GI tract Anatomy-Histology Correlate

GI tractAnatomy-Histology Correlate By: Michael Lu, Class of ‘07

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 squamous epithelium. 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 stratified squamous epithelium (blue) is present where flexibility is required, as is the case of the lip and cheeks aforementioned. • Keratinized stratified squamous epithelium (red) is required where abrasion occurs frequently and the lining epithelium needs to be more rigid. This is the case of the hard 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 stratified squamous 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 stratified squamous epithelium (B) of the oral vestibule approaches the teeth, a transition occurs at the muco-gingival junction (C) into keratinized stratified squamous epithelium (A) of the gingiva. • The gingiva (bottom left) is very tightly attached to the tooth by the dentogingivalfibers. Free gingiva surrounds the enamel, which was removed during slide preparation (decalcification) leaving the dentin. • The periodontal ligament is anchored within the tooth cementum and inserts into the alveolar bone. These insertions, indicated in the bottom right with an arrow, are known as Sharpey’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 cellular cementum (bottom right) near the root of the tooth.

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 stratified squamous 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 are taste 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, and retromandibular vein. The parotid duct opens near the upper 2nd molar tooth. The gland is completely serous. • Submandibular gland: Sitting most posteriorly in the submandibular triangle, it is supplied by the facial artery and vein. Submandibular ducts, which cross the lingual nerves, open on both sides of the tongue frenulum. 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) the lamina 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 the submucosa, 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 the duodenum, pancreas, ascending colon, descending colon, and rectum, all with their own fusion fascia. All other sections in the peritoneum are attached via mesentery. • 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 squamous epithelium, with an underlying dense layer of elastic tissue (blue brackets).

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’s plexus 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 stratified squamous. The muscularis mucosae (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 (involuntary). - IMPORTANT: Remember, the esophagus has secretory glands in the submucosa.

Note that the esophagogastric junction is located approximately at the level of the diaphragm. Contractions of the diaphragm create sphincter-like effects, preventing reflux of stomach acids and content. The esophagogastric junction is a functional, not anatomical, sphincter. • Note the abrupt transition of epithelium at the esophagogastric junction, from the non-keratinized stratified squamous epithelium of the esophagus to the columnar gastric surface epithelium. • Once again, there is no evident muscular sphincter at the junction. • In the following slides, we will review the anatomical features of the stomach, followed by a histological comparison of the stomach mucosa.

NOTE: • The stomach sits in the upper left quadrant of the abdomen. It can be divided into 4 parts: the cardia, the fundus, the body or corpus, and the pylorus. • The lesser curvature of the stomach is connected to the liver via the hepatogastric ligament, which comprise the lesser omentum with the hepatoduodenal ligament. On the other side, the greater curvature is connected to the greater omentum 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 the right gastric artery • 2) splenic artery – supplies the spleen, giving off the left gastro-omental artery which 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. • In short, the stomach is supplied by the right and left gastric arteries at the lesser curvature and the right and left gastro-omental arteries at the greater curvature. • The lesser curvature is drained by the coronary vein, while the greater curvature is drained by the right and left gastro-omental veins.

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. • Note the cardiac glands (gastric glands in the cardia; blue box & bottom right) are mucus secreting, and the gastric pits extend approximately half (50%) the depth of the mucosa.

The gastricglands 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. • In the bottom left panel, some gastric pits are indicated. In the gastric glands, the left bracket is the parietal cell zone and on the right is the chief cell zone. • The bottom right panel magnifies the base of a gastric gland. The black arrows are parietal cells, which are roughly oval to pyramidal in shape with a round, central nucleus. The red arrowheads indicate chief cells, with granular apical cytoplasm and empty granules.

The gastric pits of the pyloric glands (bottom left & blue box) extend at least half way to two-thirds down the depth of the mucosa. The base of the gastric pits are indicated by the vertical line with the arrow. • The bottom right panel shows the gastroduodenal junction. The thickened muscle mass, indicated by the arrow, is the pyloric sphincter. Unlike the esophagogastric junction, which is a functional sphincter, the gastroduodenal junction is an anatomical sphincter. The boxed region in the duodenum indicates submucosal Brunner’s glands, which will be discussed next.

