The esophagus is a strong muscular tube, about 25 cm long, that delivers food to the stomach. In the relaxed state, the mucosa is deeply folded, allowing it to stretch when a bolus of food is delivered. It was not possible to get a sufficiently low power view of the entire esophagus with its folded mucosa to create an image here. For such a view, consult an atlas, or better yet, your slides.
Figure 1 (from slide #37) shows a low power view of part of the wall of the human esophagus. The dip in the middle of the field of view represents a mucosal fold. The mucosa (= epithelium, lamina propria and muscularis mucosae), submucosa and part of the inner circular layer of the muscularis externa are shown.
The epithelium is stratified squamous non-keratinized epithelium. Keratohyalin granules may be present in some of the surface cells. A closer examination of the epithelium would show that the cells in the lower to middle layers are cuboidal or polygonal, but the surface layers are always flattened, hence the designation squamous. Stratified squamous epithelium is typical of surfaces that will encounter friction.
The appearance of the lamina propria is typical although lymphocytes cant be distinguished at this magnification. Note the abundance of blood vessels, only a few of which are indicated. The boundary between the lamina propria and epithelium is distinct, but marked by irregularities where connective tissue papillae push into the epithelium. In some of the papillae, such as those indicated on the right, the continuity with the underlying lamina propria can be seen. Others are sectioned obliquely, and look like islands of connective tissue within the epithelium. The asterisk at the left of the figure is between two such islands. Throughout, most of the esophagus, no glands are present in the lamina propria. However, at its terminal part, and sometimes at its beginning, mucous glands resembling the cardiac glands of the stomach (see below) are present. Cardiac glands are not present in the segment of esophagus shown in Figure 1.
The muscularis mucosae consists of longitudinal muscle fibres and is quite prominent.
The mucosa of the esophagus has the simplest structure of anywhere in the alimentary canal. Use it to familiarizing yourself with its the three component layers: epithelium, lamina propria, and muscularis mucosae.
The appearance of the submucosa is typical as described under General Structure. Tubulo-alveolar mucous secreting glands are scattered throughout the submucosa of the esophagus but are not seen in section shown in Figure 1. The submucosa has folds which follow those of the mucosa.
The muscularis externa of the esophagus is typical as far as the orientation of the muscle layers is concerned (inner circular, outer longitudinal). However, the muscularis externa of the upper third of the esophagus contains skeletal muscle rather than smooth muscle and is under voluntary control. The middle third has both skeletal and smooth muscle, while the muscularis externa of the lower third consists entirely of smooth muscle. Figure 1 shows a segment from the lower third.
Figure 2 shows a high power view of the epithelium of the esophagus. The irregularly shaped cells of the lower layers become more flattened toward the surface. Although keratohyalin granules may be present in surface cells, these cells retain their nuclei and are not keratinized. Two nuclei in the upper layers are indicated. [In contrast, the upper layers of the cells of the epidermis (skin) are enucleated cells filled with keratin (cornified).]
Figure 3 shows the loose CT of the lamina propria lying between the epithelium and the muscularis mucosae. The general abundance of blood vessels is evident. A particularly large one, filled with RBCs, is seen in the muscularis mucosae. The abundant lymphocytes appear as little purple specks. Here, some have aggregated to form a lymph nodule. No cardiac-like glands are present in the lamina propria, as this section was not made from the upper- or lowermost parts of the esophagus. The structure of the lamina propria is best seen in the esophagus, as the LP of the stomach, small intestine and colon is filled with galnds.
The muscularis mucosae is quite thick in the esophagus and consists mainly of longitudinal muscle fibres.
Figure 4 shows the submucosa of the esophagus lying between the muscularis mucosae and the part of the inner circular layer of the muscularis externa. The connective tissue is denser than that of the lamina propria, and is considered moderately dense irregular CT. Blood vessels are also abundant. No ganglion cells or nerve bundles are seen in this section (no Meissners plexus). Mucous glands (called esophageal glands) are scattered throughout the submucosa of the esophagus, but none are seen in this section.
Figure 5 shows a low power view of the muscularis externa of the esophagus. The two layers, inner circular and outer longitudinal, are easily distinguished. (Not infrequently, these layers get sectioned more obliquely.) The muscle bundles in each layer are separated by connective tissue and are well vascularized (not evident at this magnification). A slightly larger layer of CT lies between the circular and longitudinal bands of muscle. Although difficult to discern at low power, a ganglion of the Auerbachs plexus can be seen in this CT. A bit of the adventitia is seen at the bottom right. It contains some adipose cells (extreme bottom) and 2 blood vessels.
Figure 6 shows a higher power view of the Auerbachs plexus seen in Figure 5. A number of cell bodies (or somata) of a ganglion can be seen. They stain more darkly than the surrounding fibres. Sometimes a pale nucleus can be seen in the nerve cell bodies, as is the case in the cell body at the right. When a nucleolus is sectioned (not here), it appears as a brightly staining spot in the paler nucleus. The nuclei of satellite cells, which surround the nerve cell bodies, can be seen. The nuclei seen among the fibres belong to Schwann cells or to fibroblasts of the endoneurium. (Even unmyelinated fibres are enveloped in Schwann cell cytoplasm.) If you scan along the CT separating the two layers of the muscularis externa, you are likely to come across a few ganglia. You might also come across some nerve fibres alone. Meissners (submucosal) plexus is similar to Auerbachs but smaller.
Figure 7 shows a low power view of the adventitia of the esophagus. The band of connective tissue constituting the adventitia blends in with the CT separating the muscle bundles of the muscularis externa, and with the CT of surrounding structures. A number of large blood vessels can be seen in the adventitia; the connective tissue with which it blends contains abundant adipose tissue, some blood vessels and some large nerve bundles. The Auerbachs plexus from the previous Figure is also in the field of view, but does not appear very distinct at this magnification.
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