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 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 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.

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.

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.

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.

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.