In a bustling crowd, one can always see a few people with thinning hair or even balding. Why do most people have good hair and some people have “shedding”? Why does the hair on the back of the head not fall out? Why don’t women see the same thing? The most common form of hair loss is male pattern baldness (abbreviated as MPB), which is also known as premature baldness, alopecia areata, or “shedding”. It is also known as androgenetic alopecia (AGA) because it is influenced not only by family history, but also by androgens. Why does androgenetic alopecia occur? It is believed that androgenetic alopecia is related to genetics and endocrinology. Genetics is the prerequisite for the development of the disease, but its genetic characteristics can only manifest under the action of androgens. I. Genetic Factors 1. When asked about male pattern baldness, the majority of patients have a family history of hair loss. Most people believe that the disease is an autosomal dominant disorder, while others believe that it is polygenic. Men with a family history of the disease do not develop it before puberty, but can develop hair loss again after administration of androgen therapy. This indicates the role of genetics and androgens in this disease. 2. There are significant differences in the onset of the disease among different races, with Caucasians, especially Caucasians, being the most common, with an incidence of up to 80% in men. Second, how do androgens work? 1. The skin contains more androgen-dehydroisandrosterone, and there is a type II 5αC reductase in the scalp, mainly in the sebaceous glands and the outer root sheaths of hair follicles, as well as in the surrounding tissues of hair follicles, which can convert the weak androgen-dehydroisandrosterone into strong androgen. Dehydrotestosterone is converted to testosterone and dihydrotestosterone (DHT) by the action of this enzyme. dHT binds to the receptor five times more strongly than testosterone, and when testosterone and DHT bind to the receptor, a complex enzymatic reaction occurs. Phosphorylation and receptor sulfhydryl reduction produce androgen-receptor complexes that enter the nucleus and bind to genetic loci, thereby stimulating or altering the internal processes that mediate hair growth, inhibiting adenylate cyclase activity, and shortening the anagen phase of the hair follicle and making the diameter of the follicle smaller. 2. 5αC reductase in the hair follicles on the top of the head of women is more than 3 times less than in men. There is a cytochrome P450 aromatase specifically present in the outer hair root sheath of the hair follicle in the scalp, and its content is 2-5 times higher than that of men. The role is to convert testosterone and androstenedione into estradiol and estrone. This is why women have relatively little baldness, and even when they do lose hair, they don’t experience the “shedding” that men do. Most hair loss is scattered, showing varying degrees of thinning of the hair. 3. The androgen receptors and 5αC reductase in the frontal and top scalp are higher than in the occipital area, and there are more cytochrome P450 aromatase in the occipital scalp. Therefore, the occipital hair will not fall off, and the hairs transplanted to the forehead and the top of the head will be the same as the occipital hair and will not fall off. The blood supply of the scalp is not the main cause of hair loss. In the past, some people thought that the local vasoconstriction of the scalp reduced the blood supply, causing malnutrition of the hair follicles and hair loss. In the past, some people thought that the constriction of local blood vessels in the scalp reduced the blood supply, causing malnutrition of hair follicles and hair loss. In contrast, when hair follicles from non-alopecia areas were transplanted to both alopecia and non-alopecia areas, the hair grew well and maintained its original growth characteristics. The experiment shows that hair loss is not related to the local blood supply to the scalp, but to the hair follicle itself. In Europe, some doctors have tried to improve the blood supply to the alopecia area through vascular bypass, but no satisfactory hair growth has occurred. Male pattern baldness is often associated with increased sebum production, increased dandruff, and a greasy, itchy scalp. However, increased sebum secretion is not the main cause of the disease; its increased secretion is also associated with androgens. Hair loss not only affects the appearance, but also brings a certain degree of psychological stress to those who suffer from hair loss and affects self-confidence. There are many medications available to treat hair loss, but the only proven treatment that can really improve the appearance is hair transplantation.