The stem cells of the liver ? a selective review

Kurt Aterman, MUDR, MB, BCh BAO HONS, DCH, MRCP, PhD, DSc, FRCPath: "A Small Man With a Very Large Cerebrum and a Soul to Match"

Kurt Aterman was raised in the Czech-Polish portions of the former Austro-Hungarian Empire during World War I and the interwar period. After completing medical school and beginning postgraduate pediatrics training in Prague, this Jewish Czech physician fled to England as a refugee when the Nazis occupied his homeland in 1939. He repeated/completed medical training in Northern Ireland and London, working briefly as a pediatrician. Next, he served in the Royal Army Medical Corp in India, working as a pathologist. After the war and additional pathology training, he spent the next decade as an experimental pathologist in Birmingham, England. After completing a fellowship with Edith Potter in Chicago, Aterman spent the next 2 decades as a pediatric-perinatal pathologist, primarily working in Halifax, Canada. Fluent in many European languages, he finished his career as a medical historian. Aterman published extensively in all 3 arenas; many of his pediatric pathology papers were massive encyclopedic review articles, accurately recounting ideas from historical times. Aterman was a classical European scholar and his papers reflected this. Aterman was one of the founding members of the Pediatric Pathology Club, the predecessor of the Society for Pediatric Pathology. This highly successful refugee's writings are important and memorable.

Allogeneic Hepatocyte Transplantation Using FK 506

Hepatocyte transplantation is an intriguing alternative to orthotopic liver transplantation. While engraftment of syngeneic hepatocytes can be achieved with relative ease, engraftment of allogeneic hepatocytes has been far more complicated. We used FK 506 (Tacrolimus), a novel and highly efficient immunosuppressant, which has been reported to augment liver regeneration in rats. Recipients of isolated syngeneic (LEW) and allogeneic (Wistar F.) rat hepatocytes (major histocompatibility barrier) recieved different immunosuppressive regiments with FK 506 or Cyclosporine A (CsA). Mature syngeneic hepatocytes could be retrieved up to post op day 300 with the lowest number of hepatocytes on post op day 20. Following allogeneic transplantation, no mature hepatocytes could be identified after post op day 10, though ductular like structures within the spleen were found in FK 506 but not CsA-treated animals. The epithelial cells of ductular like structures exhibit cytological features of CK-19 positive cells. Our results suggest that under CsA or FK 506 immunosuppression long-term survival of mature allogeneic hepatocytes within the spleen cannot be achieved across a major histocompatibility barrier though FK 506 allows engraftment of allogeneic donor type ductular cells. Copyright © 1997 Elsevier Science Inc.

Morphometrical and immunohistochemical study of intrahepatic bile ducts in biliary atresia

BACKGROUND

Extrahepatic biliary atresia results from a progressive destruction of the bile ducts by an inflammatory fibrosing process which leads ultimately to cirrhosis of biliary type. The etiology of the disorder remains unknown. The histological features include cholestasis, ductular proliferation, eventual loss of intrahepatic bile ducts, and ducts with primitive embryonic shape (ductal plate malformation).

PURPOSE

To examine the morphological changes of the biliary intrahepatic ducts, we aimed at investigating the cell proliferation and the diameter of the interlobular bile ducts in extrahepatic biliary atresia, and in normal liver children.

METHODS

Liver samples from 35 patients with biliary atresia and 10 from control normal children were used. Immunoexpression of cytokeratin 19 was evaluated and a double-staining procedure was performed with cytokeratin 8/proliferating cell nuclear antigen. The stereological measurements of the intrahepatic bile ducts diameter were evaluated by a computerized system of image analysis.

RESULTS

The patterns of intrahepatic cholangiopathy in biliary atresia were obstructive features (42.86%), paucity of intrahepatic bile ducts (20%), ductal plate malformation (28.57%), and ductal plate malformation associated with paucity of intrahepatic bile ducts (8.57%). The average external diameter of interlobular bile ducts in biliary atresia was smaller than that of the control infant livers. Among the four patterns of biliary atresia cholangiopathies, those associated with ductopenia showed the smallest bile duct diameter. There was a negative correlation between the bile duct to portal space ratio and the age of the child at the time of Kasai portoenterostomy. Only in biliary atresia are the bile duct cells stained with proliferating cell nuclear antigen.

CONCLUSION

(i) In biliary atresia, both ductular metaplasia and ductular proliferation were observed; (ii) biliary atresia associated with ductopenia showed narrowing of interlobular ducts, probably as a consequence of degeneration with atrophy and fibrosis.

Immunohistochemical classification of ductular reactions in human liver

AIMS

  Ductular reactions occur in a wide variety of liver diseases. Their origin and function is still debated. Our understanding of these histological reactions is impaired by their great diversity; therefore rational classification should precede further detailed analysis. The aim was to achieve a reproducible classification of hepatic ductular reactions based on their immunophenotype.

METHODS AND RESULTS

  Sixty-nine liver specimens with ductular reactions were analysed by immunohistochemistry. The majority of the samples could be classified into three categories based on their immunophenotype. Type P(rimitive) reaction is characterized by CD56 immunoreactivity. Most primary biliary cirrhosis and focal nodular hyperplasia samples fall into this group; these ductules do not show any sign of differentiation. Type D(ifferentiating) ductules are positive for CD56, epithelial membrane antigen (EMA) and CD10. Cirrhotic samples and regenerating livers following fulminant hepatic failure contain such ductular reactions; this immunophenotype indicates hepatocytic differentiation. Biliary obstruction results in EMA-positive type O(bstructive) reactions; these ductules are similar to the normal interlobular bile ducts.

CONCLUSION

  Ductular reactions can be classified based on their immunophenotype. Our results may initiate further, similar, studies resulting in a generally accepted rational classification. We believe that such categorization is necessary for elucidating their biological and clinical significance.

Alpha-fetoprotein, stem cells and cancer: how study of the production of alpha-fetoprotein during chemical hepatocarcinogenesis led to reaffirmation of the stem cell theory of cancer

Identification of the cells in the liver that produce alpha-fetoprotein during development, in response to liver injury and during the early stages of chemical hepatocarcinogenesis led to the conclusion that maturation arrest of liver-determined tissue stem cells was the cellular process that gives rise to hepatocellular carcinomas. When the cellular changes in these processes were compared to that of the formation of teratocarcinomas, the hypothesis arose that all cancers arise from maturation arrest of tissue-determined stem cells. This was essentially a reinterpretation of the embryonal rest theory of cancer whereby tissue stem cells take the role of embryonal rests. A corollary of the stem cell theory of the origin of cancer is that cancers contain the same functional cell populations as normal tissues: stem cells, transit-amplifying cells and mature cells. Cancer stem cells retain the essential feature of normal stem cells: the ability to self-renew. Growth of cancers is due to continued proliferation of cancer transit-amplifying cells that do not differentiate to mature cells (maturation arrest). On the other hand, cancer stem cells generally divide very rarely and contribute little to tumor growth. However, the presence of cancer stem cells in tumors is believed to be responsible for the properties of immortalization, transplantability and resistance to therapy characteristic of cancers. Current therapies for cancer (chemotherapy, radiotherapy, antiangiogenesis and differentiation therapy) are directed against the cancer transit-amplifying cells. When these therapies are discontinued, the cancer reforms from the cancer stem cells. Therapy directed toward interruption of the cell signaling pathways that maintain cancer stem cells could lead to new modalities to the prevention of regrowth of the cancer.

