Sweat glands are classified on the bases of their structure and the nature of their secretion. Two types of sweat glands are recognized:  Eccrine sweat glands are distributed over the entire body surface except for the lips and part of the external genitalia. Apocrine sweat glands are limited to the axilla, areola, and nipple of the mammary gland; skin around the anus; and the external genitalia. The ceruminous glands of the external acoustic meatus canal and the apocrine glands of eyelashes (glands of Moll) are also apocrine-type glands.
Eccrine sweat glands are simple coiled glands that regulate body temperature.
Eccrine sweat glands are independent structures, not associated with the hair follicle that arises as a downgrowth from the fetal epidermis. Each eccrine gland is arranged as a blind-ended, simple, coiled tubular structure. It consists of two segments: A secretory segment located deep in the dermis or in the upper part of the hypodermis and a directly continuous, less coiled duct segment that leads to the epidermal surface (Fig. 15.15 and Plate 44, page 519).
Eccrine sweat glands play a major role in temperature regulation through the cooling that results from the evaporation of water from sweat on the body surface. The secretory portion of the glands produces a secretion similar in composition to an ultrafiltrate of blood. Resorption of some of the sodium and water in the duct results in the
FIGURE 15.15 • Photomicrograph of an eccrine sweat gland.
This photomicrograph of a H&E–stained section of human skin shows profiles of both the secretory component and the duct of an eccrine sweat gland. The secretory component appears as a double layer of cuboidal epithelial cells and peripherally, within the basal lamina, a layer of myoepithelial cells. The duct portion of the gland has a narrower outside diameter and lumen than the secretory portion of the gland. It consists of a double layer of small cuboidal cells without the myoepithelial cells. 320.
release of a hypotonic sweat at the skin surface. This hypo-tonic watery solution is low in protein and contains varying amounts of sodium chloride, urea, uric acid, and ammonia. Thus, the eccrine sweat gland also serves, in part, as an excretory organ.
Excessive sweating can lead to the loss of other electrolytes such as potassium and magnesium, as well as significant water loss. Normally, the body loses about 600 ml of water a day through evaporation from the lungs and skin. Under conditions of high ambient temperature, water loss can be increased in a regulated manner by an increased rate of sweating. This thermoregulatory sweating first occurs on the forehead and scalp, extends to the face and to the rest of the body, and occurs last on the palms and soles. Under conditions of emotional stress, however, the palms, soles, and axillae are the first surfaces to sweat. Control of thermoregulatory sweating is cholinergic, whereas emotional sweating may be stimulated by  adrenergic portions of the sympathetic division of the autonomic nervous system.
The secretory segment of the eccrine sweat gland contains three cell types.
Three cell types are present in the secretory segment of the gland: clear cells and dark cells, both of which are secretory epithelial cells, and myoepithelial cells, which are contractile epithelial cells (Fig. 15.16 and Plate 45, page 521). All of the cells rest on the basal lamina; their arrangement is that of a pseudostratified epithelium.