and the density of androgen receptors in the dermal papillae. Both must reach a threshold for the proliferation of hair follicle cells. It occurs to varying degrees in most human populations.

From childhood onward, regardless of sex, vellus hair covers almost the entire area of the human body. Exceptions include the lips, the backs of the ears, palms of hands, soles of the feet, certain external genital areas, the navel, and scar