Histology of GIT 3

-The duodenum is mostly retroperitoneal and divided into 4 parts – the ampulla (no circular folds), descending (papillae), horizontal (crossed by superior mesenteric artery), and ascending (duodenojejunal flexure and suspensoryligament) parts.

- The duodenum is supplied by anterior and posterior superiorpancreaticoduodenalarteries (celiac trunk) and anterior and posterior inferior pancreaticoduodenal arteries (superior mesenteric artery).

- The gastroduodenal junction (bottom left) connects the stomach (S) with the duodenum (D). The muscular pyloric sphincter and outer muscle layers are shown.

- A distinct characteristic of the duodenum, which differs from the other parts of the small intestine, are mucus-secreting Brunner’s glands (G) within thesubmucosa (just like the esophagus).
-Another important characteristic of the small intestine (in general) is the presence of numerous villi. These finger-like projections extend out from the mucosal surface into the intestinal lumen, increasing surface area for absorption. The inset indicates permanent folds in the intestinal wall known asplicae.

- The jejunum and ileumare attached to the posterior abdominal wall via mesentery. Within the mesentery are arcades and straight arteries.Jejunal arteries are shorter than ilealarteries. In addition, the jejunum mucosa has many more circular folds than the ileum, showing that the jejunum absorbs most of the nutrients.

- Histologically, the jejunum and ileum are very similar. Note once again the numerous villi. Extending into the lamina propria from the mucosa are intestinal glands, better known as intestinal crypts or crypts of Lieberkuhn.

- The ileum ends in the right lower quadrant of the abdomen and connects to thececum, which then leads into the ascending colon.

- The ileocecal region is supplied by theileocolic artery, which branches off the superior mesenteric artery. The ileocolicartery gives off a colic branch which supplies beginning of the ascending colon, and an ileal branch that supplies the end of the ileum.
-The first third of the colon is supplied by the superior mesenteric artery via the ileocolic,right colic, andmiddle colic arteries. The rest of the colon is supplied by the left colic,sigmoid, and rectal arteries all branching off the inferior mesenteric artery. Note also the marginal artery running the colonic border and the arteriaerectae.

-Generally, the appendix has the same histological appearance as the large intestine. The main difference is the appendix contains a complete outer layer of longitudinal muscle, instead of bands of teniae coli.

-

- The mucosa resembles that of the colon. There is simple columnar epithelium with numerous goblet cells. The glands or crypts of Lieberkuhn are straight andunbranched, but there are no villi.

-The rectum differs from the rest of the colon in that the lower one-third has nothing to do with the peritoneum, and the upper two-thirds are considered retroperitoneal. In addition, the teniae coli expand and unite to form the longitudinal muscle layer.

- The external anal sphincter is composed of voluntary, skeletal muscle. In contrast, the internal anal sphincter is not under conscious control.

- The anal columns, between each are anal valves. They mark the pectinateline, where there is an abrupt transition from simple columnar epithelium of intestine to keratinized stratified squamous epithelium of skin.

- The pectinate line also divides arterial supply.Superior to the line is supplied by thesuperior rectal arteries and drained by superior rectal veins into the portal system back to the liver. Inferior to the line, the inferior rectal arteries supply blood and middle andinferior rectal veinsdrain into the cavalsystem to the vena cava.

 The large number of veins in this region, which may become dilated and varicose, commonly known as hemorrhoids. External hemorrhoids occur below thepectinate line and can be very painful. Internal hemorrhoids, on the other hand, are usually painless

Histology of GIT 2

-The tongue is specialized for moving food around in the oral cavity and mostly composed of skeletal muscle.  The ventral surface is covered by non-keratinized stratifiedsquamous epithelium. The dorsal surface, shown on the left, is covered by various papillae.

- The filiform papillae (bottom left) look like “hooks” that are composed of hard keratinized epithelium.

- The fungiform papillae(bottom middle), easily identified, are mushroom-shaped and slightly higher than surrounding filiform papillae. To the naked eye, they appear as red spots on the tongue. The paler staining regions aretaste buds.

- The circumvallate papillae are much larger than fungiform papillae, with numerous taste buds. In addition, they are surrounded by deep trenches, which are continually flushed by secretions from the underlying lingual (von Ebner’s) glands.

- Note the 3 major salivary glands. Below, from left to right, are the parotid,submandibular, and sublingual glands.

- Parotid gland: In the parotid fossa, three main structures transverse this gland – facial nerve, external carotid artery, andretromandibular vein. The parotid duct opens near the upper 2nd molar tooth. The gland is completely serous.

- Submandibular gland: Sitting most posteriorly in the submandibulartriangle, it is supplied by the facial artery and vein.Submandibular ducts, which cross the lingual nerves, open on both sides of the tonguefrenulum. It is mostly serous but partially mucus, with many serous demilune cells.

