The Gastrointestinal System can be broken up in different ways. Here, it is broken up into the Upper Digestive Tract, including the oral cavity, parotid gland, submandibular gland, sublingual gland, and oropharynx; the Middle Digestive Tract including the esophagus, stomach, and proximal duodenum; the Lower Digestive Tract including the small & large intestines and anal canal; and the Accessory Organs which include the liver, pancreas, and gall bladder. Depending on the course you are taking, please pay attention to which slides you should focus on for your current lab session.
Overview
First, it is best to understand the general structure of the GI tube which includes the esophagus, stomach, small intestine (duodenum, jejunum, ileum) and large intestine or colon (cecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum). Starting from the lumen of the tube, the mucosa consists of the epithelial lining (mucosal epithelium), loose connective tissue layer (lamina propria), and a thin layer of smooth muscle (mucularis mucosae). The next layer, deep to the muscularis mucosae is the submucosa, a thick layer of connective tissue. Next, there is a thick layer of muscle known as the muscularis externa (the majority of the GI tract is smooth muscle with the exception of part of the esophagus discussed below). Typically, there is an inner circular layer and an outer longitudinal layer. Finally, the outer layer of the GI tract is the adventitia or serosa layer. If this layer contains mesothelium from visceral peritoneum, it is known as serosa. This would include all intraperitoneal organs and portions of organs that are secondarily retroperitoneal such as the ascending colon where only the anterior surface is covered by visceral peritoneum. You should be familiar with the variations of each of these layers found in different parts of the GI tract.
Upper Digestive Tract
Overview
In this section, you will focus on specimen slides from tissues of the oral cavity, including the upper lip, tongue, and developing tooth. You will also focus on the glands including the submandibular, sublingual, and parotid glands. And finally, you will focus on the oropharynx. These structures all play a role in the initial preparation of food. The lips and tongue aid in ingestion of the food while the teeth help break up the food. The glands moisten and provide enzymes that begin the initial digestion of food to create the food bolus which passes through the oropharynx to reach the esophagus which provides transit to the stomach.
Salivary Gland Morphology:
These glands include the parotid, sublingual, and submandibular glands. Each of these glands has either serous-secreting (parotid), mucous-secreting (sublingual), or mixed-secreting (submandibular) acini. Acini are surrounded my myoepithelial cells which help with secretion. The acini are found within lobules and lobules are divided by connective tissue septa. Ducts found within a lobule are intralobular while ducts found outside the lobules in the septa are interlobular.
Intralobular ducts include intercalated ducts (closest to the acini and lined by cuboidal epithelium) and striated ducts (receive product from intercalated ducts, lined by striated columnar epithelium).
Interlobular ducts are line by stratified cuboidal epithelium.
Upper Digestive Tract Slides: Oral Cavity, Salivary Glands, Oropharynx
Use the following slides to review Upper Digestive Tract Content.
The outer portion of the upper lip can be verified by the presence of hair follicles. Look for the vermilion border near the “anterior” label where you will find more blood vessels in the dermis. The entire surface of the lip is covered with stratified squamous keratinized epithelium. Notice the difference in thickness of the stratum corneum which is thinner at the inner portion of the lip in comparison with the outer portion. The inner portion of the lip starts the beginning of the lining mucosa of the oral cavity which is found associated with the inner lips, inner cheeks, alveolar surface, floor of the mouth, inferior surfaces of the tongue, and soft palate. In some regions of this mucosa, the epithelium is nonkeratinized while other portions are described as parakeratinized which is similar to keratinized epithelium except that the superficial cells do not loose their nuceli. These nuclei are highly pyknotic (condensed) and the cytoplasm does not stain with eosin as intensely as keratinized superficial cells. Labial salivary glands are located in the inner portion of the lip. These are mixed glands with mucous acini and serous demilunes. You can identify the ducts of these glands by looking for stratified columnar epithelium. You can also identify the skeletal muscle of the orbicularis oris muscle.
