The Microcirculatory Acinar Concept of Normal and Pathological Hepatic Structure

Beitr. Path. Bd. 157,215-243 (1976)

Review

Department of Physiology, University of Toronto

The Microcirculatory Acinar Concept of Normal and Pathological Hepatic Structure Das mikrozirkulatorisch-azinare Konzept der normalen und pathologischen Leberstruktur A. M. RAPP APORT With

12

Figures' Received October I4, I975 . Accepted December I, I975

Key words: Structural and functional liver unit - Microcirculatory zones and enzymes - Patterned lesions - Zonal lesions - "Central(?)" necrosis - "Bridging"

Introduction The idea stressed by Heidenhain (1937) that organs are composed of small units is generally accepted since the introduction of developmental physiology. In the liver such units (acini) have been observed in vivo already by Malpighi (1666), in close connection with the afferent vessels of the hepatic microcirculation. About two hundred years later the physiologist Johannes Muller (1844), described the secretory units of the liver as an "agglomeration of primitive cells that secrete bile into the ductules, the cells filling the spaces between the vascular loops of the lobule". These functional lobules organized around the supplying blood vessels and biliary channels differ from the static hexagonal lobule surrounding the efferent venule described by Kiernan (1833). He based his concept on the microscopic aspect of dead liver tissue of the pig. Unfortunately Kiernan's lobule because of its geometric regularity still serves as a frame of reference in clinical pathology when one describes a lesion as "pericentral" or "periportal". Our investigations (Rappaport et al., 1954; Rappaport, 1973) however, revealed that the hepatocytes in vivo are not organized around 15 Beitr. Path. Bd. 157

2I6

. A. M. Rappaport

the hepatic venules ("central" veins) but along the microcirculatory path that with its pressure gradient leads from the hepatic arterioles and portal venules towards the hepatic end venules. The territory of a hexagon becomes thus subdivided into smaller irregularly-shaped sectors, each of them being a different liver acinus (see also differently colored areas in Fig. 8). The hexagonal lobule is supplied by a number of arterioles and portal end-venules, each of them originating from different parent branches, thus there is no microcirculatory unity in Kiernan's hepatic unit. Its supplying vessels deliver their blood into at least two hexagonal fields and are from the viewpoint of circulatory physiology the centre of the hepatic microcirculation but not the "periphery". We will describe the structural units of the liver in accordance with the microcirculation as observed in vivo.

The in vivo Structural Units of the Liver and their Microvasculature I. Structural Units

The simple liver acinus (Fig. I) represents a microscopic parenchymal mass irregular in size and shape arranged around an axis consisting of a terminal hepatic arteriole (THA), terminal portal venule (TPV), bile ductule(s), lymph vessels and nerves which grow out together from similar preterminal structures in a small triangular portal field (Rappaport et al., 1954)· An acinus lies between two or more terminal hepatic venules (Th V), the "central" veins, with which its vascular and biliary axial channels interdigitate. The interdigitations of these terminal branches originating from three or more triangular spaces around one Th V may create a vascular pattern simulating a hexagon. This spatial relationship of afferent vessels to the efferent ones is the result of developmental ingrowth of the hepatic parenchyma with its bile ductules and supplying vessels into the omphalomesenteric venous plexus, the remnant of which are the sinusoids and hepatic veins. Interdigitation is therefore present at macroscopic level too and the polygonal areas delimited by macroscopic afferent vessels are equally devoid of structural, microcirculatory and secretory unity. Shortly, the "hexagon" is not a unit (Rappaport, 1963). A simple liver acinus is subdivided into three circulatory zones (Fig. I) which surround the axial structures like layers of a bulb. The cells in zone I are situated close to the supplying vessels; they are bathed by blood of a composition similar to that in the afferent vessels. The cells in zone 3

Structure, Microcirculation, Pathology of Liver Acini.

21 7

are the most distant from their own supplying vessels as well as from those feeding the neighbouring acini. Hence, zone 3 is situated at the microcirculatory periphery of the acinar unit and receives blood that has already exchanged gases and metabolites with cells in zones I and 2. Cells of zone 3 of several adjacent acini lie close to a Th V, their common drainage center, located at the most peripheral parts of the hepatic microcirculation; these cells form the acra of the acini. It is evident that the cells

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Fig. r. Blood supply of the simple liver acinus, zonal arrangement of cells and the microcirculatory periphery. The acinus occupies adjacent sectors of neighboring hexagonal fields. Zones I, 2 and 3, respectively represent areas supplied with blood of first, second, and third quality with regard to oxygen and nutrient contents. These zones center about the terminal afferent vascular branches, bile ductules, lymph vessels and nerves and extend into the triangular portal field from which these branches crop out. Zone 3 is the microcirculatory periphery of the acinus since its cells are as remote from their own afferent vessels as from those of adjacent acini. The perivenular area is formed by the most peripheral portions of zone 3 of several adjacent acini. In injury progressing along this zone, the dam age d area assumes the shape of a seastar (heavy crosshatching around a ThV in the center). I, 2, 3 = microcirculatory zones; r', 2', 3' = zones of neighbouring acinus; - - - - boundaries of acini; _ _ _ _ afferent vessels of acini outlining the hexagons.

