FIGURE 19.8 • Scanning electron micrograph of the luminal surface of a bronchus. The nonciliated cells are the goblet cells (G). Their surface is characterized by small blunt microvilli that give a stippled appearance to the cell at this low magnification. The cilia of the many ciliated cells occupy the remainder of the micrograph. Note how all are “synchronously” arrayed (i.e., uniformly leaning in the same direction) appearing just as they were when fixed at a specific moment during their wavelike movement. 1,200.
especially numerous in the cartilage-free gap on the posterior portion of the trachea. Some penetrate the muscle layer at this site and, therefore, also lie in the adventitia. The submucosal layer ends where its connective tissue fibers blend with the perichondrium of the cartilage layer.
The tracheal cartilages and trachealis muscle separate submucosa from adventitia.
The tracheal cartilages, which number about 16 to 20 in humans, represent the next layer of the tracheal wall. As noted, the cartilages are C shaped. They sometimes anastomose with
FIGURE 19.9 • Photomicrograph of tracheal epithelium. Three major cell types are evident in the tracheal epithelium (Ep): ciliated columnar cells; mucus-secreting goblet cells (G) interspersed between the ciliated cells; and basal cells, which are close to the basement membrane (BM ). The ciliated columnar cells extend from the basement membrane to the surface. At their free surface they contain numerous cilia that, together, give the surface a brushlike appearance. At the base of the cilia is a dense eosinophilic line. This is owing to the linear aggregation of structures referred to as basal bodies, located at the proximal end of each cilium. Although basement membranes are not ordinarily seen in H&E preparations, a structure identified as such is seen regularly under the epithelium in the human trachea. The underlying lamina propria (LP ) consists of loose connective tissue. The more deeply located submucosa (SM) contains dense irregular connective tissue with blood and lymphatic vessels, nerves, and numerous mucus-secreting tracheal glands. 400.
adjacent cartilages, but their arrangement provides flexibility to the tracheal pipe and also maintains patency of the lumen. With age, the hyaline cartilage may be partially replaced by bone tissue (see Fig. 19.6), causing it to lose much of its flexibility.
The adventitia, the outer layer, lies peripheral to the cartilage rings and trachealis muscle. It binds the trachea to adjacent
FIGURE 19.10 • Diagram of a brush cell and small granule cell. a. The brush cell, as illustrated here, is interposed between type I and type II alveolar cells of an alveolus. Blunt microvilli are distinctive features of the brush cell. The cytoplasm typically shows a Golgi apparatus, lysosomes, mitochondria, and glycogen inclusions. b. This small granule cell is shown located between two Clara cells, as in a terminal or respiratory bronchiole. The cell contains small secretory vesicles, most of which are in the basal portion of the cell. In addition to the vesicles, the most conspicuous organelles of the cell are rough-surfaced endoplasmic reticulum (rER), a Golgi apparatus, and mitochondria. A nerve terminal is shown within the epithelium adjacent to the cell.
structures in the neck and mediastinum and contains the largest blood vessels and nerves that supply the tracheal wall, as well as the larger lymphatics that drain the wall.