Hair is a symbol of vitality and health, as well as being a key feature of the individuality and identity of a person. A gleaming hair in good condition is one of the main attributes of beauty and contributes to having high self-confidence and self-esteem.
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.
Structure Of Hair
Hair is a complex organ that includes the external part, known as the hair shaft, and the internal part, called the hair follicle. The following parts can be distinguished inside the follicle, depending on the current phase of the hair cycle.
Multipotent stem cells are present in the follicle. These cells are self-renewing and differentiate into more mature but less potent specialized cells. The division and differentiation of these cells can restore hair. For this reason, they are directly related to hair growth and absence of hair loss.Recent studies have shown that this group of stem cells is not homogenous, because it consists of two biochemically and functionally distinct populations:
The first one, situated in the bulge, consists of the stem cells which are responsible for the differentiation and growth of various cell types to produce pigmented hair.
The second one, which niche is called the secondary germ, consists of the progenitor cells. They are the direct descendants of the bulge stem cells and are in contact with the dermal papilla, which activates them to initiate and control hair growth.
The dermal papilla (DP) is a conglomerate of specialized fibroblasts that is involved in the morphogenesis and hair cycle through the regulation of various cell types in the follicle. It contains stimulating factors for the proliferation and differentiation of follicular keratinocytes, which can induce the formation of a new hair follicle.
During the initial stages of hair regeneration (end of the telogen phase/beginning of the anagen phase), the hair follicle stem cells are quiescent.
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 then matrix cells quickly proliferate to produce the hair shaft. Hair pigmentation is due to melanocytes inserted between these cells.
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.
Approximately 85-90% of the follicles are in the anagen phase , which usually lasts from 2 to 8 years. 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.
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.
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 re-enter 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.