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of zone 3 are most sentitive to damage through ischaemia, anoxia, congestion and nutritional deficiency (Rappaport, I 96 3; Rappaport et aI., I954)' The entire zone 3 is affected when there is a major degree of circulatory or nutritional deficiency. Zone 2 (Z2) is a transitional stretch of tissue between zone I and zone 3. With increased microcirculation zone I expands over Z2; in decreased states zone 2 becomes part of zone 3. This shifting depends mainly on the fluctuations in arteriolar activity. The simple liver acinus is also the secretory unit of the liver; the produced bile is delivered into the terminal bile ductule, part of the corresponding axial triad. The watershed of bile flow is the dividing line between two adjacent acini. The complex ilcinus (Fig. 2) is a microscopic clump of tissue composed of at least three simple acini and a sleeve of parenchyma around the preterminal portal, arterial, and biliary branches, lymph vessels and nerves giving origin to the terminal axial structure of the simple acini that constitute this larger unit. The preterminal arterial and portal branches have thus ramified in three directions. Each of the ramifications forms the axis of a simple acinus, whose cells occupy the intersinusoidal spaces. The sinusoids drain into at least two terminal hepatic venules situated between zones 3 of the acini forming the complex acinus. Such distribution of vessels is seen in vivo in the transilluminated liver of small mammals. There is in addition a distinct sleeve of parenchyma surrounding the preterminal vascular and biliary structures of the complex acinus. This sleeve of parenchyma may consist of tiny acini (acinuli) that are supplied by arterioles and by small portal venules branching off from the preterminal vessels located in the triangular portal spaces. Such small vessels have been demonstrated radiologically by Daniel and Prichard (I951). A cut through the complex acinus shown in Fig. 2 at the level of division of the preterminal vessel into its terminal branches would have to be inclined toward the left in order to lay bare in one cross section all three terminal branches cropping out from the preterminal vessels in the triangular portal space in the left half of figure I. The latter are the nutrient vessels of the entire complex acinus. The subdivision of the complex acinus into microcirculatory zones can be obtained by extending the zonal subdivision from the left simple acinus to the other acini. Zone I would appear then as a trident while zone 3 would best be simulated by three adjacent and opened umbrellas whose central sticks form the trident representing zone I. Lesions occupying such three vaulted areas can be seen in the liver damaged by a fair degree of ischemia or nutritional deficiency. The complex acini are parts of the largest microscopic acinar unit, the acinar agglomerate, consisting of at least three complex acini.

Structure, Microcirculation, Pathology of Liver Acini. 2I9

Fig. 2. Human complex acinus composed of simple acini. Three terminal portal venules branch out from a pre-terminal parent stem; each of them together with its sinusoids irrigates a simple acinus. In the left acinus the zonal arrangement of the hepatocytes situated in the intersinusoidal spaces is indicated. Zone I (Zl) harbors the tissue close to the terminal afferent vessels. Cells at the microcirculatory periphery (Z3) are more vulnerable to ischemia and dietary deficiency. Human liver injected with india ink; thick cleared section X 60 (from A. M. Rappaport, Microvasc. Res., 1973, courtesy Acad. Press Inc. N.Y.).

II. Microvasculature and the Microcirculatory Hepatic Unit

Each liver acinus is supported by a microvascular framework consisting of THA(s), TPV, a glomus of sinusoids, Th V and their nerves forming together with lymph vessels the microcirculatory unit (Rappaport, 1973). (a) The Hepatic Arterioles range from 100 ftm to 50 [tm in diameter. The THA vary in diameter between 50 ftm and 15 ftm and they still have an elastica interna and a single layer of smooth muscle cells (Burkel, 1970). Some of the arterial capillaries have a "precapillary sphincter" (Rhodin, 1967), and their non-fenestrated endothelium has a basement membrane. These arterioles and their capillaries (8-IO ftm wide) lace a dense plexus around the bile ductules (Fig. 3). Efferent arterioles and venules originate from the periductular plexus where the arterial blood pressure has been

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Fig. 3. Microcirculatory hepatic unit. The unit consists of; (a) the terminal portal venule (TPV) with the sinusoids branching off it and forming a glomus; (b) the hepatic arteriole (THA) lacing with its branches a plexus around the terminal bile ductule (BD). The arterioles empty either directly (r) or via the peribiliary plexus (2) into the TPV and sinusoids. The sinusoids run along the outside of cell plates and cords inside which are the capillaries of the secretory and excretory system of the liver. The glomus of sinusoids is drained by at least two terminal hepatic venutes (ThV); Ly = lymphatics (from A. M. Rappaport; Microvascular Research, r973, courtesy Acad. Press Inc. N.Y.).

substantially lowered by the high resistance in this arteriolar plexus; they then join the sinusoids. Some arterioles bypass the periductular plexus and empty directly with strongly pulsating jets into the TPV (Fig. 3 and 4); they have been demonstrated recently in the film "Normal Microcirculation of the Mammalian Liver" (Rappaport, 1972). All arteriolar openings are found in zone I only (Wakim and Man, 1942; Reeves et aI., 1966;

Structure, Microcirculation, Pathology of Liver Acini.

