Esophagus histology

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.This condition is called Barrett esophagus, which is a form of metaplasia.Unfortunately though, this metaplastic change also increases the risk of developing an esophageal adenocarcinoma.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.The mucus is excreted through ducts that reach the mucosa’s epithelium in order to help lubricate the surface.These ducts typically have a cuboidal or stratified cuboidal cells.The next main layer is the muscularis propria, which varies depending on the portion of the esophagus we’re looking at, but throughout the entire esophagus there’ll be an inner circular layer of muscle and an outer longitudinal layer of muscle.The proximal portion of the esophagus is under voluntary control, so the upper third of the esophagus will have primarily skeletal muscle.The muscle type will gradually transition from skeletal to smooth muscle.So as a result, the middle third will have both skeletal and smooth muscle present, and the lower third will consist entirely of just smooth muscle.This cross-section of the esophagus was taken from the middle third, so if we zoom in to 40x magnification we can see that both skeletal and smooth muscle are present.The skeletal muscle can be differentiated from the smooth muscle by the typical striations we’d expect to see in the skeletal muscle fibers.Between the inner and outer layers of muscle there’s also a network of nerves called the Auerbach’s plexus or myenteric plexus.In this image, a ganglion or a group of neuron cell bodies can be seen with its pink outer capsule visible as well.These nerves innervate the muscles and are responsible for producing the organized rhythmic contractions that result in the peristaltic movement of the esophagus.The majority of the esophagus is surrounded by its outermost layer of connective tissue called the adventitia.In this image, the underlying muscularis propria can still be seen at the top.The adventitia is attached to nearby structures, such as retroperitoneal organs, which keeps the esophagus in a relatively fixed position.The last 1-2 cm of the esophagus is within the abdominal cavity, after crossing the diaphragm.This portion of the esophagus has an outer layer of connective tissue called the serosa.The serosa is covered by a simple squamous epithelium or mesothelium that’s continuous with the abdominal mesenteries.If we increase the magnification, we can see some of the additional structures within the connective tissue, such as a large artery, lymphatic vessels, and adipocytes.These structures would be found in both the adventitia as well as the serosa.##SummaryAlright, as a quick recap.The wall of the esophagus has 4 main layers; the mucosa, submucosa, muscularis propria, and either the serosa or adventitia.The mucosa consists of 3 parts, the stratified squamous epithelium, its supporting lamina propria, and a thin layer of smooth muscle called the muscularis mucosa.The submucosa consists mostly dense irregular connective tissue, but also contains unique mucous glands that are only seen within the submucosa in the esophagus and duodenum.The muscularis propria is the main layer of both smooth and skeletal muscle with an inner circular layer of muscle and an outer longitudinal layer of muscle.The last 1-2 cm of the esophagus is surrounded by an outer layer of serosa, but the majority of the esophagus has an outermost layer of connective tissue called the adventitia.