(C1-7) SLIDE 76: Liver, pig (trichrome)

This slide shows almost every feature of the liver very well. Its only disadvantage is that it is, of course, pig and not human. At low power identify classic liver lobules, outlined by greenish connective tissue. These lobules vary from three sided to seven or eight sided. Hexagonal lobules, such as the one shown in C1 are only encountered occasionally. Look for central veins in the center of the lobules (C in C1) and portal canals at the corners of the lobules (P in C1). Find a spot where there are three central veins in three adjacent classic lobules and draw an imaginary triangle between these central veins (C2). These lines define the "portal lobule", which has a bile duct in a portal canal at its center, and emphasizes bile secretion. In a similar region, find two adjacent central veins and draw imaginary lines from them to the nearest shared portal canals (C3). This identifies the "liver acinus" which emphasizes blood supply. Note also that the connective tissue capsule of the liver (Glisson's capsule) is present on two edges of this slide. At higher power, find a central vein in which sinusoids are draining (C in C4). In the pig, there is considerably more connective tissue around central veins than in the human. Now find a good portal canal and identify its basic constituents; portal vein, hepatic artery, bile duct and lymphatic (V, A, B and L in C5). Bile ducts are lined by simple cuboidal epithelium, and the hepatic "artery" is usually an arteriole. Note that several examples of each may be present in each portal canal. Next, find examples of sublobular veins (S in C6). These are larger than central veins, are often found between lobules, and are surrounded by connective tissue. Next, examine the parenchyma at high power. Note the hepatocytes, many of which are binucleate. Look for lipocytes, which look like isolated, miniature unilocular fat cells (arrow, C7). The lipocytes are storage sites of vitamin A in the liver. These only show well in the pig, not in the human. Note also the endothelial cells lining the sinusoids. Also notice that there is a faint green stain in the wall of the sinusoids. This is due to the presence of reticular fibers. Bile canaliculi can also be seen in this slide, but are inconspicuous, compared to their appearance in other slides (especially slide 65).