Liver Hepatic lobule

The term parenchyma referring to the liver tissue was coined by Erasistratos. The liver lobules were first described in the pig in 1664 by XX Wepfer (using microscopic techniques) while the lobular structure was confirmed by M. Malpighi in 1666. The term acinus was coined by S.Th. Sommering in 1796. However, it was F. Kiernan (1833) who first gave a classic definition of the lobule in pig liver ( hepatic lobule ). Today, such anatomical clarity can only be found in the livers of the camel, polar bear and seal. (s. fig. 1.18)... 

The liver contains an enormous number of hepatocytes that perform the various functions noted above. The hepatocytes are contained within minute units known as hepatic lobules, in which the cell layers (which are one or two cells thick) are in contact with networks of minute blood channels - the sinusoids - which ultimately join the venous capillaries. Capillaries carrying blood from the hepatic artery and the portal vein empty separately into the sinusoids. The walls of sinusoids and liver cells are incomplete, and blood is brought into direct contact with the hepatocytes. 

Hepatic lobule and hepatic acinus are relatively well-accepted models to describe the structure and functional aspects of the liver. Histologically, the hepatic lobule is a hexagonal region of the liver parenchyma around the central vein. Typically, six portal triads, consisting of branches from the portal vein and hepatic artery as well as bile ductules, border the edge of the lobule. Cords of hepa-tocytes are arranged radially around the central vein and blood sinusoids form between them. The hepatic parenchyma is divided into three zones based on the proximity to... 

Fig. 2.11 Diagram of the traditional ( classic ) hepatic lobule according to the lobular structure (F. Kiernan, 1833) (s. fig. 1.18) and as stereogram (H. Elias, 1949) the liver cell columns run radially from the limiting plate to the central vein (10) (s. fig. 2.16)...

The portal vein lobule was first recognized in the description of the portal unit given by RP. Mall (1906). It resembles a hexagon. The periportal field constitutes the axis at the centre while the central veins form the limiting points, (s. fig. 2.13) The glandular character of the liver is the main criterion of differentiation of the portal vein lobule. Thus the direction of blood flow is from the centre towards the periphery (centrifugal) and the direction of bile flow from the periphery towards the centre (centripetal). It could also be demonstrated that the lobule periphery is enclosed by basket-like ramifications of the portal vein (= corbicula portalis). (77) This further emphasizes the significance of the hepatic lobule. 

Elias, H. A reexamination of the structure of the mammalian liver. II. The hepatic lobule and its relation to the vascular and biliary system. Amer. J. Anat. 1949 85 379-456... 

Classification The classification of the hepatobiliary enzymes essential for enzyme diagnostics is based on their characteristic nature - i. e. excretory, secretory and indicator enzymes, (s. tab. 5.5) They are located predominantly within the liver cells and the biliary ducts as well as within the hepatic lobules. The speed of enzyme elimination does not depend on the blood enzyme levels, but follows an exponential curve. This allows the computation of the half-life of enzymes within the plasma, which is not influenced either by gender or age and is a typical enzyme characteristic. The velocity of enzyme elimination is largely constant, (s. tab. 5.5) However, in chronic diseases of the liver, it is known, for example, that GPT is usually eliminated faster than GOT despite its longer half-life. 

Autosomal recessive cystinosis is caused by an enzyme-induced blockage of cystine degradation, particularly in the RES lysosomes of the bone marrow, liver, spleen and kidneys. Especially in the stellate cells of the spleen and to a lesser extent of the hepatic lobule centres, hexagonal and rectangular cystine crystals are found, pointing at an early stage to cystinosis. There is evidence of hepatosplenomegaly and microvesicular steatosis. The clinical picture of the infantile type presents as a Fan-coni syndrome, (s. pp 593, 597) The children affected die in the first five years of life. 

