<P> The function of hair in humans has long been a subject of interest and continues to be an important topic in society, developmental biology and medicine . Of all mammals, humans have the longest growth phase of scalp hair compared to hair growth on other parts of the body . For centuries, humans have ascribed esthetics to scalp hair styling and dressing and it is often used to communicate social or cultural norms in societies . In addition to its role in defining human appearance, scalp hair also provides protection from UV sun rays and is an insulator against extremes of hot and cold temperatures . Differences in the shape of the scalp hair follicle determine the observed ethnic differences in scalp hair appearance, length and texture . </P> <P> There are many human diseases in which abnormalities in hair appearance, texture or growth are early signs of local disease of the hair follicle or systemic illness . Well known diseases of the hair follicle include alopecia or hair loss, hirsutism or excess hair growth and lupus erythematosus . </P> <P> The position and distribution of hair follicles changes over the body . For example, the skin of the palms and soles do not have hair follicles whereas skin of the scalp, forearms, legs and genitalia have abundant hair follicles . There are many structures that make up the hair follicle . Anatomically, the triad of hair follicle, sebaceous gland and arrector pili muscle make up the pilosebaceous unit . </P> <P> A hair follicle consists of: </P>

The glands which are normally associated with hair follicles are called