The structural, or pilosebaceous, unit of a hair follicle consists of the hair follicle itself with an attached sebaceous gland and arrector pili muscle.
The hair follicle begins at the surface of the epidermis. For follicles that produce terminal hairs, the hair follicle extends into the deep dermis, and sometimes even subcutis. Meanwhile, follicles producing vellus hairs extend only to the upper reticular dermis. There are three important segments of hair follicles found on the head: the infundibulum, the isthmus, and the lower follicle/inferior segment (which includes the bulb).
The infundibulum segment is the upper portion of the follicle. It begins at the surface of the epidermis and extends to the opening of the sebaceous duct. The isthmus is the area between the sebaceous duct opening and the bulge. The bulge is an area of the follicle marked by the insertion of the arrector pili muscle. Also, the bulge contains several epidermal stem cells that are part of the outer root sheath and stain with CK19, CK15, and CD200. Finally, the inferior segment of the hair follicle extends from the bulge to the base of the follicle. This segment includes the bulb, which contains the follicular matrix surrounding the sides and top of the dermal papilla. The dermal papilla contains capillaries. The papilla interacts with the matrix, which has the highest mitotic rate of any organ. The matrix is the part of the hair follicle where matrix keratinocytes proliferate to form the hair shaft of growing hair. Melanocytes are mixed amongst the matrix cells to provide the hair shaft with color.
The hair shaft consists of an inner core known as the medulla. This is surrounded by the cortex, which makes up the bulk of the hair. Moving outwards, there is a single layer of cells making up the shaft cuticle. The shaft cuticle is then encased in three layers that form the inner (internal) root sheath. The inner sheath is important in shaping the hair shaft as it grows upwards from the matrix. The inner sheath keratinizes from the outside-in, and will eventually disintegrate mid-follicle, around the level of the isthmus. Finally, the outer (external) root sheath encases the entirety of the hair shaft. This layer undergoes trichilemmal keratinization around the level of the isthmus.
Sebaceous glands are holocrine glands closely associated with hair follicles, especially in certain areas of the skin such as the face. These glands open onto the hair follicles, except in areas such as the lips, where they empty directly onto the mucosa surface because lips do not contain 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 barrier that can serve as protection.
Arrector pili muscles insert at the level of the bulge and also attach to the papillary layer of the dermis. In cold climates, sympathetic stimulation causes these muscles to contract. This raises the level of the skin slightly and causes the hair to stand erect, which is commonly referred to as “goose-bumps.”
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