School of Anatomy and Human Biology - The University of Western Australia
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Lab Guides and Images
Exocrine Pancreas - H&E
Endocrine Pancreas - Island of Langerhans
Gall Bladder - H&E
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The exocrine pancreas consists of tubuloacinar glands. A single layer of pyramidal shaped cells forms the secretory acini. The apical cytoplasm (towards the lumen of the acini) is filled with secretory vesicles containing the precursors of digestive enzymes. The first portion of the duct system extends into the centre of the acini, which is lined by small centroacinar cells. These cells form the first part of intercalated ducts. Intercalated ducts are lined by low columnar or cuboidal epithelium. They empty into interlobular ducts, which are lined by a columnar epithelium. Interlobular ducts in turn empty into the main pancreatic duct (of Wirsung), which is lined by a tall columnar epithelium.
The main pancreatic duct opens into the summit of the major duodenal papilla, usually in common with the bile duct. A duct draining the lower parts of the head of the pancreas, the accessory pancreatic duct (of Santorini), is very variable. If present, it may open into the minor duodenal papilla ~2 cm above the major papilla in the duodenum.
Pancreatic juice is a clear alkaline fluid which contains the precursors of enzymes of all classes necessary to break down the main components of the diet:
Proteolytic enzymes are secreted as zymogens - inactive precursors of the enzymes. They are activated in the lumen of the digestive canal. The enzyme enteropeptidase is associated with the brush border of enterocytes. It catalyses the conversion of trysinogen into trypsin. Trypsin can activate a number of the other pancreatic zymogens.
While the enzymes are secreted by the secretory cells of the pancreatic acini, the bulk of fluid and bicarbonate ions of the pancreatic juice are secreted by the cells which form the intercalated ducts of the pancreas. Bicarbonate ions in the pancreatic juice neutralize the acidic contents which the stomach empties into the duodenum.
Pancreas, human - H&E
Look at the slide at low magnification and note the subdivision of the pancreas into numerous lobes and lobules. Identify the connective tissue between the lobes and lobules and try to find interlobar or interlobular excretory ducts. Their outline is often irregular and their lumen is lined by a tall columnar epithelium. If you find a large duct you may see a number of smaller ducts streaming towards the larger duct and, occasionally, connecting with it.
The overall appearance of the pancreas is that of a serous gland. At a first glance it may be possible to confuse the pancreas with other serous glands, e.g. the parotid gland. Note that the pancreas does not contain structures resembling the large intralobular ducts of the salivary glands and that the interlobular and interlobar secretory ducts of large salivary glands are lined by ... which type of epithelium?
Draw the excretory duct.
Now have a closer look at the secretory tissue within the lobules. At low magnification
most of the tissue appears to be composed of small reddish packages, the secretory
acini. Intercalated ducts are difficult to find and so are the initial segments
of the (non-secretory) intralobular ducts (cuboidal epithelium). You may try
to find them and include them in your drawing, but don't get upset if you or
the demonstrators have difficulties locating them.
Draw at the largest magnification a number of exocrine secretory acini. If possible, include in your drawing some centroacinar cells.
Components of the Endocrine Pancreas
Islands of Langerhans, usually containing several hundred endocrine cells, are scattered throughout the exocrine tissue of the pancreas. The vascularization, composed of many fenestrated capillaries, is more extensive than that of the exocrine tissue.
Although the quantitative cellular composition of the islands is quite variable, we find typically:
Pancreas, human - H&E
and Pancreas, rat - ICC
If you scan over the secretory tissue at low or medium magnification, you may be able to identify areas of tissue with a slightly different hue and texture. These areas are likely to represent the islands of Langerhans.
Draw at low or medium magnification a part of the pancreas in which you see an island of Langerhans. Make sure that the difference in texture and/or hue between the endo- and exocrine pancreas is visible (at least to you).
The portal vein, hepatic artery and bile duct enter the liver through the porta hepatis. These three vessels travel together through the liver parenchyma. If one of these vessels gives off a branch it is usually accompanied by branches of the other two vessels. Terminal branches of one of the vessels will consequently be accompanied by terminal branches of the other two vessels.
These groups of three tubes - a branch of the portal vein, a branch of the hepatic artery and a branch of the bile duct - are called portal triads. Portal triads are a key feature of the organization of the liver. Portal triads are embedded in interlobular connective tissue.