Cellular origin of hepatocellular carcinomas

There are four levels of cells in the hepatic lineage which may respond to different carcinogenic regimens: (1) the mature hepatocyte, which responds to diethylnitrosamine (DEN) hepatocarcinogenesis. (2) The bile duct progenitor cells, which give rise to cholangiocellular carcinomas when the furan model is used or when hamsters infected with liver flukes (Clornorchis sinensis) are exposed to dimethylnitrosamine. (3) The ductular 'bipolar' progenitor cell which gives rise to hepatocellular carcinomas (HCC) in several N-2-acetylaminofluorene (N-2-AAF) based regimens, and (4) the periductular stem cell, which is the cell of origin of HCC induced by the choline deficiency models of hepatocarcinogenesis. Extrahepatic (bone marrow) origin of the periductular stem cells is supported by recent data showing that hepatocytes may express genetic markers of donor hematopoietic cells after bone marrow transplantation.

Cholangiocytic apoptosis in chronic ductopenic rejection

The significance of cholangiocytic apoptosis as a mechanism of ductopenia in liver rejection remains controversial. In a previous study, the presence but not the extent of ductal apoptosis was assessed by electron microscopy. Other previously published studies using an in situ hybridization method (in situ end labeling) produced conflicting results (no apoptosis v massive apoptosis). We studied 47 liver needle biopsies from 8 patients with chronic ductopenic rejection confirmed by pathologic examination of the failed grafts. These biopsies were performed because of graft dysfunction, during a period of several months before retransplantation, and they showed cholangiocytic injury with progressive ductal paucity. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick-end labeling (TUNEL) was used to detect apoptosis (tissue digestion with proteinase K 20 microg/mL for 20 minutes). The interlobular bile ducts did not show labeling, even in lymphocytic cholangitis with obvious epithelial injury. However, there was minimal staining of ductular nuclei. Lymphocytic nuclei were also labeled. Apoptosis was not detectable in the vanishing interlobular bile ducts, even when more representative samples were studied and a more sensitive method was used. Unless apoptosis of cholangiocytes is an exceptionally rapid process escaping detection by conventional methods, ductopenia results mainly from ordinary, nonprogrammed cholangiocytic death. Apoptosis could still be involved in the pathogenesis of ductopenia by depleting cholangiocytic precursors, generally presumed to reside in ductules. This is a possible mechanism suggested by the following: (1) the established role of apoptosis in the homeostatic control of immature/progenitor cells, (2) the paucity of ductular proliferation in chronic rejection, (3) the previously reported decrease of ductular bcl-2 expression in rejection, and (4) the sporadic ductular TUNEL labeling seen in this study.

The facultative stem cell: A new star in liver pathology

Although the unlimited capacity of hepatocytes to divide has been recently proven, more and more evidences support the existence of a primitive stem cell compartment in the liver. These cells probably do not participate in the usual maintenance of the liver mass, but they are activated in case of extensive hepatocyte injury. In vivo the oval cells show deep similarly to the primitive cells of the embryonic liver and seem to be the amplification compartment of the hepatic stem cells. A primitive epithelial cell population can be isolated from the normal liver and maintained in vitro. Studies of these two experimental systems provide most of the data about liver stem cells, which may become important for the clinical practice if we understand how their growth is regulated.

Interrelationship of apoptosis, mutation, and cell proliferation in N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced medaka carcinogenesis model

The present study examined the interrelationship of GSH depletion, apoptosis, mutation, and cell proliferation following carcinogen exposure. Medaka (Oryzias latipes) were investigated following a 28 day, three times/week pulse exposure to N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). Fish (5 weeks old) were exposed to MNNG at concentrations of 0, 0.5, and 1 mg l(-1) and reared for 3, 5 and 7 more months after the last day of exposure. GSH levels were decreased in the higher concentration groups and longer-reared groups. Flow cytometric analysis revealed that fish from the groups reared 3 and 5 months showed active apoptotic changes in the dose- and time-dependent manner, but the group reared 7 months had fewer apoptotic, rather showed more necrotic and carcinogenic alterations. Mutational responses were detected by an arbitrarily primed polymerase chain reaction (AP-PCR) fingerprinting method using whole body DNA samples as templates and pBR primer. A mutational change was expressed by a loss or gain of a band. There was a time-dependent mutational change, but no distinctive concentration-dependent one. A band from normal fish sample that disappeared after treatment of MNNG was excised and sequenced. The band had an 869 base pair-long sequence, however, there was no putative protein-coding region based on an analysis by DNAsis. Spindle cell sarcomas invading muscle were detected on the whole body sections from three of ten fish examined, and immunohistochemical analysis with PCNA showed that tumor cells were actively proliferating. However, terminal deoxynucleotidyl transferase (TdT) assay showed that tumored fish still had active apoptotic cell changes in the tissues without tumor. This study shows not only the interrelationship of GSH depletion, apoptosis, mutation and cell proliferation, but also indicates that medaka is appropriate as a fish model for research on the passage of carcinogenesis.

Brown bullhead (Ameiurus nebulosus) skin carcinogenesis

Alternative models using fish species have been tested in liver toxicity and carcinogenesis bioassays. Similar models have not been developed for skin. The brown bullhead (Ameiurus nebulosus) has shown potential as a model for skin carcinogenesis studies due to its sensitivity to environmental chemical pollutants. The present study is an initial morphologic and biochemical characterization of the normal and neoplastic brown bullhead skin to assess its suitability as a model of skin carcinogenesis. Brown bullhead were removed from Back River in the Chesapeake Bay region, an area historically polluted with heavy metals and polycyclic aromatic hydrocarbons. Histology, histochemistry, and electron microscopy were used to stage the morphologic development and progression of neoplasia in skin. The distribution of keratin, a family of structural proteins with altered expression in mammalian tumorigenesis, was analyzed with one and two dimensional gel electrophoresis and nitrocellulose blots of extracts from normal skin. Keratin expression in skin and other organs was also assessed with immunohistochemistry using AE1, AE3, and PCK 26 antibodies, and the proliferation index in skin and neoplasms with PCNA antibody. Skin lesions appeared to progress from hyperplasia through carcinoma, and the proliferation index was increased in papilloma. Also in papilloma, intercellular interdigitations appeared increased and desmosomes decreased which may in future studies correlate with changes in expression of other molecular markers of neoplastic progression. Both Type I and Type II keratin subfamilies were detected in skin using gel electrophoresis with the complimentary keratin blot-binding assay. For further development of the brown bullhead model, future studies can compare and relate these baseline data to alterations in expression of keratin and other markers in fish neoplasms and to molecular events which occur in man.

