Epidermal appendages are intradermal epithelial structures lined with epithelial cells with the potential for division and differentiation. These are important as a source of epithelial cells, which accomplish reepithelialization should the overlying epidermis be removed or destroyed in situations such as partial thickness burns, abrasions, or split-thickness skin graft harvesting.

Epidermal appendages include the following:

Sebaceous glands

Sweat glands

Apocrine glands

Mammary glands

Hair follicles

They often are found deep within the dermis and in the face may even lie in the subcutaneous fat beneath the dermis. This accounts for the remarkable ability of the face to reepithelialize even the deepest cutaneous wounds. [1]

Sebaceous Glands

Sebaceous glands, or holocrine glands, are found over the entire surface of the body except the palms, soles, and dorsum of the feet. They are largest and most concentrated in the face and scalp where they are the sites of origin of acne. The normal function of sebaceous glands is to produce and secrete sebum, a group of complex oils that include triglycerides and fatty acid breakdown products, wax esters, squalene, cholesterol esters, and cholesterol. Sebum lubricates the skin to protect it against friction and makes the skin more impervious to moisture.

Sweat Glands

Sweat glands, or eccrine glands, are found over the entire surface of the body except the vermillion border of the lips, the external ear canal, the nail beds, the labia minora, and the glans penis and the inner aspect of the prepuce. They are most concentrated in the palms and soles and the axillae.

Each gland consists of a coiled secretory intradermal portion that connects to the epidermis via a relatively straight distal duct. The normal function of the sweat gland is to produce sweat, which cools the body by evaporation. The thermoregulatory center in the hypothalamus controls sweat gland activity through sympathetic nerve fibers that innervate the sweat glands. Sweat excretion is triggered when core body temperature reaches or exceeds a set point.

Apocrine and Mammary Glands

Apocrine glands are similar in structure, but not identical, to eccrine glands. They are found in the axillae, in the anogenital region, and, as modified glands, in the external ear canal (ceruminous glands), the eyelid (Moll's glands), and the breast (mammary glands). They produce odor and do not function prior to puberty, which means they probably serve a vestigial function. The mammary gland is considered a modified and highly specialized type of apocrine gland.

Hair Follicles

Hair follicles are complex structures formed by the epidermis and dermis. (See the image below.) They are found over the entire surface of the body except the soles of the feet, palms, glans penis, clitoris, labia minora, mucocutaneous junction, and portions of the fingers and toes. Sebaceous glands often open into the hair follicle rather than directly onto the skin surface, and the entire complex is termed the pilosebaceous unit. [1, 4, 5]

Caucasian hair follicles are oriented obliquely to the skin surface, whereas the hair follicles of black persons are oriented almost parallel to the skin surface. Asian persons have vertically oriented follicles that produce straight hairs. These anatomic variations are an important consideration in avoiding alopecia when making incisions in the scalp.

The base of the hair follicle, or hair bulb, lies deep within the dermis and, in the face, may actually lie in the subcutaneous fat. This accounts for the remarkable ability of the face to re-epithelialize even the deepest cutaneous wounds. A band of smooth muscle, the arrector pili, connects the deep portion of the follicle to the superficial dermis. Contraction of this muscle, under control of the sympathetic nervous system, causes the follicle to assume a more vertical orientation.

Hair growth exhibits a cyclical pattern. The anagen phase is the growth phase, whereas the telogen phase is the resting state. The transition between anagen and telogen is termed the catagen phase. Phases vary in length according to anatomic location, and the length of the anagen phase is proportional to the length of the hair produced. At any one time at an anatomic location, follicles are found in all 3 phases of hair growth. This is extremely important for laser hair removal, because follicles in the anagen phase are susceptible to destruction, whereas resting follicles are more resistant. This explains why multiple treatments of an area may be necessary to ensure adequate hair removal.