LIVER AND PANCREAS
Objectives
You should be able to identify the following in the
class slide sets:
| Liver | Pancreas | |||
| hepatocytes | lobules | |||
| sinusoids | connective tissue septa | |||
| endothelial cells | interlobular ducts | |||
| Kupffer cells (vitally stained) | acini | |||
| lobules | acinar cells | |||
| central vein | centroacinar cells (maybe) | |||
| portal areas | intercalated duct | |||
| hepatic portal vein | islets of Langerhans | |||
| hepatic artery | ||||
| bile duct | ||||
| space of Disse | ||||
| Gallbladder | ||||
| columnar epithelium | ||||
| lamina propria | ||||
| mucosal folds | ||||
| smooth muscle | ||||
| serosa / adventitia | ||||
 |
Blue Histology |
Slides
 | D-117 | Liver (Masson) | |
| D-118 | Liver (Trypan blue + Carmine) | |
| D-119 | Liver, monkey (H&E) | |
| D-122 | Pancreas (H&E) | |
| D-124 | Gallbladder (Van Giessen) |
Optional
slides
| D-120 | Liver, rabbit (H&E) | |
| D-121 | Pancreas (Trichrome) | |
| D-123 | Pancreas (Gomori-Masson stain) | |
| D-116 | Gallbladder (H&E) |
Slide descriptions
Liver
D-117 Liver (Masson)
Connective tissue (collagen) is stained green in this slide. With your 3.5X objective, scan this slide to appreciate that there is very little of it except as branching trabeculae which surround the major blood vessels. With your 10X objective, find a portal area of connective tissue containing branches of the portal vein, the hepatic artery and the bile duct. Concentrate on cross-sections. Turn to your 43X objective, and identify the structures in it, (illustration). Some structures may be duplicated because they are continually branching (illustration). Now find a central vein running by itself among the hepatocytes (second example). It has little more than a sheet of endothelium for its wall, with a minimum of connective tissue (collagen).
If you look around carefully you may find an area where a lobule has been sectioned transversely, (illustration). At its center is a central vein and surrounding it are about three portal areas. The cords of hepatocytes radiate from the center outwards. At this point read the section in your book "Three concepts of liver lobules".
Observe the parenchyma. It is composed of irregular lines of hepatocytes, which look like "cords" of cells. The real arrangement in three dimensions is that of anastomosing sheets of cells. Between these sheets are blood sinusoids. Note the flattened nuclei of the cells lining them. They belong to endothelial cells and to Kupffer cells (macrophages). Very important metabolic exchanges occur directly between the hepatocytes and the blood, so that in this sense the liver has endocrine functions. In fact, Claude Barnard originated the word endocrine to describe the secretion of glucose into the blood by hepatocytes.
The liver is also an exocrine gland with bile as its exocrine secretion. Every hepatic cell is in contact with at least one bile canaliculus, which it secretes bile into. These canaliculi form an interconnected network which eventually drains into a bile duct in the portal area. The canaliculi actually are very narrow channels between every two adjacent hepatocytes and do not have any other wall structure. If you look closely under 43X power (or substantially better, under oil) you may be able to see them running between adjacent hepatocytes. Longitudinally they look like a double line running between cells (illustration). In cross section you see them as a tiny donut in the middle of the boundary between cells.
D-119 Liver, monkey (H&E)
Repeat your examination of the liver on this slide stained with H&E. The portal areas do not stand out as readily as they do with Masson stain, especially at low power. However, you can still recognize them as areas of connective tissue within the parenchyma without any real difficulty, (illustration). At this stage in your histology education you no longer need the crutch of trichrome staining.
The slide includes a very large portal area with vessels that you can see with the naked eye. Look at this very large bile duct. The epithelium has gotten taller as the bile ducts have fused into larger and larger ducts. Here it is tall columnar.
Now, how about those yellow granules in some of the hepatocytes (high magnification)? Last year I offered a chocolate chip cookie to anyone who could tell me what they are (I didn't know and the text books are silent of them). John Albus (class '06) took the prize with a Google search for "liver yellow granules hepatocytes". They apparently are granules of bile associated with hepatitis infection and prolonged jaundice. Thanks John. The granules show several interesting features. First, they are restricted to hepatocytes near central veins - zone III of the liver acini, remember from your textbook? In fact, they offer a nice landmark for picking out small central veins. Also, here and there the Kupffer cells (high mag.)have taken them up. How do you suppose that happened?
D-118, Liver (Trypan blue + Carmine)
This slide demonstrates the capacity of Kupffer cells for phagocytosis even better than the yellow granules in the last slide. Even with your 3.5X objective, you can see blue dots scattered around the field. Particles of trypan blue were injected intravenously into this animal and the macrophages gorged themselves. Change to the 43X objective and satisfy yourself that these macrophages help form the sinusoidal lining, (illustration). Remember, this is only a six-micron section of liver, so there must be a lot of these cells in the liver. What do you think their function is, way down here inside the body?
The carmine stain does not show up the structures in the portal areas very well. However, you can find them if you look. The red blotches on some slides are staining artifact. Do not worry about them.
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Gallbladder
D-124 Gallbladder (Van Giessen)
With the low power objective, observe that the majority of tissue on this slide is the liver, which is adjacent to the gallbladder. This slide clearly shows the relationship of these two organs. The gallbladder is attached like a bag to the underside of the liver. Thus, one side of the gall bladder is covered with a serosa, the other with an adventitia. Find the irregular mucosa of the gallbladder and turn to 10X to observe it, noting that its folds are not villi, (illustration). Find its simple columnar lining epithelium, the lamina propria and the subjacent smooth muscle (muscularis). The main function of the gallbladder is to concentrate and store bile produced by the liver cells. This concentrate is released in response to the digestive hormone, cholecystokinin.