This is a section through the tip of the tongue. The sulcus of the tongue divides the tongue into a posterior 1/3 and anterior 2/3. The superior (or dorsal) surface of the anterior portion is covered by lingual papillae and associated taste buds making up specialized mucosa only found on the dorsal surface of the tongue. 4 types of lingual papillae include filiform (smallest and most numerous), fungiform, circumvallate, and foliate. Taste buds can be found on 3 out of the 4 papillae, which one doesn’t contain taste buds? The tongue consists of one extrinsic muscle (one attachment outside of the tongue) and an intrinsic muscle consisting of striated bundles that run in three planes arranged at right angles to each other. Lining mucosa would be found on the inferior surface of the tongue but this portion of the tongue is not present in this specimen.
This is a thin, plastic section taken from the root of the tongue, posterior to the sulcus. It contains a single circumvallate papilla on the dorsal surface where you can look for contents including taste buds and sensory nerves. Look for the small serous gustatory (von Ebner) glands just inferior to the circumvallate papilla. There are numerous serous and mucous minor salivary glands throughout the specimen. Intrinsic tongue muscle bundles can also be seen throughout the specimen.
The parotid salivary glands are located just anterior to the ears and each gland has multiple lobules separated by connective tissue septa. Each lobule contains parotid glands which are serous acini with 2 types of intralobular ducts found within the lobules: long intercalated ducts and short striated ducts. These striated ducts lead to excretory ducts usually found between the lobes so they are also called interlobular ducts. Excretory ducts collectively empty into the large parotid duct which empties in the oral cavity through the inner cheek near the second maxillary molar.
The submandibular glands are located just inferior to the jaw in the more lateral aspects. This salivary gland contains a mixture of many serous acini and few mucous acini with serous demilunes. It has a similar duct pattern as the parotid gland. In this specimen, there are short intercalated ducts that may be difficult to find but you can see clear examples of striated ducts. Look for simple cuboidal cells (they become taller and more columnar as they get closer to the excretory ducts) with striations (lines) coming from the basement membrane.
The sublingual salivary glands are located inferior to the tongue and each gland consists mostly of mucous acini. You can find clear examples of myoepithelial cells at the edges of the acini. Unlike the previous glands, both the intercalated ducts and striated ducts are poorly developed and difficult to see.
The oropharynx is the region just posterior to the oral cavity and leads to the esophagus. The luminal mucosa is covered by stratified squamous unkeratinized epithelium, similar to the esophagus. However, this structure does not contain a muscularis mucosae or submucosa layer like we see in the esophagus which means there is no boundary between the mucosa (epithelium + lamina propria) muscularis externa. The lamina propria contains many elastic fibers that can be seen with special stains. There are skeletal muscle fibers in the outer layers (muscularis externa) arranged as inner longitudinal and outer circular layers. These muscle fibers represent the constrictor muscles of the pharynx.
This slide contains a developing tooth from a pig that has not yet broken through the surface of the gum. The tooth is embeded in a socket of bone. The pulp cavity is filled with embryonic mesenchyme. The darkly stained portion of the tooth is the crown. Look for odontoblasts, predentin, dentin, dentinal tublues, enamel, and ameloblasts.
Middle Digestive Tract
Overview
In this section, the focus is on the esophagus, stomach, and proximal duodenum. The esophagus transmits food from the oral cavity to the stomach. The stomach has a very acidic environment to digest food before moving the food bolus to the small intestine where nutrients can be absorbed. Use the cartoon figure below to help identify the layers discussed at the top of the page.
Middle GI Tract Slides: Esophagus, Stomach, Proximal Duodenum
This is a cross section of the esophagus. Review the 4 basic layers (mucosa, submucosa, muscularis externa, and adventitia/serosa) using this slide. Notice the mucosa of the esophagus has transient folds which flatten as the food bolus passes through. You may notice some lymphocytes in the lamina propria. The esophagus has a prominent muscularis mucosae. You will also notice mucous glands and ducts in the submucosa layer. The myenteric (Auerbach) plexus can also be identified between the two layers of muscle in the muscularis externa.
