The mucosa that lines the length of the esophagus has a nonkeratinized stratified squamous epithelium (Fig. 17.3 and Plate 54, page 606). In many animals, however, the epithelium is keratinized, reflecting a coarse food diet. In humans, the surface cells may exhibit some keratohyalin granules, but keratinization does not normally occur. The underlying lamina propria is similar to the lamina propria throughout the alimentary tract; diffuse lymphatic tissue is scattered throughout, and lymphatic nodules are present, often in proximity to ducts of the esophageal mucous glands (see page 573). The deep layer of the mucosa, the muscularis mucosae, is composed of longitudinally organized smooth muscle that begins near the level of the cricoid cartilage. It is unusually thick in the proximal portion of the esophagus and presumably functions as an aid in swallowing.
The submucosa consists of dense irregular connective tissue that contains the larger blood and lymphatic vessels, nerve fibers, and ganglion cells. The nerve fibers and ganglion cells make up the submucosal plexus (Meissner’s plexus). Glands are also present (see page 571). In addition, diffuse lymphatic tissue and lymphatic nodules are present mostly in the upper and lower parts of the esophagus where submucosal glands are more prevalent.
The muscularis externa consists of two muscle layers, an inner circular layer and an outer longitudinal layer (Plate 54, page 606). It differs from the muscularis externa found in the rest of the digestive tract in that the upper one third is striated muscle, a continuation of the muscle of the pharynx. Striated muscle and smooth muscle bundles are mixed and interwoven in the muscularis externa of the middle third of the esophagus; the muscularis externa of the distal third consists only of smooth muscle, as in the rest of the digestive tract. A nerve
FIGURE 17.2 • Photomicrograph of the esophagus. This low-magnification photomicrograph shows an H&E–stained section of the esophagus with its characteristically folded wall, giving the lumen an irregular appearance. The mucosa consists of a relatively thick stratified squamous epithelium, a thin layer of lamina propria containing occasional lymphatic nodules, and muscularis mucosae. Mucous glands are present in the submucosa; their ducts, which empty into the lumen of the esophagus, are not evident in this section. External to the submucosa in this part of the esophagus is a thick muscularis externa made up of an inner layer of circularly arranged smooth muscle and an outer layer of longitudinally arranged smooth muscle. The adventitia is seen just external to the muscularis externa. 8.
FIGURE 17.3 • Photomicrograph of the esophageal mucosa.
This higher-magnification photomicrograph shows the mucosa of the wall of the esophagus in an H&E preparation. It consists of a stratified squamous epithelium, lamina propria, and muscularis mucosae. The boundary between the epithelium and lamina propria is distinct, although uneven, because of the connective tissue papillae. The basal layer of the epithelium stains intensely, appearing as a dark band because the basal cells are smaller and have a high nucleus-to-cytoplasm ratio. Note that the loose connective tissue of the lamina propria is very cellular, containing many lymphocytes. The deepest part of the mucosa is the muscularis mucosae that is arranged in two layers (inner circular and outer longitudinal) similar in orientation to the muscularis externa. 240.