Improved Kupffer's gold chloride method for demonstrating the stellate cells storing retinol (vitamin A) in the liver and extrahepatic organs of vertebrates

The Space of Disse: The Liver Hub in Health and Disease

Since it was first described by the German anatomist and histologist, Joseph Hugo Vincenz Disse, the structure and functions of the space of Disse, a thin perisinusoidal area between the endothelial cells and hepatocytes filled with blood plasma, have acquired great importance in liver disease. The space of Disse is home for the hepatic stellate cells (HSCs), the major fibrogenic players in the liver. Quiescent HSCs (qHSCs) store vitamin A, and upon activation they lose their retinol reservoir and become activated. Activated HSCs (aHSCs) are responsible for secretion of extracellular matrix (ECM) into the space of Disse. This early event in hepatic injury is accompanied by loss of the pores—known as fenestrations—of the endothelial cells, triggering loss of balance between the blood flow and the hepatocyte, and underlies the link between fibrosis and organ dysfunction. If the imbalance persists, the expansion of the fibrotic scar followed by the vascularized septae leads to cirrhosis and/or end-stage hepatocellular carcinoma (HCC). Thus, researchers have been focused on finding therapeutic targets that reduce fibrosis. The space of Disse provides the perfect microenvironment for the stem cells niche in the liver and the interchange of nutrients between cells. In the present review article, we focused on the space of Disse, its components and its leading role in liver disease development.

The stellate cell system (vitamin A-storing cell system)

Past, present, and future research into hepatic stellate cells (HSCs, also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells, or Ito cells) are summarized and discussed in this review. Kupffer discovered black-stained cells in the liver using the gold chloride method and named them stellate cells (Sternzellen in German) in 1876. Wake rediscovered the cells in 1971 using the same gold chloride method and various modern histological techniques including electron microscopy. Between their discovery and rediscovery, HSCs disappeared from the research history. Their identification, the establishment of cell isolation and culture methods, and the development of cellular and molecular biological techniques promoted HSC research after their rediscovery. In mammals, HSCs exist in the space between liver parenchymal cells (PCs) or hepatocytes and liver sinusoidal endothelial cells (LSECs) of the hepatic lobule, and store 50-80% of all vitamin A in the body as retinyl ester in lipid droplets in the cytoplasm. SCs also exist in extrahepatic organs such as pancreas, lung, and kidney. Hepatic (HSCs) and extrahepatic stellate cells (EHSCs) form the stellate cell (SC) system or SC family; the main storage site of vitamin A in the body is HSCs in the liver. In pathological conditions such as liver fibrosis, HSCs lose vitamin A, and synthesize a large amount of extracellular matrix (ECM) components including collagen, proteoglycan, glycosaminoglycan, and adhesive glycoproteins. The morphology of these cells also changes from the star-shaped HSCs to that of fibroblasts or myofibroblasts.

Senescence in hepatic stellate cells as a mechanism of liver fibrosis reversal: a putative synergy between retinoic acid and PPAR-gamma signalings

Hepatic stellate cells (HSCs), also known as perisinusoidal cells, are pericytes found in the perisinusoidal space of the liver. HSCs are the major cell type involved in liver fibrosis, which is the formation of scar tissue in response to liver damage. When the liver is damaged, stellate cells can shift into an activated state, characterized by proliferation, contractility and chemotaxis. The activated HSCs secrete collagen scar tissue, which can lead to cirrhosis. Recent studies have shown that in vivo activation of HSCs by fibrogenic agents can eventually lead to senescence of these cells, which would contribute to reversal of fibrosis although it may also favor the insurgence of liver cancer. HSCs in their non-active form store huge amounts of retinoic acid derivatives in lipid droplets, which are progressively depleted upon cell activation in injured liver. Retinoic acid is a metabolite of vitamin A (retinol) that mediates the functions of vitamin A, generally required for growth and development. The precise function of retinoic acid and its alterations in HSCs has yet to be elucidated, and nonetheless in various cell types retinoic acid and its receptors (RAR and RXR) are known to act synergistically with peroxisome proliferator-activated receptor gamma (PPAR-gamma) signaling through the activity of transcriptional heterodimers. Here, we review the recent advancements in the understanding of how retinoic acid signaling modulates the fibrogenic potential of HSCs and proposes a synergistic combined action with PPAR-gamma in the reversal of liver fibrosis.

