 Basal cells serve as a reserve cell population that maintains individual cell replacement in the epithelium. Basal cells tend to be prominent because their nuclei form a row in close proximity to the basal lamina. Although nuclei of other cells reside at this same general level within the epithelium, they are relatively sparse. Thus, most of the nuclei near the basement membrane belong to basal cells.
A thick “basement membrane” is characteristic of tracheal epithelium.
Located beneath the tracheal epithelium is a distinctive layer typically referred to as a basement membrane (see Fig. 19.9). It usually appears as a glassy or homogeneous light-staining layer approximately 25 to 40 m thick. Electron microscopy reveals that it consists of densely packed collagenous fibers that lie immediately under the epithelial basal lamina. Structurally, it can be regarded as an unusually thick and dense reticular lamina and, as such, is part of the lamina propria. In smokers, particularly those who experience chronic coughing, this layer may be considerably thicker, a response to mucosal irritation.
The lamina propria, excluding that part just designated as basement membrane, appears as a typical loose connective tissue. It is very cellular, containing numerous lymphocytes, many of which infiltrate the epithelium. Plasma cells, mast cells, eosinophils, and fibroblasts are the other cell types readily observed in this layer. Lymphatic tissue, in both diffuse and nodular forms, is consistently present in the lamina propria and submucosa of the tracheal wall. It is also present in other parts of the respiratory system involved primarily with air conduction. This lymphatic tissue is the developmental and functional equivalent of the bronchus-associated lymphatic tissue (BALT).
FIGURE 19.7 • Electron micrograph of human trachea. This electron micrograph shows the three main cell types of this respiratory epithelium. They are represented by ciliated epithelial cells extending to the surface, where they possess cilia; goblet cells with mucinogen granules; and basal cells, which are confined to the basal portion of the epithelial layer near the connective tissue. 1,800.
(Courtesy of Dr. Johannes A. G. Rhodin.)
The boundary between mucosa and submucosa is defined by an elastic membrane.
Interspersed among the collagenous fibers are numerous elastic fibers. Where the lamina propria ends, the elastic material is more extensive, and in specimens stained for these fibers, a distinct band of elastic material is seen. This band or elastic membrane marks the boundary between the lamina propria and submucosa. In H&E preparations, however, the boundary is not obvious.
The submucosa is unlike that of most other organs, in which this connective tissue typically has a dense character. 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