Epidermal Derivatives: Sweat Glands

There are two types of sweat glands:

1. Eccrine (merocrine) sweat glands.

2. Apocrine (merocrine) sweat glands.

The eccrine sweat glands are simple coiled tubular glands with a role in the control of body temperature. Eccrine sweat glands are innervated by cholinergic nerves. The secretory portion of the eccrine sweat gland is a convoluted tube composed of three cell types:

1. Clear cells.

2. Dark cells.

3. Myoepithelial cells.

The clear cells are separated from each other by intercellular canaliculi, show an infolded basal domain with abundant mitochondria, rest on a basal lamina, and secrete most of the water and electrolytes (mainly Na⁺ and Cl^-) of sweat.

The dark cells rest on top of the clear cells. Dark cells secrete glycoproteins, including AMPs human beta-defensins (BD1 and BD2), cathelicidin, and dermicidin. Together with the secretion of sebaceous glands and the aqueous product of the clear cells, AMPs are produced under steady-state and inflammatory conditions.

Myoepithelial cells are found between the basal lamina and the clear cells. Their contractile activity assists in the release of secretion into the glandular lumen.

The excretory portion of the eccrine sweat gland is lined by a bilayer of cuboid cells that partially reabsorb NaCl and water under the influence of aldosterone. The reabsorption of NaCl by the excretory duct is deficient in patients with cystic fibrosis. The duct follows a helical path when it approaches the epidermis and opens on its surface at a sweat pore. Within the epidermis, the excretory duct loses its epithelial wall and is surrounded by keratinocytes.

Apocrine sweat glands are coiled and occur in the axilla, mons pubis, and circumanal area. Apocrine sweat glands contain secretory acini larger than those in the eccrine sweat glands.

The secretory portion is located in the dermis and hypodermis. The excretory duct opens into the hair follicle (instead of into the epidermis as in the eccrine sweat glands). Apocrine sweat glands are functional after puberty and are supplied by adrenergic nerves.

Two special examples of apocrine sweat glands are the ceruminous glands in the external auditory meatus and the glands of Moll of the margins of the eyelids.

The ceruminous glands produce cerumen, a pigmented lipid; the excretory duct opens, together with the ducts of sebaceous glands, into the hair follicles of the external auditory meatus.

The excretory duct of the glands of Moll opens into the free surface of the epidermis of the eyelid, or the eyelashes.