Anatomy, Hair Follicle

Excerpt

The pilosebaceous unit is the hair follicle's structural unit, comprised of the hair follicle and its associated sebaceous gland and arrector pili muscle (see Image. The Common Integument, Section of Skin). The hair follicle originates from the surface of the epidermis. Follicles that produce terminal hairs extend into the deep dermis and, sometimes, even the subcutis. Meanwhile, follicles producing vellus hairs extend only to the upper reticular dermis. Three essential segments of hair follicles are found on the head: the infundibulum, isthmus, and lower follicle. The lower follicle is also known as the inferior segment and includes the bulb.

Sebaceous holocrine glands are associated with hair follicles, especially in some skin regions, such as the face. These glands open onto hair follicles, except in areas like the lips, where they empty directly onto the mucosal surface due to the absence of hair follicles. When stimulated by hormones such as androgens, sebaceous glands secrete a lipid-rich sebum that protects the hair and provides the skin with a hydrophobic protective barrier.

The arrector pili muscle is a bundle of smooth muscle fibers that connects to the bulge area of the external root sheath of the hair follicle and extends obliquely to its superior attachment point just below the epidermis (papillary dermis). Sympathetic stimulation causes these muscles to contract in cold climates, slightly raising the skin and making the hair stand erect, an occurrence commonly referred to as "goose-bumps."