This slide contains the junction of the esophagus with the stomach. You can see a drastic change in the epithelium where the labels above are located. Notice the change from stratified squamous unkeratinized epithelium to simple columnar epithelium. You will also notice a thick layer of muscularis externa where the sphincter is located. In this lower portion of the esophagus, there is only smooth muscle. In the stomach region, you will notice transient submucosal folds (rugae) which flatten as the stomach fills. This region of the stomach is known as the cardiac region and contains a lymph nodule, an aggregation of lymphocytes that is infiltrating both the mucosa and submucosa. The cardia region of the stomach contains gastric pits closer to the lumen and cardiac glands with larger mucous cells closer to the muscularis mucosa.
The cells that make up the simple columnar epithelium of the stomach include many mucous cells (surface and neck region), scattered parietal cells (large, oval eosinophilic cells with central nucleus), chief cells (small basophilic cells), and very few enteroendocrine cells closer to the base. See the image below to help identify these different cell types.
This specimen is found at the junction between the pyloric stomach and duodenum. Look through the epithelium of the slide to help determine which parts belong to the the stomach (gastric pits) and which parts belong to the duodenum (villi). The pyloric stomach has generally shorter/shallower glands compared to the fundus and body/corpic regions of the stomach. Look for the Bruner glands that can only be found in the duodenum deep to the muscularis mucosae. Notice there there is a very thick region of the muscularis externa where the sphincter is located. The slide is stained with Masson trichrome which stains collagen fibers green.
Lower Digestive Tract
Overview
The small intestines are about 11m long. The first (and shortest) part is the duodenum. The duodenum receives acidic chyme from the stomach, neutralizes it, and mixes chyme with bile (from the liver) and digestive enzymes (from the pancreas). The duodenum has small plicae circulares, short villi, and duodenal submucosal (Brunner’s) glands. The second (and longest) part is the jejunum. Most of the digestion and absorption of nutrients occurs here. It has large plicae circulares and long villi. The third part is the ileum. The plicae circulares are again smaller, the villi are shorter, and there is an abundance of lymphoid tissue in the lamina propria and submucosa (Peyer patches). From the ileum, digested food passes into the large intestine (cecum, ascending colon, transverse colon, descending colon, sigmoid colon). Water and residual nutrients are absorbed here and waste is mixed with mucus to form feces. The feces are then expelled periodically through the rectum and anal canal.
The small intestines are dedicated to digestion and absorption. They have great length (and therefore great surface area), submucosal folds (plicae circulares-more surface area), villi (more surface area), villi covered by an epithelium with apical microvilli (more surface area), and a fuzzy coat of glycocalyx on microvilli (still more surface area). Villi (finger-like projections), extending from the free anatomical surface of the organ into the lumen, are uniquely found in the small intestines. In addition, there are deep invaginations from the free anatomical surface called crypts (intestinal glands). In the duodenum only, these crypts penetrate the muscularis mucosae (into the submucosa) to form duodenal submucosal (Brunner’s) glands. In the jejunum and ileum, the crypts end at the muscularis mucosae. The large intestines are dedicated to formation of feces. All gross anatomical segments are histologically similar. They have deep cryptic invaginations of the free anatomical surface but no villi. There is a large number of goblet cells in the mucosal epithelium. The muscularis externa has a uniformly thick inner circular layer and an outer longitudinal layer with variable thickness. In most locations, the outer longitudinal layer is thin, but contains three thick bands (teniae coli), which are equally spaced around the periphery of the colon. Their contractions form segmented pouches of the wall called haustra.
