Histochemical studies on metabolic zonation of the liver in the trout (Salmo gairdneri)

Unveiling the power of microenvironment in liver regeneration: an in-depth overview

The liver serves as a vital regulatory hub for various physiological processes, including sugar, protein, and fat metabolism, coagulation regulation, immune system maintenance, hormone inactivation, urea metabolism, and water-electrolyte acid-base balance control. These functions rely on coordinated communication among different liver cell types, particularly within the liver's fundamental hepatic lobular structure. In the early stages of liver development, diverse liver cells differentiate from stem cells in a carefully orchestrated manner. Despite its susceptibility to damage, the liver possesses a remarkable regenerative capacity, with the hepatic lobule serving as a secure environment for cell division and proliferation during liver regeneration. This regenerative process depends on a complex microenvironment, involving liver resident cells, circulating cells, secreted cytokines, extracellular matrix, and biological forces. While hepatocytes proliferate under varying injury conditions, their sources may vary. It is well-established that hepatocytes with regenerative potential are distributed throughout the hepatic lobules. However, a comprehensive spatiotemporal model of liver regeneration remains elusive, despite recent advancements in genomics, lineage tracing, and microscopic imaging. This review summarizes the spatial distribution of cell gene expression within the regenerative microenvironment and its impact on liver regeneration patterns. It offers valuable insights into understanding the complex process of liver regeneration.

Comparative study on a novel lobule structure of the zebrafish liver and that of the mammalian liver

The mammalian liver has a lobule structure with a portal triad consisting of the portal vein, hepatic artery, and bile duct, which exhibits zonal gene expression, whereas those of teleosts do not have a portal triad. It remains to be demonstrated what kind of the unit structures they have, including their gene expression patterns. The aims of the present study were to demonstrate the unit structure of the teleost liver and discuss it in terms of evolution and adaptation in vertebrates and the use of teleosts as an alternative model for human disease. The zebrafish liver was examined as a representative of teleosts with respect to its morphological architecture and gene expression. A novel, polygonal lobule structure was detected in the zebrafish liver. In it, portal veins and central veins were distributed at the periphery and center, respectively. Sinusoids connected both veins. Anxa4-positive preductules were incorporated into the tubular lumen of two rows of hepatocytes in sections. Intrahepatic bile ducts resided randomly in the liver lobule. Zebrafish livers did not have zonal gene expression for metabolic pathways examined. The lobules of the zebrafish liver with preductules located in the tubular lumina of hepatocytes may resemble the oval cell reaction of injured livers of mammals and might convey bile to the intestine more safely than mammalian livers. The gene expression pattern in liver lobules and our liver lobule model of the zebrafish may be important to discuss data obtained in experiments using this animal as an alternative model for human disease.

Daily rhythms in the morphometric parameters of hepatocytes and intestine of the European sea bass (Dicentrarchus labrax): influence of feeding time and hepatic zonation

The digestive system presents daily rhythms at both physiological and histological levels. Although cell morphology rhythms in mammals have been reported, they have scarcely been investigated in fish. The aim of the present research was to investigate the existence of daily rhythms in the morphology of cells in the liver and intestine of a teleost fish, the European sea bass (Dicentrarchus labrax), and how feeding time influences them. Regarding liver, we also focused on differences between the two metabolic zones: perivenous and periportal. For this purpose, fish were divided into two groups: fish fed once a day in the mid-light phase (ML) or the mid-dark phase (MD). After 1 month under each feeding regime, liver and intestine samples were collected every 4 h during a 24-h cycle, and different parameters were studied by light microscopy and image analysis. Daily rhythms occurred in most of the parameters evaluated in the liver. The effect of feeding time depended on the metabolic zone: the rhythms in the periportal zone were synchronized mainly by the light/dark cycle regardless of feeding time, whereas in the perivenous zone, rhythms were influenced more by feeding time. In the intestine, a daily rhythm in villi height was found with acrophases coinciding with feeding time in each group. These findings show for the first time the existence of cellular morphological rhythms in fish liver and intestine, and highlight the interactions between light and feeding cycles in the different metabolic zones of the liver.

