs a day.
The percentage of follicles in the scalp in the telogen phase is from 5 to 15%. An increase in this percentage leads to excessive hair loss (Paus & Cotsarelis, 1999).
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