Hepatic lobules

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... 

Description of the bile microcapillaries in peripheral hepatic lobules by J. Gerlach. 

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)... 

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)...

Fig. 2.12 Hepatic lobules with central vein ( f ) and Glisson s triangles (G). Slight distortion of the lobular architecture...

The classic central vein lobule accords with the traditional description of lobular structure (F. Kiernan, 1833 H. Eppinger, 1937 H. Elias, 1949). The hepatic lobule... 

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. 

Fig. 2.13 Diagram of the classic hepatic lobule (I), the portal vein lobule (II) and the hepatic acinus (III) CV = central vein ( ), P = portal tract ( ). Flow direction venous blood (= blue arrow), arterial blood (= red arrow) and bile (= green arrow), with the microcirculatory acinus zones 1, 2, 3. (cf. W Ekataksin et af, 1992 the microvascular unit is regarded as an area in which all hver cells receive blood from a common terminal vessel)...

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. 

The inflammatory mesenchymal reactions in the portal field and lobular periphery induce extremely low-cell perilobular and tylotic fibrosis. (425, 442) Slowly, starshaped portal fibrosis, so-called holly-leaf fibrosis, develops. This morphological picture, similar to that of chronic hepatitis, was termed chronic siderophile hepatitis by H. Kalk (1962). The hepatic lobules are gradually... 

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)... 

Micronodular cirrhosis Monolobular (monoacinar) cirrhosis consists of individual hepatic lobules separated by connective tissue, with the central vein maintained in the interior. In pseudolobular cirrhosis, no elements of the lobular architecture and no central vein can be detected the nodules in this type of cirrhosis are small. The monolobular and pseudolobular forms are classified as micronodular cirrhosis, or Nagayo type C (with node size <0.3 cm). A striking feature is the uniformity of the nodules, (s. figs. 35.4, 35.5)... 

Fig. 35.7 Scheme of possibilities for hepatic transformation after parenchymal necroses (1) formation of pseudolobuli, (2) nodes with enclosed portal field, (3) formation of parenchymal garlands, (4) development of a coarse node from several hepatic lobules which are, in part, completely intact, (5) scar formation after total lobular necrosis (H. Thaler, 1975) (179)... 

Circulatory disturbances within the hepatic lobules are of central pathophysiological importance three different forms can be differentiated ... 

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. 

Schematic representation of structures within the hepatic lobule. H = hepatocyte, BC = bile canaliculus, KC = Kupffer cell, EC = endothelial cell, N = nerve fiber, F = reticulin fibers, S = sinusoid, D = space of Disse, X = gap between sinusoid lining cells, and RBC = red blood cell.

Ammonia contained in the blood flowing through the hepatic lobule is removed by the hepatocytes and converted into urea. Periportal hepatocytes are the predominant sites of urea formation. Any ammonia that is not converted to urea may be incorporated into glutamine catalyzed... 

Arterial and venous blood from the portal triad passes through the hepatic lobules to the central veins via the hepatic sinusoids. After passing through the hepatic lobules, blood collects in the central veins, which ultimately coalesce into the hepatic veins, which then enter the inferior vena cava. 

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... 

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