221

Kaman, 1965; Hase and Brim, 1966; McCuskey, 1967; Rappaport et al., 1966). Thus the periportal area is the site where the arterial and portal streams merge and empty into the sinusoids. (b) The Terminal Portal Venules are about 20 [lm wide; their wall does not have smooth muscle fibres, only endothelium, basement membrane and scant connective tissue. Consequently at the site of the origin of the sinusoids there are no muscular inlet sphincters, only large endothelial cells which by swelling or shrinking can modify inflow of blood into the sinusoids, (McCuskey, 1966). ( c) The Sinusoids, the venous capillaries of the liver, have an average width of 14 [lm, but they can widen and permit the simultaneous passage of 3-4 erythrocytes. The length of the sinusoid varies with the species and is 250 [lm in the rat. The endothelial lining of the sinusoids is thin and fenestrated and permits an easy passage of the plasma fluid into and from the Disse space surrounding the sinusoids and continuing into similar space around the TPV and Th V. Loss of fenestration and permeability transforms the sinusoids into capillaries similar to those seen in other organs (Schaffner and Popper, 1963). (d) The Terminal Hepatic Venule is at the TERMINAL of the hepatic microcirculation and it is misleading to call this peripheral venule a "central" vein. The wall structure of the Th V is similar to that of the TPV but has fewer vascular pericytes and reticular fibres. Its endothelial lining has a basement membrane. III. Metabolic Zones of the Acini

Different arterial irrigation and p02 in zone 1 and 3 create microenvironments suitable for specific enzymic activities (Fig. 4). From the data collected in the literature on histochemistry and enzymology and tabulated in figure 4 we conclude that in the liver acinus there is metabolic organization in close connection with the direction of blood flow. As indicated by the high levels of UDP-glucose: a-4 glucosyltransferase (UDPGGT), phosphorylase and glucose-6-phosphatase activities, the cells in zone 1 are geared to glycogen synthesis and glycogenolysis (Sasse, 1969a; Sasse and Kohler, 1969). These cells contain numerous long mitochondria and here the oxidative processes operate at a high level via the Krebs cycle; here too the activity of respiratory enzymes such as succinic dehydrogenase and cytochrome oxidase is increased. The abundance of lysosomes rich in acid phosphatase facilitates a higher rate of pinocytosis and uptake of materials from the nutrient-laden portal blood. Le Bouton (1968) has demonstrated that zone I is the prime area of protein metabolism and

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Structure, Microcirculation, Pathology of Liver Acini· 223

formation of plasma proteins. He established that length and width of the active area coincides with the extent of zone 1. Zone 3 is the site of NAD and NADH tetrazolium reductase activity, of glycogen storage, of fat and pigment formation. With increased fat formation the lipid laden cells are seen also at the perivenular (Th V) site where zones 3 of several adjacent acini adjoin. The cells in zone 3 are rich in microsomes involved in drug metabolism. Enzymatic specificity and metabolic heterogeneity of cells in different circulatory zones must be implicated in the selective toxic injury of cells in different parts of the liver acinus (Stoner, 1956; Wilson, 1958). The difference in susceptibility to and degree of damage by anoxia or malnutrition in diverse zones enabled us to delimit these zones before their enzymic pattern was established (Rappaport et al., 1954). However, the outlined enzymic activities should not be regarded as a fixed map. The hepatic cells are capable of multiple metabolic functions; pathologic changes in structure and microcirculation may cause an enzymic shift from one acinar zone to another (Eger, 1961; Sasse and Kohler, 1969; Seawright and Hrdlicka, 1972). The zonal distribution of drug induced toxic hepatic lesions is due to the location of enzymes involved in the metabolism of the offending substance. Glucuronization of some drugs proceeds at a more efficient pace in zone I, but the microsomal biotransformation and detoxication of other drugs occurs in zone 3. Ohnhaus et al. (1971) have reported that in rats hepatic blood flow increases by an average of 100% during the 4th day

Fig. 4. Metabolic areas in the acini. This acinus can be considered as corresponding to the one on the right-hand side of the complex acinus (Fig. 2). Specific enzymic activities indicate predominant metabolic functions in each of the microcirculatory zones of the acinus. The references in brackets indicate corresponding source. (a) = (Burstone, 1959); (b) = (Eger, 1961; Klein, Widmer and Grossman, 1952); (c) = (Novikoff and Essner, 1960); (d and e) = (Novikoff, Hausman and Podber, 1958); (f) = (Padykula and Herman, 1955); (g) = (Rutenberg and Seligman, 1955); (h) = (Schepers, 1961); (i) = (Wachstein, 1959); (j) = (Schumacher, 1957); (k) = (Greenberger, Cohen and Iselbacher, 1965); (1) = (Isselbacher and Jones, 1964); (m) = (Sasse, 1969; Sasse and Kohler, 1969); (n) = (Hayashi, 1964); (0) = (Albert, Orlowska, Orlowski and Szewczuk, 1964); (p) = (Mizutani, 1968); (q) = (Wachstein, Meisel and Falcon, 1961); (r) = (Johnson, 1967); (s) = (Balogh, 1966); (t) = (Reith, Schuler und Yogell, 1968) ; (u) = (Grisham, 1960); (v) = (Pette and Brandau, 1966); (w) = (Nolte and Pette, 1972); (x) = (Swick, Tollaksen, Nance and Thomson, 1970); (y) = (Shank, Morrison, Cheng, Karl and Schwartz, 1959). P.Y. = portal vein; ThY = terminal hepatic venule; BD = bile ductule; h ep. art. = hepatic arteriole; 2 1 = periportal area; 23 = mic rocirculatory periphery (from Rappaport, A. M., Anatomic Consideration. Chap. r. In: L. Schiff (ed.), Diseases of the Liver. Lippincott, Philadelphia 1974).