As review, the mucosae of the cardia, body, and pylorus are compared. The vertical lines indicate the approximate end of the gastric pit and start of the gastric gland. • Note the paler staining of the cardiac and pyloric glands compared to the parietal and chief cells of the gastric glands proper. • Approximate pit depth: cardia – 50%; body – 25~33%; pylorus – 50~66%

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 suspensory ligament) parts. • The duodenum is supplied by anterior and posterior superior pancreaticoduodenal arteries (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 the submucosa (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 as plicae. • The 4 layers of the GI tube are shown again in the bottom right. The villi consist of epithelium and lamina propria of the mucosa. The small arrows point to muscularis mucosa. The submucosa, muscularis externa, and serosa are also labeled. • Note the arteries of the small intestine, all supplied by the superior mesenteric artery off the aorta. Jejunal arteries are shorter than ileal arteries. They anastomose as arcades and give off arteriae rectae.

Here we take a closer look at the intestinal villi. The villus core contains loose connective tissue, smooth muscle from the muscularis mucosae, blood vessels, lymphatic vessels, and nerves. The blue arrowheads indicate intraepithelial lympthocytes. Epithelial cells are shed at the villus tip, where they are shed or exfoliated. • The epithelium consists of absorptive, columnar enterocytes and goblet cells. The black arrowheads point to the apical surfaces of enterocytes, forming a striated border. These are the thousands of microvilli which increase surface area for absorption. • At the base of intestinal crypts, we can find enteroendocrine cells, which are identified by cytoplasmic granules at the basal instead of apical surface, releasing hormones into the bloodstream. • There are also Paneth cells that secrete lysozyme to kill bacteria. A – goblet cell B – enterocyte (absorptive) C – Paneth cell D – enteroendocrine cell

Once again, the 4 layers of the GI tract are shown – mucosa, submucosa, muscular layers, and serosa. • The jejunum and ileum are attached to the posterior abdominal wall via mesentery. Within the mesentery are arcades and straight arteries. Jejunal arteries are shorter than ileal arteries. 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. • REMEMBER: Only the esophagus and duodenum have submucosal glands.

The ileum ends in the right lower quadrant of the abdomen and connects to the cecum, which then leads into the ascending colon. • The ileocecal region is supplied by the ileocolic artery, which branches off the superior mesenteric artery. The ileocolic artery gives off a colic branch which supplies beginning of the ascending colon, and an ileal branch that supplies the end of the ileum. • Note the abrupt transition in the epithelial lining from the small intestinal (S) villi to the glandular form of large intestine (L). The ileocecal valve contains considerably thickened muscularis propria (M) with some lymphoid tissue (Ly). • Note the appendix and appendicular artery shown here. We look in more detail in two slides.

Note the distinct structures of the large intestine – haustra, omental appendages, and teniae coli (3 distinct bands of longitudinal muscle). The colon can be divided into the cecum, ascending, transverse, descending, and sigmoid colons, and the rectum. The ascending and descending portions are retroperitoneal; all other portions have their mesentery. • The first third of the colon is supplied by the superior mesenteric artery via the ileocolic, right colic, and middle 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 arteriae rectae. • Rule of thumb: all intestinal arteries should be identified by where they are running to, not the order of which the branches come off. • Note the main differences between colonic versus intestinal epithelium: there are only glands, no villi, and more goblet cells.

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 and unbranched, but there are no villi. • The border between mucosa and submucosa, or namely the muscularis mucosae, may be difficult to identify. The submucosa are often heavily infiltrated with lymphoid follicles (F). The lymphoid tissue may even extend into the mucosa, almost approaching the luminal surface. • The adventitia, or serosa (S), and mesoappendix (M) are also indicated.

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. • Note the anal columns, between each are anal valves. They mark the pectinate line, 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 the superior 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 and inferior rectal veins drain into the caval system to the vena cava. • Note the large number of veins in this region, which may become dilated and varicose, commonly known as hemorrhoids. External hemorrhoids occur below the pectinate line and can be very painful. Internal hemorrhoids, on the other hand, are usually painless.

GI tract Anatomy-Histology Correlate