Microscopically, AGA is characterised by progressive shrinking of scalp hair follicles.8 In many patients, AGA is accompanied by an acceleration of the hair growth cycle, as reflected by decrease of anagen and increase of telogen hair in the trichogram. However, some patients have a normal anagen/telogen ratio despite slowly progressive AGA. Whether and when a scalp hair follicle miniaturises is dependent on two factors: genetics and androgens. The genes responsible for shrinkage of a scalp hair follicle are not known. Each scalp hair follicle carries individual genetic information that determines whether and when it will develop a sensitivity towards androgens. Once a scalp hair follicle has become sensitive to androgens, it will progressively shrink over the next years. In men, the most important androgen driving AGA is dihydrotestosterone (DHT). Within the cells of the hair follicle, DHT is derived from its precursor testosterone by two enzymes: the 5-a-reductase types I and II.9 DHT seems to be less important in women than in men.10 In general, androgens can be considered potentially harmful and oestrogens potentially beneficial for scalp hair growth in women.
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