Digestive Organs Lab

Learning Objectives

  • Describe the organization of and roles played by the salivary glands, pancreas, liver, and gall bladder in initiating and facilitating particular digestive processes
  • Contrast the three types of salivary glands, their component cells, and secretions
  • Name the exocrine cells of the pancreas and explain how they are organized
  • Describe the structure of the liver lobule and its component cell, the hepatocyte, and how this structure is related to the flow of blood and bile
  • Draw a connection between the structure of the pancreas and salivary glands
  • Identify some key pathological examples involving these organs

Keywords

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Pre-Lab Reading

Introduction

In the previous laboratory, we discussed in detail the different portions of the GI tract and their component cellular structures. This laboratory will focus on the accessory glands of the GI tract, whose secretions are primarily responsible for the digestion of food. These glands include the salivary glands, pancreas, liver, and gall bladder.

Salivary Glands

Salivary glands secrete saliva, a lubricant composed of mucous, lysozyme, antibodies, inorganic ions, and amylase. Saliva is released in response to parasympathetic stimulation, and up to 1500 milliliters can be produced each day. The most important component of saliva for digestion is amylase, which hydrolyzes dietary carbohydrates into disaccharides.

There are three major pairs of salivary glands, and each secretes saliva of a slightly different composition based upon their primary cell types:

  • The parotid gland is composed of serous cells, and secretes watery saliva with many proteins, including enzymes and antibodies.
  • The submandibular gland is composed of serous acini and mucous cells, and secretes saliva that contains more mucous than that of the parotid.
  • The sublingual gland is composed primarily of mucous acini, and secretes thicker, mucoid saliva.

The salivary glands are structured as groups of acini connected by tubules. These acini are often surrounded by myoepithelial cells that contract to expel the saliva. In glands with primarily mucous acini, such as the sublingual glands, serous cells surround the terminal ends of the acini and form caps known as serous demilunes.

Exocrine Pancreas

The pancreas consists of two functionally distinct parts: an exocrine part that produces digestive secretions that are discharged into the duodenum via a system of ducts, and an endocrine part consisting of the islets of Langerhans, which secrete insulin and glucagon to regulate carbohydrate metabolism. This latter part will be discussed in detail in the Laboratory on Endocrine Systems.

The exocrine pancreas is a compound tubulo-acinar gland composed of serous secretory cells. The duct system branches extensively, with each branch terminating in a luminal space bound by the secretory acinar cells. These cells make up the bulk of the organ's parenchyma and are organized in clusters within capsules of connective tissue. Groups of acini form lobules, which are separated by connective tissue septa that contain blood vessels, lymphatics, nerves, and excretory ducts.

Centroacinar cells have a characteristic location in the center of the acini and secrete bicarbonate ions into the pancreatic fluid. These cells are unique to the pancreas and can be useful in distinguishing it from other glands with a tubulo-acinar structure.

Liver

The liver is the largest organ of the body. Like the pancreas and salivary glands, it releases secretory products into the digestive tract. The liver has numerous functions.

  • Secretes bile into the duodenum via the common bile duct.
  • Plays a major biosynthetic and degradative role in regulating the macromolecular composition of the blood plasma.
  • Serves as a highly efficient filter of portal blood from the intestine.
  • Participates in large-scale synthesis and discharge of plasma proteins and lipoproteins.
  • Detoxifies drugs and toxins.
  • Is an important central organ for the synthesis and storage of glycogen and its mobilization and release as glucose in response to hormonal signals.

The basic building block of the liver is the hepatocyte. Hepatocytes are parenchymal cells that together form hepatic lobules, which appear hexagonal in cross section. At the center of each lobule is a central vein, and at each angle are portal triads composed of branches of the hepatic artery, portal vein, and bile duct system.

