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 brush-like appearance. At the base of the cilia is a dense eosinophilic line. This is due 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 submu-cosa (SM) contains dense irregular connective tissue with blood and lymphatic vessels, nerves, and numerous mucus-secreting tracheal glands, x 400.
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).
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, where this connective tissue typically has a dense charac-
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