Lower GI Tract Slides: Small & Large Intestines
This is a cross section of about half of the duodenum with a small piece of the pancreas (bottom right) included. Start at the lumen, you should be able to identify the four basic layers. At a higher magnification, observe villi, with many absorptive cells and a few goblet cells, the discontinuous muscularis mucosae and submucosal Brunner glands. The muscularis externa has a clear inner circular and an outer longitudinal layer. Darker pink stripes in inner circular layer are contraction bands, a fixation artifact. You will be able to see the myenteric plexus clearly between the two layers of the muscularis externa. Scan along the deep part of the crypts. Here and there you will find Paneth cells with large, bright pink granules. These lie at the deepest part of the crypts, at the mucosal-submucosal boundary. Thin slips of smooth muscle here are the muscularis mucosae. Deep to the muscularis mucosae, the crypts lead into Brunner glands with mucous cells in them.
The jejunum has very large plicae circulares (PC). Observe the villi facing the lumen (L). They are long and their distal tips can be flattened. They are covered by absorptive and goblet cells. There are no Brunner glands and the muscularis externa (ME) has two neat layers with a myenteric plexus between. Paneth cells are numerous at the bases of the crypts.
This section of the jejunum has been stained with silver salts and counterstained with a pink dye. You can tell that you are looking at the jejunum because there are huge submucosal folds (plicae circulares) and long, distally expanded villi. The silver salts are reduced to silver metal (black) by the chromaffin granules in enteroendocrine cells and the reducing power of collagen fibers that encapsulate autonomic ganglia. This slide contains examples of enteroendocrine cells and examples of the parasympathetic myenteric (Auerbach) and submucosal (Meissner) plexus. Along with the submucosal plexus, you will notice lots of blood vessels in the submucosa of this specimen as well. Notice the villi are cut parallel to their long axis and have a core of CT covered by a mucosal epithelium. Scattered throughout the mucosal epithelium, you can find many cells with small black granules in them. These are enteroendocrine cells. There are several different types of enteroendocrine cells present here but they all look more or less alike. Specific immunohistochemical techniques would be required to identify subtypes of enteroendocrine cells. Now find the muscularis mucosae and look along its deep (submucosal) side. You may find scattered, sparse, poorly-fixed neuron cell bodies (or small clusters of them). These are not easy to find but they are there. These neurons have large round nuclei and prominent nucleoli. These are neurons of the submucosal (Meissner) plexus. Next, move out to the boundary between the inner muscularis externa and outer muscularis externa. Along this boundary there are prominent collections of neurons encapsulated by a thin CT capsule of black fibers. You may also be able to find nerve fibers connecting these ganglia. This is the myenteric (Auerbach) plexus. Although not obvious in this slide, the ganglia of the myenteric plexus and the submucosal plexus are connected by thin nerve fibers.
The Ileum contains the four layers we see throughout the GI tract. The most obvious feature of this slide is the superabundance of lymphoid nodules (Peyer patches) in the submucosa. There is also massive lymphoid infiltration into the lamina propria. You may be able to identify plasma cells and lymphocytes in the lamina propria but don’t spend a lot of time on this, because this is a thick section with relatively poor fixation. Note also that there are many more goblets cells in the mucosal epithelium. The layering on the muscularis externa is particularly clear here and you can find the myenteric plexus.
This slide is a cross section through the vermiform appendix, a cul de sac of the cecum adjacent to the ileocecal valve. It has features characteristic of the colon, namely a simple columnar epithelium with mostly goblet cells, crypts rather than villi, a thick inner circular and a thin outer longitudinal muscularis externa, extensive lymphoid infiltration of the lamina propria, and numerous lymph nodules in the submucosal. It differs from the ileum because it has crypts rather than villi. Does this organ have an adventitia or a serosa? Why? If an inflamed appendix were to rupture, the bacteria in its lumen would be spilled into which body cavity?