An idealized representation of the "classical" liver lobule is a six-sided prism about 2 mm long and 1 mm in diameter. It is delimited by interlobular connective tissue (only little, if any, visible in humans; plentiful in e.g. pigs). In its corners we find the portal triads. In cross sections, the lobule is filled by cords of hepatic parenchymal cells, hepatocytes, which radiate from the central vein and are separated by vascular sinusoids.
There are other ways of dividing the parenchyma of the liver into units. Two common ways are divisions into portal lobules and liver acini. Portal lobules emphasize the afferent blood supply and bile drainage by the vessels of the portal triads. The secretory function of the liver is emphasized by the term liver acinus. Acini are smaller units than portal or "classical" liver lobules and relate structural units to terminal branches formed by the vessels of the portal triad but not the portal triad itself. Representations of portal lobules and liver acini vary in different textbooks.
Hepatocytes are separated from the bloodstream by a thin discontinuous simple squamous epithelium, which lines the sinusoids. Between the hepatocytes and the epithelial cells is a narrow perisinusoidal space (of Disse). Contents of the blood plasma can freely enter the perisinusoidal space through the openings in the epithelium lining the sinusoids. Fixed macrophages, Kupffer cells, are attached to the epithelium .
The liver lobule is drained by the central vein, which open into the intercalated or sublobular veins of the liver. These in turn coalesce to form the hepatic veins. They run alone through the tissue, are usually covered by connective tissue and eventually empty into the inferior vena cava.
Adjoining liver cells form the walls of the bile canaliculi , which form a three dimensional network within the sheets of hepatocytes. Bile canaliculi connect via very short canals (of Hering; formed by both hepatocytes and cells similar to those in the epithelium of bile ducts) to terminal bile ducts (cholangioles) which empty into the interlobular bile ducts found in the portal triads.
Liver - H&E and Liver,
rabbit - trichrome & carbon
The Kupffer cell section does not show the detailed organization of the liver very well. It is however the best section to identify liver lobules. Scan over the tissue at low magnification and identify lobules. Once you have recognized them, change to the H&E stained section. It is difficult, if not impossible, to clearly identify liver lobules in the H&E stained section. The best indication of a liver lobule are the large central veins and the strands/sheets of hepatocytes, which seem to radiate out from the central veins. Change to a higher magnification in the region of a central vein and try to identify the epithelial cells forming the walls of the liver sinusoids.
Draw a central vein and adjacent sheets of liver cells and sinusoids. Try to find and draw a portal triad. Identify in your drawing the branches of the portal vein, hepatic artery and bile duct. Their size will very much depend on how close you are to the terminal branches of these structures. Try to avoid the largest portal triads you see.
Liver, rabbit - trichrome
& carbon and Liver - reticulin
Bile contains both organic components (e.g. lecithin, cholesterol and bilirubin - the latter is a breakdown product of haemoglobin and accumulates in the blood in jaundice) and inorganic components (bile salts). The bile salts facilitate the digestion and absorption of fat in the small intestine.
Terminal bile ducts merge to form interlobular, intrahepatic bile ducts, which eventually coalesce to form first the left and right hepatic ducts and then the common hepatic duct, which connects to the cystic duct and the bile duct (ductus choledochus). The bile duct carries the bile to the duodenum. The cystic duct leads to the gall bladder.
Terminal bile ducts are lined by a cuboidal epithelium. All other parts of the bilary system are lined by a tall columnar epithelium. In the gall bladder the epithelium is often folded and "caved". The gall bladder functions in the storage and concentration of bile. Microvilli on the apical surface of the epithelial cells facilitate the resorption of water from the bile.
The epithelium lining the biliary system does not contain mucus-producing cells and a muscularis mucosae is absent. These features allow you to distinguish the gall bladder from other parts of the gastrointestinal tract.
Many of the components of bile are not secretory products of the hepatocytes in a strict sense. They are reabsorbed in the gut and return to the liver through the portal vein. Here they are taken up by the hepatocytes and excreted again - a phenomenon called enterohepatic circulation.
Gall Bladder, human - H&E
Have a look at the slide at low magnification. Note the irregular outlines of the epithelium, the relatively dense irregular connective tissue beneath it, and the irregular appearance of the muscular layer of the gall bladder. Now take a close look at the epithelium.
Draw the epithelium and part of the underlying connective and muscular tissues.
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last updated: 5/08/09