- Be able to describe the pathways of blood and bile flow through the liver.
- Know what organelles are prominent within the hepatocyte, and be able to relate different organelles to specific liver functions.
- Describe what surrounds the hepatocyte at various surfaces.
- Explain the organization of the bile canaliculus, space of Disse and liver sinusoid, including Kupffer cells.
- Know the organization of the classical liver lobule and Rappaport’s lobule and understand their relationship to liver functions.
- Be able to identify the gall bladder by the structure and arrangement of the various tissue layers in the wall.
- Be able to distinguish the pancreas from salivary glands at the light microscope level.
- Be able to identify acinar cells, centroacinar cells, intralobular (intercalated) ducts and interlobular ducts in the pancreas, and name the major secretory products of the first two cells.
- Identify the islets of Langerhans and cells that produce major hormones (i.e. insulin, glucagon, etc.).
I. LIVER AND GALL BLADDER
(W pgs 288-298)
Slide 1 40x (liver, H&E) WebScope ImageScope
Slide 194 40x (liver, H&E) WebScope ImageScope
Slide 195 40x WebScope ImageScope
Slide 198-1 40x ("even" slide collections, liver, Golgi silver) WebScope ImageScope
Slide 198-2 40x ("odd" slide collections, liver, reticulin stain) WebScope ImageScope
Examining slide 1 at low magnification, observe numerous small, pale spots in the parenchyma, most of which are either central veins or small branches of portal veins (in portal canals) (W pg 292-3, 15.7). There may be a few larger channels, which are larger veins either entering or leaving this region of the liver. Now examine the section with the intermediate objective of your microscope, and distinguish central veins from portal veins.
The central veins (also referred to as terminal hepatic venules) are surrounded intimately by hepatocytes similar to those that make up the bulk of the liver tissue (W pg 294, 15.8). Portal veins are usually surrounded by visible connective tissue that also contains sections through one or more bile ducts and branches of the hepatic artery. Bile ducts at medium power appear in section as a circle of rather prominent nuclei. In small branches of the hepatic artery you will see primarily the ring of smooth muscle that makes up their wall. The three components together (portal vein, hepatic artery, bile duct) constitute a portal triad or "portal canal" [example] (W pg 290, 15.3 ). Look for good examples of portal canals where all three components are seen well. Of course, if you look carefully, you'll see that there is often a very thin-walled lymphatic vessel within the portal canal as well. In addition, these structure twist and turn so there may be more than one cross section of a bile duct, artery, vein, or lymph vessel, so it's not always a "triad" of structures that you'll see in the portal canal. Now, see if you can define a classic liver lobule at low power (W pg 294, 15.8).
In the hepatic parenchymal tissue (W pg 294, 15.8), note the plates of hepatocytes (the arrangement of these cells in plates is not always clear, due to plane of section and the frequent interconnections of plates). Occasional hepatocytes are binucleate. Between the plates of hepatocytes are intervening sinusoids lined by a thin endothelium (W pg 294, 15.9). Larger eosinophilic cells lining the sinusoids are mostly Kupffer cells [example] (a type of macrophage, part of the mononuclear phagocyte system) (W pg 294, 15.9). Look for Kupffer cells using slide 194 as these cells are not readily recognized in slide 1. You should be able to distinguish Kupffer cells from endothelial lining cells.
The space between the endothelial cells and hepatocytes is called the “space of Disse” and is somewhat artificially enlarged in conventional sections. Remember that blood flows from the portal veins and hepatic arteries (of the portal canals) through the sinusoids to the central veins. A classical liver lobule has a central vein in its center and has several portal triads at its periphery. Bile flows through the bile canaliculi (too small to see) to the canals of Hering to bile ducts in portal canals, to hepatic ducts of increasing sizes and to the common hepatic duct, eventually to be emptied into the duodenum via the common bile duct. If you really want to find a canal of Hering, look for a line of low cuboidal cells immediately adjacent to a portal canal [example] --the canal of Hering connects canaliculi to the bile duct.
While there are two sources of blood input to the liver (hepatic arteries and portal veins), there is only one output starting with central veins which empty into sublobular veins and into collecting veins of various sizes and eventually into the hepatic veins (remember from Gross Anatomy the 3 large veins that empty into the inferior vena cava!). One characteristic of the hepatic outflow system is that it cuts through the liver parenchyma without respecting the organization of the liver lobules. The portal inflow system, on the other hand, is always located at the periphery of each liver lobule. Answer (DG6)
Slide 195 is stained with Masson trichrome and shows the distribution of collagen (blue) in connective tissue of the portal canals and less around the central veins. Staining is also visible in thin layers around the sinusoids. Note that this slide also includes some of the gall bladder, so make sure you're looking at liver.