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of enzymic induction with phenobarbitone; the increase in blood flow parallels the augmentation of the microsomal mass. Also, the rate of blood flow determines the quantity of substance brought to the cells to be metabolized. In a dog with an ischemic liver the detoxication of thiopental is delayed. If the ischemic liver is perfused in situ with additional blood from the dog's own aorta the rate of detoxication of thiopental is increased (Rappaport et aI., 1956). These findings should remind us of the intimate connection between microcirculation and cellular function. Toxic injury to the cells is often aggravated by the associated damage to the microcirculation (Himsworth, 1948), and the rapid spread of injury has been demonstrated by in vivo studies and film recordings of the pathologic microcirculation induced with Fulvine (Rappaport et aI., 1969) and with Paracetamol, (Rappaport and Macdonald, 1975). Thus, the histology based on the orientation of the parenchyma around the nutrient vessels and bile ductules facilitates the understanding of the chemomorphology of the acinus.

IV. Pathophysiology of the Liver Acini In the liver acini the afferent vessels, particularly the arterioles, are the organizing principle of the hepatic parenchyma, and the microcirculatory zones with their different tissue p02 and enzymic activity are bound to display different susceptibility to damage. The patterns of hepatic lesions (Rappaport and Hiraki, 1958) will be contingent upon the action of the injurious agent as such or, after its biotransformation, on the parenchymal cells. Thus the site of the lesions will depend to a large degree on the circulatory pathways and will be either in zone I, i.e. "periportal" or at the microcirculatory periphery of the acinus, particularly at its acra, i.e. the peri venular portions of zone 3 (Fig. I). The view of a hexagonal lobule as the smallest liver unit organized around a ThV ("central vein") offers to the pathologist only 2 or 3 landmarks for the orientation of lesions he observes. These are: "central veins", "portal periphery", and "midzone". The acinar concept to the contrary conveys a number of orienting lines and patterns in accordance with normal or impaired function of the hepatic acini and their 3-dimensionally arranged flow patterns. There are six dynamic double supply lines (Fig. I, heavy broken lines) at the periphery of a regular hexagonal field by which adequate or deficient nutrients are delivered to the parenchyma, and an additional channel through which the produced bile is carried away. Therefore in liver biopsies it is easier to orient the tissue by studying first the parenchyma in the

Structure, Microcirculation, Pathology of Liver Acini . 225

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Fig. 5. (A) Hepatic lesions limited to single acini. Crosshatched area I = "paracentral" necrosis revealed by a tangential cut of a lesion in the peripheral part, zone 3, of an acinus. 2 = transverse section of an entirely diseased acinus close to its axial vessels; the injury extends therefore into twO sectors of neighboring hexagonal fields and reaches their ThV. (B) Perivenular ("pericentral") necrosis fatty or fibrous change result from mild damage to the parts of Z3 most remote from the terminal afferent vessels of the complex acini occupying parts of at least three adjacent hexagonal fields. The lesion around the ThV assumed a triangular shape. The triangularity is due to the extension of the injury that has crept along zone 3, it joins up with similar lesions around neighboring acini and leads finally to the linkage of Th V to Th V. The uniform parenchyma has been reduced to disjoined clumps of tissue, the size of complex acini. (C) When the damage progressed peri venular lesions become periacinar. They developed in zones 3 and 2, have extended close to the portal spaces containing the pre-terminal afferent vessels. Small clumps of parenchyma have been isolated; they are surrounding the terminal afferent vessels as remnants of zone I of simple acini . The arrows indicate the path of the advancing injury bridging the PS to the Th V and breaking through the site where the capillarized .axial structures of neighboring acini about •. The lesion also bridges Th V to Th V and the perivascular regions; the bands of injury curve and return to the initial portal fields. Z1, Z2, Z3 = circulatory zones; PS = portal space; ThY = terminal hepatic venule (modified after Rappaport and Hiraki, Acta Anatom. 32, 126, Karger S. A., Basel 1958).

226 . A. M. Rappaport

vicinity of these lines in the smallest portal triads, as in any slide the triads are more numerous than the Th V's. Noxious agents causing pathological changes are brought to the parenchyma of the acini by the triadal pathways; these changes include biliary obstruction and ascending infections in the ductules and in the lymph vessels. Lesions may therefore be present in some acini participating in the formation of an hexagonal field, while adjacent acini in the same field may be unaffected. The injury can completely wipe out the parenchyma or may affect only cellular enzyme systems in certain zones of the simple acini and complex acini, Thus, various patterns of lesions will result and reveal the shape of parts or of whole acini. The recognition of these patterns is made easier by always keeping in mind the fact that the tissue contained within one hexagon has no unity. A hexagonal area is composed of unequal portions of several acini that happened to be exposed by the random histological section. Lesion I in figure 5 A exemplifies a cut through the damaged tip of an acinus, i.e. the part contained within a hexagon. Lesion 2 is centered around the terminal afferent vessels, that interdigitate with two Th V's. It represents the cross section of a liver acinus entirely diseased, and can be seen in phosphorous poisoning or in fatty change due to severe starvation (Rappaport and Hiraki, I 958). Figure 5 B shows a lesion affecting only the most peripheral portions of zone 3 in complex acini. The perivenular lesion occupies a triangular area around the Th V but it already disrupts the contiguity between the complex acini by stellate projections extending along zone 3. However, the simple acini within the damaged complex acini are still in close microcirculatory contact with each other, and function together. Such lesions can be caused by a fair degree of ischemia (Rappaport et aI., I 9 54a, b; see also Fig. 7), hepatitis (Desmet and De Groote, I974) fatty change (Best et aI., I 9 55; Sellers and You, I 9 5 I), carbon tetrachloride intoxication (Cameron and Karunaratne, I936; Himsworth, I948), in early dietary cirrhosis (Hartroft, I 9 53; Hoffbauer, I 9 59), sclerosing hyaline necrosis (Edmondson et aI., I967) alcoholic cirrhosis (Lischner et aI., I 97I; Galambos, I 972) and veno-occlusive disease due to senecio or Fulvine poisoning (Bras and Hill, I956). Such lesions disjoining the complex acini at their periphery are called by pathologists "bridging". But these bridges of injury are only descriptively interconnecting the Th V's, anatomically however, the injury has reduced the number of microcirculatory pathways that allowed intercommunication between neighbouring hepatic venules. With progression of the damage within the complex acini the normally invisible peripheries of the simple acini become conspicuous through the