If it is not possible to remove the obstruction and achieve defect healing (i.e. with fibrous residues) of the chronic (relapsing) cholangitis, the inflammatory destruction of periportal liver parenchyma will result in portoportal bridge formations and thus isolation of hepatic lobules by means of connective tissue. Monolobular, mostly micronodular biliary cirrhosis develops. (4l, 54) (s. fig. 32.5)... 

Liver. - The detailed mapping of glucose and lactate metabolism along the radius of the hepatic lobule has been investigated in situ in rat livers perfused with 1.5 mM lactate before and during perfusion with 5 mM fructose. 

Blood from both the portal vein and hepatic artery empty into a common con-dnit, mixing their contents as they enter the liver sinnsoids (Fig. 46.1). The sinusoids are expandable vascnlar channels that rnn throngh the hepatic lobules. They are lined with endothelial cells that have been described as leaky because, as blood flows through the sinusoids, the contents of the plasma have relatively Ifee access to the hepatocytes, which are located on the other side of the endothelial cells. 

The diverse metabolic activities of the liver make it susceptible to solvent induced in-jiuy, particularly from reactive intermediates which damage cellular macromolecules. The microscopic anatomy of the liver provides an explanation for this susceptibility. The basic unit of the liver is the hepatic lobule which consists of a central vein surrounded radially by sinusoids of liver cells (hepatocytes). Portal triads consisting of a hepatic artery, a hepatic vein and a bile canniliculus are located at the periphery. Liver cells closest to the vascular... 

On gross examination of a liver section, one recognizes red dots corresponding to the central vein, surrounded by brown polygonal zones made of hepatic cells and alternating with gray areas of the portal spaces. This one-dimensional description of the hepatic lobule does not adequately describe the functional unit, and anatomists and physiologists have coordinated their efforts to reconstitute the functional unit... 

The acinus is that portion of liver supplied by a primary afferent branch of the portal vein and hepatic artery. If we look at a bell-shaped hepatic lobule hanging from and traversed by a branch of the hepatic vein, the portal space runs on its side containing the artery, the vein, and the bile duct (see Figs. 9-21 and 9-22). At some point the vein and the arteriole yield three branches, two lateral and one apical, that are the primary afferent branches. They leave the connective tissue of the portal tract to penetrate in the substance of the lobule, where they further ramify into smaller branches that enter the sinusoid of the lobule. The territory irrigated by these primary afferent arterioles, the acinus, forms a somewhat irregular ovoid mass of tissue inserted between two central veins. The tissue mass is composed mainly of hepatic cells and sinusoids lined by Kupffer s cells. The central zone of the acinus (zone 1) receives the fresh blood supply and may therefore be the first to be injured by toxins in blood. The peripheral zone (zone 3) receives a blood supply partially exhausted in its oxygen and nutrients and is therefore more susceptible to anoxemia. 

Classically the liver has been divided into hexagonal lobules centred around the terminal hepatic venules. Blood enters the liver through the portal tracts that are situated at the corners of the hexagon. The portal tracts are triads of a portal vein, an hepatic artery, and a common hepatic bile duct. The vast expanse of hepatic tissue, mostly consisting of parenchymal cells (PC) or hepatocytes, is serviced via terminal branches of the portal vein and hepatic artery, which enters the tissue at intervals. The hepatocytes are organized into cords of cells radially disposed about the central hepatic venule. Between these cords are vascular sinusoids that transport the blood to the central hepatic venules. The blood is collected through the hepatic venules into the hepatic vein which exits the liver into the inferior vena cava (Figure 4.1). 

The bile canaliculi form a network, which feed into ductules, which become bile ducts (Fig. 6.3). The structural and functional unit of the liver is the lobule, which is usually described in terms of the hepatic acinus (Fig. 6.5), based on the microcirculation in the lobule. When the lobule is considered in structural terms, it may be described as either a classical or a portal lobule (see "Glossary"). The acinus comprises a unit bounded by two portal tracts and terminal hepatic or central venules, where a portal tract is composed of a portal venule, bile ductile, and hepatic arteriole (Fig. 6.5). Blood flows from the portal tract toward the central... 

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