The basic helix-loop-helix transcription factor, heart and neural crest derivatives expressed transcript 2, marks hepatic stellate cells in zebrafish: analysis of stellate cell entry into the developing liver

Hepatic stellate cells (HSCs) are liver-specific mesenchymal cells that play vital roles in liver development and injury. Our knowledge of HSC biology is limited by the paucity of in vivo data. HSCs and sinusoidal endothelial cells (SECs) reside in close proximity, and interactions between these two cell types are potentially critical for their development and function. Here, we introduce a transgenic zebrafish line, Tg(hand2:EGFP), that labels HSCs. We find that zebrafish HSCs share many similarities with their mammalian counterparts, including morphology, location, lipid storage, gene-expression profile, and increased proliferation and matrix production, in response to an acute hepatic insult. Using the Tg(hand2:EGFP) line, we conducted time-course analyses during development to reveal that HSCs invade the liver after SECs do. However, HSCs still enter the liver in mutants that lack most endothelial cells, including SECs, indicating that SECs are not required for HSC differentiation or their entry into the liver. In the absence of SECs, HSCs become abnormally associated with hepatic biliary cells, suggesting that SECs influence HSC localization during liver development. We analyzed factors that regulate HSC development and show that inhibition of vascular endothelial growth factor signaling significantly reduces the number of HSCs that enter the liver. We also performed a pilot chemical screen and identified two compounds that affect HSC numbers during development.

CONCLUSION

Our work provides the first comprehensive description of HSC development in zebrafish and reveals the requirement of SECs in HSC localization. The Tg(hand2:EGFP) line represents a unique tool for in vivo analysis and molecular dissection of HSC behavior.

Hepatic stellate cell (vitamin A-storing cell) and its relative--past, present and future

HSCs (hepatic stellate cells) (also called vitamin A-storing cells, lipocytes, interstitial cells, fat-storing cells or Ito cells) exist in the space between parenchymal cells and liver sinusoidal endothelial cells of the hepatic lobule and store 50-80% of vitamin A in the whole body as retinyl palmitate in lipid droplets in the cytoplasm. In physiological conditions, these cells play pivotal roles in the regulation of vitamin A homoeostasis. In pathological conditions, such as hepatic fibrosis or liver cirrhosis, HSCs lose vitamin A and synthesize a large amount of extracellular matrix components including collagen, proteoglycan, glycosaminoglycan and adhesive glycoproteins. Morphology of these cells also changes from the star-shaped SCs (stellate cells) to that of fibroblasts or myofibroblasts. The hepatic SCs are now considered to be targets of therapy of hepatic fibrosis or liver cirrhosis. HSCs are activated by adhering to the parenchymal cells and lose stored vitamin A during hepatic regeneration. Vitamin A-storing cells exist in extrahepatic organs such as the pancreas, lungs, kidneys and intestines. Vitamin A-storing cells in the liver and extrahepatic organs form a cellular system. The research of the vitamin A-storing cells has developed and expanded vigorously. The past, present and future of the research of the vitamin A-storing cells (SCs) will be summarized and discussed in this review.

Human liver autofluorescence: an intrinsic tissue parameter discriminating normal and diseased conditions

BACKGROUND AND OBJECTIVE

Autofluorescence (AF) emission is an intrinsic parameter that can provide real-time information on morpho-functional properties of biological tissue, being strictly related with their biochemical composition and structural organization. The diagnostic potentials of AF-based techniques have been investigated on normal, fibrotic, and steatotic liver tissues, in reference to histological features as evidenced by specific histochemical stainings.

