Fine structure of hepatic sinusoids and their development in human embryos and fetuses

Liver Organoids: Recent Developments, Limitations and Potential

Liver cell types derived from induced pluripotent stem cells (iPSCs) share the potential to investigate development, toxicity, as well as genetic and infectious disease in ways currently limited by the availability of primary tissue. With the added advantage of patient specificity, which can play a role in all of these areas. Many iPSC differentiation protocols focus on 3 dimensional (3D) or organotypic differentiation, as these offer the advantage of more closely mimicking in vivo systems including; the formation of tissue like architecture and interactions/crosstalk between different cell types. Ultimately such models have the potential to be used clinically and either with or more aptly, in place of animal models. Along with the development of organotypic and micro-tissue models, there will be a need to co-develop imaging technologies to enable their visualization. A variety of liver models termed "organoids" have been reported in the literature ranging from simple spheres or cysts of a single cell type, usually hepatocytes, to those containing multiple cell types combined during the differentiation process such as hepatic stellate cells, endothelial cells, and mesenchymal cells, often leading to an improved hepatic phenotype. These allow specific functions or readouts to be examined such as drug metabolism, protein secretion or an improved phenotype, but because of their relative simplicity they lack the flexibility and general applicability of ex vivo tissue culture. In the liver field these are more often constructed rather than developed together organotypically as seen in other organoid models such as brain, kidney, lung and intestine. Having access to organotypic liver like surrogates containing multiple cell types with in vivo like interactions/architecture, would provide vastly improved models for disease, toxicity and drug development, combining disciplines such as microfluidic chip technology with organoids and ultimately paving the way to new therapies.

Differential Location and Distribution of Hepatic Immune Cells

The liver is one of the main organs in the body, performing several metabolic and immunological functions that are indispensable to the organism. The liver is strategically positioned in the abdominal cavity between the intestine and the systemic circulation. Due to its location, the liver is continually exposed to nutritional insults, microbiota products from the intestinal tract, and to toxic substances. Hepatocytes are the major functional constituents of the hepatic lobes, and perform most of the liver’s secretory and synthesizing functions, although another important cell population sustains the vitality of the organ: the hepatic immune cells. Liver immune cells play a fundamental role in host immune responses and exquisite mechanisms are necessary to govern the density and the location of the different hepatic leukocytes. Here we discuss the location of these pivotal cells within the different liver compartments, and how their frequency and tissular location can dictate the fate of liver immune responses.

Hepatic antigen-presenting cells and regulation of liver transplant outcome

In the steady state, hepatic antigen (Ag)-presenting cells (APC) generally dampen systemic inflammatory responses to gut-derived Ags. Our studies focus on the role of specific liver APC populations, both non-parenchymal cells (dendritic cells [DC], Kupffer cells, and hepatic stellate cells [HSC]) and parenchymal cells, in the molecular regulation of tissue damage (ischemia and reperfusion [I/R] injury) and immunity following liver transplantation. We focus on factors that either promote or overwhelm the natural tendency of the liver to suppress inflammatory/immune responses. We are also examining molecular mechanisms that regulate liver DC maturation and function and that determine their role in the control of allogeneic T-cell function and the fate of the transplanted liver. Our studies are also aimed at elucidating mechanisms by which HSC regulate DC and T-cell function. These investigations may provide new targets for therapeutic intervention in liver inflammation.

Expression of LYVE-1 in sinusoidal endothelium is reduced in chronically inflamed human livers

BACKGROUND

LYVE-1, a specific marker of lymphatics, is expressed in hepatic sinusoidal endothelium. A previous study revealed that LYVE-1 expression in sinusoidal endothelium was reduced in cirrhosis. However, it is still obscure how LYVE-1 expression in sinusoidal endothelium was reduced during the disease progression. To elucidate whether there were relationships among LYVE-1 attenuation, sinusoidal capillarization, and disease progression, we performed immunohistochemical investigations based on frozen human livers.

METHODS

Frozen liver sections of chronic hepatitis (n = 8), cirrhosis (n = 13), and normal/control liver (n = 10) were examined by immunostaining for lymphatic endothelial markers (LYVE-1 and D2-40) and vascular endothelial markers (CD31 and von Willebrand factor). Computer-aided morphometry was applied to obtain objective data. The diseased liver tissues were also examined ultrastructurally.