pathological change. This happens most commonly in congestive heart failure (Fig. 6) ischemic necrosis, acute viral hepatitis, (Desmet et aI., I 972;

Structure, Microcirculation, Pathology of Liver Acini.

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Fig. 6. Liver of a patient who died of myocardial infarction. A seastar-shaped area in the center of the figure is formed by severe congestion and necrosis in zone 3 of adjacent acini. One can easily recognize the remnants of 4 simple acini with the portal field in their center (courtesy Dr. S. Ritchie, Dept. of Pathology, TGH, University of Toronto).

Desmet and De Groote, 1974; Weinbren, 1966) fatty or fibrous damage. The bands of injury form a perivenular seastar-like pattern (Th V left center, Fig. 5 C) by extending towards the portal triad from which the afferent vessels of the affected acinus have originated. The bands have also extended along zone 3 of the simple acinus to the site where the tips of the nutrient vessels of serveral neighbouring acini have dwindled down to capillaries (Fig. 5 C, e), the so-called "nodal" points (Mall, 1906). From this point the band of damage curves along zone 3 and returns to the original portal triad. The surviving parts of the acini (zone I and 2) remain thus enclaved in a layer of damaged tissue, which in the repair process may become replaced by a densely woven scar. According to recent views these scars were not due to reticular collapse but are the result of new formation of fibres (Popper and U denfried, 1970). This is most frequently the case in alcoholic hepatitis with cirrhosis. In the sclerosed acini the scar is acting as a peri acinar dam to the outflow of blood into the systemic veins (see Fig. 12). Any dam, however, raises the level upstream and so the post-

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Fig. 7. (A) Hepatic damage affecting simple acini (r), complex acini (2) and an acinar agglomerate (3). The lesion is most severe in the right half of the diagram, where the simple acini have lost most of their parenchyma and have been reduced to small "periportal" rims of tissue (r) around the nutrient vessles. The injury is less in the center of the figure where complex acini (2) have survived. However, they are already separated from each other and from the bulk of a well preserved acinar agglomerate (3) by strands of damaged tissue (broken line) linking the portal spaces (PS) to terminal hepatic venules (ThV) situated at the microcirculatory periphery of this acinar agglornerate.

(B) Regeneration of the acinar remnants. Regeneration and hyperplasia, starting from the surviving remnants of the acini have created a nodular pattern of the hepatic paren-

Structure, Microcirculation, Pathology of Liver Acini . 229

sinusoidal barriers raise the pressure level in the portal stream. Portal hypertension will ensue. A closer view of the histogenesis of the various forms of cirrhosis is facilitated by figure 7. Periportal rims, islets or clumps of tissu surviving in ischemic necrosis, CC1 4 poisoning, chronic hepatitis (Schmid, I966) the fatty liver of the alcoholic and experimental cirrhosis (Best et aI., I 955) are found to be oriented around the terminal (Fig. 7 A, I) and preterminal (Fig. 7 A, 2) afferent axial vessels and bile ducts. Conversely the Th V ("central veins") have become isolated, "divorced" from the portal afferents by bands of damaged tissue. The same figure 7 A, 2 depicts complex acini, with their centre of surviving tissue around the preterminal vessels united and the vaulted areas of damage at their periphery. A band of injured tissue (the broken line in figure 7 A) connecting several hepatic venules to each other and to portal fields may arch over many hexagonal fields to surround clumps of tissue that represent the cross sections of acinar agglomerates (Fig. 7 A, 3). Within this area the hepatic parenchyma may show slight damage without a pattern because the pathological change was not yet severe enough to injure the microcirculatory peripheries of the complex or simple acini. Such relationship between damaged and intact clumps of tissue is not always visible in a single section but can be disclosed on serial sectionmg. When the damaged tissue is replaced by fibrous bands they will crisscross the parenchyma annihilating completely the design of hexagons resulting from a regular interdigitation between afferent and efferent vessels. Instead of polygonal figures, a nodular pattern becomes conspicuous in which parenchymal clumps of various sizes are centered around preterminal (Fig. 7 B, 2) or terminal (Fig. 7 B, I) afferent vessels. This "pseudolobulation" represents in fact the regenerated parenchymal cores of the damaged acini, the "pseudolobules" differing in size and shape according

chyma. The mono acinar nodules (1) developed from the remnants of simple acini can easily be distinguished from small nodules (2) formed out of surviving remnants of complex acini. The large node (3) is the result of hyperplasia in a preserved acinar agglomerate. Note that the hepatic veins are in their normal position, i.e. at the periphery of the acini and their regenerated remnants. PS = portal space, HV = hepatic vein, ThV = terminal hepatic venule (from Rappaport and Hiraki, Acta Anatom. 32, 126, Karger S. A., Basel 1958).