MATERIALS AND METHODS

AF emission under excitation at 366 nm has been examined on cryostatic tissue sections obtained from biopsies collected during surgical operation, by means of fluorescence imaging and microspectrofluorometric techniques.

RESULTS

NAD(P)H, collagen, and vitamin A were found to be the endogenous fluorophores characterizing normal, fibrotic, and steatotic liver tissue AF, respectively. The differences of their photo-physical properties, in terms of emission amplitude, spectral shape, and response to irradiation, give rise to modifications of overall AF signal collected from tissues that allow the liver conditions to be distinguished.

CONCLUSION

The study provides a valid premise for a development of AF-based optical biopsy techniques for a real-time discrimination of liver anatomo-pathological patterns.

Liver autofluorescence properties in animal model under altered nutritional conditions

Autofluorescence spectroscopy is a promising and powerful approach for an in vivo, real time characterization of liver functional properties. In this work, preliminary results on the dependence of liver autofluorescence parameters on the nutritional status are reported, with particular attention to vitamin A and lipid accumulation in liver tissue. Normally fed and 24 h starving rats were used as animal models. Histochemical and autofluorescence analysis showed that lipids and vitamin A colocalize in the liver parenchyma. Fasting condition results in a parallel increase in both lipids and vitamin A. Autofluorescence imaging and microspectrofluorometric analysis carried out on unfixed, unstained tissue sections under 366 nm excitation, evidenced differences in both spectral shape and response to continuous irradiation between liver biopsies from fed and starving rats. As to photobleaching, in particular, fitting analysis evidenced a reduction of about 85% of the signal attributable solely to vitamin A during the first 10 s of irradiation. The tissue whole emission measured in fed and starving rat livers exhibited reductions of about 35% and 52%, respectively, that are closely related to vitamin A contents. The findings open interesting perspectives for the set up of an in situ, real time diagnostic procedure for the assessment of liver lipid accumulation, exploiting the photophysical properties of vitamin A.

Distribution of retinylester-storing stellate cells in the arrowtooth halibut, Atheresthes evermanni

Hepatic stellate cells play a major role in retinylester storage in mammals, but the retinoid-storing state in nonmammalian vertebrates remains to be elucidated. In this study, we examined retinoids and retinoid-storing cells in the arrowtooth halibut, Atheresthes evermanni. High-performance liquid chromatography analyses revealed the highest concentrations of stored retinoids (retinol and retinylester, 6199 nmol/g) in the pyloric cecum, a teleost-specific organ protruding from the intestine adjacent to the pylorus. Considerable amounts of retinoids were also stored in the intestine (3355 nmol/g) and liver (1891 nmol/g), and small amounts in the kidney (102 nmol/g). Very small amounts or no retinoids were detected in the heart, gill, skeletal muscle, and gonads (less than 2 nmol/g). Use of gold chloride staining and fluorescence microscopy to detect retinoid autofluorescence showed that, in the pyloric cecum and intestine, retinoid-storing cells were localized in the lamina propria mucosae. Under electron microscopy, cells containing well-developed lipid droplets, which are common morphological characteristics of the hepatic stellate cells of mammals, were observed in the lamina propria mucosae of the pyloric cecum. Thus, the distribution of stellate cells with retinoid-storing capacity differs between this halibut and mammals, suggesting that the retinoid-storing site has shifted during vertebrate evolution.

Postnatal development of rat vocal folds

OBJECTIVES

In this study we aimed to determine the feasibility of using a rat model for the study of postnatal vocal fold (VF) development.

METHODS

Eighteen male rats that were 3 days old, 3 weeks old, or 8 months old were analyzed histologically with Alcian blue stain used for detecting hyaluronic acid, elastin-van Gieson stain for elastin, Oil Red O and gold chloride stains for vitamin A-containing lipid droplets, and immunohistochemistry for vimentin (general fibroblast marker) and collagen types I and III.

