1. Identify and distinguish the three extrinsic salivary glands.

2. Identify the components of the secretory end-pieces.

3. Describe the flow of saliva from its synthesis to secretion.

4. Identify the glands and bladder associated with the lower digestive system.

5. Describe the organization of the parenchyma in the pancreas and its duct system.

6. Define and characterize the cellular components found in the acini of the pancreas.

7. Describe the organization of the liver parenchyma.

8. Describe/diagram the flow of blood and bile through the liver.

9. Define and characterize the cellular components of the liver.

10. Define the wall of the gall bladder and compare and contrast it to that found in the large intestines in relation to structure and function.

Slides:

s44 parotid gland
s45 submandibular gland
s51 sublingual gland
s55 duodenum/pancreas
s63 liver and gallbladder (monkey)
s64 pancreas (human)
s67 liver (human)
s79 liver (pig)
Demonstration slide liver (mouse)

Index of images

The extrinsic glands associated with the digestive system secrete components that aid in the digestion of foodstuffs in the mouth and intestine. The liver is also involved in the further processing of nutrients for the body. Digestion starts with saliva which is a mucous secretion containing the enzyme amylase. The mucous wets the food and the amylase starts the digestion of simple carbohydrates. There are three extrinsic salivary glands found peripheral to the oral cavity. These are the parotid, submandibular, and sublingual gland. In these glands, the parenchyma is segregated into lobules by connective tissue septa. Distinguishing between these salivary glands can be done histologically by the relative amount of serous, mucous, and fat cells in the gland. The saliva is synthesized in the exocrine cells then released into intercalated ducts from which it flows to intralobular ducts (ducts within the lobule; in specific glands these are also called striated cutes) then into interlobular ducts (also called excretory ducts) which release the contents onto the mucosal surface of the mouth. The pancreas is both an endocrine and exocrine gland. The endocrine portion of the gland involves the islets of Langerhans that secrete insulin, glucagon and somatostatin. The exocrine portion of the gland synthesizes many proteases, lipases, and nucleases that are released by the serous cells into intercalated ducts which feed into intralobular ducts which feed into interlobular ducts which carry the secretions out of the organ for final release into the duodenum. The serous cells are arranged in acini.  The liver is a unique structure in that it has exocrine function in terms of bile processing and secretion, endocrine function in terms of synthesis and secretion of serum proteins, and metabolite processing in terms of further processing of absorbed glucose, amino acids, fatty acids, and detoxification of substances. The hepatocyte is the cell in the liver that performs these numerous functions. The liver receives portal blood (venous) which is drained from the spleen, gaster and intestines; this blood is nutrient rich and oxygen poor. The portal blood is mixed directly with arterial blood which is nutrient poor but oxygen rich. This blood mixture then flows through sinusoidal capillaries where it percolates past hepatocytes. These cells process the blood and it returns to the systemic circulation via the central vein (terminal hepatic venule). The hepatocytes also produce bile which is transported out of the liver through a separate drainage system. The parenchyma of the liver is loosely organized into lobules that are defined by the arrangement of vessels found in the liver. The last organ of the digestive tract is the gall bladder whose primary function is to store and concentrate bile that it receives from the liver. The lumen of the bladder has a unique topology and is covered by simple columnar epithelia.

We will start our study of the glands associated with the digestive system with the extrinsic salivary glands. The different salivary glands can be characterized by their relative level of serous, mucous and fat cells. The secretory parenchyma of the parotid gland is mostly serous cells in terms of its exocrine cells and contains numerous fat cells (about an equal amount in volume to the serous cells). The submandibular gland has few fat cells and the secretory parenchyma is about equal in terms of mucous and serous cells. The secretory parenchyma of the sublingual gland is mostly mucous.

