|Atlas||Wheater's, pgs 288-298, Liver and Gallbladder|
Ross and Pawlina, Chapter 18 Digestive System III: Liver, Gallbladder, Pancreas
Using the low power objective observe numerous small, pale spots in the parenchyma, most of which are either central veins or small branches of portal veins (in portal canals). There may be a few larger channels, which are larger veins either entering or leaving this region of the liver. Try to identify classic liver lobules vs. portal lobules vs. acini of Rappaport.
The central veins #001 central veins, terminal hepatic venules Webscope ImageScope (also referred to as terminal hepatic venules) are surrounded intimately by hepatocytes similar to those that make up the bulk of the liver tissue. Portal veins #001 portal vein Webscope ImageScope at medium power appear in section as a circle of rather prominent nuclei. In small branches of the hepatic artery #001 hepatic artery Webscope ImageScope 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 . Look for good examples of portal canals where all three components are seen well. Keep in mind that these structures twist and turn so there may be more than one cross section of a bile duct, artery, or vein, 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.
In the hepatic parenchymal tissue, 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. Larger eosinophilic cells lining the sinusoids are mostly Kupffer cells #194 Kupffer cells Webscope ImageScope (a type of macrophage, part of the mononuclear phagocyte system). 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 #001 canal of Hering Webscope ImageScope immediately adjacent to a portal canal --the canal of Hering connects canaliculi to the bile duct.
This portal inflow system can be distinguished from the portal outflow system which lacks accompanying arteries and bile ducts. The hepatic outflow system starts 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.
One of the difficult concepts in the study of this organ is to understand the three-dimensional arrangement of the bile canaliculi. Slide 198-1 is a rather thick section of liver that has been treated with silver salts in a manner that specifically stains these structures. 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.