Immunohistochemical study on phenotypical changes of hepatocytes in liver disease with reference to extracellular matrix composition

AIMS/BACKGROUND

Extracellular matrix (ECM) may affect the function and phenotype of hepatocytes. Phenotypic changes of hepatocytes in diseased liver were investigated with reference to ECM composition.

METHODS

Immunohistochemistry was performed on biopsied liver samples from chronic viral hepatitis (CVH), primary biliary cirrhosis (PBC), primary sclerosing cholangitis (PSC) and normal patients, using monoclonal antibodies for laminin, type IV collagen, cytokeratin 19 (CK19) and epithelial glycoprotein (EGP), a protein homologous to nidogen.

RESULTS

In normal controls, both EGP and CK 19 were expressed exclusively on biliary epithelia. Laminin and type IV collagen were expressed around portal bile ducts and blood vessels. Although type IV collagen was expressed in Disse's space, laminin was scarcely expressed. In all pathological livers, both EGP and CK 19 were expressed in proliferated bile ductules. In CVH with piecemeal necrosis, EGP was expressed on periportal hepatocytes, while CK19 expression was limited to a few hepatocytes. Laminin was expressed in Disse's space of periportal sinusoids, where EGP was expressed on hepatocytes. EGP expression on hepatocytes and laminin deposition in Disse's space were rare in PBC and PSC liver.

CONCLUSION

These results suggest that hepatocytes transform into a phenotype similar to biliary epithelia and, laminin deposition in Disse's space (capillarization of sinusoids) may play a role in this phenotypic change.

Experimental studies on the stem cell concept of liver regeneration. II

There is increasing evidence in support of the existence of a hepatic stem or progenitor cell system as well as its participation in the physiological as well as reparative regeneration of the liver and in carcinogenesis. In the present paper it will be demonstrated that under the condition of a simultaneous intoxication with allyl alcohol and tetrachlorcarbon a regenerative process occurs, which is composed of two distinct partial processes: 1. a "proliferative wave" of the persisting, non-necrotic differentiated hepatocytes of the acinus, starting at 24 h after intoxication and ending after >48 h, and 2. a proliferation of cells at the rim of the portal fields, which afterwards enter the acinus and differentiate into hepatocytes, starting at 48 h after intoxication. These results support the participation of a stem or progenitor cell system in the reparative regeneration of the liver.

Diagnostic implications of albumin messenger RNA detection and cytokeratin pattern in benign hepatic lesions and biliary cystadenocarcinoma

Cytokeratin (CK) patterns and albumin messenger RNA (mRNA) are investigated in 24 patients with benign hepatic lesions (7 patients with focal nodular hyperplasia [FNH], 10 with hepatocellular adenomas [HA], 1 with biliary hamartoma, 4 with biliary cysts, 2 with cystadenomas) and in 8 patients with cystadenocarcinoma, a rare liver malignancy. The lesions and surrounding tissue of the hepatocytic components expressed CK 8 and 18 at immunohistochemistry, whereas the biliary elements evidenced CK 8 and 18 and CK 7 and 19. The albumin mRNA, as detected by in situ hybridization (ISH), revealed different distributions in the hepatocytes of FNH and HA. In the benign biliary lesions, the normal hepatocytes surrounding the tumors expressed albumin mRNA, whereas the biliary structures did not. Interestingly, in the cystadenocarcinomas, albumin mRNA was observed not only in the hepatocytes of residual parenchyma, but also in neoplastic bile duct cells lining the carcinomatous cysts; no signal was identified in the nonneoplastic biliary elements. This indicates that cystadenocarcinomas have a mixed biological phenotype and suggests they could arise either from pluripotent cells or from neoplastic cells that reacquire epigenetic features. Our results suggest two possible diagnostic applications for albumin ISH: on routine sections, it could represent an important tool for distinguishing between cystadenoma and cystadenocarcinoma; and on fine needle biopsy specimens, it could reduce uncertainty between FNH and HA.

Allogeneic hepatocyte transplantation: Contribution of Fas-Fas ligand interaction to allogeneic hepatocyte rejection

Hepatocyte transplantation is a potential therapeutic modality for overcoming the shortage of liver donors, and the clinical application of allogeneic hepatocyte transplantation has been considered. However, there are two major problems with allogeneic hepatocyte transplantation: protection of transplanted hepatocytes from rejection and stimulation of the rapid proliferation of surviving cells. Without immunosuppression, allogeneic hepatocytes are rapidly rejected within a few days after transplantation, even though it is relatively easy to induce immunotolerance after allogeneic whole liver transplantation. Accordingly, different rejection mechanisms seem to operate after allogeneic hepatocyte transplantation and whole liver transplantation. To overcome the rejection of transplanted hepatocytes, induction of donor-specific unresponsiveness to graft without compromising the host immune system would be ideal. We previously reported that the Fas-Fas ligand system plays a critical role in the CD28-independent pathway of hepatocyte rejection. Therefore, blockade of rejection using CTLA4 immunoglobulin (CTLA4Ig) or anti-CD80/86 monoclonal antibodies and anti-FasL monoclonal antibody may prolong the survival of transplanted allogeneic hepatocytes. Furthermore, administration of hepatocyte growth factor (HGF) can promote the proliferation of allogeneic hepatocytes and this may lead to the development of a functioning liver substitute.

Wound healing in the liver with particular reference to stem cells

The efficiency of liver regeneration in response to the loss of hepatocytes is widely acknowledged, and this is usually accomplished by the triggering of normally proliferatively quiescent hepatocytes into the cell cycle. However, when regeneration is defective, tortuous ductular structures, initially continuous with the biliary tree, proliferate and migrate into the surrounding hepatocyte parenchyma. In humans, these biliary cells have variously been referred to as ductular structures, neoductules and neocholangioles, and have been observed in many forms of chronic liver disease, including cancer. In experimental animals, similar ductal cells are usually called oval cells, and their association with impaired regeneration has led to the conclusion that they are the progeny of facultative stem cells. Oval cells are of considerable biological interest as they may represent a target population for hepatic carcinogens, and they may also be useful vehicles for ex vivo gene therapy for the correction of inborn errors of metabolism. This review proposes that the liver harbours stem cells that are located in the biliary epithelium, that oval cells are the progeny of these stem cells, and that these cells can undergo massive expansion in their numbers before differentiating into hepatocytes. This is a conditional process that only occurs when the regenerative capacity of hepatocytes is overwhelmed, and thus, unlike the intestinal epithelium, the liver is not behaving as a classical, continually renewing, stem cell-fed lineage. We focus on the biliary network, not merely as a conduit for bile, but also as a cell compartment with the ability to proliferate under appropriate conditions and give rise to fully differentiated hepatocytes and other cell types.

