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

Liver Zonation - Revisiting Old Questions With New Technologies

Despite the ever-increasing prevalence of non-alcoholic fatty liver disease (NAFLD), the etiology and pathogenesis remain poorly understood. This is due, in part, to the liver's complex physiology and architecture. The liver maintains glucose and lipid homeostasis by coordinating numerous metabolic processes with great efficiency. This is made possible by the spatial compartmentalization of metabolic pathways a phenomenon known as liver zonation. Despite the importance of zonation to normal liver function, it is unresolved if and how perturbations to liver zonation can drive hepatic pathophysiology and NAFLD development. While hepatocyte heterogeneity has been identified over a century ago, its examination had been severely hindered due to technological limitations. Recent advances in single cell analysis and imaging technologies now permit further characterization of cells across the liver lobule. This review summarizes the advances in examining liver zonation and elucidating its regulatory role in liver physiology and pathology. Understanding the spatial organization of metabolism is vital to further our knowledge of liver disease and to provide targeted therapeutic avenues.

Hematopoietic origin of hepatic stellate cells in the adult liver

Hepatic stellate cells are believed to play a key role in the development of liver fibrosis. Several studies have reported that bone marrow cells can give rise to hepatic stellate cells. We hypothesized that hepatic stellate cells are derived from hematopoietic stem cells. To test this hypothesis, we generated chimeric mice by transplantation of clonal populations of cells derived from single enhanced green fluorescent protein (EGFP)–marked Lin−Sca-1+c-kit+CD34− cells and examined the histology of liver tissues obtained from the chimeric mice with carbon tetrachloride (CCl4)–induced injury. After 12 weeks of CCl4 treatment, we detected EGFP+ cells in the liver, and some cells contained intracytoplasmic lipid droplets. Immunofluorescence analysis demonstrated that 50% to 60% of the EGFP+ cells were negative for CD45 and positive for vimentin, glial fibrillary acidic protein, ADAMTS13, and α-smooth muscle actin. Moreover, EGFP+ cells isolated from the liver synthesized collagen I in culture. These phenotypes were consistent with those of hepatic stellate cells. The hematopoietic stem cell–derived hepatic stellate cells seen in male-to-male transplants revealed only one Y chromosome. Our findings suggest that hematopoietic stem cells contribute to the generation of hepatic stellate cells after liver injury and that the process does not involve cell fusion.

Ito cell morphology, alpha-smooth muscle actin and collagen type IV expression in the liver of patients with gastric and colorectal tumors

The alteration in sinusoidal collagen type IV occurrence, and myofibroblastic (alpha-SMA-positive) Ito cellular transformation are described in the liver of patients with malignant gastric and colorectal tumors, using electron microscopy as well as light microscopical and ultrastructural immunohistochemistry. The ultrastructural finding revealed transformation of Ito cells mostly into transitional cells in highly differentiated primary tumors and into transitional and myofibroblast-like cells with expressed changes in the other sinusoidal cells in poorly differentiated tumors. Ito cell numbers increased significantly in the livers of cancer patients. A highly significant statistical association was obtained between Ito cell numbers on the one hand and collagen type IV and alpha-SMA immunoreactivity on the other hand in the pericentral zone of the liver lobule. Ultrastructural immunohistochemistry showed increased collagen IV immune deposits in the space of Disse, assembled for the most part around and inside transitional cells. Alpha-SMA immunoreactivity was detected in activated Ito cells diffuse in the lobule, with stronger expression in the intermediate and pericentral zones. It is suggested that stimuli which can influence Ito cell transformation are produced by tumor cells from the primary tumor (TGF-beta1, TNF-alpha, PDGF-beta etc.) and from the metastasizing gastric or colorectal tumor cells--matrix metalloproteinase-2 (MMP-2). It is suggested that sinusoidal extracellular matrix deterioration creates a barrier for cancer invasion on the one hand, or possibly facilitates metastasizing by ensurance of matrix for adhesion on the other hand.