We begin our study of the salivary glands with the submandibular gland (s45) as it has good examples all the components of salivary glands. Study the section at low power noting the remnants of the capsule and the lobular arrangement of the parenchyma. The connective tissue septa defining the lobules are readily apparent. Within the septa are nerve, blood and lymph vessels and excretory ducts (= interlobular ducts). The cells lining the lumen of the larger excretory ducts are cuboidal to columnar and the epithelium becomes pseudostratified or stratified the closer the duct is to the oral mucosa. You will also find smaller excretory ducts but the duct-lining cells are more cuboidal in nature. The excretory ducts receive saliva from the striated ducts (= intralobular ducts). The striated ducts have little to no obvious connective tissue around then and they are lined with cuboidal cells with an eosinophilic cytoplasm. The "striations" are due to in-foldings of the basal plasma membrane to increase the surface area for ion transport. There are also abundant mitochondria (to supply ATP for the active ion transport) within the cells. These features allow for conditioning (removing Na+ and adding K+) of the saliva as it passes through the duct. The striated ducts receive saliva from intercalated ducts. These are so called because they are intercalated (interposed) between the secretory end-units and the striated duct. These ducts are an absolute pain in the colorectal junction to find. The intercalated ducts are small tubules within the secretory parenchyma. The cells of the duct, when observed in cross-section are small cuboidal cells with a poorly-staining cytoplasm. In longitudinal section, the ducts are seen to connect directly to the secretory end-pieces and again, show poor cytoplasmic staining and round nuclei. The exocrine parenchyma is made mostly of mucous and serous cells arranged into secretory end-pieces. The mucous cells are arranged into tubes and are cuboidal to columnar with a heterochromatic nucleus at the base of the cell and a poorly-staining, vacuolated cytoplasm. In cross section, the mucous tubes are delimited by a basement membrane and within this basement membrane you may observe an elongate nucleus between the base of a mucous cell and the basement membrane. This is the nucleus of a myo-epithelial cell that is contractile in nature and has stellate processes that wrap around the tube and squeeze the mucous out of the tube. You will also observe groups (acini) of serous cells with their round nuclei at the outer aspect of the acini and obvious eosinophilic granules within their cytoplasm. Lastly, you will find groups of cells (tubulo-acini) that are a mix of mucous and serous cells.  The serous cells are typically associated with the outer aspect or end of a mucous cell tube and these serous cell are arranged in a crescent. This group of serous cells is called a serous demilune (or demilunes of Giannuzzi) whereas the entire mixed secretory unit is called a seromucous demi-lune.  Careful examination of the stroma surrounding the secretory end-pieces will reveal the presence of numerous plasma cells which impart mucosal immunity.  

We next study the parotid gland (s44). View the section at low power and you should observe connective tissue septa, serous acini, and fat cells. Study the interlobular connective tissue and find excretory ducts with their cuboidal cells lining the lumen. Within the lobules, you will find fat cells, serous acini, striated ducts but essentially no mucous cells. The striated ducts are lined with simple cuboidal epithelia as seen in the submandibular gland. Intercalated ducts must be present, but I have not found any obvious examples yet.

The last salivary gland that we will study is the sublingual gland (s51). The tissue in this section has been poorly preserved and thus does not present a good example of this gland. Look at the section briefly and note the remnants of the capsule, connective tissue septa, and excretory ducts. The parenchyma is difficult to study because of the poor preservation. The gland is mostly a mucous gland with scattered serous cells.

The pancreas is the next digestive gland that we will study. Begin by observing s55 in which there is a section of tissue containing both the pancreas and a cross section of the duodenum. The parenchyma is encased within a connective tissue capsule and organized into lobules via thin connective tissue septa. At low magnification, you will observe areas of connective tissue invested with ducts and blood vessels. Most of the area of the lobule contains dense eosinophilic staining with cluster of lighting staining regions. The parenchyma contains both endocrine and exocrine cells. The endocrine cells are observed as clusters of cells (islets) that have lighter staining than the surrounding exocrine cells. Further study of the islets of Langerhans shows that they contain cords of cuboidal cells and have a heavy investment of capillaries within the islet. This facilitates the release of the islet endocrine hormones into the blood stream. Most of the cells of the pancreas are exocrine cells that are serous cells arranged in acini. The exocrine products are secreted into intercalated ducts that connect to interlobular ducts. The intercalated ducts are small, squamous or low cuboidal lined tubules with the duct cells having a lighter staining cytoplasm than the surrounding exocrine acini. These ducts feed into the more cuboidal intralobular ducts.  However, the intralobular duct lining cells do not have striations as observed in the intralobular ducts (or striated ducts) of the serous salivary glands. The nuclei of the intralobular duct cells are euchromatic and cuboidal and when seen in longitudinal section, also the cell margins are difficult to identify. The interlobular ducts are readily apparent in the interlobular connective tissue in association with blood vessels. As with the interlobular (excretory) ducts in the major salivary glands, they are lined with a cuboidal to columnar epithelia. You can also study s64, a section of the human pancreas. The interlobular connective tissue is more apparent than s51 and contains an investment of nerves, blood vessels and interlobular ducts. The islets are apparent but intralobular and intercalated ducts are difficult to identify in the endocrine parenchyma.
 

