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In humans, sweat glands generally are found as two types, (1) eccrine and (2) apocrine. Eccrine-gland sweat allows the body to control its internal temperature in response to thermal stress. Apocrine gland function is more obscure but likely includes pheromone production.
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Humans have approximately 2–4 million sweat glands.1 Sweat glands are found over nearly the entire body surface, and are especially dense on the palms, soles, forehead, and upper limbs.2 Analgen of eccrine sweat glands first appear in the 3.5-month-old fetus on the palms and soles (see Chapter 7), then develop in the axillary skin in the fifth fetal month, and finally develop over the entire body by the sixth fetal month.3 The analog of the eccrine sweat gland, which developed from the epidermal ridge, is double layered, and develops a lumen between the layers between the fourth and eighth fetal months. By the eighth fetal month eccrine secretory cells resemble those of the adult; by the ninth fetal month myoepithelial cells form.
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Two distinct segments, the secretory coil and the duct, form the eccrine sweat gland. The secretory coil secretes an isotonic sweat, while the duct resorbs Na+ and Cl−, thus producing sweat to cool the body while preserving Na+ and Cl– body stores.
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The secretory coil contains three distinct cell types: (1) clear (secretory), (2) dark (mucoid), and (3) myoepithelial.4 The clear and dark cells occur in approximately equal numbers but differ in their distribution (Fig. 83-1). While the dark cells border the apical (luminal) surfaces, the clear cells rest either directly on the basement membrane or on the on the myoepithelial cells. The clear cells directly access the lumen by forming intercellular canaliculi (Fig. 83-2). Spindle shaped contractile myoepithelial cells lie on the basement membrane and abut the clear cells. The adult secretory coil is approximately 2–5-mm long, and approximately 30–50 μm in diameter. Heat accumulation results in larger sweat glands and ducts, and their dimensions, in turn, correlate with enhanced sweat output.5 Clear cells contain abundant mitochondria and an autofluorescent body, called the lipofuscin granule, in the cytoplasm. The clear cell plasma membrane forms many villi. The clear cell ...