 Mucous cells are similar in appearance to intestinal goblet cells and are thus often referred to by the same name. They are interspersed among the ciliated cells and also extend through the full thickness of the epithelium (see Fig. 19.9). They are readily seen in the light microscope after they have accumulated mucinogen granules in their cytoplasm. Although the mucinogen is typically washed out in hematoxylin and eosin (H&E) preparations, the identity of the cell is made apparent by the remaining clear area in the cytoplasm and the lack of cilia at the apical surface. In contrast to ciliated cells, the number of mucous cells increases during chronic irritation of the air passages.
 Brush cells have the same general features as those described for the respiratory epithelium of the nasal cavity (Fig. 19.10). They are columnar cells that bear blunt microvilli. The basal surface of the cells is in synaptic contact with an afferent nerve ending (epitheliodendritic synapse). Thus, the brush cell is regarded as a receptor cell.
 Small granule cells (Kulchitsky cells) are respiratory representatives of the general class of enteroendocrine cells of the gut and gut derivatives (see Fig. 19.10). Their presence is explained by the development of the respiratory tract and lungs from an evagination of the primitive foregut. Small granule cells usually occur singly in the trachea and are sparsely dispersed among the other cell types. They are difficult to distinguish from basal cells in the light microscope without special techniques such as silver staining, which reacts with the granules. The nucleus is located near the basement membrane; the cytoplasm is somewhat more extensive than that of the smaller basal cells. With the transmission electron microscope (TEM), a thin, tapering cytoplasmic process is sometimes observed extending to the lumen. Also, with the TEM, the cytoplasm exhibits numerous, membrane-bounded, dense-core granules. In one type of small granule cell, the secretion is a catecholamine. A second cell type produces polypeptide hormones such as serotonin, calcitonin, and gastrin-releasing peptide
FIGURE 19.6 • Photomicrograph of a section of the trachea and esophagus. a. This specimen, obtained from an elderly individual, shows the relationship between the trachea and the esophagus at the base of the neck. The cartilaginous tracheal rings, which keep the trachea patent, have a C-shaped appearance. The cartilage gap, where the trachea is adjacent to the esophageal wall, is spanned by a fibroelastic membrane. It contains the trachealis muscle and numerous seromucous glands. In this specimen, the tracheal ring has been transformed, in part, to bone, a process that occurs in aging. The darker-staining material represents cartilage, whereas the lighter-staining material has been replaced by bone tissue. The very light areas (arrows) are marrow spaces. 3.25. b. This high-magnification photomicrograph shows an area of the tracheal ring that has partially transformed into bone. The top of the micrograph shows the tracheal mucosa and submucosa. Below is part of the tracheal ring. In this particular region, however, a substantial portion of the cartilage has been replaced by bone tissue and marrow. The bone tissue exhibits typical lamellae and osteocytes. The cartilage tissue, in contrast, exhibits nests of chondrocytes. 100.
(bombesin). Some small granule cells appear to be innervated. The function of these cells is not well understood. Some are present in groups in association with nerve fibers, forming neuroepithelial bodies, which are thought to function in reflexes regulating the airway or vascular caliber.