Next, we proceed to the liver. There are several slides of the liver in your collection. We will limit our study to those that present good examples of liver structure. Begin with s79 which contains a section of suid liver. This section shows the lobular nature of the liver in which the lobules are delimited by thin connective tissue septa. At the center of each lobule is the central vein (terminal hepatic venule) which appears as a lumen with a very thin wall. The hepatocytes radiate out from the central vein in plates or columns of cells. Hepatocytes are cuboidal cells with a central nucleus. Some cells will be binucleate. Switch to s63 which contains both the liver and the gall bladder. Here, we will focus our study on the organization of the vessels and ducts of the liver. Study the section at low magnification and you should readily find a central vein (terminal hepatic venule). Study the central vein (terminal hepatic venule) at high magnification and you should note that the wall of the vein (more like a very large capillary) is lined with simple squamous epithelia (endothelium) and how the sinusoidal capillaries of the lobule empty into the central vein. Progress out from the central vein (terminal hepatic venule) at high magnification and you will observe plates or cords of hepatocytes with capillaries between the plates. The hepatocytes are cuboidal cells with a round nucleus, a prominent nucleolus and eosinophilic cytoplasm. Further examination of the capillary lumen will show that they are lined with endothelial cells and you may find cuboidal cells within the lumen with an obvious cytoplasm. These latter cells are likely macrophages of the liver (Kupffer cells). The blood that enters these capillaries is from the hepatic arteriole and portal venule of the portal triad. Arterial and portal blood are mixed together and percolate through the sinusoidal capillaries/hepatocytes towards the central vein. A portal triad consists of three vessels:  a hepatic arteriole, a portal venule, and an [interlobular] bile duct. The hepatic arteriole contains a few layers of smooth muscle cells around the endothelial cells delimiting the vessel lumen. The portal  venule is a vessel with a very thin wall made of endothelial cells and maybe one layer of smooth muscle. The [interlobular] bile duct is lined by a single layer of cuboidal epithelial cells with microvilli on the lumenal side (not apparent). The flow of bile is in the opposite direction of the flow of blood within the classic lobule. These three vessels run together within a connective tissue septum; the portal triad along with its connective tissue investment are referred to collectively as the portal canal.

The abundance of hepatic macrophages (Kupffer cells, also called stellate sinuosoidal macrophages) within the hepatic sinusoids is difficult to appreciate in routine H&E sections.  [You possibly spent several minutes looking for a good example in slide 63.]  Examine the demonstration slide of mouse liver.  This animal had been injected with India ink prior to sacrifice and its macrophages took up the ink, thus revealing their distribution (low power, high power).  Although this method clearly effectively labels macrophages, it does so at the cost of obscuring their morphology.

The last organ of the digestive system that we will study is the gall bladder (vesica fallea) found in s63. Study the wall of the gall bladder at low magnification. The wall of the gall bladder is formed by a mucosa, muscularis and adventitia / serosa.  The gall bladder demonstrates the difference between and an adventitia and a serosa.  The part of the gall bladder surrounded by the liver is covered externally by an adventitia; the part of the gall bladder not surrounded by the liver is exposed to the abdominal (peritoneal) cavity and is covered with a serosa, in this case the visceral peritoneum.  Look closely and you might be able to observe its mesothelial covering, unless, of course (probably) it was removed in tissue collection.   The smooth muscle of the muscularis is oriented such that it wraps around the lumen and its contraction results in expelling bile into the cystic duct. Next, you will observe the lamina propria of the mucosal wall. It protrudes into the lumen wall in a plicae-like fashion as observed in the small intestine. The lamina propria is as observed in the large intestine with it being made of loose irregular connective tissue. Finally, the lumenal epithelium is made of simple columnar epithelia when observed in good section. These columnar cells (cholecystocytes) have microvilli at their apex and abundant mitochondria rendering the cytoplasm eosinophilic. These are characteristic features of cells involved in fluid transport, in this case resulting in concentration of the bile.

Lab:  Digestive System III - Extrinsic Glands