Stem cells and rat liver carcinogenesis: contributions of confocal and electron microscopy

Microscopic analysis in combination with cytochemistry and immunocytochemistry has revealed the presence of four cell types not previously described in the portal area and parenchyma of the liver from an experimental rodent hepatocarcinogenic rat model. Within the intrahepatic bile ductules, which proliferate after administration of chemical carcinogens and partial hepatectomy, small, undifferentiated nonpolarized, nonepithelial cells with a blast-like phenotype and polarized epithelial cells different from the polarized epithelial cells that typically line the walls of the bile ductules were found. In the connective tissue stroma surrounding the bile ductules, nonpolarized epithelial cells with hepatocyte phenotype were found. In the parenchyma, subpopulations of bile ductule epithelial cells that established ATPase-positive bile canalicular structures, including the formation of desmosomes and tight junctions, with parenchymal hepatocytes within the hepatic lobule were found. These observations raise the following questions in this model. Are there undifferentiated progenitor cells with stem cell-like properties within bile ductules? What are the interrelations of the newly described cell types with each other, with parenchymal hepatocytes, with preneoplastic nodules, and with hepatomas? Do the heterogeneous cell types within the bile ductules, in the surrounding connective tissue, and within the hepatic cords represent intermediate stages of single or multiple cell lineage pathways leading to hepatocyte differentiation, liver regeneration, and/or preneoplastic nodule formation?

Maturation-dependent gene expression in a conditionally transformed liver progenitor cell line

We have isolated a conditionally transformed liver progenitor cell line with phenotypic similarities to both hepatoblasts (bipotent embryonic liver cells that give rise to hepatocytes and intrahepatic biliary epithelial cells) and liver epithelial cells (primitive hepatic cells isolated from adult livers capable of generating both hepatocytic and biliary lineages). Cell line L2039 was derived from E14 fetal mouse liver after transformation with temperature-sensitive SV-40 large T antigen. At 33 degrees C, these cells have an epithelial morphology with a high nucleocytoplasmic ratio and express both hepatocytic and biliary genes, including albumin, alpha-fetoprotein, glutamine synthetase, insulinlike growth factor II receptor, fibronectin and laminin, and cytokeratins 8 and 19, a set of markers characteristic for hepatoblasts. The presence of cytokeratin 14, vimentin, and several oval-cell antigens link cell line L2039 to nonparenchymal liver epithelial cell populations thought to contain progenitor cells. Serum-free, hormonally defined media conditions and extracellular matrix requirements were determined for growth and differentiation of this cell line. During culture on type IV collagen at 39 degrees C, L2039 cells cease dividing and demonstrate hepatocytic differentiation with the assumption of a hepatocytelike morphology and glucocorticoid-dependent regulation of liver-specific genes, including albumin, alpha-fetoprotein, phosphoenolpyruvate carboxykinase, and liver-enriched transcription factors. The number of albumin-positive cells increases during culture at 39 degrees C, indicating that L2039 cells convert from a prehepatocytic to a hepatocytic phenotype. Under conditions specific for hepatocytic differentiation, C/EBPs were expressed and differentially regulated, with C/EBPbeta and C/EBPdelta upregulated early and C/EBPalpha only slightly expressed after 7 d, indicating that C/EBPalpha may not be a crucial factor in commitment to the hepatocytic phenotype.

Experimental studies on the stem cell concept of liver regeneration. I

There are increasing references for the existence of a hepatic stem or progenitor cell system as well as its participation in the physiological as well as reparative regeneration of the liver and in carcinogenesis. For the physiological regeneration the existence of a dynamic "cell-renewal" system finds increasing consideration and in the "streaming liver concept" (Zajicek et al. 1985) its functional expression. This concept is still under discussion. The present paper tries to check this animal-experimentally (Wistar rats) under use of two different thymidine analogues (3H-thymidine and Bromodeoxyuridine). In different time intervals after labelling (1 h, 14, 30, 60, 90, 120 d) a shift of the labelling bias or a migration of the hepatocytes in the liver acinus (Rapaport) in portovenous direction could be shown. The average migration speed is 0.575 microm or 0.0315 cell positions per day, the cell production rate is one in 31.5 days. The present paper results support the inclusion of a stem or progenitor cell system into the physiological regeneration of the liver and allow the classification into the "streaming liver concept" (Zajicek et al. 1985).

Comparison of liver progenitor cells in human atypical ductular reactions with those seen in experimental models of liver injury

The ultrastructural characteristics of liver progenitor cell types of human atypical ductular reactions seen in chronic cholestasis, in regenerating human liver after submassive necrosis, in alcoholic liver disease, and in focal nodular hyperplasia are compared with liver progenitor cell types seen during experimental cholangiocarcinogenesis in hamsters; during hepatocarcinogenesis in rats; and in response to periportal liver injury induced by allyl alcohol in rats. Three types of progenitor cells have been identified in human atypical ductular reactions: type I: primitive, has an oval shape, marginal chromatin, few cellular organelles, rare tonofilaments, and forms desmosomal junctions with adjacent liver cells; type II: bile duct-like, is located within ducts, has few organelles, and forms lateral membrane interdigitations with other duct-like cells; and type III: hepatocyte-like, is located in hepatic cords, forms a bile canaliculus, has tight junctions with other hepatocyte-like cells, prominent mitochondria and rough endoplasmic reticulum, and some have lysosomes and a poorly developed Golgi apparatus. Each type is seen during cholangiocarcinogenesis in hamsters, but the most prominent cell type is type II, duct-like. A more primitive cell type ("type 0 cell"), as well as type I cells, are seen in the intraportal zone of the liver within 1 to 2 days after carcinogen exposure or periportal injury in the rat, but both type II and type III are seen later as the progenitor cells expand into the liver lobule. After allyl alcohol injury, type 0 cells precede the appearance of type I and type III cells, but most of the cells that span the periportal necrotic zone are type III hepatocyte-like cells showing different degrees of hepatocytic differentiation. Some type II cells are also seen, but these are essentially limited to ducts. It is concluded that there is a primitive stem cell type in the liver (type 0) that may differentiate directly into type I and then into type II, duct-like or or type III hepatocyte-like cells. The terms oval cell, transitional hepatocyte, biliary hepatocyte, hepatocyte-like cell, atypical ductular cell, neocholangiole, etc., are used to describe these cells. Although these terms are useful as general descriptive terms for liver precursor cells at the light microscopic level, the cells included in these descriptive categories may be very different from one another biologically and ultrastructurally.

