Periportal zone

Fig. 2.15 Diagram of the liver lobule and the acinus arranged like a clover leaf around the portal field according to the acinar structure (modified from D. Sasse, t986) central hepatic vein (CV) or terminal hepatic vein, periportal field (P). Circulatory and meta-bolically different zones zone t (periportal), zone 2 (intermediate), zone 3 (perivenous)...

Periportal zone (zone 1) Perivenous zone (zone 3) ... 

The urea cycle, also called ornithine cycle, was first described by H.A. Krebs and K. Henseleit in 1932. (quot. 51) The principle of ammonia detoxification in the urea cycle is based on the conversion of ammonium and bicarbonate in the mitochondria under ATP consumption into carbamoyl phosphate (by means of carbamoyl phosphate synthetase). It enters the urea cycle, which is localized mainly - yet with a low affinity for ammonium - in the periportal zone of the liver lobule. In the urea cycle alone, about two thirds of the amino nitrogen of ammonia are irretrievably lost to the organism (= definitive ammonia detoxification), (s. fig. 3.12)... 

Periportal inflammation Periportal hepatitis is characterized by penetration of the limiting plate. The border between the portal field and the lobule can appear irregular sometimes it assumes the shape of a maple leaf In this periportal zone (i.e. zone 1), piecemeal necroses may develop. They are, however, not true necroses , but apoptoses. Today, periportal inflammation with piecemeal necrosis is termed interface hepatitis. This condition is not always accompanied by piecemeal necrosis the inflammatory infiltrate can also enter the lobule without causing liver cell necroses. The composition of the inflammatory infiltrates is similar to that in the portal fields. Periportal infiammation contributes to the grading of chronic hepatitis, (l)... 

Fibrosis As a consequence of the necroinflammatory process, fibrogenesis is activated (s. p. 403) it begins in the portal fields and leads to their fibrotic dilation. Fibrosis extends to the periportal zones and can ultimately link portal tracts to other portal tracts and to terminal hepatic venules. Reliable staging requires con-... 

Periportal zone/Zone 1 Glucose release Oxidative energy metabolism Amino acid utilization Protection against oxidants Bile acid uptake and excretion Bilirubin excretion... 

GLD is more concentrated in the central areas of the liver lobules than in the periportal zones. This pattern of distribution is the reverse of that of ALT. Pronounced release of GLD is therefore to be expected in conditions in which cen-trilobular necrosis occurs (e.g., as a result of ischemia or in halothane toxicity). 

To understand the possibilities and limitations of liver in vitro systems it is crucial to be aware of the organization principles of this organ. The smallest functional unit of the liver is the lobule (Fig. la). The human liver is composed of approximately one million lobules. Each lobule is supplied by branches of the portal vein which carries blood from the intestine (about 80 % of the liver s blood). Moreover, arterial blood is supplied by branches of the liver artery (about 20 %). The blood enters the lobules in the periphery, passes through microvessels where it is in close contact with hepatocytes, is finally drained off into the central veins, and leaves the liver by the hepatic vein. The oxygen concentration is about 13 % v/v (60-65 mmHg) in the periportal zone and drops to about 4 % v/v (30-35 mmHg) in the central vein [3]. 

The toxic effects of aflatoxin in animals includes gross liver damage, development of a necrosis in the periportal zone [246], and haemorrhage in the intestinal tract and peritoneal cavity. Aflatoxins also affect several cell culture systems in vitro [247—249], and cause lesions in chick embryos [249—251 ]. The finding of multiple liver tumours and lung metastases [252] was the first indication of the hepatotoxicity of this group of mycotoxins. This observation has been several times confirmed [253-257] and only 0.005 p.p.m. failed to induce liver tumour in rats [258]. Mice are more resistant [259—261], whereas rainbow trouts are considerably more sensitive to the action of aflatoxins [262—266]. 

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