Androgenic hair
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Body hair, or more precisely androgenic hair, is a common term for the terminal hair on the human body developing during and after puberty in contrast to the head hair and the less visible vellus hair. The medical term androgenic hair refers to the fact that its growth depends on the level of androgens (male hormones) in the individual human organism. Due to a normally higher level of androgens men tend to have more androgenic hair than women so that this type of hair can be seen as a sign for sexual dimorphism and therefore a secondary sex characteristic.
From childhood onward, vellus hair covers the entire human body regardless of sex or race except in the following locations: the lips, the backs of the ears, the palms of hands, the soles of feet, certain external genital areas, the navel and scar tissue. The density of the hairs (in hair follicles per square centimeter) varies from one person to another.
[edit] Development and growth
The rising level of androgens due to puberty causes a transformation process of vellus hair into terminal hair on several parts of the human body. The hair follicles respond to androgens, primarily testosterone and its derivatives. The rate of hair growth and the weight of the hairs increase. Different areas respond with different sensitivities. As testosterone levels increase, the sequence of appearance of androgenic hair reflects the gradations of androgen sensitivity. The pubic area is most sensitive, and heavier hair usually grows there first in response to androgens.
Areas on the human body that develop terminal hair growth due to rising androgens in both sexes, are the axillary hair and the pubic hair. In contrast to that, normally only men grow androgenic hair in other areas. There is a sexual dimorphism in the amount and distribution of androgenic hair, with males having more terminal hair (particularly facial hair, chest hair, abdominal hair, leg and arm hair) and females having more vellus hair, which is less visible. The genetic disposition determines the sex-dependent and individual rising of androgens and therefore the development of androgenic hair.
[edit] Evolution
The evolutionary theory suggests that human androgenic hair is the remnant of body hair of the type found in all other "great apes", which was lost in the course of the history of the Homo genus (human evolution) roughly 3-2 million years ago. Loss of body hair is coupled to the evolution of dark skin colour, which was complete by 1.2 million years ago, judging from the numbers and spread of variations among human and chimpanzee MC1R nucleotide sequences.de:Körperbehaarung es:Vello androgénico nl:Lichaamsbeharing pl:Owłosienie ciała