This is a portion of the wall of the colon. It has a folded mucosa (m) caused by contraction of the small muscle in the inner muscularis externa. Notice that this layer is considerably thicker than the much thinner outer muscularis externa. The teniae coli are three longitudinal thickenings of this otherwise thin layer but are not in this specimen. There is extensive lymphoid infiltration of the lamina propria and a small lymphoid nodule (l)that extends well into the submucosa. The mucosal epithelium consists of numerous deep glands (not villi) lined mostly by goblet cells. These surface invaginations end abruptly at the muscularis mucosae.
This slide passes through the junction between the rectum, which has a well-defined muscularis mucosae and a simple columnar epithelium with goblet cells and the anal canal, which has a less well-defined or even lacks a muscularis mucosae and has a stratified squamous unkeratinized epithelium. The large pink structures at the top right are dilated hemorrhoidal vessels filled with blood. The internal anal sphincter is a thickening of the circular inner smooth muscle of the muscularis externa of the rectum. The external anal sphincter consists of skeletal muscle. Neither structure is included in this specimen. Think about the embryonic origin of these tissues leading to the drastic change we see in the epithelium.
GI Accessory Organs: Liver, Gallbladder, and Pancreas
Overview: Liver
The liver is one of the largest organs in the body and is vital for many functions including synthesis of blood proteins, synthesis of apolipoproteins, detoxification of drugs and metabolic wastes, and, of course, processing the products of digestion. It has a dual blood supply, one from the systemic circulation and a second from the hepatic portal system. The portal component is functionally crucial. In general, a portal system consists of two capillary beds connected by large drainage vessels. Basically, nutrient-laden (portal) vessels drain from the stomach and small intestines, carrying the products of digestion directly to the liver, where they are processed and stored. After hepatic processing, central veins drain blood into the hepatic veins, inferior vena cava, and thence to the systemic circulation. The best way to learn about liver function is to master hepatic blood flow.
Structure:
- Parenchyma- organized plates of hepatocytes
- Connective tissue stroma- CT separating the parenchyma and is continuous with the fibrous capsule of Glisson. Blood vessels, nerves, lymphatic vessels, and bile ducts can be found throughout the stroma.
- Sinusoidal capillaries- vascular channels between plates of hepatocytes.
- Perisinusoidal spaces (spaces of Dissse)- lie between sinusoidal endothelium and hepatocytes, barely visible in light microscopy.
Liver lobules:
Histologists have defined several sorts of structural subunits of the liver. The easiest to find is the classical lobule. This is a hexagonal structure with portal triads at all six of angles of the hexagon. In the middle of the classical lobule is the central vein. There is another way to describe lobulation in the liver, namely using the portal lobule. This is a triangular structure with three central veins at the points of the triangle and a portal triad in the middle. Some histologists also speak of a liver acinus. This is an ellipsoidal structure with portal triads at opposite ends of the ellipsoid along the major axis. While examining the liver specimens below, keep these different ways of describing the microscopic anatomy of the liver in mind. In the specific directions for the slides, we will tell you precisely what you can and can’t see in the slides.
Overview: Gallbladder
The gallbladder is a cystic dilation at the end of the cystic duct. It receives bile from the liver, stores and concentrates it, and empties bile into the duodenum when needed for digestion. Bile is a complex mixture of heme breakdown products (e.g., bilirubin) and bile salts (e.g., deoxycholate). The bile salts are potent detergents that help emulsify dietary fats and aid their digestion.
Structure:
- Mucosa– Simple columnar epithelium with microvilli, lamina propria with mucin-secreting glands, especially in the neck region (increase with inflamation); mucosal folds are present. Rokitansky-Aschoff sinuses are diverticula of the mucosa that develop due to hyperplasia and herniation of epithelial cells through the muscularis externa.