One of the difficult concepts in the study of this organ is to understand the three-dimensional arrangement of the bile canaliculi (W pg 295, 15.10). Slide 198 (even) is a rather thick section of liver that has been treated with salts that react specifically with naturally occurring deposits of silver (being a heavy metal, silver naturally accumulates here) causing a precipitate to form in the canaliculi. The liver cells are unstained and so are not seen. Try to gain some understanding of the “chicken-wire” arrangement of the canaliculi as they extend between all cells in the plate of hepatocytes, eventually leading to the portal canal, where the bile is delivered to bile ductules and then to bile ducts. Slide 198 (odd) is stained with silver nitrate to show reticular (type III collagen) fibers. In this case, the silver is from the stain and it reacts with carbohydrate groups on the reticular fibers causing a precipitate to form here. Compare the distribution of the stains in these two slides
B. Gall Bladder (W pg 298, 15.13)
Slide 194 H&E 40x WebScope ImageScope
Slide 195 trichrome 40x WebScope ImageScope
Upon gross examination of slides 194 and 195 (i.e. with the naked eye or at the lowest power on the virtual microscope) you will see a portion of the gall bladder wall nestled in an indentation of the liver tissue. Examine the wall of the gall bladder with your microscope. Extensive folds of the mucosa extend into the lumen. The mucosa consists of a tall, simple columnar epithelium and its underlying connective tissue (constituting a lamina propria). No submucosa is defined. The muscularis externa consists of scattered bundles of smooth muscle. Beyond the muscularis is an adventitia consisting of rather dense connective tissue that binds the gall bladder to the liver. Where the surface of the gall bladder faces the abdominal cavity there is a serosa.
II. PANCREAS (W, pgs. 283-285)
A. Exocrine Pancreas
Slide 188B (pancreas, H&E) WebScope ImageScope
WU Slide 98 (pancreas, thin section, H&E) WebScope ImageScope --note, since this is a thin section (from Western University's digital slide collection), features of the endocrine and exocrine pancreas may be easier to find here, but you should definitely look at both slides.
Examine slides 188B and 98 at the lowest power and note that most of the section appears purple or bluish. This is the parenchyma (or functional tissue) of the exocrine pancreas (W pg 299, 15.14). You will note that the parenchyma is rather indistinctly divided into smaller areas by slits of open space or by pink connective tissue (stroma). The smallest of these areas constitute the lobules of this gland. You may see a few circular structures of various size between the lobules. These are cross sections either through branches of the pancreatic duct or through blood vessels. If you observe the parenchyma carefully you will note scattered small spots that are a lighter blue-gray. These are the islets of Langerhans, which comprise the endocrine pancreas.
First observe the parenchyma, noting that it is made up of large numbers of acini (W pg 300, 15.15a), although you may also see occasional fat cells in the parenchyma. Each acinus is a cluster of secretory cells arranged around a small lumen (which is generally collapsed and therefore not visible in your sections). The acini may vary considerably in shape, since they are cut randomly in the section. Note that the peripheral region of each acinus, which represents the basal portions of the individual acinar cells, stains more blue or purple. The hematoxylin component of the H&E stain is staining the ribosomal RNA in the abundant rough (or granular) endoplasmic reticulum found in this portion of the secretory cells (W pg 300, 15.15b).
This “cytoplasmic basophilia” is the reason why the whole section appears purple or blue. The central region of the acinus, representing the apical portions of the acinar cells, is pink (acidophilic) because of the presence of the Golgi complex and numerous secretory granules in this part of the cell (you will probably not be able to make out the individual granules). Here and there you may see a smaller cell, or cluster of cells, with pale cytoplasm in the central region of an acinus. These are centroacinar cells [example] and represents the initial portion of the excurrent duct that extends up into the acinus (W pg 300, 15.15b). These slender ducts extending from the acini to larger excretory ducts located outside the lobule are called intercalated ducts [example --found by Sahar Aminipour] and may be found by looking for small clusters of 3-5 slightly elongated nuclei lying between the acini; the cytoplasm of the duct cells is very pale, and you may or not be able to make out the lumen. As in salivary glands, intercalated ductal cells in the pancreas contribute bicarbonate ions (sodium and water follow passively) to the exocrine secretory product. However, unlike salivary glands, there are no striated ducts in the pancreas to recover sodium, so the final product is rich in both sodium and bicarbonate (as opposed to saliva in which the sodium content is about one tenth that of plasma).
Using intermediate or low power, observe the larger ducts that are located in the connective tissue septa between the lobules. These interlobular ducts can be distinguished from blood vessels by their lining epithelium, which is either simple cuboidal or, in the larger ducts, simple columnar.
B. Endocrine Pancreas (W pg 299, 15.14; pgs 342-3, 17.21-3)
Slide 188B (pancreas, H&E) WebScope ImageScope
Observe the islets of Langerhans (W pg 299, 15.14) in slide 188, occurring as pale areas of cells here and there in the parenchyma (you can find them most easily under low power). Note the scattered distribution of the islets and their variation in size. You will not be able to distinguish the various cell types in the islets in this routine H&E preparation.