- Sublingual gland: The smallest salivary gland sits beneath the oral mucosa in the floor of the mouth. It has multiple small openings. This gland is almost completely mucus-secreting.
-The intercalated duct carries the acinar secretion to the striated duct. Shown below, it is characterized by the faint vertical striations in the cytoplasm of the duct cells. They are elaborate membrane infoldings and aligned mitochondria, allowing the striated duct to pump sodium and chloride out of the lumen and exchanging for potassium and bicarbonate. As a result, the secretions become hypotonic.

- Within the salivary glands, the lobules are composed of numerous acini.

- Secretions produced by the acinar cells are released into intralobular ducts, which converge into larger ducts leading out of the salivary glands.

- The panel above shows an intercalated duct lined with thin, low cuboidal epithelium.

- Within the acini, the secretions are hypertonic. In the intercalated duct, they are modified to be isotonic.

- As a review – the parotid gland is completely serous, the submandibular gland is mostly serous and partly mucous, and the sublingual gland is mostly mucous.

- In addition to the relative ratio of serous acini to mucous acini, the submandibular and sublingual glands are also characterized by its serous demilunes. These are serous cells capping mucous acini, indicated by the arrows in the panel above.

- The gastrointestinal tube  is composed of 4 distinct layers.

- A) The mucosa is the innermost layer near the lumen of the GI tube. It is further subdivided into 1) the epithelium, 2) thelamina propria, and 3) the muscularis mucosae. Depending on the location in the GI tract, there may also be glandular tissue.

- B) The next layer is thesubmucosa, which contains extensive connective tissue. Only the esophagus and the duodenum have secretory glands within the submucosa.

-  C) The third layer is the muscularis propria or externa. As shown in the diagram, there is an inner circular layer and an outer longitudinal layer.

- D) The outermost layer is the serosa or adventitia. The former refers to the visceral layer of the peritoneum.  The latter consists of loose connective tissue.

- Note in the abdomen, the retroperitoneal parts of the GI tract include theduodenum, pancreas, ascending colon, descending colon, and rectum, all with their own fusion fascia. All other sections in the peritoneum are attached viamesentery.

 The kidneys are also retroperitoneal.

- The pharynx connects the nasal and oral cavities superiorly with the larynx and esophagus inferiorly. It sorts food, water, and air to arrive at their destinations.

- In the pharynx, the paths of food and air cross. Food travels from the mouth (anterior) to the esophagus (posterior). Air travels from the choanae (posterior) to the trachea (anterior).

- The pharynx contains 2 layers of muscles – outer circular and inner longitudinal.

- The outer circular muscles include the superior, middle, and inferior pharyngeal constrictor muscles. One easy landmark to identify them is the tip of the greater horn of the hyoid bone, to which the middle pharyngeal constrictor attaches. The 3 muscles contract serially to push a bolus down the esophagus.

- The inner longitudinal muscles include the stylopharyngeus, salpingopharyngeus, and palatopharyngeus muscles, which elevate and widen the pharynx to accommodate a bolus when swallowing.

- The levator veli palatini and tensor veli palatini (not shown here) muscles elevate the soft palate to seal off the nasopharynx when swallowing. The epiglottis closes off the larynx and trachea.

- The interior fascia is the pharyngobasilar fascia, an area which does not have any muscle tissue.

- The pharyngeal mucosa is covered by non-keratinized stratified squamousepithelium, with an underlying dense layer of elastic tissue.

- The esophagus is posterior to the larynx and trachea in the neck region and upper thorax. It travels on the right side of the descending aorta, passes through the diaphragm, and connects with the stomach.

- Note the esophageal plexus with the main anterior and posterior vagal trunks from the left and right vagus nerves, respectively. Within the submucosa is the Meissner’splexus and in between the muscular layers is the myenteric or Auerbach’s plexus.

- The histological slides are good examples of the 4 layers of the GI tube. The epithelium (E) is non-keratinized stratifiedsquamous. The muscularismucosae (MM) is indicated by the arrows. There are also inner circular and outer longitudinal muscle layers.

 The upper third is skeletal muscle (voluntary), middle third is mixed, and lower third is smooth muscle

- the esophagogastricjunction is located approximately at the level of the diaphragm. Contractions of the diaphragm create sphincter-like effects, preventing reflux of stomach acids and content. Theesophagogastric junction is a functional, not anatomical, sphincter.

-abrupt transition of epithelium at the esophagogastric junction, from the non-keratinized stratified squamous epithelium of the esophagus to the columnar gastric surface epithelium.

- The stomach sits in the upper left quadrant of the abdomen. It can be divided into 4 parts: the cardia, the fundus, the body orcorpus, and the pylorus.

- The lesser curvature of the stomach is connected to the liver via thehepatogastric ligament, which comprise the lesseromentum with thehepatoduodenal ligament. On the other side, thegreater curvature is connected to the greateromentum of the abdomen. Note the other surrounding structures.