Characterization of growth factor responsiveness and alterations in growth factor homeostasis involved in the tumorigenic conversion of mouse oval cells

Five mouse oval cell lines were investigated in regards to their growth and differentiation factor (GDF) responsiveness and to changes in their GDF responsiveness following tumorigenic conversion. In all 59 GDFs and 11 comitogens were evaluated with variable responsiveness, depending on the mouse oval cell line under study, observed. Analysis of oval cell GDF responsiveness during tumorigenic conversion revealed that tumorigenic variants displayed alterations in GDF responsiveness which correlated with tumorigenicity. In addition, analysis of autocrine/paracrine growth factor production demonstrates that most tumorigenic variants produce growth factors. These studies demonstrate for the first time that (1) mouse oval cells respond to a wide variety of GDFs including various members of the interleukin, chemokine, stem cell factor, EGF, FGF, PDGF, TGF-beta, VEGF, insulin, CSF, TNF, HGF, and IFN growth and differentiation factor families in addition to multiple comitogens and (2) during tumorigenic conversion mouse oval cells undergo alterations which result in both alterations in GDF responsiveness and the autocrine/paracrine production of multiple GDFs.

Substitution of 15(S)hydroxyeicosatetraenoic acid in phosphatidylinositol alters the growth of liver epithelial cells

We investigated the substitution of 15(S)-hydroxyeicosatetraenoic acid (15(S)HETE) in phospholipid signaling pathways and its consequences on the growth of non-transformed (NT-) and spontaneously transformed (T-) rat liver epithelia cells (RLEC). 15(S)HETE was selectively incorporated into the sn-2 position of phosphatidylinositol (PI) and at a higher rate into T-RLEC. RLEC rapidly mobilized the resulting 15(S)HETE-containing PI (15(S)HETE-PI) and produced 1-acyl,2-[1(S)HETE]-glycerol. Although total diacylglycerol levels were similar in both cell types, the ratio 1-acyl,2-[15(S)HETE]-glycerol / 15(S)HETE-PI was higher in NT-RLEC, suggesting a lower mobilization of 15(S)HETE-PI in T-RLEC. Using rat brain protein kinase C, 1-stearoyl,2-[15(S)HETE]-glycerol was as potent an in vitro protein kinase C activator as 1-stearoyl,2-arachidonoyl-glycerol. Finally, selective substitution of 15(S)HETE in PI altered DNA synthesis in T-RLEC: whereas low concentrations of 15(S)HETE (1 nM and 10 nM) in these cells were mitogenic, higher concentrations resulted in a 30% inhibition of DNA synthesis.

Electron microscopic identification of putative liver stem cells and intermediate hepatocytes following periportal necrosis induced in rats by allyl alcohol

The ultrastructural characteristics of the putative liver stem cells that repopulate the necrotic periportal zones after allyl alcohol induced liver injury are described. Periportal liver cell necrosis was induced in adult female Sprague-Dawley rats by i.p. injection with 0.62 mmol/kg of allyl alcohol. Electron microscopic examination of the livers was carried out at 33, 57, 81 and 129 h after injection. After periportal necrosis small nondescript intraportal cells (putative liver stem cells) as well as three type of "progenitor" cells are seen: type I, immature "precursor" cells; type II, bile duct-like; and type III, hepatocyte-like, with numerous cells of intermediate type between type I and type III. The periportal necrotic zone (zone I) is reconstituted largely by an increase in hepatocyte-like cells containing mitochondria, lysosomes, lipid-filled vacuoles, rare peroxisomes, prominent endoplasmic reticulum and lateral microvilli (type III cells) with a relatively small number of type I (immature) cells participating. The type III cells display different degrees of differentiation; the less mature are termed "restitutive" and the more mature "transitional" hepatocytes to emphasize the probable relationship between these cell types. Immature ductular cells (type II cells) are seen located basally within hyperplastic ducts in the periportal zone. It is postulated that hepatocyte restitution after periportal necrosis is accomplished by proliferation and differentiation of stem cells with both biliary and hepatic potential that specifically differentiate into hepatic cells through "restitutive" and "transitional" intermediates. These postulated liver stem cells may be intraportal cells seen 33-57 h after injury that precede the type I and type III hepatic precursors seen later.

Immunocytochemical study of parafollicular cells of the thyroid and ultimobranchial remnants of the European bison

The aim of the present study was to compare parafollicular cells in the bison thyroid and its ultimobranchial remnants. The thyroid of 26 European bisons was fixed in Bouin's fluid, 5 microns thick paraffin sections were stained with hematoxylin and eosin. Azan or silver Grimelius methods. For immunocytochemical analysis specific rabbit antisera were used against human calcitonin (CT), human calcitonin gene-related peptide (CGRP), bovine (b) or rat (r) neuron-specific enolase (NSE), human synthetic somatostatin (ST), and porcine chromogranin. Strongly positive reactions in the majority of parafollicular cells were observed after application of antisera against CT, CGRP, bNSE and rNSE only. ST-immunopositive cells were found in small numbers. Immunopositive parafollicular cells were also present outside typical structures of the thyroid within persistent ultimobranchial remnants. In persistent ultimobranchial bodies, parafollicular cells were frequently observed in groups between ultimobranchial follicles in form of solid cell nests. Many of these cells did not react with any of the antisera used and showed features of immature cells. It is concluded that histomorphologic analysis and immunocytochemical examination reveals a heterogeneous population of parafollicular cells in the bison thyroid, and this heterogeneity was particularly clear in persistent ultimobranchial bodies.

