The hair cycle
Hair grows, falls and repels according to a cycle called the capillary cycle that will occur about 25 times in the life of the hair. Each hair cycle is made up of three phases: Anagen Telogen and Catagen. Anagen Phase The Anagen Phase is the growth phase of hair. This is the longest part of the hair cycle as it lasts from 2 to 5 years (4 years for men and 6 years for women). The vast majority of hair (around 90%) present on our head is in the anagen phase. Catagen phase The catagen phase is a resting phase during which the hair stops evolving. This rather short phase is on the order of one to three months, which is considerably less than the previous phase. Telogen Phase In the telogen phase, the hair eventually falls out and gives way to another hair that begins its anagen phase. This three-month phase is the renewal of the hair's life. This is thus distributed over a new life of several years. This is the telogen phase of hair that we find every day on our brush or at the bottom of the sink, but that we also lose during the day, so it is normal to see an estimated 150 hair loss per day. seasonal changes (seasonal fall in spring and autumn). It is during this phase that the hair renews itself the most. Hair loss is considered pathological when a person loses more than 100 hairs per day for a fairly long period of up to two months. Diagnosis this fall requires a consultation and sometimes an exam called a "trichogram" (we take a bit of hair from different areas of the scalp and look at it under a microscope). Sometimes a hormonal evaluation is necessary in the woman. The results of these tests point to one of the following problems.
Classification
There are five types of alopecia:

Hereditary androgenetic alopecia: The most common, manifested by a decrease in hair volume, including baldness, and affects 50 to 70% of men (20% of men between the ages of 20 and 30, hair loss usually begins at age 20 and stabilizes at age 30). Alopecia classification Acute alopecia: May be related to chemotherapy treatment, stress, significant nutritional deficiencies, iron deficiency, hormonal disorders, acute irradiation;
Localized alopecia: It can be caused by skin problems (tumor, burns, alopecia areata), radiotherapy or parasites (ringworm, lichen);
Congenital alopecia: Alopecia areata (alopecia areata)
Alopecia areata appears to be of autoimmune origin (cellular mediation mechanism) characterized by a more or less large "patch" attack in one or more places. This form of alopecia can reach the entire head and we talk about total alopecia and sometimes the whole body: it is universal alopecia, and in this case, there is no hair or hair on the body. whole body
Androgenetic Alopecia: Androgenetic alopecia is a gradual loss of hair due to the influence of male hormones. In general, it is between 30 and 40 years old, in 70% of men on average, but in some cases, it can appear from the age of 16. The causes that cause hair loss are multiple, the hormone traditionally involved is dihydrotestosterone (DHT). However, the scientific community agrees that it is not the only one responsible, so epitestosterone could play an important role in the process. This hair loss usually starts at the temporal glands and temples (at the front of the head) and at the vertex (top of the head). It is characterized by a thinning of the hair and a loss that can be sudden in some individuals and slower in others. The modified Hamilton Norwood scale allows for baldness graduated from 1 to 7, where 1 is the absence of baldness and 7 is maximum alopecia (loss of hair in the total and vertex gulfs).