NMR-based metabolomics reveals compartmental metabolic heterogeneity in liver of Arctic char (Salvelinus alpinus)

Metabolomics involves systematic study of low-molecular-mass metabolites in cells, tissues, or biofluids and is nowadays widely applied to characterize the physiological status of aquatic organisms under a set of conditions, such as disease and toxin exposure. Liver, an important metabolic center in the fish body, is often used for metabolomics analysis. Compared with the whole fish liver, the proportion of liver sample needed for metabolomics analysis is relatively small. The homogeneity of metabolites in liver is thus an important issue, especially for comparative studies and biomarker discovery. This study examined the homogeneity of the metabolic profile in liver of Arctic char (Salvelinus alpinus (L., 1758)) using a NMR-based metabolomics approach. For the analysis, whole liver samples were cut into four parts along the direction of gall bladder and at right angles to this, and metabolites in each part of the liver were extracted and analyzed by multivariate and univariate data analyses. Although the multivariate model was not significant due to variation within the data, the metabolic differences in polar portion of liver extract between the parts were seen, indicating non-homogeneity of Arctic char liver. Therefore, when sampling fish liver for further metabolomics studies, this heterogeneity should be taken into consideration.

A Brief Overview of Nonneoplastic Hepatic Toxicity in Fish

Biochemical assays are not routinely used to assess liver damage in fish, therefore, a histopathological evaluation is usually required to determine the existence or extent of nonneoplastic liver toxicity. Many mammalian pathologists may be uncomfortable when requested to identify and interpret subtle liver changes in these unfamiliar animals. It may be reassuring to note that there are more similarities than differences between fish and mammals in terms of their macro- and microanatomy, physiological and biochemical characteristics, and pathologic responses to hepatotoxic substances. This brief overview addresses several topics pertaining to hepatotoxicity in fish, including: anatomic considerations, that is, how the anatomy of the fish liver may be predictive of its metabolic capacity, and also its microscopic appearance, following exposure to toxins; physiologic considerations, including comparisons between mammalian and fish livers regarding the uptake, elimination, toxification, or detoxification of xenobiotic compounds; morphologic responses to toxicity, in which some of the general types of findings that are most commonly observed in cases or studies of fish hepatotoxicity are highlighted; and last, responses of the fish liver to specific hepatotoxins.

Morphologic study of the liver of lambari (Astyanax altiparanae) with emphasis on the distribution of cytokeratin

Studies on the morphology of the liver of teleosts reflect some controversy in the interpretation of the data, but also provide confirmation of variations in the structure of the organ in several species. Thus, we intend to understand the specific structural organization of the liver of Astyanax altiparanae. Specimens were collected in the city of Andirá, Paraná, Brazil. The livers were processed according to histological routine for inclusion in Paraplast, and the sections were stained with HE and Mallory's trichrome or followed the protocol for fluorescence immunohistochemistry, anti-cytokeratin. The liver of A. altiparanae was covered by a capsule of connective tissue, without delimiting lobes. The hepatocytes had an arrangement in cords around sinusoids. Melanomacrophage centers were observed. The vascular components and intrahepatic pancreatic acini were distributed between hepatocytes. Presence of cytokeratin was detected in tissues that lined the liver and endothelial cells of sinusoids. The comparison of the liver of A. altiparanae to other characids corroborates with the fact that there is variation in the morphology of the liver even between closely related species. Moreover, it appears that in this species, endothelial cells of sinusoids can synthesize the cytokeratin filaments required for the regulation of blood flow in capillaries in adults.

