Skin Disorders Guide

Apocrine Sweat Glands

Apocrine sweat glands develop, like sebaceous glands, from follicular epithelial cells. For this reason their ducts open into the follicular lumina. They are phylogenetic remnants of the sexually important scent glands of animals but seem to have no direct usefulness in humans today. Apocrine glands are found primarily in the axillae, perineum, and on the areolae of the breasts. A few ectopic glands may be found elsewhere. Greatly modified apocrine glands are found in the ear (ceruminous glands) and eyelid (Moll’s glands). The glandular portion of breast tissue is also derived from apocrine glands. Apocrine sweat can be produced continuously or can appear in response to psychologic stimuli. Apocrine sweat has no intrinsic odor but odor is quickly produced when the sweat delivered to the surface is acted on by resident skin bacteria.

Eccrine Sweat Glands

Unlike sebaceous and apocrine glands, eccrine sweat glands develop directly from the overlying epidermis such that their coiled ducts open onto the smiace epithelium rather than into the hair follicle. Sweat glands occur over the entire skin surtace. Sweat glands located on the palms, soles, axillae, and forehead are under both psychologic and thermal control, whereas those located elsewhere respond solely to thermal stimuli. Ecrine sweat glands are innervated by sympathetic nerve fibers, but paradoxically, physiologic mediation of sweating is due to acetylcholine release.

Stimulation of sweating results in the formation of isotonic plasma-like fluid in the secretory portion of the gland. As a result of processing in the ductal system, however, the sweat delivered to the skin surface is usually low in sodium and chloride and high in potassium, urea, ammonia, and some amino acids.

Eccrine sweat glands have the potential to deliver astounding amounts of fluid to the skin surface. For short periods more than 1 liter/hr can be elaborated and up to 10 liters of sweat can be produced in a single day. Sweat delivered 10 the surface of the skin cools the skin (and thus the blood running through the skin) through evaporation. This represents the most important thermoregulatory mechanism available to the body. Unfortunately, little or no significant loss of toxic substances can be accomplished by sweating and thus sweat glands cannot be viewed as miniature kidneys.

Sebaceous Glands

Sebaceous glands are formed from epidermally derived cells that bud outward from the side of the hair follicle . Sebum, formed from holocrine secretion of the sebaceous cells within the gland, travels to the surface of the skin via the lumen of the hair follicle. Essentially, all hair follicles have sebaceous glands, but over most of the body the glands are small and relatively inactive. In the skin of the face, scalp, and upper trunk, however, the sebaceous glands enlarge and produce copious amounts of sebum as a result of androgenic stimulation.

Sebum accounts for the majority of lipid found on the face and scalp, but elsewhere on the body much of the surface lipid is formed by keratinocytes. The lipids produced by sebaceous cells and by keratinocytes are made up largely of triglycerides and related fatty acids, but sterol synthesis in sebum is directed toward squalene, whereas sterol synthesis in keratinocytes is directed toward cholesterol. Sebum elaboration is a continuous process, and the rate at which it is delivered to the surface of the skin is seemingly unaffected by temperature, sweating, or sebum removal.

The physiologic role of lipids produced by sebaceous glands and keratinocytes is uncertain. It is likely, however, that surface lipids retard evaporative moisture loss from epidermal cells and thus are important in skin hydration and lubrication.