Acinus hepatic

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... 

Figure 6.5 Schematic representation of a hepatic acinus. PT represents the portal tract, consisting of branches of the portal vein and hepatic artery and a bile duct. CV represents a branch of the central vein. The areas 1, 2, and 3 represent the various zones draining the terminal afferent vessel. Source Adapted from Ref. 5.

Although the classic lobular model describes the microscopic appearance of the liver well, a better way to understand how the liver functions is to subdivide it into regions of hepatocytes irrigated by a single portal tract - an area referred to as the hepatic acinus. 

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... 

The functional and microcirculatory hepatic unit forms the basis for assessing the hepatic acinus (A. M. Rappaport, 1954). (41-43) The portal vascular bundle, with the terminal branches of the hepatic artery and portal vein diverging fan-shaped after penetrating the lobules, is at the centre of the acinar structure. These vessels represent the central axis for the circular blood supply of the related liver parenchyma. This area is roughly the shape of a rhombus, the outer angles of which are formed by the two central veins of the adjacent lobules while the diagonal... 

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)...

Fig. 9-22. Hepatic acinus. Modified after L.G. Whitley from A Companion to Medical Studies . Blackwell Scientific Publications, Oxford and Edinburgh...

The hepatic lobule and hepatic acinus are relatively well accepted models to describe the structure and functional... 

Gumucio, J. J. Miller, D. L. Krauss, M. D. ZanoUi, C. C. Transport of fluorescent compounds into hepatocytes and the resultant zonal lahehng of the hepatic acinus in the rat. Gastroenterology 1981,80, 639-646. 

Han Nl, Lee YS, Choi H, Choi JY, Yun SK, Cho SH, Han JY, Yang JM, Ahn BM, Choi SW, Lee CD, Cha SB, Sun HS, Park DH (2002) PCNA expression and electron microscopic study of acinus-forming hepatocytes in chronic hepatitis B. Korean J Intern Med 17 100-106 Herve F, Urien S, Albengres E, Duche JC, TiUement IP (1994) Drug binding in plasma. A summary of recent trends in the study of drug and hormone binding. Qin Pharmacokinet 26 44-58... 

Figure 28.1. Comparison of structural liver lobule with functional acinar regions. The liver lobule is centered on the terminal hepatic venule (THV), also called the central vein, and assumes a roughly hexagonal shape with its vertices at the portal triads, which contain the portal vein (PV), hepatic artery (HA), and bile duct (BD). The liver acinus is centered upon the tract of blood vessels that branch from the hepatic artery and portal vein of the portal triads. Hepatocytes within the acinus are grouped within functional zones 1,2, and 3 located at increasing distances from the vascular tracts that interconnect adjacent portal triads.

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 zones of the acinus differ in their blood supply corresponding to the distance of the hepatic cells from the periportal field and from the terminal blood vessels, respectively. Zone 1 has the best supply of oxygen and substrates it comprises the (lobule-peripheral) parenchyma adjacent to the limiting lamella. Toxins are most damaging in zone 1. Zone 2 corresponds to the inter-... 

Fig. 2.14 Diagram of the functional (microcirculatory) liver unit ( simple acinus ) (A.M. Rappaport, 1960, 1963) terminal hepatic vein (thV), periportal field (P), zones of different blood supply A, B, C and 1, 2, 3. Zone 1 (afferent zone) zone richest in O2, nutrients and hormones. Zone 3 (efferent zone) zone poorest in O2, nutrients and hormones, but enriched with CO2 and metabolites from zones 1 and 2. (Direction of blood flow is from 1 to 3 and from A to C)...

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)...

Figure 2 Diagram illustrating the basic functional unit of the liver, the acinus, consisting of the tissue centered around the terminal branches of the hepatic artery and portal vein. The cells in zone 1 nearest these vessels receive the highest concentrations of oxygen and nutrients, while those in zones 2 and 3 are exposed to progressively depleted blood. (Reproduced from Bloom W and Fawcett DW (eds.) (1968) A Textbook of Histology, 9th edn. Philadelphia Saunders, with permission redrawn from Rappaport A et al. (1954) The Anatomical Record 11.)...

The functional anatomical unit of the hver is the acinus, adjacent to the portal triad, which consists of a branch of the portal vein, hepatic artery, and bile duct. Each acinus is a diamond-shaped mass of liver parenchyma that is supplied by a terminal branch of the portal vein and of the hepatic artery and drained by a terminal branch of the bile duct. The blood vessels radiate toward the periphery, forming sinusoids, which perfuse the liver and ultimately drain into the central (terminal) hepatic vein (Figure 47-2). The sinusoids... 

The blood supply to each acinus consists of three zones (Figure 47-3). Zone 1 is the area immediately adjacent to the portal tract and is enriched with lysosomes and mitochondria. The periphery of the acinus, zone 3, is enriched with endoplasmic reticulum, is very active meta-bolically, and has relatively low oxygen tension. Tills area is most susceptible to injury, although zone 1 appears to be involved with protecting the liver from external injury and providing a base for hepatic regeneration (by means of the oval cells of periportal bile ductules, the apparent hepatic stem cells). 

The lobes of the liver are divided into smaller lobules with a roughly hexagonal arrangement of hepatocytes around a central vein. At the vertices of the lobules are bile ducts, terminal branches of the hepatic artery, and portal veins—termed the portal triad. Connective stromal tissues extend throughout the liver, providing support for cells and routes for blood vessels, lymphatic vessels, and bile ducts. The hepatocytes form groups of cells around small branches of the portal vein, hepatic arteriole, bile duct, lymph vessel, and nerves this functional unit is called an acinus (plural acini). The acini form part of a larger structure, which can be divided into three zones ... 

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. 

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