Temporal repeatability of metabolic rate and the effect of organ mass and enzyme activity on metabolism in European eel (Anguilla anguilla)

Intraspecific variation in metabolic rate of fish can be pronounced and have been linked to various fitness-related behavioural and physiological traits, but the underlying causes for this variation have received far less attention than the consequences of it. In the present study we investigated whether European eels (Anguilla anguilla) displayed temporal repeatability of body-mass-corrected (residual) metabolic rate over a two-month period and if variations in organ mass and enzyme activity between individual fish could be the cause for the observed variation in metabolic rate. Both standard metabolic rate (SMR; Pearson's r=0.743) and routine metabolic rate (RMR; r=0.496) were repeatable over the two-month period. Repeatability of RMR is an interesting finding as it indicates that the level of spontaneous activity in respirometer-confined fish is not random. Cumulative organ mass (liver, heart, spleen and intestine; mean 1.6% total body mass) was found to explain 38% of the variation in SMR (r=0.613) with the liver (one of the metabolically most active organs) being the driver for the correlation between organ mass and metabolic rate. No relationships were found for either liver citrate synthase or cytochrome oxidase activity and metabolic rate in the European eels. Reasons for, and contributions to, the observed variation in metabolic rate are discussed.

Spatial relationships of the intrahepatic vascular–biliary tracts and associated pancreatic acini of Nile tilapia, Oreochromis niloticus (Teleostei, Cichlidae): A serial section study by light microscopy

Reports on teleost liver morphology reflect both controversial and confirmed interspecies variations. Choosing Nile tilapia as a model, we described the histology and 3D organization of all types of vascular-biliary tracts and their spatial relationships from the organ hilum toward the hepatic vein opening(s). The portal tracts entering the hilum, termed pancreatic-venous-biliary-arteriolar tracts (P-VBAT), are associated with pancreocytes and have an afferent axially located vein, plus biliary duct(s) and small artery(ies). The P-VBAT gradually disappears toward the anterior (efferent) end of the liver; those tracts ramify and originate new types of tracts, which may carry one type of element (vascular or biliary) or groups of two, in all possible combinations. Most tracts carrying afferent veins had pancreocytes, thus forming (pancreatic-venous tracts (P-VT), pancreatic-venous-biliary tracts (P-VBT), and pancreatic-venous-arteriolar tracts (P-VAT). There were terminal (and smaller) afferent isolated veins that had no associated pancreocytes. Also, the pancreatic sleeve of a vein could end abruptly or attenuate and disappear, reappearing in distal portions of the same vein. Thus, veins without pancreatic covering as seen in sections are not always efferent. Small arterioles can enter the liver retrogradely, via the adventitia of efferent hepatic veins, thus forming venous-arteriolar tracts (VAT). In comparison with the salmonid-liver type, there were no VBAT without associated pancreocytes and there was a smaller degree of ambiguity in identification of the afferent vs. efferent veins. Thus, the tilapine-liver type is proposed to be a more promising model for studying hepatic metabolic zonation in fish, defined not as in mammals, but eventually considering a gradient radiating from the hilum. Our data and differences from mammals supported the adequacy of the previously proposed nomenclature for the vascular-biliary tracts of fish livers, extending it to those that contain the exocrine pancreas.

In vitro-in vivo extrapolation of quantitative hepatic biotransformation data for fish. I. A review of methods, and strategies for incorporating intrinsic clearance estimates into chemical kinetic models

Scientists studying mammals have developed a stepwise approach to predict in vivo hepatic clearance from measurements of in vitro hepatic biotransformation. The resulting clearance estimates have been used to screen drug candidates, investigate idiosyncratic drug responses, and support chemical risk assessments. In this report, we review these methods, discuss their potential application to studies with fish, and describe how extrapolated values could be incorporated into well-known compartmental kinetic models. Empirical equations that relate extrapolation factors to chemical log K(ow) are given to facilitate the incorporation of metabolism data into bioconcentration and bioaccumulation models. Because they explicitly incorporate the concept of clearance, compartmental clearance-volume models are particularly well suited for incorporating hepatic clearance estimates. The manner in which these clearance values are incorporated into a given model depends, however, on the measurement frame of reference. Procedures for the incorporation of in vitro biotransformation data into physiologically based toxicokinetic (PBTK) models are also described. Unlike most compartmental models, PBTK models are developed to describe the effects of metabolism in the tissue where it occurs. In addition, PBTK models are well suited to modeling metabolism in more than one tissue.

