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.
ter. In the trachea, the submucosa is a relatively loose connective tissue similar in appearance to the lamina propria, which makes it difficult to determine where it begins. Diffuse lymphatic tissue and lymphatic nodules characteristically extend into this layer from the lamina propria. The submucosa contains the larger distributing vessels and lymphatics of the tracheal wall.
Submucosal glands composed of mucus-secreting acini with serous demilunes are also present in the submucosa. Their ducts consist of a simple cuboidal epithelium and extend through the lamina propria to deliver their product, largely glycoproteins, on the epithelial surface. The glands are 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 adventi-tia. 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 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. 18.5), 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 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.
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