RESULTS

The macula flava (MF) was observed as a mass of cells that expressed vimentin intensively in the cytoplasm. The MF showed denser hyaluronic acid and collagen type I than did the midmembranous portion of the VF lamina propria. Clear developmental changes were evident in the MF and other regions. The vimentin-positive cells of the 3-day-old MF were mainly oval-shaped and had less cytoplasm, whereas those of the 8-month-old MF were spindle- and stellate-shaped and had more cytoplasm, similar to that reported in humans. Vitamin A-containing lipid droplets were limited to the 3-week-old and 8-month-old MFs and were not present in the 3-day-old VF.

CONCLUSIONS

These results suggest that a rat model is useful in studying VF development and that vitamin A is related to the maturity of the VF.

Retinoic acid metabolism and signaling pathways in the adult and developing mouse testis

As a first step in investigating the role of retinoic acid (RA) in mouse testis, we analyzed the distribution pattern of the enzymes involved in vitamin A storage (lecithin:retinol acyltransferase), RA synthesis (beta-carotene 15,15'-monoxygenase and retinaldehyde dehydrogenases) and RA degradation (cytochrome P450 hydroxylases) as well as those of all isotypes of receptors transducing the RA signal [RA receptors (RARs) and rexinoid receptors (RXRs)]. Our data indicate that in adult testis 1) cytochrome P450 hydroxylase enzymes may generate in peritubular myoid cells a catabolic barrier that prevents circulating RA and RA synthesized by Leydig cells to enter the seminiferous epithelium; 2) the compartmentalization of RA synthesis within this epithelium may modulate, through paracrine mechanisms, the coupling between spermatogonia proliferation and spermatogenesis; 3) retinyl esters synthesized in round spermatids by lecithin:retinol acyltransferase may be transferred and stored in Sertoli cells, in the form of adipose differentiation-related protein-coated lipid droplets. We also show that RARalpha and RXRbeta are confined to Sertoli cells, whereas RARgamma is expressed in spermatogonia and RARbeta, RXRalpha, and RXRgamma are colocalized in step 7-8 spermatids. Correlating these expression patterns with the pathological phenotypes generated in response to RAR and RXR mutations and to postnatal vitamin A deficiency suggests that spermiation requires RXRbeta/RARalpha heterodimers in Sertoli cells, whereas spermatogonia proliferation involves, independently of RXR, two distinct RAR-mediated signaling pathways in both Sertoli cells and spermatogonia. Our data also suggest that the involvement of RA in testis development starts when primary spermatogonia first appear.

Transdifferentiation of vocal-fold stellate cells and all-trans retinol-induced deactivation

The maculae flavae of the human vocal folds include dense extracellular matrices and compacted cells with a stellate morphology. These vocal-fold stellate cells are thought to participate in the metabolism of extracellular matrices essential in maintaining vocal-fold viscoelasticity required for phonation. We have isolated and cultured these new cells and have tested the hypothesis that they maintain a distinct cellular and biochemical phenotype. We have compared proliferation rates, changes on immunophenotype, and intracellular lipid and vitamin A storage. Vocal-fold stellate cells undergo culture-induced transdifferentiation to a myofibroblast-like phenotype with an altered phenotype resembling, but not identical to, activated hepatic and pancreatic stellate cells. Our results reveal that these cells are capable of responding to exogenous all-trans retinol in culture. Exposure to this synthetic co-factor causes deactivation characterized by decreased proliferation, loss of the activated stellate cell marker, alpha-smooth muscle actin, and restoration of intracellular lipid and vitamin A metabolite storage. These data establish a new and distinct cellular target for future investigations of the viscoelastic properties of the vocal-fold mucosa during normal phonation, aging, vocal-fold scarring, laryngeal fibrosis, and myofibroblastoma.