Slide 189 (pancreas, vascular inj.) WebScope ImageScope (W pg 342, 17.22)
Before histological preparation, the arterial supply of this pancreas was injected with a red material. The main point of the slide is to show you how much richer the vascular supply is to the endocrine tissue, the islets of Langerhans, compared to the surrounding exocrine pancreas. Find islets in the parenchyma, and observe the denser concentration of capillaries. Many of these slides in our collection (including the virtual slide) contain some pieces of lymph nodes, so make sure that you are looking at the pancreatic tissue [ORIENTATION].
Slide 190 (pancreas, Chrome-alum hematoxylin & phloxine) WebScope ImageScope
UCSF slide 294 (pancreas, Chrome-alum hematoxylin & phloxine) WebScope ImageScope
Scan the parenchyma of this slide to find islets of Langerhans [example]. The staining procedure used here allows you to differentiate the two principal cell types found in the islets in slide #190 and UCSF294. Although the nuclei in both a and b cells are reddish, the insulin secretory granules in the beta (or B) cells cause the cytoplasm to stain a pale blue-green with the chrome-alum hematoxylin. The alpha (or A) cells, containing secretory granules of glucagon, are stained reddish. Note that the beta cells are usually more numerous and occur in the interior of the islet, while the alpha cells are found more peripherally. You will not be able to distinguish delta (or D) cells, the source of somatostatin, but you should know that they are there. Incidentally, the secretory granules of the acinar cells stain quite well in these slides and can be seen clearly in the exocrine pancreas.
Electron Micrograph Wall Charts
#122 LIVER - PORTAL AREA WebScope ImageScope
In the center of the field observe the portal vein, hepatic artery and bile duct that make up the portal “triad”, and note the connective tissue that surrounds them. In the liver tissue around the portal area you will see plates of hepatocytes, with sinusoids between them. Bile canaliculi can be seen as small white spots between hepatocytes. The sinusoids are lined by endothelial cells and occasional Kupffer cells.
#123 LIVER SINUSOID
In the Kupffer cell note occasional lysosomes, which are involved in the phagocytic activities of this cell type. The endothelial lining of the sinusoid is discontinuous, allowing free passage of materials into the space of Disse (note the numerous short microvilli extending from the surface of hepatocytes into this space). There is no organized basal lamina along the endothelial cells or hepatocytes.
Most of the typical organelles are well developed in liver cells, reflecting the many functions of these cells. Note the nucleus, rough (granular) endoplasmic reticulum, smooth endoplasmic reticulum (not labeled here), mitochondria, Golgi complex, lysosomes, peroxisomes and occasional lipid droplets. The liver cell stores glycogen and lipid. The cytoplasm contains clusters of glycogen particles (black), which can be metabolized to glucose for release into the blood when needed by the body. The glycogen occurs primarily in areas rich in smooth endoplasmic reticulum. The diverse secretory and absorptive functions of the hepatocyte take place primarily across two surfaces, shown clearly here: (1) The cell surface facing the blood in the space of Disse and adjacent sinusoid. (2) The cell surface involved in the bile canaliculus. Note the junctional complexes that seal the two sides of the bile canaliculus, and keep the bile products isolated from the blood.
#125 EXOCRINE PANCREAS
In this low power electron micrograph, observe the organization of the acini, composed of acinar cells. Within the acinar cells you will see the basal rough endoplasmic reticulum, and the numerous secretory granules in the apical region of the cells, facing the small lumen of the acinus. Note the centroacinar cell in one of the acini. The intercalated duct shown here is of intermediate size.
Review Question Answers
DG6: What are the three classifications of liver lobules and what structures define each?
1. Which of the following statements regarding the 3 zones comprising the liver acinus (of Rappaport) is CORRECT?
- Zone 1 is closest to the central vein.
- Zone 2 is the first to undergo necrosis if circulation is impaired.
- Zone 3 is closest to branches of the hepatic artery.
- Zone 1 is the first to receive nutrients delivered by the portal vein.
- Zone 2 is the last to receive any toxins that may be in the blood.
click here to view slide
2. The region of the GI tract shown is:
- lower esophagus
- cardia of stomach
- pylorus of stomach
- gall bladder
Click here to view medium magnification image.
Click here to view high magnification image.
3. The cell indicated:
- produces bile.
- is in the space of Disse.
- produces pancreatic pro-enzymes (such as trypsinogen).
- adds bicarbonate and water to the pancreatic exocrine secretion.
- removes sodium from the pancreatic exocrine secretion.
- secretes insulin.
- secretes glucagon.
Click here to view image.
4. The asterisk is in:
- liver sinusoid
- space of Disse
- central vein
- branch of hepatic artery
- bile duct
- pancreatic intercalated duct
- pancreatic interlobular duct
- parotid gland intercalated duct
- parotid gland interlobular duct