RESULTS

In controls, sinusoidal endothelium was positive for LYVE-1 and CD31, but negative for D2-40 and von Willebrand factor. In chronic hepatitis and cirrhosis, LYVE-1 expression in sinusoidal endothelium was attenuated, especially in areas adjacent to active inflammatory or fibrotic lesions, while von Willebrand factor was reciprocally expressed in sinusoidal endothelium. The morphometric analyses revealed that LYVE-1 positivity in sinusoidal endothelium was significantly (P < 0.0001) decreased in chronic hepatitis and cirrhosis compared to controls and was negatively correlated (Rs = -0.72, P < 0.0001) to von Willebrand factor positivity. Furthermore, LYVE-1 positivity was negatively correlated to inflammatory grade (Rs = -0.51, P = 0.021) and fibrosis severity (Rs = -0.46, P = 0.038). Sinusoidal endothelium in the diseased livers showing LYVE-1 attenuation had lost fenestrations.

CONCLUSIONS

These findings indicated that LYVE-1 attenuation in sinusoidal endothelium was one of the manifestations of capillarization, and was associated with hepatic disease progression.

Immunohistochemical analysis of development of desmin-positive hepatic stellate cells in mouse liver

Development of desmin-positive hepatic stellate cells was studied in mice using double immunofluorescent techniques and in vitro cultures with special attention given to their cell lineages. Several studies recently reported on the presence of cells that are immunologically reactive with both antidesmin and anticytokeratin antibodies in young fetal rat livers, and suggested the possibility that these cells give rise to hepatocytes and hepatic stellate cells. At early stages of mouse liver development, stellate cells with desmin-positive filaments were scattered in the liver parenchyma. However, the stellate cells definitely differed from hepatoblasts and hepatocytes in terms of their morphology and expression of desmin and hepatoblast and hepatocyte-specific E-cadherin in the liver. Fetal hepatoblasts and hepatocytes did not react with antidesmin antibodies, nor did desmin-positive stellate cells express E-cadherin in vivo and in vitro. Thus it is likely that desmin-positive stellate cells and hepatoblasts belong to different cell lineages. In the fetal liver, the desmin-positive stellate cells surrounded blood vessels, and extended their processes to haematopoietic cells and megakaryocytes. Many, but not all, hepatoblasts and hepatocytes were observed to be associated with the stellate cells. At fetal stages, cellular processes positive for desmin in the stellate cells were also thick compared with those in the adult liver, in which desmin-positive stellate cells lay in Disse's space and were closely associated with all hepatocytes. These developmental changes in the geography of desmin-positive cells in the liver parenchyma and their morphology may be associated with their maturation and interactions with other cell types.

Development of the vitamin A-storing cell in mouse liver during late fetal and neonatal periods

Vitamin A-storing cells in perinatal mouse liver were studied by chemical and autoradiographic analyses of exogenous vitamin A. The amount of retinyl palmitate in the fetal liver increased significantly following oral administration of retinyl acetate to the mother, suggesting the existence of storage sites of the vitamin in fetal liver. Light microscope semi-serial autoradiography of the fetal liver on the 15th day of gestation showed that 3H-vitamin A administered to the mother was incorporated into cells distributed exclusively along the hepatic blood vessels and the blood islands. Mitotic figures of the labeled cells were frequently observed. Electron microscope autoradiography revealed that the vitamin was incorporated into lipid droplets, rough endoplasmic reticulum and Golgi apparatus of the fibroblast-like cells in close apposition to the endothelial cells. The labeled cells differed in their ultrastructure from the vitamin A-storing cells (Ito cells) of the adult liver. In the later gestational period, silver grains tended to be more concentrated in lipid droplets, and the cytological features of the labeled cells became similar to those of the vitamin A-storing cells. Both retinyl palmitate content and the labeling of lipid droplets increased rapidly in the liver of neonates after commencement of suckling. The labeled cells had the same appearance as the vitamin A-storing cells (Ito cells). It is concluded that vitamin A transported across the placenta is taken up in the fetal liver by the cells distributed along the blood vessels, and that these cells proliferate in accordance with vascular development and gradually take on the characteristics of vitamin A-storing cells during the perinatal period. A defensive role of the vitamin A-storing cell against the toxic effects of vitamin A is also suggested.