Each hair follicle represents an invagination of the epidermis in which a hair is formed.
Hair follicles and hairs are present over almost the entire body; they are absent only from the sides and palmar surfaces of the hands, sides, and plantar surfaces of the feet, the lips, and the region around the urogenital orifices. Hair distribution is influenced to a considerable degree by sex hormones;  FOLDER 15.3 Functional Considerations: Hair Growth and Hair Characteristics
Unlike the renewal of the surface epidermis, hair growth is not continuous, but a cyclic process. A period of growth (anagen) in which a new hair develops is followed by a brief period in which growth stops (catagen). Catagen is followed by a long rest period (telogen) in which the follicle atrophies, and the hair is eventually lost. Epidermal stem cells found in the follicular bulge are capable of pro-viding stem cells that give rise to mature anagen follicles. During the hair growth cycle, mature anagen hairs period-ically undergo apoptosis and regress to the catagen stage. In this phase, entire follicles retract toward the epidermal layer. As the base of the  retracted follicle approximates the follicular bulge, the hair shaft is no longer supported by the nutrient-rich anagen bulb and eventually is ejected from the resting telogen follicle. This makes room for a new shaft that will grow during anagen regeneration. More than 80% of the hair present in the normal scalp is in the anagen phase. In catagen, the germinative zone is reduced to an epithelial strand still attached to a remnant of the dermal papilla. In the telogen phase, the atrophied follicle may contract to one half or less of its original length. The hair may remain attached to the follicle for several months during this stage and is called a club hair because of the shape of its proximal end. Hairs vary in size from long, coarse terminal hairs that may reach a meter or more in length (scalp hair and beard hair in males) to short, fine vellus hairs that may be visible only with the aid of a magnifying lens (vellus hairs of the forehead and anterior surface of the forearm). Terminal hairs are produced by large-diameter, long follicles; vellus hairs are produced by relatively small follicles. Terminal hair follicles may spend up to several years in anagen and only a few months in telogen. In the balding individual, large ter-minal follicles are gradually converted into small vellus folli-cles after several growth cycles. The ratio of vellus follicles to terminal follicles increases as baldness progresses. The “completely bald” scalp is not hairless but is populated by vellus follicles that produce fine hairs and remain in telogen for relatively long periods.
these include, in the male, the thick, pigmented facial hairs that begin to grow at puberty and the pubic and axillary hair that develops at puberty in both genders. In the male, the hairline tends to recede with age; in both genders, the scalp hair thins with age because of reduced secretion of estrogen and estrogenlike hormones.
The hair follicle is responsible for the production and growth of a hair. Coloration of the hair is attributable to the content and type of melanin that the hair contains. The follicle varies in histologic appearance, depending on whether it is in a growing or a resting phase. The growing follicle shows the most elaborate structure; thus, it is described here.