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 include enzymes and other macromolecules that facilitate digestion of food and absorption of nutrients. 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: parotid, sublingual and submandibular. All three have a similar architecture of secretory cells arranged in clusters called acini leading to tubes or ducts that collect the secretions. This structure is commonly referred to as a tubulo-acinar gland, and will be found in other parts of the body. Although the structure of the three glands is similar, each secretes saliva of a slightly different composition based upon their primary cell types.
The parotid gland is produces saliva that is watery and rich in enzymes (amylase and lysozyme) and antibodies. Two types of cells are visible in this section. The abundant serous exocrine cells make up the bulk of the gland and synthesize the enzymes. The cells are arranged into clusters called acini. The cells secrete protein at their apical surface into a central lumen within the acinus. The acini are surrounded by contractile myoepithelial cells; when they contract, they squeeze the saliva out of the acinar lumen into the ducts that transport saliva out of the gland. The ducts in the parortid and other salivary glands are lined by a simple cuboidal epithelium. The cells in the epithelium actively absorb sodium to produce a hypotonic saliva.
The sublingual gland is composed primarily of mucous cells. Similar to the parotid gland, the mucous cells are arranged into acini. Note the presence of a serous cell (serous demilune) that caps some of the acini. These cells produce lysozyme which digests the cell walls of bacteria. Although the serous demilune cells appear out of line with the mucous cells, this is an artifact of the fixation and preparation of the sample. In fact, the demilune cells are part of the same epithelium as the mucous cells.
The submandibular gland is composed of both serous acini and mucous cells, and secretes saliva that contains more mucous than that of the parotid.
The ducts in all three glands are primarily responsible for adjusting the ionic composition of saliva. Sodium and chloride are absorbed to generate a hypotonic solution.
The pancreas produces a solution rich in digestive enzymes and bicarbonate which is transported via a duct into the lumen of the duodenum. Digestive enzymes breakdown the macromolecules in food to facilitate absorption by enterocytes, whereas bicarbonate neutralizes the acid of the chyme coming from the stomach. Structurally, the pancreas consists of two functionally distinct parts: an exocrine part that produces digestive enzymes and an endocrine part consisting of the islets of Langerhans that secrete insulin and glucagon to regulate carbohydrate metabolism. The islets of Langerhans stain more lightly by H&E than the exocrine portion.
Similar to the salivary glands, 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.
This H&E section of the exocrine pancreas shows several of its characteristic features. The exocrine cells show a strongly basophilic cytoplasm that represents the area occupied by the rough endoplasmic reticulum. The apical side of the cells is filled with zymogen granules that contain a variety of digestive enzymes; most prominent are two proteases: trypsin and chymotrypsin. Intercalate ducts are also visible. The ducts are lined by a simple cuboidal epithelium. The cells which line the ducts secrete bicarbonate in response to the hormone secretin that is produced by cells in the duodenum. When bicarbonate reaches the duodenum, it neutralizes the pH of the acid coming from the stomach. Also visible are centroacinar cells that form the terminal lining of the intercalated ducts.
This electron micrograph shows in detail the acinar cells of the pancreas. These cells have basally located nuclei and numerous zymogen granules at their apical pole. They also have abundant endoplasmic reticulum in the basal portion of the cytoplasm. These cells will secrete their granule contents into the lumen of the duct, which will carry the enzymes out of the pancreas and to the duodenum.
The liver is the largest organ of the body and performs many essential functions. First, it secretes products that aid digestion of macromolecules and absorption of nutrients in the small intestine. The liver produces bile that is released into the duodenum via the common bile duct. Bile performs two major functions. First, it contains molecules that help solubilize, digest and absorb lipids. Second, bile contains many waste products, including bilirubin and cholesterol.
In addition, the liver receives and processes many of the nutrients that are absorbed by the small intestine. The liver is the first organ exposed to many of the nutrients absorbed by the small intestine. Blood that has perfused the small intestine is delivered directly to the liver via the portal vein. This blood is rich in nutrients. but because it has flowed through capillary beds in the small intestine, it contains less oxygen. The liver also receives fully oxygenated blood via the hepatic artery.
The liver also synthesizes and degrades plasma proteins, detoxifies drugs and toxins, stores glycogen, and releases glucose in response to hormonal signals.
Remarkably, all of the functions listed above are performed by one type of cell: the hepatocyte.
Hepatocytes are arranged in lobules which in cross section appear hexagonal. At the points of the lobule are a structure called the portal triad which contains the hepatic artery (freshly oxygenated blood), portal vein (blood from small intestine) and the bile duct. At the center of each lobule is the central vein. Hepatocytes reside between the portal triads and central vein.
Lobules define the histological arrangement of hepatocytes, but functionally, hepatocytes are separated into zones based on their proximity to a portal triad. Hepatocytes closest to the portal triads (Zone 1) are the first to receive oxygenated blood from the hepatic artery. As the blood percolates into zones 2 and 3, the concentration of oxygen decreases. Consequently, hepatocytes in zone 3 receive less oxygen than those in zone 1, and the hepatocytes in the three zones have different metabolic activities and perform different functions.
Also note that blood and bile flow in opposite directions between portal triads and central vein. Blood moves from the hepatic artery and portal vein toward the central veins, where as bile flows from hepatocytes towards the portal triads.
Portal triads are composed of three major tubes. Branches of the hepatic artery carry oxygenated blood to the hepatocytes, while branches of the portal vein carry blood with nutrients from the small intestine. The structure of these blood vessels is similar to those in other organs, but note that the portal venule is much larger than the hepatic artery and consequently the hepatocytes receive more partially oxygenated blood than fully oxygenated blood. The bile duct carries bile products away from the hepatocytes, to the larger ducts and gall bladder. The bile duct is lined by a simple cuboidal epithelium.
Blood from the hepatic artery and portal venule flows toward the central vein through special vessels called sinusoids. Sinusoids are lined by a discontinuous endothelium. This image shows the close proximity between the blood in the sinusoids and hepatocytes. 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.
Also visible is a Kupffer cell. Kupffer cells are the resident macrophages of the liver and are typically found within the lumen of the sinusoids.
The sinusoids have a discontinuous endothelium with large gaps and no basement membrane. Consequently, sinusoids are permeable to most macromolecules including proteins and lipoproteins. 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.
Hepatocytes secrete bile into canaliculi that are defined by junctions between adjacent hepatocytes. Bile flows through theses narrow tubes toward the bile duct. Bile then flows into the biliary tree, out of the liver, and to the gall bladder or intestine.
The gall bladder stores and concentrates bile from the liver. It has several important characteristic features that can be used to distinguish it from other organs in the GI system. These include irregularly shaped villi that are lined by abnormally tall columnar epithelial cells. The smooth muscle in the wall of the gall bladder contracts under the influence of the hormone cholecystokinin to expel the bile into the duodenum.