Like eccrine glands, apocrine glands are coiled tubular glands. They are sometimes branched. The secretory portion of the gland is located deep in the dermis or, more commonly, in the upper region of the hypodermis.
The secretory portion of apocrine glands has a wider lumen than that of eccrine glands and is composed of a single cell type.
The secretory portion of apocrine glands differs in several respects from that of eccrine glands. The most obvious difference, readily noted in the light microscope, is its very wide lumen (Fig. 15.17 and Plate 44, page 519). Unlike  eccrine glands, apocrine glands store their secretory product in the lumen. The secretory portion of the gland is composed of simple
FIGURE 15.16 • Electron micrographs of an eccrine sweat gland. a. This micrograph shows myoepithelial cells (My) and two distinctive gland cell types, dark cells (D) and clear cells (C). The apical portion of the dark cell is broad; it faces the lumen (L) of the gland and contains numerous secretory granules. The dashed line marks the boundary of one dark cell. The clear cell is more removed from the lumen of the gland. Its base rests on the myoepithelial cells or directly on the basal lamina. Most of the free surface of the clear cell faces an intercellular canaliculus (IC). Clear cells contain numerous mitochondria, extensive infoldings of the plasma membrane, and large numbers of electron-dense glycogen inclusions. 5,600. (Courtesy of Dr. John A. Terzakis.) b. At higher magnification, dark cells display rER (arrow) and a Golgi apparatus (G) in addition to secretory granules. Clear cells show large amounts of folded membrane, mitochondria, and glycogen. The myoepithelial cells (My) contain large numbers of contractile actin filaments. Short stubby arrows (upper right) mark the boundary of a clear cell. 17,500. (Courtesy of Dr. John A. Terzakis.)
FIGURE 15.17 • Photomicrograph of an apocrine sweat gland. This section of adult skin from the area around the anus shows several apocrine (anal) sweat glands, which are easily identified by the large lumen of their secretory components. This apocrine sweat gland is close to a hair follicle (center of photomicrograph) and deep to the dense, irregular connective tissue of the dermis. 45. Inset. Higher magnification of secretory component shows the cell types of the apocrine gland. The gland consists of a simple epithelium whose cells are either cuboidal or columnar and myoepithelial cells located in the basal portion of the epithelial cell layer. 230.
epithelium. Only one cell type is present, and the cytoplasm of the cell is eosinophilic. The apical part of the cell often exhibits a bleblike protrusion. It was once thought that this part of the cell pinched off and was discharged into the lumen as an apocrine secretion, thus the name of the gland. However, TEM studies confirm that the secretion is a merocrine type. The apical cytoplasm contains numerous small granules, the secretory component within the cell, which are discharged by exocytosis. Other features of the cell include numerous lysosomes and lipofuscin pigment granules. The latter represent secondary and tertiary lysosomes. Mitochondria are also numerous. During the refractory phase, after expulsion of the secretion, the Golgi apparatus enlarges in preparation for a new secretory phase.
Myoepithelial cells are also present in the secretory portion of the gland and are situated between the secretory cells and the adjacent basal lamina. As in eccrine glands, contraction of the processes of myoid cells facilitates expulsion of the secretory product from the gland.