Figure 1415

Electron micrograph of a sebaceous gland. Basal cells (1) close to the connective tissue (CT) are small and undifferentiated. Among these cells are dividing cells, one of which (P) appears to be in early prophase. From this peripheral position the cells move toward the opening of the gland (2 to 4) and produce a fatty secretory product. This oily product is first seen in the cytoplasm as small lipid droplets (5) that gradually fuse (6). The cells perish (7) during the secretion of the secretory product, producing sebum, x6,800. (Courtesy of Dr. Bryce L. Munger.)

rectly continuous, less coiled duct segment that leads to the epidermal surface (Fig. 14.16).

Eccrine sweat glands play a major role in temperature regulation through the cooling that results from 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 release of a hypotonic sweat at the skin surface. This hypotonic 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 loss of other electrolytes, such as potassium and magnesium, and to 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, while 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. 14.17). All of the cells rest on the basal lamina; their arrangement is that of a pseu-dostratified epithelium.

• Clear cells are characterized by abundant glycogen. The glycogen is conspicuous in Fig. 14.17a because of its amount it would stain intensely with the periodic adipose tissue

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