Use of polyclonal antibodies for the detection of changes induced by cadmium in lysosomes of aquatic organisms

Lysosomal responses are widely accepted cellular effect biomarkers of general stress. Up to now, these biomarkers have been analysed by means of conventional techniques based on enzyme histochemical methods, where lysosomal enzymes such as acid phosphatase and beta-glucuronidase (beta-GUS) have been employed as markers of lysosomes. The aim of the present work was to develop more advanced and sensitive methods based on the use of polyclonal antibodies to measure lysosomal enzymes in different sentinel organisms. For this purpose, we have studied the cross-reactivity of two commercial polyclonal antibodies against the lysosomal enzymes acid phosphatase and beta-GUS with molluscan digestive gland by means of immunoblotting and immunohistochemistry. The antibody against acid phosphatase cross-reacted specifically with the lysosomal fraction of the digestive gland, while unspecific immunoreaction occurred with digestive gland whole homogenates and tissue sections. The antibody against beta-GUS cross-reacted specifically with digestive gland whole homogenates and tissue sections. The cross-reactivity of this antibody was tested also in crab hepatopancreas and mullet liver where the same successful results were obtained. The second aim of the present study was to test if the immuno-based approach was sensitive enough to detect lysosomal alterations provoked by contaminants. For this purpose two experiments were carried out with mussels treated with cadmium in two ways: in vivo treatment by injection and in vitro treatment using digestive gland explants. Afterwards immunoblotting studies with the antibody against beta-GUS were applied and immunoreactive bands were quantified by means of a gel analysis programme. We found that beta-GUS protein levels were higher in treated mussels when compared with controls in either in vivo or in vitro treatments. All these data suggest that the polyclonal antibody against beta-GUS is adequate to be used in immuno-based approaches to detect contaminant-induced lysosomal alterations.

Architectural pattern, tissue and cellular morphology in livers of fishes: relationship to experimentally-induced neoplastic responses

The teleost liver is one of the most sensitive organs to show alteration in biochemistry, physiology and structure following exposure to various types of environmental pollutants. Despite the importance of this organ to environmental toxicology and to ecotoxicology where biomarkers of exposure and of deleterious effect are found, the architectural pattern is not well known. This chapter reviews an architectural plan for teleost liver and compares that to the often cited mammalian pattern. Hepatic tubules composed principally of hepatocytes and biliary epithelial cells are in close proximity to lacunae which are of mesodermal origin. As is described, the tubule and lacunae concepts provide a means to better interpret morphologic alterations following exposure. These concepts are used to illustrate features of the chronic toxicity following exposure to proven carcinogens.

Impact of starvation-refeeding on kinetics and protein expression of trout liver NADPH-production systems

Herein we report on the kinetic and protein expression of glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase, and malic enzyme (ME) in the liver of the trout (Oncorhynchus mykiss) during a long-term starvation-refeeding cycle. Starvation significantly depressed the activity of these enzymes by almost 60%, without changing the Michaelis constant. The time response to this nutritional stimulus increased with fish weight. The sharp decline in G6PDH and ME activities was due to a specific protein-repression phenomenon, as demonstrated by molecular and immunohistochemical analyses. Also, the dimeric banding pattern of liver G6PDH shifted from the fully reduced and partially oxidized forms, predominant in control, to a fully oxidized form, more sensitive to proteolytic inactivation. Refeeding caused opposite effects in both protein concentration and enzyme activities of about twice the control values in the first stages, later reaching the normal enzyme activity levels. Additionally, the partially oxidized form of G6PDH increased. The kinetics of these enzymes were examined in relation to the various metabolic roles of NADPH. These results clearly indicate that trout liver undergoes protein repression-induction processes under these two contrasting nutritional conditions.

Liver of the brown trout, Salmo trutta (Teleostei, Salmonidae): a stereological study at light and electron microscopic levels

BACKGROUND

A quantitative study was undertaken for the first time on the normal liver of male and female 2-year-old brown trout, Salmo trutta.

