The gastrointestinal or GI tract extends as a single tube from the esophagus all the way to the distal portion of the anal canal.
Although different parts of the tract may appear to have very different structures and functions, the wall still maintains 4 main layers all throughout the GI tract: the mucosa, submucosa, muscularis propria, and either an outer serosa or adventitia.
Even in this low power cross-section of the esophagus, we can see the inner mucosa, submucosa, and muscularis propria, although the outer adventitia isn’t present in this image.
All 4 layers have variations to their structure and function in different regions of the GI tract, but the mucosa is the layer that typically has the most significant changes.
The mucosa of the esophagus consists of 3 main layers.
At 20x magnification, we can see each of the layers more clearly.
The epithelium, lamina propria, and muscularis mucosa.
The thick epithelial layer lines the lumen of the esophagus and consists of stratified squamous non-keratinized cells, which has their typical appearance of flat, overlapping cells that are more flat as they move away from the base or basal cell layer.
The lamina propria is a much thinner layer of dense irregular connective tissue.
It provides a supporting function to the epithelium, such as the blood vessels within the connective tissue that supply blood to the epithelium.
The muscularis mucosa is the outermost layer of the mucosa and is comprised of smooth muscle.
The muscle fibers in this image have a circular or dot-like appearance because the fibers run longitudinally or in the same direction as the esophagus.
The lower esophageal sphincter or LES for short is actually not an anatomical sphincter, which means histologically, there is no well-defined thickening or muscle that controls the LES.
Instead, the LES is considered a physiological or functional sphincter.
But when this sphincter isn’t functioning properly, it can lead to gastric acid reflux into the lower esophagus, which can cause heartburn.
Also, over time, prolonged exposure to gastric acid can cause damage to the epithelium of the esophagus.
The body attempts to adapt by transforming the normal stratified squamous cells of the lower esophageal lining into a mucus-secreting epithelium with simple columnar cells, which are better at protecting itself from gastric acid.
These changes can also be identified by using an endoscope, where we’d see “Salmon-colored” patches at the lower segment of the esophagus.
The presence of both endoscopic and histologic changes are needed for the diagnosis of barrett esophagus.
The next main layer is the submucosa, which consists mostly of dense collagenous connective tissue that stains pink, but the submucosa also contains mucous glands, blood vessels, lymphatic vessels, and nerves.
Within the connective tissue there’s also elastin fibers, which allows the esophagus to expand when food passes through it.
The mucous gland is visible, which are small, compound, tubuloalveolar glands that secrete mucus that stains purple.
There are typically more mucous glands present in the upper and lower thirds of the esophagus.
The location of mucous glands within the submucosal layer is actually a unique feature specific to both the esophagus and the duodenum.