Portal and arterial blood moves from the portal vein and hepatic artery, respectively, into the sinusoids, which are capillaries characterized by a discontinuous endothelium. Recall from the Laboratory on Blood Vessels that discontinuous endothelia have no basement membrane. In the liver, there is a gap between the endothelium and the hepatocytes known as the space of Disse. The blood enters this space and percolates around the hepatocytes, which perform their filtration and secretion functions. It then enters the central vein and drains into the hepatic vein, which drains into the inferior vena cava.

Bile runs through the portal triads in the opposite direction from blood. Hepatocytes take up the contents of bile from the sinusoidal blood and secrete bile pigments and salts into the bile canaliculi, which drain into the ducts of the portal tracts. Bile then flows into the biliary tree, out of the liver, and to the gall bladder or intestine.

It is crucial to understand the membrane topology of the hepatocyte: the apical surface of the hepatocyte is where bile secretion occurs and faces the lumen of the bile canaliculus; the basolateral surface of the hepatocyte faces the sinusoid and is where materials are absorbed from and secreted into the blood.

Aside from hepatocytes, the liver also has a population of Kupffer cells. These cells are the equivalent of liver macrophages.

Gall Bladder

The gall bladder is a muscular sac lined by a simple columnar epithelium. Its cells manipulate the bile and concentrate it. The gall bladder is recognizable because it has irregularly shaped villi and abnormally tall columnar cells.

Pre-Lab Quiz

  1. Draw a hepatocyte. Indicate the apical and basolateral surfaces of this cell. Which surface faces the bile duct and which surface faces the sinusoids?
  2. Answer:
  3. List the salivary glands in order of increasing mucous content of their secretions.
  4. Answer:
  5. Trace the path that a bile pigment takes from the hepatic artery to the gall bladder.
  6. Answer:
  7. What is the difference between an acinar and centro-acinar cell, and how would you expect them to appear differently in the light and electron microscope?
  8. Answer:

Slides

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  1. Parotid Gland
  2. Sublingual Gland
  3. Submandibular Gland
  4. Pancreas
  5. Pancreatic Acinar Cells
  6. Pancreatic Acinar Cells EM
  7. Liver Organization
  8. Portal Triad
  9. Hepatocytes and Sinusoids
  10. Hepatocytes EM
  11. Sinusoid EM
  12. Gall Bladder

Virtual Microscope Slides

  1. Pancreas
  2. Locate acinar cells and distinguish their apical and basolateral surface.
  3. Liver
  4. At low magnification, identify the key landmarks of the liver lobule: portal triad and hepatic venule. Which way does the blood flow through the sinusoids? Which way does bile flow?
  5. Gall Bladder
  6. Observe the gall bladder and comment on its surface villi and epithelial cells.

Pathology

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  1. Cirrhosis
  2. Steatosis
  3. Cholestasis

Quiz

  1. Identify the organ and list three types of cells in this image..
  2. Answer: Organ = Liver; Cells = Hepatocytes, endothelial cells and red blood cells.
  3. Name the three vessels that are found in this region?
  4. Answer: Portal venule, hepatic arteriole and bile duct
  5. In what direction does the blood flow?
  6. Answer: Towards the center of the image
  7. Identify the organ.
  8. Answer: Pancreas. Note the acinar cells surrounding an islet of Langerhans.
  9. What is the function of the pale-staining cells?
  10. Answer: Bicarbonate Secretion
  11. In this image of the liver sinusoids, identify A, B, C, D, and E.
  12. Answer: A = space of Disse, B = sinusoidal endothelial cell, C = sinuoid lumen, D = hepatocyte, E = bile canaliculus
  13. You see a patient who is experiencing severe abdominal pain and diarrhea. A liver biopsy shows this image. Hypothesize what is afflicting the patient. Where within the liver lobule are these deposits located?
  14. Answer: This patient has an obstruction of his bile duct, also known as cholestasis. These are accumulations of bile pigment in the liver parenchyma. Specifically, bile accumulations are located in the canaliculi, which are on the apical side of the hepatocytes. Pigments can also be seen in some hepatocytes, because once they are filtered from the blood they cannot be secreted into the canaliculus.