Degradation of extracellular matrix by extrahepatic stellate cells in the intestine of the lamprey,Lampetra japonica

To investigate the mechanisms involved in the atrophy of the intestines in lampreys (Lampetra japonica) during the spawning migration stage, we examined by morphological methods their intestines with special reference to degradation of extracellular matrix (ECM) components. Stellate cells are known to be distributed not only in the liver (hepatic stellate cells) but also in other organs, such as the pancreas, intestine, lung, and kidney (extrahepatic stellate cells). Hepatic stellate cells are well known to be able to biosynthesize, secrete, and degrade ECM. Therefore, we investigated the cellular and molecular mechanisms involved in the atrophy of the intestines by focusing on these intestinal extrahepatic stellate cells. The cells were found to contain phagocytosed and degraded collagen fibrils, which are one of the ECM components. A positive reaction for trimetaphosphatase (TMPase, a cytochemical marker of lysosomes) was preferentially detected in round or elongated vesicles in the intestinal extrahepatic stellate cells and the deposits of the reaction products coexisted with the degraded collagen fibrils. However, the basement membrane of the intestine, which membrane is also an ECM component, was preserved throughout the spawning migration stage of the lamprey and accumulated as a mass of thick membrane, suggesting the existence of a special mechanism for selective digestion of ECM components. These results indicate that the intestinal extrahepatic stellate cells in Lampetra japonica during its spawning migration stage might play an important mechanistic role in the atrophy of lamprey intestines by phagocytizing collagen fibrils and digesting the phagocytized collagen fibrils in their lysosomes.

Retinal is the essential form of retinoid for storage and transport in the adult of the ascidian Halocynthia roretzi

Retinoids in the organs (gonad [GND], body wall muscle [BWM], hepatopancreas [HP], gill, hemolymph cells and hemolymph plasma) of the adult ascidian Halocynthia roretzi were analyzed by high performance liquid chromatography. Retinal (RAL) occurred in every organ examined, and most of RAL (>/=99%) was localized in the GND and BWM. None of the organs contained significant amounts of retinol (ROL) or retinyl ester (RE). Lipid droplets, which are characteristic of stellate cells (RE-storing cells of vertebrates), could not be found in the GND, BWM and HP by microscopic observations. These results indicate that this ascidian lacks the RE-storing mechanism, which is ubiquitous in adult vertebrates. The amount and localization of RAL showed the annual change in relation to the reproductive cycle. During summer, the growing season, RAL was present in both GND and BWM at a ratio of about 3:2. From summer to winter, RAL in the GND gradually increased, concomitant with the decrease of RAL in the BWM. In winter, the spawning season, most of RAL was present in the GND (ca. 98%). RAL appears to be accumulated first in the BWM and transported to oocytes accompanying yolk accumulation. ROL and RE were not implicated in the storage and transport of retinoids. The results in the present research strongly suggest that retinoic acid (RA) is produced by the two-step enzymatic reaction: carotenoid cleavage to RAL followed by RAL oxidation to RA and that the prevertebrate chordate lacks ROL-metabolizing systems.

Lung retinol storing cells synthesize and secrete retinoic acid, an inducer of alveolus formation

Retinoids play a key role in the formation of pulmonary alveoli. Lipid interstitial cells (LICs) of the alveolar wall store retinol and are concentrated at sites of alveolus formation, suggesting they are an endogenous source of retinoids for alveolus formation. We show in cultured rat lung cells that LICs synthesize and secrete all-trans retinoic acid (ATRA); its secretion is halved by dexamethasone, an inhibitor of alveolus formation. In a second alveolar wall cell, the pulmonary microvascular endothelial cell (PMVC), ATRA increases expression of the mRNA of cellular retinol binding protein-I (CRBP-I), a protein involved in ATRA synthesis. Serum-free, exogenous ATRA-free medium conditioned by LICs rich in retinol storage granules caused a 10-fold greater increase of CRBP-I mRNA in PMVCs than media conditioned by LICs with few retinol storage granules. This action of medium conditioned by retinol storage granule-rich LICs is decreased by a retinoic acid receptor pan-antagonist and by a retinoid X receptor pan-antagonist, suggesting the responsible molecule(s) is a retinoid and that retinoid signaling occurs in a paracrine fashion.

