Histology of Esophagus
The esophagus, like other parts of the gastrointestinal tract, is made up of a mucosa, a submucosa, a muscularis externa, and an adventitia. The mucosa is subdivided into epithelium, lamina propria, and muscularis mucosae. The epithelium that lines the lumen of the esophagus is stratified squamous epithelium, which is continuous with the epithelial lining of the pharynx. The surface cells of this epithelium are flattened and contain a few keratohyaline granules, but do not form a keratinized layer. An abrupt transition takes place between the stratified squamous epithelium of the esophagus and the simple columnar epithelium of the stomach along an irregular zigzag line, known as the Z line, situated slightly superior to the cardiac region of the stomach. The lamina propria is a layer of loose connective tissue deep to the epithelium. The muscularis mucosae is a thin layer of smooth muscle deep to the lamina propria and is continuous with the pharyngeal aponeurosis. A transition from connective tissue to muscular tissue takes place in this aponeurosis at about the level of the cricoid cartilage. It contains both longitudinal smooth muscle fibers and some elastic tissue and is thicker at the lower end of the esophagus.
The submucosa is formed by dense irregular connective tissue and contains both elastic and type I collagen fibers, as well as blood vessels and nerves supplying the mucosa. Lymphocytes are scattered in moderate numbers through both the lamina propria and the submucosa, and occasionally these may be found in isolated concentric groups. In its contracted state, the esophageal mucosa is thrown into irregular longitudinal folds. The submucosa extends into these folds, but the more superficial muscular layers do not.
The muscularis externa consists of an inner circular layer and an outer longitudinal layer. A thin layer of connective tissue exists between the two layers, in which is embedded the myenteric ganglia and plexus (of Auerbach). Between the muscularis externa and the submucosa are the submucosal ganglia and plexus (of Meissner) and several blood vessels. The musculature of the upper one fourth of the esophagus is generally striated in character, the second fourth contains both striated and smooth muscle, and the lower half is composed entirely of smooth muscle. The adventitia is a layer of loose connective tissue on the surface of the esophagus that anchors the organ to the surrounding structure.
Two types of glands can be recognized in the esophagus. The esophageal glands proper (of Brunner) are irregularly distributed throughout the entire length of the tube. They are small, compound mucous glands. Their ducts penetrate the muscularis mucosae and their branched tubules lie in the submucosa. They are some what more prominent in the superior part of the esophagus. The other type of glands is known as the esophageal cardiac glands because they closely resemble or are identical with the cardiac glands of the stomach. They are found just above the cardiac region of the stomach, in the distal esophagus. They are also occasionally found proximally, a few centimeters below the level of the cricopharyngeus muscle. They differ from the esophageal glands proper in that their ducts do not penetrate the muscularis mucosae and their branched and coiled tubules are located in the lamina propria, not in the submucosa.