- No muscularis mucosae or submucosa
- Muscularis externa– randomly oriented bundles of smooth muscle with collagen and elastic fibers
- Adventitia & Serosa– the surface of the gallbladder attached to the liver is covered by adventitia, the exposed surface is covered by serosa
Overview: Pancreas
The pancreas is both an exocrine and an endocrine gland. In the acini, pancreatic acinar cells synthesize and secrete a complex family of digestive enzymes. These are released from the pancreas via a complex system of ducts, into the duodenum, where they contribute mightily to digestion of foods. In addition, the endocrine portion of the pancreas, the islets of Langerhans, secretes insulin, glucagon, and other peptides with complex regulatory roles in carbohydrate and lipid metabolism. The islet tissue is intimately associated with a glomerulus of fenestrated capillaries that carry the hormonal secretions into the systemic circulation. The pancreas consists of a head (surrounded by duodenum), body, and tail. A main pancreatic duct (of Wirsung) extends through the length of the pancreas and empties into the duodenum. Some individual have an accessory pancreatic duct (of Santorini), maintained from embryogenesis.
Stucture:
- Capsule formed of thin layer of loose connective tissue, septa from the capsule extend into the gland creating ill-defined lobules
- Connective tissue found separating lobules, surrounding large ducts, vessels and nerves
- Mucous glands empty into main pancreatic duct
- Exocrine gland– serous glands with acinar cells filled with acidophilic zymogen granules containing digestive enzymes. Acinar cells empty into intercalated ducts. Centroacinar cells (squamous) are duct cells found within the acinus. Intercalated ducts are short and drain into intralobular collecting ducts followed by larger interlobular ducts (low columnar with enteroendocrine cells and some goblet cells). Interlobular ducts drain into the main pancreatic duct and (if present) the accessory pancreatic duct.
- Endocrine gland– islets of Langerhans are filled with polygonal cells arranged in irregular cords with a network of fenestrated capillaries. 3 principal cells types are visible with special stains and fixation (Zenker-formal fixation with Mallory-Azan stain), however, these cells are not distinguishable with typical H&E stains- A (alpha, secrete glucagon), B (beta, secrete insulin), and D (delta, secrete somatostatin) cells.
GI Accessory Organs Slides
This is a thin plastic section of human liver. Scan the slide at low magnification. You will see plates and cords of granulated cells with round nuclei, peripheral heterochromatin, and prominent nucleoli. These are hepatic parenchymal cells. Between the plates of parenchymal cells you will find vascular sinusoids lined by endothelial cells. There are few RBCs in sinusoids, but they are marked by neutrophils. You will also find large vascular spaces filled with RBCs, in the center of masses of hepatic parenchymal cells. These are central veins. They are located in the center of a classical lobule and receive blood from the sinusoids and drain into the hepatic vein. Now find CT on the edge of the classical lobule. Here you can find the three components of the portal triad: 1) hepatic arterioles (a smaller diameter vessel: an endothelium, and a few layers of smooth muscle; 2) branches of the portal vein (a venule with larger diameter: an endothelium, and only scant evidence of mural smooth muscle; and 3) a hepatic (bile) duct, lined by a simple cuboidal epithelium.
You should also be able to find examples of liver acini and portal lobules.
You will see hepatic parenchymal cells at high magnification. Among clusters of parenchymal cells, you can see pink, refractile, branching spaces. These are bile canaliculi, formed by tight junctions (seen as spaces surrouned by dark pink lines in light microscopy) between parenchymal cells. Now look on the opposite side of the parenchymal cells. Here, they face the lumen of sinusoids. There is a space of Disse between the basal surface of the parenchymal cells and the vascular endothelial cells (better viewed with electron microscopy. With a little imagination, you can see it but don’t be alarmed if you can’t find it. The lumina of the sinusoids have very few RBCs but are marked by scattered neutrophils. The endothelial cell nuclei are highly flattened and darkly stained. You may also be able to find stellate sinusoidal macrophages (Kupffer-Browicz cells) with their large round nuclei appearing to protrude into the sinusoidal lumen.
In this slide, you can clearly see the outlined classic hepatic lobules. The stain turns the connective tissue between the lobules blue. Use this slide to identify the classic lobule, liver acini, and portal lobules, check with faculty in the lab to see if you have correctly identified each structure. Discuss what makes up these structures and the function of the structures.
