 Clear cells are characterized by abundant glycogen. The glycogen is conspicuous in Figure 15.16a because of its amount; it would stain intensely with the periodic acid–Schiff (PAS) method. In routine H&E preparations, the cytoplasm of clear cells stains poorly. Membranous organelles include numerous mitochondria, profiles of sER, and a relatively small Golgi apparatus. The plasma membrane is remarkably amplified at the lateral and apical surfaces by extensive cytoplasmic folds. In addition, the basal surface of the cell possesses infoldings, although they are considerably less complex than the cytoplasmic folds. The morphology of these cells indicates that they produce the watery component of sweat.
 Dark cells are characterized by abundant rER and se cretory granules (see Fig. 15.16). The Golgi apparatus is relatively large, a feature consistent with the glyco protein secretion of these cells. The apical cytoplasm contains mature secretory granules and occupies most of the luminal surface (see Fig. 15.16a). Clear cells have considerably less cytoplasmic exposure to the lumen; their secretion is largely via the lateral surfaces of the cell, which are in contact with intercellular canaliculi that allow the watery secretion to reach the lumen. Here it mixes with the  proteinaceous secretion of the dark cells.
 Myoepithelial cells are limited to the basal aspect of the secretory segment. They lie between the secretory cells, with their processes oriented transversally to the tubule.
The cytoplasm contains numerous contractile filaments (actin) that stain deeply with eosin, thus making them readily identifiable in routine H&E specimens. Contraction of these cells is responsible for rapid expression of sweat from the gland.
The duct segment of eccrine glands is lined by stratified cuboidal epithelium and lacks myoepithelial cells.
The duct segment of the gland continues from the secretory portion with coiling. In histologic sections, multiple duct profiles typically appear among the secretory profiles. As the duct passes upward through the dermis, it takes a gentle  spiral course until it reaches the epidermis, where it then continues in a tighter spiral to the surface. When the duct enters the epidermis, however, the duct cells end and the epidermal cells form the wall of the duct. The duct is composed of stratifed cuboidal epithelium, consisting of a basal cell layer and a luminal cell layer. The duct cells are smaller and appear darker than the cells of the secretory portion of the gland. Also, the duct has a smaller diameter than the secretory portion. In contrast to the secretory  portion of the eccrine gland, the duct portion does not possess myoepithelial cells. These features are useful in distinguishing the duct from the secretory portion in a histologic section (see Fig. 15.15).
The basal or peripheral cells of the duct have a rounded or ovoid nucleus and contain a prominent nucleolus. The cytoplasm is filled with mitochondria and ribosomes. The apical or luminal cells are smaller than the basal cells, but their nuclei are similar in appearance. The most conspicuous feature of the luminal cells is the deeply stained, glassy (hyalinized) appearance of the apical cytoplasm. The glassy appearance is attributable to the presence of large numbers of aggregated tonofilaments in the apical cytoplasm.
Apocrine glands are large-lumen tubular glands associated with hair follicles.
Apocrine sweat glands develop from the same down-growths of epidermis that give rise to hair follicles. The connection to the follicle is retained, allowing the secretion of the gland to enter the follicle, typically at a level just above the entry of the sebaceous duct. From here, the secretion makes its way to the surface.