Extracellular matrix composition and integrin expression in early hepatocarcinogenesis in human cirrhotic liver

Extracellular matrix (ECM) plays a major role in cell differentiation, proliferation, and gene expression, both in physiological and in pathological conditions. Immunohistochemistry has been used to investigate modifications of ECM and related receptors, the integrins, in 26 small nodular lesions developed in human cirrhotic livers, on the basis that these lesions could represent sequential steps of hepatocarcinogenesis: the lesions were 16 macroregenerative nodules (MRNs), either of ordinary (n = 5) or atypical (n = 11) type, and ten small (< 15 mm) hepatocellular carcinomas (HCCs). Data were compared with those obtained in the surrounding cirrhotic tissue, in large HCCs, and in normal liver. The results indicate similarities between ordinary MRNs and cirrhosis, on the one hand, and between atypical MRNs and small HCCs, on the other. Strong and homogeneous deposition of collagen type IV and laminin in sinusoids and overexpression of alpha 6 integrin by sinusoidal cells and hepatocytes were especially noticeable in dysplastic areas characteristic of atypical MRNs, as in small HCCs. In addition, the staining of alpha 2 and alpha 6 integrins in MRNs revealed the presence of widespread atypical ductular proliferation expanding from periportal and perinodular areas, containing epithelial cells with transitional (hepato-biliary) phenotype. These findings suggest a transition from atypical MRNs to small HCCs and a possible role for liver epithelial precursor cells ('stem cells') in the development and evolution of MRNs.

Wholesale hepatocytic differentiation in the rat from ductular oval cells, the progeny of biliary stem cells

BACKGROUND/AIMS

Biliary epithelial cells (ductular oval cells) migrate into the periportal and midzonal parenchyma when hepatocyte regeneration after injury is significantly impeded. The potential of oval cells to differentiate into hepatocytes has been questioned. We have sought to resolve this issue using the modified Solt-Farber procedure in which 2-acetylaminofluorene is used to block hepatocyte regeneration in partially hepatectomized rats.

METHODS

Rats received 2-acetylaminofluorene by oral gavage for 6 days before and up to 7 days after a two-thirds hepatectomy. The cellular reaction was visualized by the immunohistochemical localization of intermediate filaments cytokeratins 8 and 19 and vimentin, cytochrome P450 enzymatic proteins and alpha-foetoprotein. Expression of albumin and alpha-foetoprotein mRNA transcripts were observed in situ using antisense riboprobes.

RESULTS

During the first 9 days after partial hepatectomy long strings of ductular cells spread outwards from the portal areas. These cells exhibited strong diffuse cytoplasmic staining with the anticytokeratin 8 and 19 antibodies, like authentic bile ducts, but in addition also expressed vimentin and alpha-foetoprotein (protein and mRNA)-collectively termed the "oval cell phenotype". Thereafter, these ducts rapidly vanished to be replaced by basophilic hepatocytes which lacked the oval cell phenotype, but which acquired strong expression of albumin mRNA. At 14 days after partial hepatectomy the oval cell phenotype was restricted to the peripheral margins of the newborn periportal hepatocytes, the distal tips of the oval cell ducts, and these too had disappeared within another 7 days.

CONCLUSIONS

Ductular oval cells will differentiate into hepatocytes under appropriate experimental conditions.

Phenotypic and genotypic analysis of rat liver epithelial cells infected with retroviral shuttle vectors

Rat liver epithelial cells (RLE) are suspected to be pluripotent hepatic stem cells that give rise to a diverse variety of liver tumors. The molecular events responsible for transformation of these cells and the diversity of the tumor phenotypes remains to be fully elucidated. We examined the genotype and phenotype of RLE cells infected with retroviral shuttle vectors carrying a neomycin resistance (neor) Ha-ras or a lacZ gene. WBneoIII, WBrasIII and WBlacZ cell lines were examined for evidence of a transformed phenotype by comparing their behavior with the parental strain (WB-344) and with WBneo-C-II and WBrasII cells. Confluent cultures of WBneo-C-II and WBrasII cells were found to contain significantly higher numbers of total cells than the other cell lines. The growth rate of WBneo-C-II and WBrasII cells were faster than that of the parental cell line. Addition of epidermal growth factor (EGF) to the medium was found to stimulate the growth rate of WBneo-C-II cells and to induce anchorage independent growth (AIG). No cell line produced tumors in nude mice (nu/nu) except WBrasII cells. Radioimmunoprecipitation studies and sequencing of the p53 exons 5-8 indicate WBneo-C-II, and WBrasII cells produce a mutant p53. Northern blot analysis showed an increased expression of c-myc mRNA in WBneo-C-II and WBrasII cells. These results demonstrate that alterations in critical growth and differentiation controlling genes have occurred in WBrasII cells which may, independent of or in conjunction with ras insertion, cause the transformed phenotype.

Reactive biliary epithelium: the product of a pluripotential stem cell compartment?

Liver parenchymal cells (hepatocytes) have a low rate of turnover, but can nevertheless mount a rapid and efficient regenerative response. However, in some cases of extreme hepatotoxicity hepatocyte proliferation is restricted or even abolished, and instead biliary epithelial cells, commonly referred to as ductular oval cells, migrate into the periportal and midzonal parenchyma. Initially these cells behave as authentic biliary epithelium with expression of the biliary cytokeratin intermediate filaments, but then show hepatocytic traits such as alpha fetoprotein and albumin synthesis. Thereafter these biliary ducts rapidly vanish to be replaced by either small hepatocytes or intestinal-type cells. The proliferation and differentiation of oval cells is probably strongly influenced by paracrine signalling from liver stellate cells. Oval cells appear to be the progeny of facultative pluripotential stem cells which have the lineage potential of uncommitted gastrointestinal stem cells; these stem cells are likely to be located in the cholangioles and small interlobular bile ducts. Oval cells thus constitute an important reserve compartment for hepatocytes when hepatocyte regeneration is compromised.

Experimental chemical carcinogenesis in fish

Experimental carcinogenesis using fish species as alternative models is a dynamic field of research. The 1940's expansion of synthetic chemical producing industries coincided with a number of pollution-associated fish neoplasia epizootics, with polycyclic aromatic hydrocarbons as significant components of contaminated sediment in several cases. Epizootics of primarily liver and skin neoplasia in benthic species near coastal urban or industrial areas indicated the sensitivity of fish species to known mammalian carcinogens. Stressing a mechanistic approach, investigators have used data compiled from epizootics as the backbone of current research efforts to define carcinogenesis in fish species. With liver as the focus, patterns of neoplastic development similar to those seen in rodent bioassays have been induced in various fish species by genotoxic carcinogens. Similarities between fish and rodent models include chemical and species-specific responses to exposure and the development of predictable preneoplastic and neoplastic lesions. The expression of molecular molecules related to carcinogenesis is currently under investigation, which includes alterations in certain proteins, enzyme activity, and oncogene/tumor suppressor gene function. The potential for the application of research findings to both human and environmental health issues makes fish species attractive and valuable alternative models in carcinogenesis and toxicity research.

