In all mammals hair works as a temperature regulator in association with muscles in the skin. If the outside temperature is cold, the arrector pili muscles pull the hair strands upright, creating pockets that trap air. This trapped air provides a warm, insulating layer next to the skin. If the temperature outside is warm, the muscles relax and the hair becomes flattened against the body, releasing the trapped air. In humans, body hair is mostly reduced; it does not play as large a role in temperature regulation as it does in other animals.
In humans, hair has no vital function; the main function of the hair shaft is its role as an important facet of appearance. Its psychological functions are extremely important, as any clinical hair transplant surgeon/dermatologist or cosmetician can readily attest from routine daily practice. Across cultures for centuries, the decoration and styling of scalp hair has been a means of social communication and display of social identity or status. Hair is so important in our society that hair loss (in men and women), as well as the overgrowth of terminal hair on the body or face, in excess of the culturally accepted norm (especially in women), has deleterious effects on self-esteem.
Moreover, hair serves other purposes: in particular, it is concerned with sexual and social communication by constructing adornments such as the mane of the lion or the beard of the human male, or assisting in the dispersal of scents secreted by complexes of sebaceous or apocrine glands.
Historically there have been many explanations of how human hair follicles form. Philosopher Aristotle in the fourth century suggested that hair formed from sooty vapors exhaled through the body’s pores that hardened on contact with air. Albertus Magnus a eleventh century scientist suggested that hair had excretory function and was a product of gross humor and phlegm divested from brain. Only with careful study of skin and hair follicles by anatomists of the nineteenth century were formation and hair follicle structure fully defined and mechanism of hair growth correctly extrapolated.
Human hair follicles appear first in the regions of the eyebrows, upper lip and chin at about 9 weeks of embryonic development,and in other regions in the fourth month. A full complement of the hair follicles is ready by the 22nd week of gestation. It is generally believed that no “new” hair follicles develop after birth.
Hair follicle formation is the result of a complex signaling mechanism between the dermal layer and the overlying epidermis. The hair follicle formation events can be divided into at least eight distinct developmental stages.
When humans are born, they have about 5 million hair follicles, only 2 percent (100,000-150,000) of which are on the head. Hair is present over the entire surface of the body except the palms, soles, glans penis and labia minora. This is the largest number of hair follicles a human will ever have. Considering the distribution of human hair in different areas of the body surface, it is possible to note that human hair growth is reduced with tiny and virtually colorless hair on most of the body surface (vellus hairs), whereas hair is longer, thicker, and heavily pigmented in other areas, such as the scalp, eyelashes, and eyebrows (terminal hairs).
As a human ages, the density of hair decreases. The mean hair follicle density depends on the skin area, because hair follicles are built in the early fetal period. After birth, the body proportions change and the hair follicles move apart according to the growth of body and skin. Because of the relatively lower growth of the head compared to the extremities, hair follicles are much more numerous on the scalp and in the face than on arms and legs.