The hormones involved in the mechanism of androgenetic hair loss are male hormones, which are called androgens. The answer to the question “How androgens cause baldness?” is not straightforward. Androgens have profound influences on several biological mechanisms. They exert their actions by interacting with specific receptors found on cell membranes or inside cells, as all other hormones do. Several types of androgens can affect a single hair follicle and different types of hair follicles in different regions of the skin respond to the same androgen in different ways. For example, hairs located on the armpit respond to androgens by growing, whereas hairs in the scalp fall out in response to them.
Two types of androgens are involved in the mechanism of male pattern hair loss. These are testosterone and dihydrotestosterone (DHT). Dihydrotestosterone is a derivative of testosterone. The enzyme called 5-alpha reductase converts the relatively inactive testosterone to a more potent form, dihydrotestosterone. Thus, the key enzyme involved in androgenetic hair loss is 5-alpha reductase, which is also found in and around the hair follicle, especially in the dermal papilla. Once formed, this potent male hormone, DHT gains ability to bind any androgen receptor on the hair follicles. Plain testosterone can also act on the follicles, but its effect of causing hair loss is weaker.
By the interaction of these two hormones and their receptors on the follicle, hairs covering the scalp undergo some changes ending with hair loss. Over time, the growth cycles of the terminal hair follicles on the scalp get progressively shorter, with reduced anagen (growing) phase. Catagen (intermediate) and telogen (shedding or resting) phases of hairs remain the same, thus, yielding a net result of increased number of resting hair follicles. So, the usual proportion of telogen follicles (10%) increases up to 20% of total. That is, more hairs enter the resting stage and the number of hairs that shed increases.The affected follicles decrease both in length and diameter, growing thinner, shorter and more brittle hairs with weaker shafts.
One may ask why do some people develop male pattern baldness while others do not, although all men and women have these hormones causing hair loss and their receptors in their bodies? There is not a satisfactory answer to this question. There are some speculations however, of which the most important ones are mentioned below:
- In the balding areas of the scalp of patients with androgenetic hair loss the number of androgen receptors in a single hair follicle is higher than normal. So that with the same level of androgens in the bloodstream, a more pronounced effect of androgens (that is, hair loss) is observed, as if their levels were higher than normal.
- In balding persons’ hair follicles sensitivity of receptors to androgens is higher than normal, thus, resulting again in a more pronounced effect of androgens (that is, hair loss) in normal levels.
- The activity of the enzyme 5-alpha reductase is higher in the balding area, thus, converting more testosterone to dihydrotestosterone. The higher the proportion of DHT to testosterone; the faster is the hair loss process.
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