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to their original anatomical order. Th V's are conspicuous at the periphery of the nodules, but these venules have not changed the position they always occupy normally, i.e. at the periphery of the acinar clumps.

Discussion The classical concept of a hexagonal liver lobule is based on a twodimensional visual impression of 6 pairs of horizontally running efferent vessels and bile ductules surrounding a Th V. This concept resulted from the study of thin tissue sections under the light microscope. Clinical pathologists basing their diagnosis primarily on such tissue sections, find it hard to dispense with the hexagonal diagram. However, tissues are tridimensional structures and stereometry is being used for their study with increasing frequency (Weibel, 1963). Similarly, the need for a tridimensional visualization for the two-dimensional electronmicroscopic data has lead to development of the scanning electron microscopy and the technique of fractured parenchymal tissue that provide visual depth in the spatial arrangement of cells and their organelles. It is useful to remember that the tissue within a hexagon represents only a two dimensional aspect of tridimensional structures exposed by one plane of section and is composed of cells belonging primarily to halves

Fig. 8. The tissue comprised within the hexagon has neither structural nor microcirculatory unity; it is composed of parts of different simple acini marked in colors. The cells adjacent to the terminal hepatic venule (Th V) are most distant from the afferent vessels of their acini. These perivenular hepatocytes are all part of zone 3 and thus susceptible to the lesions indicated.

Structure, Microcirculation, Pathology of Liver Acini .

231

of several simple acini (indicated with different colors in Fig. 8). Each of them may receive blood from different gastrointestinal venous sources as far apart from each other as stomach from large bowel (Rappaport, 1963). The structural and microcirculatory uniformity of normal hepatic tissue is only apparent. When zones 3 of the acini taking part in the formation of a hexagon are affected by pathological processes, the non-unity of the tissue becomes evident. The selective damage of cells has already been pointed out as due to their specific enzymic machinery (Seawright and Hrdlicka, 1972) and its close connection with the microcirculatory environment. However, one finds frequently quoted experiments that deny the dependence of regional enzymic activity on the microcirculatory flow pattern. These deductions have been made from experimental "reversal of intrahepatic blood flow" in dogs to prove that the enzymic topography is independent of the direction of intra-acinar blood flow. They are not valid. The " reversed liver" (Child, 1959) does not receive portal blood at all, and the venous (caval) blood flows in a direction opposing the arterial stream. Severe hepatic congestion and ascites occurred in the very few dogs surviving such an experiment. In another preparation, the neck transplanted liver lobe (Sigel, 1968) was supplied by arterial blood only via the hepatic venous system; the blood did not pass through the arteriolar gates before reaching the sinusoids. In addition, biliary secretion was totally blocked through ligation of the bile ducts. One cannot accept such severely damaged preparations for the study of the effects of reversal of normal blood flow in the liver. In the discussion of some common descriptive terminology in hepatic pathology we have to mention "central (pericentral) necrosis"; it is the most durable descriptive patho-anatomical term in liver disease and a confusing misnomer, since the pathophysiology of the lesion indicates its location at the microcirculatory periphery of the tissues. These facts are clearly exemplified in figure 9, the autopsy slide from the liver of a young man who died after resuscitation from cardiac arrest. One notices that the orientation of the necrotic areas is not based on the "central veins", the Th V's, but on the supplying vessels in the intact zone 1 of the acini (I-IV) participating in the formation of the imaginary hexagonal field. It is evident that rows of cellular plates peripheral to zone 1 have broken down and melded to form the necrotic patch (right half of Fig. 9). The patch is neither "central" nor "pericentral" but extends with irregular projections into zone I. Also, the foundered cell plates of zone 3 in the longitudinally sectioned acinus I and in the cross-sectioned acinus II interconnect the necrotic patch (A) with a neighbouring necrotic patch (B) above it. A similar connection runs between zone 1 of acini II and III. 16 Beitr. Path. Bd. 157

232 . A. M. Rappaport

Fig. 9. Liver necrosis due to circulatory failure after resuscitation from cardiac arrest. The extent of damage is different in the acini I-IV indicating the lack of structural unity within the hexagonal field. However, all surviving cells are in zone I of the acini surrounding a Th V. Acini I-III are separated from each other by "bridges" of necrosis, acini III + IV are still interconnected by a few liver cords. For further explanation see text. A, B = necrotic areas; Th V = terminal hepatic venule; PS = portal field ( X 50 approx., courtesty Dr. A. Medline, Dept. of Pathology, T.W.H., Toronto).

These interconnections, the so called bridges, will be discussed later. Thus "central necrosis" is not central but within the terminal of the hepatic microcirculation, and here the Th V's are situated. Necrosis as well as other lesions around the Th V's are: a) Perivenular, i.e., a narrow ring of injured cells next to the Th V; it represents the mildest form of damage to zone 3. The affected cells are located at the acra of this zone. (b) Triangular, i.e. a patch of greater injury with the ThV in its center and extending its processes to neighbouring Th V's, thus occupying the microcirculatory periphery of a complex acinus (see Fig. 5 B). (c) Seastar-shaped lesions affecting zone 3 of the simple acini that are part of a hexagonal area. The "seastar" has some regularity when only a few cellular rows of zone 3 are equally damaged in the simple acini surrounding a Th V (see Fig. 5 C and Fig. 6).