METHODS

Liver was fixed by controlled perfusion. Organ-level morphometry provided weight and volume. A two-stage stereological approach was performed at light and electron microscopy levels. Systematic sampling and point-counting morphometry were used for estimating the relative volumes of the structural compartments. Total absolute volumes of these components were obtained by multiplying each volume density by the volume of its reference space.

RESULTS

Liver volume was 3,423.6 mm3 for males and 3,657.4 mm3 for females. Parenchyma accounted for 95% of hepatic volume. Veins and bile ducts occupied, respectively, 76% and 17% of the stroma, whereas arteries, connective tissue, and melanomacrophages together composed only 6%. Hepatocytes occupied 88% of the parenchyma. Nonhepatocytic cells (endothelium, biliary epithelial cells, Ito cells, and macrophages) composed 4% of the parenchyma, the capillary lumen 6%, and other spaces (Disse space, canaliculi, and lumina of preductules and ductules) composed 2%. Significant sexual differences were found: (1) Females showed a greater parenchymal volume density (0.85% vs. 0.35%) and absolute volume (29.5 mm2 vs. 11.7 mm3) of Ito cells; (2) macrophages of females also presented a greater parenchymal volume density (0.94% vs. 0.46%), but not absolute volume.

CONCLUSIONS

The need to analyze both relative and absolute stereological data was stressed. Similarities and differences were detected between brown trout and other species (fishes and mammals); the findings suggest that, despite architectural differences, some quantitative parameters of liver microanatomy were retained during phylogeny. Factors mediating sexual differences in Ito cells and macrophages were discussed and the need for further studies was highlighted.

Ultrastructural alterations in liver of medaka (Oryzias latipes) exposed to diethylnitrosamine

Liver cytotoxic alterations of adult medaka (Oryzias latipes) following short-term bath exposure (48 hr) to 500 mg/L diethylnitrosamine (DEN) were studied (days 3-21) by electron microscopy and cytochemistry. Control medaka displayed hepatic sexual dimorphism as described for other sexually active fish. Following DEN exposure, decreased glycogen stores with loss of cellular compartmentation obscured sexual dimorphism. A spectrum of organelle alterations, previously not reported in livers of fish, was seen. Early changes in hepatocytes included: nuclear lipid inclusions, nucleolar changes, decreased amounts of granular endoplasmic reticulum (GER), increased fractionation and steatosis of GER, proliferation of smooth ER and lysosomes, reduction in number and content of particulate lipoproteins and vitellogenin in Golgi vesicles, and reduction in number and staining intensity of peroxisomes. At day 14 and/or 21, partial to complete reversal of the above alterations indicated hepatic recovery, and fewer necrotic cells were seen at day 21 versus day 14. Lesions that did not resolve during this study were altered mitochondria and areas of spongiosis hepatis that developed at day 8 and continued to increase throughout the study. Infiltration of lymphocytes, granulocytes, and large numbers of macrophages were late changes. The description, timing, and duration of lesions are of value for consideration as biomarkers of exposure and effect in aquatic toxicology.

Functional units in rainbow trout (Salmo gairdneri, Richardson) liver: III. Morphometric analysis of parenchyma, stroma, and component cell types

Hepatic stroma and parenchyma with its component cell types were quantitatively described in adult male and female actively-spawning 5-year-old rainbow trout (Salmo gairdneri, Richardson). Point-count morphometry of glycol methacrylate sections estimated volume compartments for stroma and parenchyma. Veins composed 85% of the stroma while arteries and bile ducts occupied approximately 6-7% each. Parenchyma accounted for 95% of hepatic volume. Point-count morphometry of transmission electron micrographs estimated volume compartments as well as numerical and surface density measurements for parenchymal components. Within the hepatic parenchymal compartment, hepatocytes occupied 85% and showed significant sex differences. Female hepatocytes were significantly more numerous but were smaller, only 60% of the volume of male hepatocytes. Since hepatocyte nuclear volume was equal in both sexes, differences were due to reduced cytoplasmic volume in females. Perisinusoidal macrophages of females occupied larger volumes of their respective parenchymal compartments, and their larger mean cytoplasmic volumes suggested activation. Biliary epithelial cells of preductules and ductules were numerous. Ratios of numerical density of hepatocytes to biliary epithelial cells were consistent with a tubular arrangement of hepatocytes. Factors possibly mediating the sexual dimorphism are discussed.

