nTwo types of sweat glands, eccrine (or merocrine) and apocrine, occur in human skin. Both develop as epidermal invaginations into the dermis, and they differ mainly in their size, distribution, and secretory products.
nA. Eccrine Sweat Glands:
n1. Distribution. The most numerous sweat glands in humans, these average about 3 million per individual. They occur over most of the body, except for the glans penis, glans clitoridis, and the vermiliion border of the lips. They are most abundant in thick skin, such as the palms, where there are about 3000 per square inch. 2. Structure. They are simply coiled tubular glands. a. Ducts. The slightly coiled ducts are lined with simple to stratified cuboidal epithelium; their lining cells are smaller than those in the secretory portions and stain darker. Each duct opens directly onto the skin surface. b. Secretory portions. These highly coiled parts of the sweat glands are located in deep reticular dermis or shallow hypodermis. Surrounding connective tissue condenses to form a sheath around the basal lamina, and there are numerous myoepithelial cells between the basal lamina and the secretory cells. The secretions are released via ex ocytosis (merocrine secretion). Secretory cells are larger and stain lighter than the duct lining cells. Two secretory cell types are seen. Dark (mucoid) cells are pyramidal and line most of the gland's secretory portion; their bases do not reach the basal lamina. They contain rodlike mitochondria, a well-developed Golgi complex, RER, many free ribo somes, and dark glycoprotein-containing granules. Clear cells are also pyramidal. They lack secretory granules, contain abundant glycogen, and surround the inner layer of dark cells. Their basal plasma membranes, which do contact the basal lamina, are highly infolded, suggesting a role in ion and water transport.3. Secretory product. Eccrine sweat is a watery secretion whose main components (besides water) include NaCI, urea, ammonia, and uric acid. The glands thus assist in excreting by-products of protein metabolism. In addition, evaporation of water from the skin surface reduces body temperature by cooling the blood in the papillary capillaries.

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