Pluripotential liver stem cells: facultative stem cells located in the biliary tree

The ability of the liver to regenerate after parenchymal damage is usually accomplished by the ephemeral entry of normally proliferatively quiescent (G0) hepatocytes into the cell cycle. However, when hepatocyte regeneration is defective, arborizing ductules which are continuous with the biliary tree, proliferate and migrate into the surrounding parenchyma. In man these biliary cells have variously been referred to as ductular structures, neoductules and neocholangioles, and have been observed in many forms of chronic liver disease, including cancer. In experimental animals similar ductal cells are usually called oval cells, and their association with defective regeneration has led to the belief that these cells represent a progenitor cell population. Oval cells are thought to take over the burden of regenerative growth after substantial hepatocyte loss, suggesting that they are the progeny of facultative stem cells. The liver is not, however, generally considered as a stem cell-fed hierarchy, although this is disputed by others. Despite this, the subject of oval cells has aroused intense interest as these cells may represent a target population for hepatic carcinogens, and they may be useful vehicles for ex vivo gene therapy. This review proposes that the liver does harbour stem cells which are located throughout the biliary epithelium, and that oval cells represent the progeny of these stem cells and function as an amplification compartment for the generation of 'new' hepatocytes. This is a conditional process which only occurs when the regenerative capacity of hepatocytes is overwhelmed and thus, unlike the intestinal epithelium, the liver is not behaving as a classical continually renewing stem cell-fed lineage. We focus on the biliary network, not merely as a conduit for bile, but also as a cell compartment with the potential to proliferate under appropriate conditions and give rise to fully differentiated hepatocytes and other cell types.

Incorporation of 12(S)-hydroxyeicosatetraenoic acid into phospholipids and active diacylglycerols in rat liver epithelial cells: effects on DNA synthesis

12(S)-Hydroxyeicosatetraenoic acid (12-HETE), the 12-lipoxygenase-derived metabolite of arachidonic acid, was incorporated into membrane phospholipids (PL) in various cells. PL are precursors of diacylglycerol (DAG), a protein kinase C (PKC) activator involved in cell-growth signaling. We studied 12-HETE incorporation into PL in non-transformed (NT-) and spontaneously transformed (T-) rat liver epithelial cells (RLEC), and its consequence on DNA synthesis. NT- and T-RLEC incorporated 12-HETE predominantly into phosphatidylcholine (PC). 12-HETE was incorporated at a greater rate, and with a higher phosphatidylethanolamine (PE)/PC ratio in T-cells. Preincubation of RLEC with 12-HETE at > or = 0.5 microM partially inhibited basal DNA synthesis in NT- and T-RLEC. Preincubation of NT-RLEC with 12-HETE (2.5 microM) also decreased bradykinin-stimulated DNA synthesis. Unstimulated RLEC produced 1-acyl-2-(12-HETE)DAG which was increased in NT-RLEC by bradykinin stimulation. Finally, 1-stearoyl-2-(12-HETE)DAG was as potent an in vitro PKC activator as 1-stearoyl-2-arachidonyl-DAG. These data demonstrate that 12-HETE incorporation into PL resulted in the production of active 12-HETE-containing DAG, together with reduced DNA synthesis.

Isolation and characterization of biliary epithelial cells from rainbow trout liver

Lectin binding and density gradient centrifugation were explored for isolating epithelial cells from trout liver. Hepatocytes exhibited preferential attachment of coverslips coated with Phaseolus vulgaris erythroagglutinin. Biliary epithelial cells attached with glycine max agglutinin; however, significant attachment of cellular debris limited the use of glycine max agglutinin. Percoll-density gradient centrifugation separated liver cells into two distinct populations with biliary cells and hepatocytes banding at densities of 1.04 and 1.09, respectively. A discontinuous gradient composed of 13% Ficoll (wt/wt) separated biliary cells from hepatocytes. The recovery of highly enriched biliary epithelial cells from trout liver using Ficoll gradients yielded approximately 8 million cells (0.1 ml packed cells) from 10 g liver. Western blot analysis demonstrated that the cytokeratin profile for extracts from biliary epithelial cell-enriched populations differ significantly from those seen with whole liver extracts or with extracts with hepatocyte-enriched populations. Ficoll-gradient purified biliary cells and hepatocytes attached to culture plates coated with trout skin extract and carried out linear incorporation of leucine into protein and thymidine into DNA for 24 h. A mixture of growth hormones (insulin, epidermal growth factor, and dexamethasone) stimulated thymidine incorporation into DNA; however, long-term culture of dividing biliary epithelial cells was not achieved. Chemical analysis of neutral and acidic glycolipids indicated that hepatocytes and biliary cells have similar glycolipid profiles with an exception in the region of GM3 mobility, which is attributed to differences in the ceramide moiety. These studies provide a starting point for further characterization of unique cell types of the trout liver that may be important in their responses to toxic and carcinogenic agents.

Evaluation of the role of mammalian thyroid parafollicular cells

In this report recent views are presented on the role of the parafollicular cells (PF) in the mammalian thyroid. Contemporary studies indicate morphological and functional heterogeneity of the PF cell population. In normal conditions most PF cells synthesize and secrete calcitonin (CT) and therefore they are frequently referred to as C cells. It seems however, that the contribution to the regional intrathyroidal regulation of secretion and growth processes is also an important role of all functionally mature PF cells of APUD (amine precursor uptake and decarboxylation) series. This has been confirmed by the latest reports on PF cells secreting numerous regulatory peptides (RP) usually defined as "paracrine" and/or "autocrine factors". These peptides are produced jointly with other RP in the same PF cells. Some of RP like CT, somatostatin, katacalcin I (CCP-I), CCP-II, gastrin-releasing peptide, thyroliberin and helodermin have been found in the PF cells, exclusively. Other RP, including calcitonin gene-related peptide, N-terminal peptide, neuromedin U, cholecystokinin and secretory peptide-I, have been simultaneously observed in the PF cells and intrathyroidal nerve fibres. Genetic mechanisms involved in RP production in the PF cells and possible path ways by which these peptides affect the adjacent follicular cells in the thyroid are discussed.

Adenomatous hyperplasia in cirrhotic livers: histological evaluation, cellular density, and proliferative activity of 35 macronodular lesions in the cirrhotic explants of 10 adult French patients

We examined 41 consecutive cirrhotic liver explants from French patients for the presence of nodules of adenomatous hyperplasia (AH) and then analyzed these lesions, together with underlying cirrhosis (C) and associated hepatocellular carcinoma (HCC), for various histological parameters, cellular density, and proliferative activity. Thirty-five AHs were identified in 10 livers (prevalence, 24%); seven of 10 were HCV positive. Hepatocellular carcinoma was more frequent in patients with AH than in patients without. The AHs consisted of 17 ordinary (OAH) and 18 atypical (AAH) adenomatous hyperplasia lesions. There was a malignant focus in five of the 18 AAHs. Wide areas of large liver cell dysplasia were frequent in OAH but never found in AAH. Obvious steatosis was frequent in HCC but exceptional in AAH and absent in OAH. There was a significant increase in cellular density in AAH and HCC as compared with C and OAH. Proliferative cell nuclear antigen immunostaining similarly showed an increase in proliferation from OAH or C to AAH and HCC. These data suggest that, in Europe as in Japan, one pathway of hepatocarcinogenesis is a multistep process in which AAH should be considered as a premalignant lesion very close to grade I HCC, while OAH seems to correspond to a regenerative nodule with limited proliferative ability.