Vitamin A distribution and content in tissues of the lamprey,Lampetra japonica

Vitamin A (retinol and retinyl ester) distribution and content in tissues of a lamprey (Lampetra japonica) were analyzed by morphological methods, namely, gold chloride staining, fluorescence microscopy to detect specific vitamin A autofluorescence, and electron microscopy, as well as high-performance liquid chromatography (HPLC). Hepatic stellate cells showed an abundance of vitamin A stored in lipid droplets in their cytoplasm. Similar cells storing vitamin A were present in the intestine, kidney, gill, and heart in both female and male lampreys. Morphological data obtained by gold chloride staining method, fluorescence microscopy, transmission electron microscopy, and HPLC quantification of retinol were consistent. The highest level of total retinol measured by HPLC was found in the intestine. The second and third highest concentrations of vitamin A were found in the liver and the kidney, respectively. These vitamin A-storing cells were not epithelial cells, but mesoderm-derived cells. We propose as a hypothesis that these cells belong to the stellate cell system (family) that stores vitamin A and regulates homeostasis of the vitamin in the whole body in the lamprey. Fibroblastic cells in the skin and somatic muscle stored little vitamin A. These results indicate that there is difference in the vitamin A-storing capacity between the splanchnic and intermediate mesoderm-derived cells (stellate cells) and somatic and dorsal mesoderm-derived cells (fibroblasts) in the lamprey. Stellate cells derived from the splanchnic and intermediate mesoderm have high capacity and fibroblasts derived from the somatic and dorsal mesoderm have low capacity for the storage of vitamin A in the lamprey.

Distribution of vitamin A-storing lipid droplets in hepatic stellate cells in liver lobules--a comparative study

To investigate the storage mechanisms of vitamin A, we examined the liver of adult polar bears and arctic foxes, which physiologically store a large amount of vitamin A, by high-performance liquid chromatography (HPLC), transmission electron microscopy (TEM) morphometry, gold chloride staining, fluorescence microscopy for the detection of autofluorescence of vitamin A, staining with hematoxylin-eosin (H&E), Masson's trichrome, and Ishii and Ishii's silver impregnation. HPLC revealed that the polar bears and arctic foxes contained 1.8-1.9 x 10(4) nmol total retinol (retinol plus retinyl esters) per gram liver. In the arctic foxes, the composition of the retinyl esters was found to be 51.1% palmitate, 26.6% oleate, 15.4% stearate, and 7% linoleate. The hepatic stellate cells of the arctic animals were demonstrated by TEM to contain the bulk of the vitamin A-lipid droplets in their cytoplasm. The liver lobules of the arctic animals showed a zonal gradient in the storage of vitamin A. The gradient was expressed as a symmetric crescendo-decrescendo profile starting at the periportal zone, peaking at the middle zone, and sloping down toward the central zone in the liver lobule. The density (i.e., cell number per area) of hepatic stellate cells was essentially the same among the zones. The gradient and the composition of the retinyl esters in storing vitamin A were not changed by differences in the vitamin A amount in the livers. These results indicate that the heterogeneity of vitamin A-storage capacity in hepatic stellate cells of arctic foxes and polar bears is genetically determined.