Structure, Microcirculation, Pathology of Liver Acini. 233

(d) Eccentric to the ThV ("central vein"), when some acini on one side of a Th V lose all zone 3 and the damage encroaches even on their zone 2 and I while the other acini show only slight damage. There is no explanation for the eccentric injury within the hexagonal lobular concept. (e) "Bridging" becomes conspicuous when lesions affect irregularly zone 3 of the acini around a Th V. The patch of necrosis containing the Th V becomes broadened, the seastar-shape of the lesion can barely be recognized (see Fig. 9) and the interconnections with neighbouring necrotic patches and their hepatic and portal venules stand out. Boyer and Klatzkin (1970) in a fine clinical study of subacute hepatitis have demonstrated statistically the grave prognostic significance of such "bridging" between two Th V's in neighbouring hexagonal fields, or between the Th V's and portal triads. The reason for the continued deterioration of liver function in these patients is now evident from the pathophysiology of the hepatic microcirculation. The circulatory intercommunication between the indivi-

Fig. 10. Liver of a patient with hepatitis who recovered from COilla. The triangular necrotic lesion around the Th V is due to the confluence of necrosis in zone 3 of the complex acini I, II, III. The arrows indicate the necrotic "bridges" that join the triangular necrotic patch with similar lesions around neighbouring terminal hepatic venules (HE; X 150 approx., courtesy Prof. Dr. V. Desmet, Pathologische Ontleedkunde, Univ. of Leuven, Belgium).

234 . A. M. Rappaport

dual simple acini is compromised; each acinus can rely only on the capacity of it's cells to resist further damage; it is limited to the supply by it's own vessels as collateral flow has been curtailed by gaps in the microvascular networks (Fig. I I b). The situation is not unlike that of a fighting unit split up by an invading enemy into small encircled yet still resisting groups with little chance of forces reuniting and thereby doomed to final destruction. The lesions shown in figure 10 is that of a patient with acute hepatitis and coma; it is commonly described as "central necrosis". However, a closer look demonstrates the triangular outline of the damaged areas around the Th V. This indicates to us a lesion occupying zone 3 of the neighbouring complex acini (see Fig. 5 B). Within the complex acinus there is still good microcirculatory intercommunication of the simple acini constituting the complex acinus. This patient recovered from coma and survived: there was more chance for recovery in the large microscopic clumps than in the tissue shown in figure I I b.

Fig. I I a. Liver of patient with serum hepatitis who died in hepatic coma. Note the seastar-shaped necrotic lesion around the Th V. This shape is produced by necrosis creeping along zones 3 + 2 of the simple acini I, II, III, IV and intercalating between them to reach the portal spaces. PS = portal space; ThV = terminal hepatic venule (HE; X 150 approx., courtesy Prof. Dr. V. Desmet, Pathologische Ontleedkunde, Leuven, Belgium).

Structure, Microcirculation, Pathology of Liver Acini . 235

Fig. I I b. Fatal serum hepatitis (same autopsy as fig. I I a) . The silver staining shows the star-shaped empty reticulum networks separating the parenchymal remnants (black) of the simple acini. Here the necrotic bridges reveal themselves as gaps in the microcirculatory interconnection of the acini (silver impregnation of reticulum fibres; X 150 approx., courtesy of Prof. Dr. V. Desmet, Pathologisdte Ontleedkunde, Leuven, Belgium).

The liver biopsy (Fig. II a) from a laboratory technician infected with serum hepatitis in a dialysis unit shows severe necrosis of zone 3 of the simple acini forming a complex acinus. The seastar-shaped lesions were caused by necrosis extending along the microcirculatory periphery and reaching the portal fields (see also Fig. I). In these simple acini the necrotic "bridges" have joined ThV to ThV and to TPV and have become periacinar. However, the general term "bridging" (Dzau et aI., 1974) when viewed from the pathophysiology of the microcirculation is a misnomer. It leads us astray from the fact that these "bridges" have created gaps in the normal continuity of the parenchyma and sinusoids of the simple acini (Fig. I I b). The blood supply is thereby diminished and hence the power of the hepatocytes to resist the onslaught of the hepatitis virus. However, the descriptive term "bridges" may be maintained if one keeps in mind that they are lanes of destruction of tissue, be they hemorrhagic, necrotic, fatty or fibrous and that the microcirculatory channels are deranged. "Bridges" between small portal fields are the result of severe mesenchymal reactions that interconnect TPV to TPV; when cut longitudi-