- The venous drainage of the lesser curvature involves the left and right gastric veins, which anastomose as the coronary vein. The greater curvature is drained by short gastric veins into the anastomoses of the left and right gastro-omental veins. They all drain into the hepatic portal vein, hepatic veins, and inferior vena cava.

-The stomach is supplied by the arteries branching off the celiac trunk.

- There are three major branches of the celiac trunk:

- 1) left gastric artery – supplies the lesser curvature and anastomoses with theright gastric artery

- 2) splenic artery – supplies the spleen, giving off the left gastro-omental arterywhich supplies the greater curvature and anastomoses with the right gastro-omental artery

- 3) common hepatic artery – supplies the liver with the hepatic artery proper. The right gastric and right gastro-omental arteries both branch off the hepatic artery proper. In addition, it also gives off the gastroduodenal artery to supply the duodenum, pancreas, and greater curvature.

- The gastric mucosa and submucosa are folded into rugae.

- The stomach surface epithelium itself is also highly folded forming gastric pits.

- Gastric glands empty into the bases of the gastric pits (bottom left). The first part of the gastric gland contains mostly parietal cells, which secrete HCl acid and intrinsic factor. The bases of the glands contain mostly chief cells, which secrete the enzyme pepsinogen.

- In addition, there are also enteroendocrine cells that secrete gastrin,somatostatin, and other hormones into the bloodstream and not the stomach lumen.

-The gastric glands proper in the corpus and the fundic glands in the fundus have the same structure. Gastric pits only extend about 25% the mucosal depth.

- The surface epithelial cells are mucus-secreting, but they are NOT goblet cells. The mucinogen granules do not distort the round or oval nuclei sitting at the base.

Histology of GIT 1

GI Tract
Anatomy-Histology Correlate

-The digestive system allows us to ingest and digest food, effectively adsorbing the nutrients required for the normal functioning of all body systems and expelling the undigested waste products.

- The digestive tract is essentially a long tube that begins from the oral cavity and continues on to the esophagus, the stomach, the small and large intestines, and finally the anus. The pancreas, liver, and gallbladder help with the digestion and absorption of nutrients.

- Beginning with the oral cavity, we will first look at the lips. There is a transition from skin to oral mucosa at the vermillion border (v.b.). The lip gets its red color from the capillaries in the high dermal papillae which are separated from the lip surface by a thin layer of epidermis, as indicated by the bracket.

- The vermillion border lacks sweat glands or sebaceous glands, making it susceptible to chapping.

- The labial vestibule of the oral cavity is lined by non-keratinized stratified squamousepithelium. The glands found in the underlying tissue are mostly mucus-secreting with some mixed muco-serous glands. The inner surface of the cheek is essentially the same.

-The oral mucosa is composed of various types of epithelia. Non-keratinized stratifiedsquamous epithelium (blue) is present where flexibility is required, as is the case of the lip and cheeks aforementioned.

- Keratinized stratified squamousepithelium (red) is required where abrasion occurs frequently and the lining epithelium needs to be more rigid. This is the case of thehard palate (bottom left) and the gingiva (next slide). The keratinized epithelium, labeled as stratum corneum, is firmly attached to the underlying bone.

- The soft palate (bottom right) is flexible and thus covered by non-keratinized stratifiedsquamous epithelium. There are numerous mucus-secreting glands amongst the skeletal muscle within the underlying tissue.

- The remainder of the oral vestibule and the ventral surface of the tongue are also covered by non-keratinized stratified squamous epithelium.

- The tongue, discussed later, contains specialized mucosa (orange) for the special sense of taste.

-Top left panel: As the non-keratinized stratifiedsquamous epithelium (B) of the oral vestibule approaches the teeth, a transition occurs at themuco-gingival junction(C) into keratinizedstratified squamousepithelium (A) of thegingiva.

- The gingiva (bottom left) is very tightly attached to the tooth by thedentogingival fibers. Free gingiva surrounds theenamel, which was removed during slide preparation (decalcification) leaving thedentin.

- The periodontal ligament is anchored within the toothcementum and inserts into the alveolar bone. These insertions, indicated in the bottom right with an arrow, are known asSharpey’s fibers. The periodontal ligament serves to attach the tooth to the bone and to absorb shock.

-The enamel , which is 96% mineral, covers the crown of the tooth. However, the dentin (80% mineral)is much thicker and forms the majority of the tooth. The black lines (top left) that run from the pulp cavity to the dento-enamel junction are dentinal tubules that were filled with odontoblasts during tooth growth.

- At the root of the tooth, the surface is covered by cementum, which has a composition similar to bone. The bottom left panel shows the cemento-enamel junction (CEJ), where the enamel ends and cementum begins. The granular dentin is also a good marker for this junction.

- Note the acellular cementum near that CEJ and compare it to the cellularcementum (bottom right) near the root of the tooth.