Hepatocellular adaptation to extreme nutritional conditions in ide,Leuciscus idus melanotus L. (Cyprinidae). A morphofunctional analysis

The adaptive response of the omnivorous ide,Leuciscus idus melanotus, to drastic metabolic conditions was analyzed on different levels of organisation investigating a variety of parameters: Organism (condition factor, liver-somatic index), organ (liver structure), cellular and subcellular level (hepatocyte structure, glycogen and lipid storage, contents and distribution of nucleic acids, enzyme alterations).During starvation, ide were able to maintain liver integrity in a biphasic process: after an initial phase of disturbance, ide established a new structural and metabolic homeostasis. Recovery from starvation was possible only with a complete diet but not with a sucrose diet. Carbohydrate overload, as evoked by sucrose refeeding, did not result in liver or carcass fattening as known from mammals.To the best of our knowledge, the present study is the first to use enzyme histochemistry in fish nutrition research. In mammals, histochemistry is particularly helpful for understanding processes of hepatic metabolic adaptation. In fish, however, on the basis of our results, enzyme histochemical studies appear to be of limited value, as long as no further data are available on a zonal distribution of enzyme activities in teleost liver parenchyma. Instead, the histochemical detection of the distribution of hepatic storage products and RNA-positive material yielded important information on liver adaptive processes.

Morphologic and histochemical analysis of the newt (Notophthalmus viridescens) liver

Architectural arrangement, ultrastructure, and selected histochemical properties of the newt (Notophthalmus viridescens) liver were examined. Although hematopoietic tissue (1-4 cells thick) invested the liver, direct vascular communication between this tissue and hepatic parenchyma was not observed. The liver was intensely positive when stained with Oil-red-O and periodic acid-Schiff reagent and connective tissue was limited to large vascular channels and the capsule. A distinctive polarity was observed in the hepatic vascular system when lobes were viewed in cross section. Dorsally, portal venules accompanied arterioles and branches of the biliary system, while tributaries of hepatic veins were observed ventrally. Following perfusion fixation, hepatocytes appeared as sheets of cells 1-5 cells thick; however, lobules as defined in adult mammalian liver were absent. Hepatocytes contained abundant smooth endoplasmic reticulum, mitochondria, electron-dense lysosomes, patches of granular endoplasmic reticulum, and lipid droplets. Continuous endothelial cells lined sinusoids and exhibited fenestrae organized into structures similar to sieve plates observed in mammalian liver. Variable numbers of melanin-containing macrophages and subendothelial macrophages were observed; however, Kupffer cells and lipid containing perisinusoidal fat-storing cells were not seen. Patterns of reaction product for glucose-6-phosphatase (G-6-Pase), glucose-6-phosphate dehydrogenase (G-6-PDH), and succinic dehydrogenase (SDH) were localized in the newt liver. All enzymes exhibited a uniform distribution pattern; however, small punctate regions of intensely positive G-6-PDH cells were noted within hepatic parenchyma. Cells comprising the hematopoietic tissue were intensely positive for G-6-Pase, G-6-PHD, and negative for SDH.

Normal versus abnormal structure: considerations in morphologic responses of teleosts to pollutants

Consideration of newer more quantitative morphologic approaches to the study of aquatic pollutants can provide opportunity for collaborative/integrated studies with other subdisciplines in toxicology. Current commonly employed morphologic approaches result largely in subjective findings difficult to analyze statistically and often are directed at levels of structural organization inconsistent with biochemical and physiological approaches. We review some of the methods and approaches available for correlated structure/function studies and present examples from normal and altered skin, gill, and liver of teleosts.