23 6 . A. M. Rappaport

nally they result in a conspicuous hexagonal design. However, the lesions and their fibrotic aftermath may break through the limiting plates and crisscross the acini with fine septa. It is easiest to understand the morphology of portal hypertension when it is considered from the viewpoint of the microcirculation. In alcoholic cirrhosis the surviving parts (zone 1 2) of the acini have become enclaved in the scar tissue that has replaced the destroyed cells of zone 3. The acinar remnants are surrounded by densely woven fibres that obstruct many sinusoids and their collecting venules. Other sinusoids that still maintain some lumen become permanently constricted. Being embedded in scar tissue, their wall looses the elasticity and capacity to adapt to variation in volume flow. A post-sinusoidal periacinar dam has been practically erected at the periphery of the microcirculation Any dam, however, raises the level upstream, therefore the pressure level in the portal stem and its venous affluents rises. One may get the impression that the liver of the chronic imbiber tries hard to stave off the repeated flooding of the systemic veins with alcohol by this fibrous dam. However, this attempt is in vain as the decrease in number of the sinusoids in the perivenular area affects zone I, its vascular pathways and collaterals. There are a few open sinusoids left that lead directly from PV to Th V; portal blood will therefore use circuitous paths and connect with some of the periacinar venous plexuses formed from remnants of sinusoidal channels. The plexuses meander upwards or downwards along the cirrhotic nodules to finally join some accessible hepatic venule. In these plexuses there are also some direct porta-caval anastomoses due to either widened pre-formed or newly formed channels. Thus it is the raised hindrance

+

factor

~ in r

Poiseuilles formula of resistance (R =

8~)

m

that is the main

cause of portal hypertension (Rappaport et aI., 1970). The length of the vascular path (1) has increased in its circuitous course, and the radius of the microvessels "r" has been narrowed. This rule applies to the post-sinusoidal as well as to the sinusoidal form of portal hypertension (Rappaport et aI., 1970; Kluge et aI, 1970; Wada et aI., 1974; Reynolds, 1969). In the latter, edema, fatty or fibrotic change of the adjacent cells narrows the lumen of the sinusoids. The morphology of the pre-sinusoidal hypertension is more complex as the factor of blood flow (F) in the formula P = F X R has to be taken into consideration. In the pre-sinusoidal area, i.e. periportal region, lies the junction of the terminal arterial vessels with the sinusoids (Fig. 12 a) and TPV's (Fig. 12 b, c). The intermittent activity of the arterioles may have been abolished by the pathological processes (reticulo-endothelial hyper-

Structure, Microcirculation, Pathology of Liver Acini. 237

Fig. 12. Peri venular fibrous dam causing sinusoidal portal hypertension. The number of sinusoidal openings into the Th V has been greatly reduced through fibrous obliteration. Sinusoidal blood under arteriolar pressure bypasses the obstacles via a porta-caval shunt (PCA) that collects the blood from venules, part of a venous plexus that has formed around the nodules. When the arteriolar activity supervenes in the microcirculation some arterial blood may bypass the sinusoids and empty directly into the portal venules (b, c) thereby reversing the direction of blood flow in them. PCA = portacaval shunt; P.V. = portal venule; A = arterial capillaries emptying into sinusoids; b, c = arteriolarportal junctions (Rappaport, A. M. "Alcoholic Liver Pathology", courtesy Addiction Research Foundation of Ontario, 1975).

plastic reactions, increased lymphopoesis) and arterial blood under high pressure may continuously gush into the portal system. On the other hand, infiltrating and scarring processes at the origin of the sinusoids and in the portal venules (Aufses et a1., 1959; Mikkelsen et a1., 1965) may increase the resistance to inflow of portal blood into the sinusoidal bed and thus raise the portal pressure. To sum up our discussion we may stress the following salient points. The architecture of the liver has been described in connection with the microcirculation as it presents itself in vivo. The smallest structural unit is the simple liver acinus, a microscopic clump of tissue that contains the microcirculatory hepatic unit and is subdivided into 3 microcirculatory zones. The hepatic arterioles empty exclusively into the periportal area (zone I). The raised pressure gradient from zone I to zone 3 in the perivenular

238 . A. M. Rappaport

area and the pulsatory discharge of arteriolar blood (Rappaport, film I972) are the determinants of the intra-hepatic blood flow. At the same time the existing p02 gradient towards the microcirculatory periphery (zone 3), is accentuated. The p02 difference in the parenchyma close to and remote from the afferent vessels creates diverse microenvironments suitable for specific enzymic activities in the respective microcirculatory zones. Thus the hepatic cells display zonal metabolic heterogeneity and different susceptibility to damage both dependant on the microcirculation. Microcirculatory zones can also be recognized in the next higher microscopic structural unit consisting of three simple acini, i.e. the complex acinus. Orientation in hepatic histopathology has been presented based on the pathophysiology of the liver acini. Tridimensional orientation and primary concentration upon zone I, the periportal area, has been stressed in the study of a liver slide, as it is the center of microcirculatory activity sustaining cellular life and function. The high pOz in zone I, the rich mesenchymal elements capable of immune- and inflammatory reactions and the prevalence of certain enzymic activities are the main elements determining the hepato-cellular reactions to injury. A zonal distribution of damage can be detected more readily in the various patterns of liver pathology if attention is paid to the acinar structure of the liver. The descriptive term "central necrosis", "bridging", "pseudolobulation" and the morphology of sinusoidal, pre- and post-sinusoidal portal hypertension have been explained by pathophysiological events in zone 3. Lesions around the Th V ("pericentral") have been classified as being either periventtlar, eccentric, triangular, or seastar-shaped and the diagnostic significance of these zone 3-lesions has been indicated. New descriptive patterns and terminology of lesions continue to appear in the literature and in them the distance of the damage from the Th V ('central vein') is made the prime point of orientation and diagnosis. This leads to arbitrary, endless grouping and subgrouping into pathological entities which have only a short lived existence, as they lack a pathophysiological foundation. Such textbook-orientation is remote from taking into account the factor vital to the liver as to any other organ: the microcirculation.

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24 I

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