Does hair renew itself, that is to say, is it normal that hair falls out?
The hair follicle doesn´t grow continuously but is produced in distinct phases which make up the hair cycle. There are three basic and well known phases of hair growth: Anagen: This is the phase of active growth. The live cells at the base of the hair follicle actively grow and eventually form a compact column which extends upwards to the surface of the skin. Afterwards a keratinized area forms on top of where the active division of the cells occurs. Live cells become dehydrated, and on death are converted into keratin. Finally the keratin filaments are cemented together by a cistine rich matrix. The area known as the dermal papilla, situated just below the area of active follicular cell division, plays an important role in controlling the growth cycle. In a scalp with hair, normally 90% of the hairs are in this phase which lasts for three months. The rate of growth in the scalp is approximately 0,035 mm per day or 1cm per month. Catagen: During the catagen phase, the base of the hair is keratinized and forms a lump called a club, which begins to separate from the dermal papilla. It then moves to the surface while remaining connected via a fine filament of connective tissue. This phase lasts for 2 or 3 weeks. Its a degenerative phase in which the hair begins to fall out to be later regenerated. Telogen: This is the rest phase. The adherence at the base of the follicle becomes fragile until the hair finally falls out. This phase lasts for 3 or 4 months and generally is what happens after a hair transplant. For this reason, there are no signs of significant growth in the grafts until after this phase. On top of that, when there is native hair present in the receptor area, some of these hairs will be affected by surgical trauma and will enter into a catagen-telogen phase. This causes a temporal but greater initial loss of hair, which is called telogen effluvium. Normally around 10% of hair follicles in a scalp with hair are in the telogen phase. When the quantity of hair being lost exceeds the percentage of hair in growth, the hair begins to look thin and bal Androgenic alopecia occurs as a result of the gradual conversion of terminal hairs (healthy and dense) into soft or down hair (pale and microscopic). It is a hereditary condition and appears to be controlled by an autosomic dominant gene which is related to sex. The expression of this gene depends on the amount of androgens circulating. Testosterone is the main androgen circulating in masculine blood, while in the female blood the steroid dehydroepiandrosterone sulfate, androstenediol sulfate and 4-androstenediol are the predominant proandrogens present. A proandrogen is a steroid which can be converted into an active androgen by enzymatic action in a specific tissue. Similarly, the enzymatic reduction of testosterone and the previously mentioned androgens by 5-alfa-reductase is necessary for the induction of androgenic hair loss, for both men and women. The initial signs of thinning are clearly related to puberty in males when the levels of testosterone begin to increase, gradually converting hair into soft or down hair. Initially, this results in the recession of the front hairline or the crown, and depending on the genetic characteristics, can progress into the parietal zone forming a strip along the temporal and occipital areas.  
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