A cross section of the wall of the esophagus is shown here. The mucosa (Muc) consists of stratified squamous epithelium (Ep), a lamina propria (LP), and muscularis mucosae (MM ). The boundary between the epithelium and lamina propria is distinct, although uneven, as a result of the presence of numerous deep connective tissue papillae. The basal layer of the epithelium stains intensely, appearing as a dark band that is relatively conspicuous at low magnification. This is, in part, due to the cytoplasmic basophilia of the basal cells. That the basal cells are small results in a high nuclear-cytoplasmic ratio, which further intensifies the hematoxylin staining of this layer.
The submucosa consists of irregular dense connective tissue that contains the larger blood vessels and nerves. No glands are seen in the submucosa in this figure, but they are regularly present throughout this layer and are likely to be included in a section of the wall. Whereas the boundary between the epithelium and lamina propria is striking, the boundary between the mucosa (Muc) and submucosa (SubM ) is less well marked, although it is readily discernable.
The muscularis externa (ME ) shown here is composed largely of smooth muscle, but it also contains areas of striated muscle. Although the striations are not evident at this low magnification, the more densely stained eosinophilic areas (asterisks) prove to be striated muscle when observed at higher magnification. Reference to the inset, which is from an area in the lower half of the figure, substantiates this identification.
The inset shows circularly oriented striated and smooth muscle. The striated muscle stains more intensely with eosin, but of greater significance are the distribution and number of nuclei. In the center of the inset, numerous elongated and uniformly oriented nuclei are present; this is smooth muscle (SM ). Above and below, few elongated nuclei are present; moreover, they are largely at the periphery of the bundles. This is striated muscle (StM ); the cross-striations are just perceptible in some areas. The specimen shown here is from the middle of the esophagus, where both smooth and striated muscle are present. The muscularis externa of the distal third of the esophagus would contain only smooth muscle, whereas that of the proximal third would consist of striated muscle.
External to the muscularis externa is the adventitia (Adv) consisting of dense connective tissue.
Mucosa, esophagus, monkey, H&E ×300.
As in other stratified squamous epithelia, new cells are produced in the basal layer, from which they move to the surface. During this migration, the shape and orientation of the cells change. This change in cell shape and orientation is also reflected in the appearance of the nuclei. In the deeper layers, the nuclei are spherical; in the more superficial layers, the nuclei are elongated and oriented parallel to the surface. That nuclei can be seen throughout the epithelial layer, particularly the surface cells, indicates that the epithelium is not keratinized.
In some instances, the epithelium of the upper regions of the esophagus may be parakeratinized or, more rarely, keratinized.
As shown in this figure, the lamina propria (LP) is a very cellular, loose connective tissue containing many lymphocytes (Lym), small blood vessels, and lymphatic vessels (LV ). The deepest part of the mucosa is the muscularis mucosae (MM ). That layer of smooth muscle defines the boundary between mucosa and submucosa. The nuclei of the smooth muscle cells of the muscularis mucosae appear spherical because the cells have been cut in cross section.