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Pancreas
Your three slides of the pancreas are pretty much similar except for the stain. Each emphasizes certain features, although all features can be seen on all slides.
Scan slide D-122 with your 3.5X objective and note that the pancreatic parenchyma has a purple color, and the connective tissue is predominantly eosinophilic (illustration). Beware; there are some areas of lymphoid tissue which also stains blue to purple, on this slide. Some slides of D-122 have more patches than others. The fairly well defined lobules of the pancreas are separated by interlobular septa of connective tissue. The pancreas has two parts, an endocrine portion and an exocrine portion. The exocrine portion is the compound alveolar gland with ducts that make up the darker staining bulk of the lobule. The endocrine portion is the series of islets of Langerhans that you can see as rounded paler areas embedded within the lobules.
While scanning this slide you may see a large interlobular duct lined with simple columnar epithelium and heavily invested with connective tissue. Large blood vessels are also present. Two months ago you might have confused these two.
Scan a lobule using the 43X objective. It is filled with irregularly shaped acini showing 2 to 10 nuclei in each. The nuclei are located at the basal end of the cell, which is distinctly basophilic. This is where the RER of the cell is located. The upper half of the cell is packed with eosinophilic secretory granules. These contain zymogens. The pancreas makes a variety of digestive enzymes for the duodenum and each acinar cell synthesizes the full range of them.
The relationship of the various ducts to the lobular structure of the pancreas is shown in comparison with those of the main salivary glands in the diagram in the glands section of your printed syllabus. The intralobular ducts are all of the "intercalated" variety. Look for them between the serous acini as small ducts lined with low columnar or cuboidal epithelium. If you don't find them in this slide (with the help of your atlas) do not fret. They are easily seen in slide D-123 when you get to it. The intralobular ducts vary substantially in size, being very small as they leave the individual acini and growing as they progressively fuse. Eventually they form an excretory duct which leaves the lobule.
The acini have an interesting specialization, centroacinar cells. When a terminal intercalated duct meets an acinus it protrudes into it so that duct cells lie right in the middle of the acinus. This is one reason that the acini are irregular in shape. The centroacinar cells appear as an extra nucleus or two in the middle of those acini which have been sectioned through their center. They and the other intercalated duct cells secrete an alkaline fluid, under command of the hormone secretin, to help neutralize the pH of the material passing into the duodenum from the stomach.
Now turn to the islets. Choose one that is large, not one that has barely gotten into the section. Note that it is quite vascular with numerous capillaries. Islets have four distinct cell types. By using appropriate stains it would be possible to distinguish them on the basis of size, nuclear appearance and position within the islet, but they all look the same here. The best way to distinguish them is immunohistochemically with antibodies to the hormones that they secrete; insulin, glucagon, somatostatin and PPP.
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Optional slides
D-120 Liver, Rabbit (H & E)
The liver has a lobular organization. However, the amount of connective tissue between lobules is minimal and the lobules are randomly oriented so, as you saw in slide D-117, subunit organization is very difficult to observe. It so happens that just under the peritoneum the blood vessels of the liver all run perpendicular towards the surface. The smallest twigs of the hepatic artery and portal veins go directly to the surface, and the central veins (and bile ducts) are returning. Therefore the lobules are all lined up in the same direction. D-120 has been carefully sectioned parallel to the surface of the liver and shows the lobular organization as well as you can hope to see it anywhere. Under low power find a central vein with cords of hepatocytes arranged radially to it, (illustration). Compare this image with the diagrammatic pictures in your textbook. Can you find portal areas? How many of them occur around the periphery of the lobule? Imagine the perimeter of a hepatic acinus. How is it related to the lobule? You can distinguish zone I areas of the acini (closest to the portal areas) by their darker staining even in the unmagnified picture of the slide in your syllabus. The zones of the acinus have both physiological and pathological significance.
Students have whinged (an Australian term for what small children and some dental students do) for the past twenty years that their slide of the gall bladder has a Van Giessen stain and yet the slide for their identification part of the final will be H&E. Geez! But, to accommodate the faint hearted a technician in Perth cut an especially fine section of gall bladder and stained it with H&E. So pick up one of these new slides and put it into your slide set if you wish. Examine it as you did D-124. Observe the wall of the gall bladder against the liver. The simple columnar epithelium shows up particularly well.
D-121 Pancreas (Trichrome)
This slide emphasizes the stroma which stains bright blue. Locate the large interlobular duct under low power and examine it at high power. What type of epithelium does it have? What are the pale round structures scattered here and there in the apical region of the cells? The connective tissue is quite vascular. What are the small denser purple masses with red circles in them, towards the outside of the connective tissue around the duct?
The alveolar structure of the gland shows up well, although the stain is too intense to see much detail of most lobules. The secretory granules are over stained far too intensely to see details within them. However, the staining is uneven and around the edges of the section there may be bluish areas that show the cells nicely. Here you can pick out centroacinar cells (high mag.) and intercalated ducts.
D-123 Pancreas (Gomori-Masson stain)
Connective tissue in this slide is blue or green. Scan the slide with your 3.5X objective. Appreciate the interlobular septa of connective tissue. Also note how the pale-staining islets of Langerhans stand out. Study one of these with your 10X and 43X objectives (illustration). Now follow general instructions for slide D-122. You can easily find intercalated ducts (intralobular) since they are rimmed by a layer of connective tissue and have a distinct, if small, lumen.