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 four main layers all throughout the GI tract the mucosa sub mucosa muscularis propria and either an outer Sosa or adventia even in this low power cross-section of the esophagus we can see the inner mucosa sub mucosa and muscularis propria although the outer adventia isn't present in this image all four layers have variations of 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 three main layers at 20 times magnification we can see each of the layers more clearly the epithelium lamin appropria and muscularis mucosa the thick epithelial layer lines the Lumin of the esophagus and consists of stratified squamous non-raised cells which has their typical appearance of flat overlapping cells that are more flat as they move away from the base or basil cell layer the lamin appropria 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 Lees for short is actually not an anatomical sphincter which means histologically Al there is no well-defined thickening or muscle that controls the Lees instead it's considered a physiological or functional sphincter but when the 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 squ cells of the lower esophageal lining into a mucus secreting epithelium with simple columnar cells which are better at protecting themselves from the gastric acid this condition is called barrit 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 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 sub mucosa which consists mostly of dense collagenous connective tissue that stains pink but the sub mucosa also contains mucus glands blood vessels lymphatic vessels and nerves within the connective tissue there's also elastin fibers which allows the esophagus to expand when food pass through it the mucus glands are visible here and they are small compound tubulo Alvar glands that secrete mucus that stains purple there are typically more mucus glands present in the upper and lower thirds of the esophagus the location of mucus glands within the sub mucosal layer is actually a unique feature specific to both the esophagus and the dadum the mucus is excreted through ducts that reach the mucose epithelium in order to help lubricate the surface surface these ducts typically have 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 will 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 smooth muscle this cross-section of the esophagus was taken from the middle third so if we zoom in to 40 times magnification we can see that both skeletal and smooth muscle are present the skeletal muscle can be different iated from the smooth muscle by the typical striations we'd expect to see in skeletal muscle fibers between the inner and outer layers of muscle there's also a network of nerves called the hour 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 par stulic movement of the esophagus the majority of the esophagus is surrounded by its outermost layer of connective tissue called the adventia in this image the underlying muscularis propria can still be seen at the top the adventia is attached to nearby structures such as retrop peraonal organs which keep the esophagus in a relatively fixed position the last 1 to 2 cm of the esophagus are within the abdominal cavity after Crossing the diaphragm this portion of the esophagus has an outer layer of connective tissue called the Sosa the Sosa 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 ad Tia as well as the Sosa all right as a quick recap the wall of the esophagus has four main layers the mucosa sub mucosa muscularis propria and either the Sosa or adventia the mucosa consists of three parts the stratified squamous epithelium its supporting lamin appropria and a thin layer of smooth muscle called the muscularis mucosa the sub mucosa consists mostly of dense irregular connective tissue but also contains unique mucous glands that are only seen within the submucosa in the esophagus and the dadum 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 to 2 cm of the esophagus are surrounded by an outer layer of Sosa but the majority of the esophagus has an outermost layer of connective tissue called the adventia helping current and future clinicians Focus learn retain and Thrive learn more