Incorporation of arachidonic and eicosapentaenoic acids into phospholipids of non-transformed and spontaneously-transformed rat liver epithelial cells: effects on DNA-synthesis

We compared the incorporation of arachidonic acid (AA) and eicosapentaneoic acid (EPA) into phospholipids of non-transformed (NT-) and spontaneously-transformed (T-) rat liver epithelial cells (RLEC), and their consequences on DNA-synthesis. In NT-cells, both radioactive fatty acids were preferentially incorporated into phosphatidylcholine (PC). In T-cells, in contrast, AA was predominantly incorporated into phosphatidylethanolamine (PE), whereas EPA remained preferentially incorporated into PC. After pulse labelling, we observed in both cell types a progressive decrease in AA- and EPA-labelled PC associated with an increase in AA- and EPA-labelled PE. Preincubation of NT-cells with increasing concentrations of AA or EPA (0.1 microM to 20 microM) resulted in a concentration-dependent DNA-synthesis stimulation with a stronger effect of AA compared with EPA. In T-cells, the same treatment had no effect on DNA-synthesis.

Constitutive over-expression of transforming growth factor-alpha in rat liver epithelial cells leads to increased cell cycling without transformation

Over-expression of transforming growth factor-alpha (TGF-alpha) is consistently seen in spontaneous transformants of rat liver derived epithelial cells (RLE phi 13) and has been implicated in the transformation of other cultured cells. We have constitutively over-expressed TGF-alpha in RLE phi 13 cells, which are known to express epidermal growth factor receptors, to determine if TGF-alpha over-expression plays a role in transformation or differentiation, or both, of these cells. Early passage RLE phi 13 cells were infected with a replication-defective murine retrovirus that expresses both the full length coding sequence for human TGF-alpha and the neomycin-resistance gene. Integration of the transcriptionally active provirus and expression of TGF-alpha mRNA were confirmed. Neither morphologic transformation nor molecular evidence for differentiation was noted in TGF-alpha-producing clones. However, these clones did exhibit an accelerated growth rate, increased expression of several cell cycle related genes including mitotic cyclic B1, proliferating cell nuclear antigen, c-myc, and p53 as well as increased expression of the preneoplastic marker enzyme, glutathione-S-transferase. This suggests that over-expression of TGF-alpha results in increased cell cycling, and that subsequent events must be necessary for cellular transformation or differentiation or both.

Expression of hepatic transcription factors during liver development and oval cell differentiation

The oval cells are thought to be the progeny of a liver stem cell compartment and strong evidence now exists indicating that these cells can participate in liver regeneration by differentiating into different hepatic lineages. To better understand the regulation of this process we have studied the expression of liver-enriched transcriptional factors (HNF1 alpha and HNF1 beta, HNF3 alpha, HNF3 beta, and HNF3 gamma, HNF4, C/EBP, C/EBP beta, and DBP) in an experimental model of oval cell proliferation and differentiation and compared the expression of these factors to that observed during late stages of hepatic ontogenesis. The steady-state mRNA levels of four (HNF1 alpha, HNF3 alpha, HNF4, and C/EBP beta) "liver-enriched" transcriptional factors gradually decrease during the late period of embryonic liver development while three factors (HNF1 beta, HNF3 beta, and DBP) increase. In the normal adult rat liver the expression of all the transcription factors are restricted to the hepatocytes. However, during early stages of oval cell proliferation both small and large bile ducts start to express HNF1 alpha and HNF1 beta, HNF3 gamma, C/EBP, and DBP but not HNF4. At the later stages all of these factors are also highly expressed in the proliferating oval cells. Expression of HNF4 is first observed when the oval cells differentiate morphologically and functionally into hepatocytes and form basophilic foci. At that time the expression of some of the other factors is also further increased. Based on these data we suggest that the upregulation of the "establishment" factors (HNF1 and -3) may be an important step in oval cell activation. The high levels of these factors in the oval cells and embryonic hepatoblasts further substantiates the similarity between the two cell compartments. Furthermore, the data suggest that HNF4 may be responsible for the final commitment of a small portion of the oval cells to differentiate into hepatocytes which form the basophilic foci and eventually regenerate the liver parenchyma.

Hepatic neoformations

The aim of the paper is an accurate histologic description and illustration of those liver lesions that are usually summarized under the heading of "hepatic tumors and related subjects". For in some cases it may be unclear or at least controversial, whether the individual lesion is indeed an autonomous neoplasia or a malformation, regeneration or hyperplasia, the indifferent master term of neoformation is introduced, based on the fact that all of them are characterized by a cellular multiplication. According to common definitory practice the survey distinguishes between mesenchymal (angiomatous and non angiomatous) and epithelial neoformations. Among the latter hepatocellular and cholangiocellular types are distinguished, the criterium for differentiation being a phenomenological one, which is by no means identical with a histogenetical statement. The definition of subgroups mostly adheres to current nomenclatory usage; only occasionally--in the group of endothelial tumors--a novel term is employed, in view of brevity and coordination with the overall system of neoformations.

Expression of HBs- and HBc-antigen in neoductular epithelium in chronic active hepatitis B. A further support for hepato-ductular metaplasia

Biopsy specimens (n = 61) from patients with chronic active hepatitis B and progressive fibrosis (n = 61) were studied immunohistochemically to obtain information about the histogenesis of neoductules. All the biopsies contained clusters of oval-shaped cells often arranged in the form of neoductular aggregates. These expressed cytokeratins 7 and 19 which in the normal liver are found only in bile duct and ductular epithelium but not in hepatocytes. Using monoclonal and polyclonal antibodies both hepatocytes and these oval neoductular cells were found to express HBs- and HBc-antigen in 15% and 20% of the biopsies, respectively. Taking into consideration the strong hepatocytotropism of the hepatitis B virus, the expression of HBV-antigens in neoductular cells suggest their development from HBV-infected hepatocytes. Using proliferating cell nuclear antigen (PCNA) as a marker of cell proliferation positive staining was detected only in hepatocytes but not in neoductular cells. Taken together findings further support the concept of hepatoductular metaplasia in the histogenesis of so-called "proliferating" ductules. In general the data show that hepatitis B virus infection does not prevent hepatocytes from undergoing ductular metaplasia.