Zonal and regional differences identified from precision mapping of vitamin A-storing lipid droplets of the hepatic stellate cells in pig liver: a novel concept of addressing the intralobular area of heterogeneity

Knowledge of hepatic heterogeneity has been strikingly increased, while an accurate means for addressing intralobular positions is still lacking. We examined pig liver preparations of the gold impregnation method for vitamin A-storing lipid droplets in hepatic stellate cells. Droplet morphometry was performed under oil immersion, and the calculated volumes plotted on computerized maps. The heterogeneous results were assessed with five concentric zones and five radial regions; the latter were determined based on midseptum visualized by portal injection. Zonation and regionation thus subdivided lobules into 5-zone/5-region (5Z/5R) compartmentalization. Distribution of values exhibited a distinct zonal gradient, heightened at peripheral zones 1 and 2, decreased over intermediate zone 3 toward centrilobular zones 4 and 5; peak was always found at zone 2. Within a single zone, variations were obvious, forming a regional gradient. Values were significantly higher at periportal than midseptal regions. Digitized mapping showed that low values filled up centrilobular zones, whereas high values concentrated in periportal regions. Along the periphery, inlet venules were quantified, revealing an occurrence rate of 60% at periportal, and 5% at midseptal regions, closely compatible with the regional gradient of vitamin A-storing capacity. The interweaving between zonal and regional gradients results in a vitamin A-low territory, a compound area composed of centrilobular zones plus extensions into midseptal regions. Because the results could account for physiological and pathological events, we regard the 5Z/5R compartmentalization a model worth routine adoption for a precise description of any morphofunctionally demonstrable heterogeneity of the liver lobules.

Structure and function of sinusoidal lining cells in the liver

The hepatic sinusoid harbors 4 different cells: endothelial cells (100, 101), Kupffer cells (96, 102, 103), fat-storing cells (34, 51, 93), and pit cells (14, 107, 108). Each cell type has its own specific morphology and functions, and no transitional stages exist between the cells. These cells have the potential to proliferate locally, either in normal or in special conditions, that is, experiments or disease. Sinusoidal cells from a functional unit together with the parenchymal cells. Isolation protocols exist for all sinusoidal cells. Endothelial cells filter the fluids, exchanged between the sinusoid and the space of Disse through fenestrae (100), which measure 175 nm in diameter and are grouped in sieve plates. Fenestrae occupy 6-8% of the surface (106). No intact basal lamina is present under these cells (100). Various factors change the number and diameter of fenestrae [pressure, alcohol, serotonin, and nicotin; for a review, see Fraser et al (32)]. These changes mainly affect the passage of lipoproteins, which contain cholesterol and vitamin A among other components. Fat-storing cells are pericytes, located in the space of Disse, with long, contractile processes, which probably influence liver (sinusoidal) blood flow. Fat-storing cells possess characteristic fat droplets, which contain a large part of the body's depot of vitamin A (91, 93). These cells play a major role in the synthesis of extracellular matrix (ECM) (34, 39-41). Strongly reduced levels of vitamin A occur in alcoholic livers developing fibrosis (56). Vitamin A deficiency transforms fat-storing cells into myofibroblast-like cells with enhanced ECM production (38). Kupffer cells accumulate in periportal areas. They specifically endocytose endotoxin (70), which activates these macrophages. Lipopolysaccharide, together with interferon gamma, belongs to the most potent activators of Kupffer cells (28). As a result of activation, these cells secrete oxygen radicals, tumor necrosis factor, interleukin 1, interleukin 6, and a series of eicosanoids (28) and become cytotoxic against tumor cells [e.g., colon carcinoma cells (19, 22, 48)]. Toxic secretory products can cause necrosis of the liver parenchyma, which constitutes a crucial factor in liver transplantation (55). Pit cells possess characteristic azurophylic granules and display a high level of spontaneous cytolytic activity against various tumor cells, identifying themselves as natural killer cells (10). The number and cytotoxicity of pit cells can be considerably enhanced with biological response modifiers, such as Zymosan or interleukin 2 (8). Pit cell proliferation occurs within the liver, but recent evidence indicates that blood large granular lymphocytes develop into pit cells in 2 steps involving high- and low-density pit cells (88). Kupffer cells control the motility, adherence, viability, and cytotoxicity of pit cells (89), whereas cytotoxicity against tumor cells is synergistically enhanced (80, 81).