This slide has a section of lymph node (round structure on the left) and liver (rectangle structure on right) together on one slide. You should focus on the liver during this lab. The sections have been stained with a silver stain for reticular fibers and a pink counterstain for nuclei. At high magnification you will see many black reticular fibers. These are especially abundant in the connective tissues that bind classical lobules. At the edges of classical lobules, you will be able to find the portal triads. Each central vein is also set off by a thin band of reticular fibers. If you look carefully at the most delicate parts of the black fibrous meshwork, you will see that reticular fibers outline and support sinusoids.
If you start at the lumen (L) and move slowly through the wall of the gallbladder, you will probably think at first that this looks very similar to small intestine. The mucosa is folded into structures that look superficially like villi but you should observe that the mucosal epithelium is simple columnar without goblet cells. More careful examination reveals that these mucosal folds are more branched than simple villi and have tortuous deep diverticula called Rokitansky-Aschoff sinuses that appear as a white space, surrounded by epithelium, surrounded by lamina propria. These are cross sections through bends in the sinuses. When the sinuses are deep and perhaps cut off from the lumen, they can retain bacteria or sludgy bile and become inflamed. They are antecedents to pathological changes in the gallbladder. There is no muscularis mucosae so there is no clear boundary between the mucosa and the submucosa. Furthermore, the mural smooth muscle is much thinner than you would find in the small intestine. So, when you see “villi” covered by a simple columnar epithelium without goblet cells and a thin muscularis externa, think gallbladder. The outer layer of the gall bladder is made up of serosa on the exposed surface but adventitia of the part of the gallbladder attached to the liver.
The pancreas is a lobulated exocrine and endocrine gland. We study the pancreas exocrine function while discussing the GI system and we will focus on the endocrine function with other endocrine organs. Note that there are three lobules present in this slide, one upper right, one upper left, and a third across the bottom of the specimen. Bright pink CT and blood vessels delineate borders of lobules. Scan a lobule to find exocrine tissue and endocrine tissue. Now see if you can find portions of the exocrine ducts. Start at an acinus. Acinar cells have many bright pink apical zymogen granules. Acinar cells secrete their digestive enzymes into a complex duct system that begins at centroacinar cells, continues down to intralobular and then interlobular ducts in CT septa, and eventually empties into the main pancreatic duct. Islet tissue is also evident in many locations. Use the next slide to study cytological details of the pancreas.
You will see some islet tissue as well (endocrine function). Depending on your course material, you may not focus on the islets while studying the GI system. You can see capillaries surrounding the endocrine tissue. With a little imagination, you may be able to define two populations of endocrine cells:
- A (alpha) cells are about 20%-30% of the total islet tissue-on periphery of islet, have large nuclei-secrete glucagon
- B (beta) cells are about 70%-80% of total islet tissue- central- have small nucleus-secrete insulin
Do not spend a lot of time at this, because the distinctions are subtle. There are also several other minor kinds of cells here but they do not show up without special stains.
This slide is a thin plastic section with cytological detail. Locate an acinus and notice the apical zymogen granules, basophilic cytoplasm in the basal portion of acinar cells, and the large, round nuclei with prominent nucleoli. Centroacinar cells and other distal portions of the ducts are visible as pale-staining cells adjacent to the acini. Centroacinar cells are the distal ends of the intercalated ducts, which empty into larger, proximal intralobular ducts, which then drain into larger interlobular ducts found in broad CT trabeculae. They are tightly joined to one another and plug into the lumen of acini, sealing this lumen from the surrounding extracellular spaces. This prevents digestive enzymes from escaping into the surrounding tissues, entering the blood, and auto-digesting an individual. In patients with inflammation of the pancreas (pancreatitis), one can find huge elevations of pancreatic enzymes such as pancreatic amylase in blood. This is not good!