Fetal hepatic drug elimination

The majority of studies of fetal hepatic elimination have concentrated on the expression and activity of the metabolizing enzymes, but the unique physiologic milieu of the fetal liver should also be considered. The basic structure of the liver is formed by the end of the first trimester. The fetal hepatic circulation differs substantially from that of the adult in that there is an extra input vessel, the umbilical vein, and there is shunting of 30-70% of hepatic blood flow via the ductus venosus. The left and right lobes of the fetal liver seem to function independently with respect to a variety of biochemical parameters, due at least in part to the lower oxygen supply to the right lobe. The zonation of drug-metabolizing enzymes along the hepatic acinus, which is prominent in the adult liver, is absent in the fetal liver. Unlike rodent species, the human fetal liver has a significant capacity for drug metabolism. Of the oxidative enzymes, CYP3A7 accounts for up to 50% of total fetal hepatic cytochrome P450 content. Expression of this enzyme decreases dramatically after birth. CYP1A1 and CYP2D6 have also been detected in human fetal liver, but whether CYP2E1 is expressed remains controversial. Several other cytochrome P450s have been identified and await characterization. Fetal hepatic drug conjugation may prolong fetal exposure to the metabolites produced, which, being more water soluble, do not readily cross the placenta back to the mother and, if excreted in fetal urine, can be recycled in the fetus via amniotic fluid and fetal swallowing. Limited activity of glucuronidation enzymes has been demonstrated in human fetal liver in contrast to the activity of sulfation enzymes, which is significant. Limited in vivo studies in fetal sheep have demonstrated significant fetal hepatic drug elimination, and this has been confirmed in studies of the isolated perfused fetal sheep liver. Our understanding of fetal hepatic elimination processes has advanced steadily over the years. Future developments, however, should consider more fully the influence of the unique physiological milieu of the fetal liver, in addition to the expression and activity of drug metabolizing enzymes.

Hepatic stellate cells (Ito cells) in veno-occlusive disease of the liver after allogeneic bone marrow transplantation

AIMS

To evaluate the role of activated hepatic stellate cells (HSCs) in hepatic veno-occlusive disease (VOD) after bone marrow transcription (BMT), we studied the distribution and area of activated HSCs by immunohistochemistry for alpha-smooth muscle actin (SMA).

METHODS AND RESULTS

We examined the liver of seven autopsy cases with hepatic VOD or without VOD after allogeneic BMT and five autopsy cases without liver disease as a control both microscopically and immunohistochemically. In normal liver tissues, SMA-positive cells were observed around the central veins, while they were more frequently noted along the sinusoidal walls as well as around the central veins in liver tissues with or without VOD after BMT. The area of activated HSCs increased significantly in zones 1 and 2, and more prominently in zone 3 of the liver tissues after BMT than normal liver tissues, and was much larger in zone 3 of liver tissues with VOD. The activated HSCs were immunohistochemically negative for the regulatory contractile proteins (heavy caldesmon and calponin).

CONCLUSIONS

These results indicated that the activated HSCs may play an important role in sinusoidal fibrosis and luminal narrowing or occlusion of the central veins in VOD after BMT.

The hepatic stellate (Ito) cell: its role in human liver disease

The hepatic stellate (Ito) cell lies within the space of Disse and has a variety of functions. Stellate cells store vitamin A in characteristic lipid droplets. In the normal human liver, the cells can be identified by the presence of these lipid droplets; in addition, many stellate cells in the normal liver express alpha-smooth muscle actin. In acute liver injury, there is an expansion of the stellate cell population with increased alpha-smooth muscle actin expression; stellate cells appear to play a role in extracellular matrix remodelling after recovery from injury. In chronic liver injury, the stellate cell differentiates into a myofibroblast-like cell with marked expression of alpha-smooth muscle actin and occasional expression of desmin. Myofibroblast-like cells have a high fibrogenic capacity in the chronically diseased liver and are also involved in matrix degradation. In vitamin A intoxication, hypertrophy and proliferation of the stellate and myofibroblast-like cells may lead to non-cirrhotic portal hypertension, fibrosis and cirrhosis. In liver tumours, myofibroblast-like cells are involved in the capsule formation around the tumour and in the production of extracellular matrix within it. The transition of stellate cells into myofibroblast-like cells is regulated by an intricate network of intercellular communication between stellate cells and activated Kupffer cells, damaged hepatocytes, platelets, endothelial and inflammatory cells, involving cytokines and nonpeptide mediators such as reactive oxygen species, eicosanoids and acetaldehyde. The findings suggest that the stellate cell plays an active role in a number of human liver diseases, with a particular reactivity pattern in fibrotic liver disorders.

Dolichol and vitamin A content in rat liver Ito (perisinusoidal) cells

Dolichol has been determined in many tissues but to date no data are available on liver Ito (fat storing) cells. In this note dolichol was determined in two subpopulations of liver Ito cells isolated from rats pretreated with vitamin A: Ito-1, vitamin A enriched and Ito-2, relatively poor of vitamin A. Differences were observed in the behaviour of the two fractions after vitamin A pretreatment of rats. In fact, in Ito-1 fraction dolichol increases with the increase of vitamin A, while in Ito-2 fraction it does not change significantly with the increase of vitamin A. These results, while confirming the heterogeneity of fat storing cells, are discussed as to the possible role of dolichol and vitamin A metabolism.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.