Light and electron microscopic analysis of liver sinusoids during hepatocarcinogenesis with 2-acetylaminofluorene in rats

To clarify the sequential changes and morphological differences of the sinusoidal structures between hepatocellular carcinoma (HCC) and hepatocellular adenoma (HA), we examined morphological changes of sinusoidal cells and related structures such as basement membrane during hepatocarcinogenesis in the rat. During continuous feeding of carcinogenic diets containing 2-acetylaminofluorene to rats, HA appeared at the 8th week in the peripheral area and then extended toward the centrolobular area. The appearance of HCC was recognized at the 27th week. In the HA lesion, the morphology of sinusoidal cells and related structures was basically the same as that of normal liver except for a slight thickening of the basement membrane and a decreased amount of vitamin A-lipid droplets of stellate cells. In HCC, the fenestrations of endothelial cells disappeared and the basement membrane became continuous, thick and often multilayered. Stellate cells contained almost no vitamin A-lipid droplets and were associated with abundant collagen fibers. Kupffer cells and pit cells were not seen inside the sinusoid. All these features of the sinusoids in HCC resembled the morphological characteristics of the capillary. The present study has revealed that HCC possesses sinusoid structures distinct from those of HA. This suggests that HCC may not derive directly from HA but may develop newly within the HA.

Intralobular heterogeneity of perisinusoidal stellate cells in porcine liver

The aim of the present investigation was to elucidate the intralobular heterogeneity of the perisinusoidal stellate cells (fat-storing cells, lipocytes) in the porcine liver. Their three-dimensional structure, desmin immunoreactivity and vitamin-A storage were studied by use of the Golgi silver, immunocytochemical and gold chloride methods. In order to locate the stellate cells, the hepatic lobules were divided into 10 zones. The stellate cells were readily identified in Golgi preparations by their striking dendritic appearance with branching processes encompassing the sinusoids. The stellate cells in the centrolobular zones were conspicuously dendritic with longer processes in comparison to those emitted by periportal elements. Such arborizations were studded with numerous thorn-like microprojections. Desmin immunoreaction in the periportal zones was stronger than that in the centrolobular zones. Vitamin-A storage in the stellate cells was well developed in zones 2-4, but reduced gradually toward the central region. The perisinusoidal stellate cells display marked heterogeneity in morphology and function based on their zonal location in the hepatic lobule.

Light microscopical demonstration and zonal distribution of parasinusoidal cells (Ito cells) in normal human liver

The parasinusoidal cells of the liver (Ito cells) were demonstrated light microscopically in autopsy specimens fixed in formalin and stained with Oil red O after dichromate treatment. The method allows examination of large samples containing numerous acini. Quantitative assessment showed a zonal gradient with 6.3 and 7.7 parasinusoidal cells per 62.5 X 10(3) micron2 in zone 1 and 3, respectively.

Stellate cells storing retinol in the liver of adult lamprey, Lampetra japonica

Distribution, localization and fine structure of the stellate cells in the liver of lamprey, Lampetra japonica, were studied during the spawning migration by use of Kupffer's gold-chloride method, fluorescence microscopy for vitamin A (retinol) and electron microscopy. The stellate cells in the lamprey liver differ in some of their properties from those in mammalian livers. Stellate cells which store abundant retinol in lipid droplets, occur not only in the hepatic parenchyma, but also in the dense perivascular and capsular connective tissue of the liver and in the interstitium of pancreatic tissue. In the hepatic parenchyma these cells are located perisinusoidally or along thick bundles of collagen fibrils. The stellate cells display a number of large retinol-containing lipid droplets, granular endoplasmic reticulum, tubular structures, dense bodies. Golgi complex, microtubules, and microfilaments. In the space of Disse, the stellate cells and extracellular fibrilar components such as collagen fibrils and microfibrils (11-12 nm in diameter) are intervened between the two layers of basal laminae. Differentiation and possible functions of the stellate cells in the lamprey liver are discussed.