The Liver and Gall Bladder

Objectives

You should be able to recognize tissue as liver based on the arrangement of hepatocytes, the presence of portal triads, and the central vein.
You should understand blood flow and duct network from bile canaliculi to gall bladder and be familiar with the various lobular definitions.
You should recognize hepatocytes and distinctive features of the liver in electron micrographs.

A knowledge of the vascular supply is essential to understand liver structure and function. The main blood and lymph vessels, bile ducts, and nerves enter or leave the liver through the hilus. Branches of these structures and their associated connective tissue bed ramify throughout the gland. Some branches of this network are known as portal areas (triads) which contain segments of the hepatic portal vein, the hepatic artery, and the bile duct, together with lymphatic vessels and nerve fibers. Terminal ramifications of the portal vein leave the portal areas and connect with capillary sinusoids which converge toward a central vein. Central veins drain into progressively larger sublobular veins, which, in turn, converge to form hepatic veins draining into the inferior vena cava.

The multiple functions of the liver are carried out mainly by two types of cells: the parenchymal cells (hepatocytes), which release substances into bile canaliculi and into blood and lymph channels, and the Kupffer cell which may form a part of the lining of blood sinusoids and are phagocytic. The remainder of the sinusoidal lining consists of squamous cells which constitute a discontinuous endothelium. The parenchymal cells are organized into anastomosing plates one or two cells thick. These plates are arranged in a radial pattern originating around the central veins, but they form a continuous network throughout many lobules at the areas around the portal vessels. Between the parenchymal cells and endothelial cells, there is an intervening narrow gap (space of Disse) best seen under the E.M.

	Examine the human liver in slide #62 beginning with low power (10x). First identify two essential landmarks: (1) the central veins which are isolated, thin-walled open vessels usually filled with blood with branched sheets of cells converging upon them in radial pattern and (2) the portal areas which are roughly triangular connective tissue spaces containing preterminal portal veins, arteries of much smaller caliber than the vein and one or more bile ducts lined with a cuboidal epithelium. Note that the central veins have several portal areas arranged around them and pick out several classical lobules (areas drained by one central vein). This pattern will probably be more clearly seen in a slide of injected liver which will follow. The hepatic veins into which central veins empty do not have sinusoids draining into them.

	Under high magnification, examine the sheets of parenchymal cells (hepatocytes) and the intervening blood sinusoids lined with endothelial and phagocytic cells. The position of these cell types, relative to each other, is most easily seen in the vicinity of the central vein. In parenchymal cells, the nuclei and nucleoli are very distinct, and the pronounced size differences of these nuclei are usually due to polyploidy (i.e., cells with exact multiples of the basic chromosome number). Binucleated and multinucleated cells may also be found in normal adult human livers. Study electron micrographs of parenchymal cells. Note the abundance of both rough and smooth ER. Endothelial cell nuclei are darkly stained and appear flattened against the parenchymal cells although actually separated by the space of Disse. Kupffer cell nuclei are rounded or triangular and bulge into the sinusoidal lumen. Some leukocytes are also found in the sinusoidal lumen.

	Now study the blood circulation pattern in a liver section from an animal whose vascular system has been emphasized by means of injection through the hepatic veins (slide #63 in odd numbered boxes). The area of the sinusoids is greatly increased relative to that of parenchymal cells owing to increased reverse pressure as well as filling of the vascular tree. This somewhat resembles the histological appearance of the liver in congestive heart failure, where there has been a backing up of blood on the drainage side of the vascular system. Identify the various types of lobules (for a description of the lobular arrangement refer to your text). In slide #63 (odd), find central veins with red tracks radiating out from them (sinusoids), and identify the shape of a classical lobule. Try to find portal areas containing portal venules, hepatic arterioles, and bile ducts. Next, identify a "functional acinus". Identify two central veins clearly separated by a terminal portal branch. The latter forms a "watershed" as the sinusoids are seen to drain on either side into one or other of the central veins. The functional acinus is a diamond with the two central veins at opposite poles and the terminal portal vessels as an axis. Note that the central veins and terminal portal veins are approximately at right angles whereas the pre-terminal portal veins are nearly parallel to the central veins. This is due to the fact that the terminal branches of the portal vein leave the parent vessel at an angle.

Biliary Tree and Gall Bladder

The liver secretes materials into both the blood [endocrine function] and the gut [exocrine function]. In the latter case, parenchymal cells secrete bile into the bile canaliculi which are continuous, tiny, tubular spaces between adjoining parenchymal cells of a given cell sheet. They lie deep in the cell sheets, never coming into contact with the sinusoids. The walls of the canaliculi are formed by the plasma membranes of adjacent cells and are sealed by tight junctions.

Bile flows towards portal areas through bile canaliculi, cholangioles, interlobular ducts and finally into bile ducts. The cholangioles are lined with squamous epithelium and as they begin, there may be squamous epithelium on one side and a parenchymal cell on the other. Near terminal portal veins the cholangioles drain into the interlobular ducts lined with cuboidal epithelium and then into larger interlobular branches, present within the portal triad. As the ducts increase in size, their epithelium increases in height and becomes surrounded by a more distinct connective tissue sheath and are considered to be extra-hepatic. The extra-hepatic ducts are lined by columnar cells with microvillous borders, interspersed with a few goblet cells. The common bile duct sheath contains smooth muscle cells.

	Bile canaliculi are difficult to find in H & E liver sections, but may be seen between adjacent liver cells in the specially stained (FeH) rat liver (slide #63 in even numbered boxes). They are smaller in diameter than nucleoli, and some appear as dots or dark-rimmed circles between adjacent parenchymal cells. Once they have been seen in cross-section (with oil immersion lens), locate some in longitudinal view (this is more difficult). You should be able to identify bile canaliculi by E.M.

	Examine the EM of liver.

	Examine the human gall bladder (slide #64). When contracted, its mucosal surface is thrown into numerous ridges and folds. Some sections may show diverticula or downward projections of lumen into the connective tissue and muscle coats. The epithelium of the gall bladder is tall columnar, with basal nuclei. The cytoplasm in the region of the free edge appears to be thickened, and under the E.M. it has numerous microvilli. The lamina propria may contain mucous glands in sections cut near the neck of the gall bladder. Beneath the lamina propria, the muscularis consists of interlacing fascicles of smooth muscle fibers and abundant intervening c.t., forming a thin irregular fibromuscular coat. The peripheral connective tissue contains rather large blood and lymphatic vessels (in view of abundant water reabsorption from bile) and nerves, and it forms an adventitia (fibrosa) on the surfaces in contact with the liver. Other surfaces are covered with a serosa.