Why people lose their hair?

Hair loss occurs in both men and women, with amounts increasing with age and varying from one person to another and from one age to another.  On men, the essential problem occurs in the temporal-frontal region, producing a line (contour) of hair in withdrawal.  The vertex hair and fronto-parietal part becomes shorter, thinner, and more fragile.  For some men, the crown hair develops short vellus hairs (fuzz), while for others, the alopecia progresses, leaving just a strip of hair, shaped like a horseshoe around the scalp. While the hair loss progresses, men notice episodes of augmented hair loss.  At the same time the texture of the hair in the pre- and post-auricular areas changes its consistency, becoming similar to that of beard hair.
On the other hand, women preserve a relatively normal frontal hair-line, but they develop a thinning of the hair in the fronto-parietal area of the scalp.  The diffuse hair loss may become extensive, although a few thin vellus hairs may be found.  The most striking change is a thinner central part.
Despite the different ways of losing hair, rapid or progressive, consistent or intermittent, the final result in both sexes is a hair loss that can be placed in a pattern. The male pattern is usually more extensive and is classified according to the Hamilton pattern, modified by Norwood.  The female pattern, after the Ludwig classification, is, usually, less severe.
Although men with balding problems seem to be more virile, they generally have a similar number of children as men without this kind of problems.
The cause of balding has not been established with certainty, but researchers generally agree that it is transmitted by multifactorial or poligenic forms of inheritance, which suggests that alopecia is the result of the interaction of different genes combined with environment factors.  Some scientists have established that baldness in men is an androgen-mediated miniaturization process of genetically susceptible hair follicles.
Women with baldness are presumed to undergo the same process as men. Hair loss in women, in general, leads to anxiety and depression. The renowned researcher Eckert believes that an inadequate directioning of women’s attention to their hair loss may be a symbolic way to emphasize their anxiety and marital problems.  Men regard hair loss as unpleasant, but cope with it in an active way and regain lightly the integrity of their personality.  Men under 55 with vertex alopecia seem to have an increased incidence of coronary artery diseases.
As already mentioned, supposedly alopecia in both cases is a miniaturzation process mediated by androgen that affects genetically susceptible follicles, at a hair level.  The follicles become smaller and smaller and have shorter periods of growth.  In time, the rough terminal hairs become thinner, less pigmented and transform into vellus hairs.
In women, this process modifies by the lack of vellus hairs on the affected scalp area.  In women with advanced alopecia, a hyperandrogenism cause must be considered.
In both sexes the area affected by alopecia is greasier than the unaffected one.  This observation has led to the suggestion that micro-organisms, altered sebum lipids, and oxidants cause and augment the process.  Gravimetric studies of the sebum level and the production of sebum on the scalp of men with and without alopecia problems reveal no difference between the two groups.  Washing the hair reduces hair loss for the next 24 hours, an observation explained by the fact that the hair wash removes the hairs, determining the end of normal telogen, thus reducing physiological hair loss for a couple of days.
So, the simple phenomenon of "hair loss" involves many different changes taking place beneath the skin, particularly changes of the hair follicle structures. The structure of hair follicles consists of highly differentiated multicompartmental cell groups. This independent "micro-organ" in the skin not only continuously manufactures hair but also undergoes a complete remodeling of its structure in continuous periodic cycles. Thus, the cyclic change of hair follicles not only brings a substitute of an old hair with new but also can modify the length, thickness, and colour of new hair. This intermittent structural remodeling of the hair follicles - from a growing (anagen) to resting (telogen) and again to new growing phases - is essential for the developmental growth and maintenance of hair as well as for the process of hair loss and for its repair.
The changed cycle of hairs in a scalp area affected by alopecia is demonstrated by a reduced number of hairs in anagen and a shorter anagen phase.  In an affected area more hairs are in telogen for a longer period and they fall much easier in the process of daily fixing than in an unaffected area.  Their reduced life-cycle leads to shorter, thinner, miniaturized hairs.  It has been demonstrated on rodents that potentially mitotic cells distributed in follicular sheaths and bulbs can be added to the diameter and weight of the hair follicles.
This non-cyclic transformation of gradual thickening or thinning of hairs on women may explain the usual loss of short vellus hairs on women with alopecia.