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At the time of puberty and even after that, the terminal hair that grows on the human body is Androgenic hair that is also commonly known as body hair. The body hair can be differentiated between head hair and vellus hair that are fine as well as light in color. Increased levels of androgens i.e. Male hormones in an individual are responsible for the growth of androgenic hair. Men naturally have higher level of androgen and thus they obviously have more androgenic hair on their body in comparison to women.
Right from the childhood people of every sex, be it male or female, they have vellus hair covered over almost the entire body excluding lip, scar tissue, palm, sole, back of the ear, navel, and the external genital areas. Every individual person has different hair density i.e. The number of hair follicles present in a single area varies in every individual. There are many situations when vellus hair on the human body starts becoming thicker and darker, for e.g. Beard hair when it starts appearing on chin of male in adolescence.
Androgenic hair follows the same growth pattern as the hair that grows on the scalp, but with a shorter anagen phase and longer telogen phase. While the anagen phase for the hair on one’s head lasts for years, the androgenic hair growing phase lasts a few months. The telogen phase for body hair lasts close to a year. This shortened growing period and extended dormant period explains why the hair on the head tends to be much longer than other hair found on the body. Differences in length seen in comparing the hair on the back of the hand and pubic hair, for example, can be explained by varied growth cycles in those two regions. The same goes for differences in body hair length seen in different people, especially when comparing men and women.
Development and Growth
While the hair we see on the outside of our bodies may appear to be actively growing, the real action takes place below the surface of our skin, or epidermis. Cells inside of our hair follicles divide and multiply, and as space fills up inside of the follicle, it pushes older cells out. After those older cells harden and exit the follicle, they form the hair shaft. The shaft is mostly comprised of dead tissue and a protein called keratin.
Hair follicles are to varying degrees sensitive to androgen, primarily testosterone and its derivatives, with different areas on the body having different sensitivity. As androgen levels increase, the rate of hair growth and the weight of the hairs increase. Genetic factors determine both individual levels of androgen and the hair follicle’s sensitivity to androgen, as well as other characteristics such as hair colour, type of hair and hair retention.
Rising levels of androgen during puberty cause vellus hair to transform into terminal hair over many areas of the body. The sequence of appearance of terminal hair reflects the level of androgen sensitivity, with pubic hair being the first to appear due to the area’s special sensitivity to androgen. The appearance of pubic hair in both sexes is usually seen as an indication of the start of a person’s puberty. There is a sexual differentiation in the amount and distribution of androgenic hair, with men tending to have more terminal hair in more areas. This includes facial hair, chest hair, abdominal hair, leg hair, arm hair, and foot hair. Women retain more of the less visible vellus hair, although leg, arm, and foot hair can be noticeable on women.
Like much of the hair on the human body, leg, arm, chest, and back hair begin as vellus hair. As people age, the hair in these regions will often begin to grow darker and more abundantly. This will typically happen during or after puberty. Men will generally have more abundant, coarser hair on the legs, arms, chest, and back, while women tend to have a less drastic change in the hair growth in these areas. However, some women will grow darker, longer hair in one or more of these regions.