Liver stem cells—prospects for clinical use
2004; Elsevier BV; Volume: 42; Issue: 1 Linguagem: Inglês
10.1016/j.jhep.2004.12.009
ISSN1600-0641
AutoresDavid Tosh, Alastair J. Strain,
Tópico(s)Pancreatic function and diabetes
Resumo1. IntroductionLiver diseases affect approximately 17.5% of the population [[1]Bellentani S. Tiribelli C. The spectrum of liver disease in the general population: Lessons from the Dionysos study.J Hepatol. 2001; 35: 531-537Abstract Full Text Full Text PDF PubMed Scopus (190) Google Scholar] with the most severe cases being treated by orthotopic transplantation. Liver transplantation has become almost a routine procedure with one-year survival rates exceeding 80% [[2]Neuberger J. Liver transplantation.J Hepatol. 2000; 32: 198-207Abstract Full Text PDF PubMed Google Scholar]. One alternative to whole organ transplantation for patients with fulminant hepatic failure and hereditary liver disease is hepatocyte transplantation. The transplantation approach has been used to re-establish liver function in a small number of cases [3Strom S.C. Chowdhury J.R. Fox I.J. Hepatocyte transplantation for the treatment of human disease.Semin Liver Dis. 1999; 19: 39-48Crossref PubMed Scopus (335) Google Scholar, 4Ohashi K. Park F. Kay M.A. Hepatocyte transplantation: Clinical and experimental application.J Mol Med. 2001; 79: 617-630Crossref PubMed Scopus (121) Google Scholar, 5Fox I.J. Chowdhury J.R. Kaufman S.S. Goertzen T.C. Chowdhury N.R. Warkentin P.I. et al.Treatment of the Crigler-Najjar syndrome type I with hepatocyte transplantation.New Engl J Med. 1998; 338: 1422-1426Crossref PubMed Scopus (902) Google Scholar, 6Fox I.J. Roy-Chowdhury J. Hepatocyte transplantation.J Hepatol. 2004; 40: 878-886Abstract Full Text Full Text PDF PubMed Google Scholar, 7Selden C. Hodgson H. Cellular therapies for liver replacement.Transplant Immunol. 2004; 12: 273-288Crossref PubMed Scopus (81) Google Scholar]. Compared to orthotopic liver transplantation, hepatocyte transplantation offers a number of advantages:1.Hepatocytes from a single donor can be used for more than one recipient.2.Cell transplantation does not involve a surgical procedure and is, therefore, associated with a lower morbidity compared to whole liver transplantation and3.Hepatocytes could be used in a temporary capacity until the patient's liver can regenerate.The ability to utilise functional hepatocytes has important therapeutic implications for other cell-based therapies including the development of bioartifical liver systems. However, there is one serious limitation to the treatment of liver diseases either by whole organ or hepatocyte transplantation or by using an extracorpeal liver support system, and that is the shortage of organ donors. Although hepatocyte transplantation generally occurs using cells isolated from livers not used for transplantation, the present in vitro culture systems cannot be expanded sufficiently to meet demand. Therefore, to overcome these problems, additional sources of hepatocytes must be found. Alternative sources of cells for transplantation have been proposed including immortalised cells. For example, the human hepatoblastoma cell line, HepG2 has been used both as a model for understanding liver function and in an artificial liver system [[8]Sussman N.L. Gislason G.T. Conlin C.A. Kelly J.H. The Hepatix extracorpeal liver assist device—initial clinical experience.Artifical Organs. 1994; 18: 390-396Crossref PubMed Scopus (271) Google Scholar]. However, HepG2 cells fail to reproduce the in vivo situation i.e. they do not express the urea cycle enzymes and cannot perform ketogenesis [[9]Agius L. Human in vitro techniques for metabolic studies.Baillieres Clin Endocrinol Metab. 1987; 1: 999-1021Abstract Full Text PDF PubMed Scopus (9) Google Scholar], so may not be an ideal model for well-differentiated human hepatocytes.There has been much interest in recent years in using stem cells to repair or regenerate damaged tissues and organs. Stem cells can be utilised as a source for cell transplantation due to their ability to differentiate into a variety of cell types. This strategy is one branch of 'regenerative medicine'. The term regenerative medicine refers to the stimulation of regeneration of damaged or defective tissues and replacement of diseased or damaged organs constitutes one of the fundamental challenges to tissue engineers. Stem cells either embryonic (derived from the inner cell mass of the blastocyst [10Thomson J.A. Itskovitz-Eldor J. Shapiro S.S. Waknitz M.A. Swiergiel J.J. Marshall V.S. et al.Embryonic stem cell lines derived from human blastocyst.Science. 1998; 282: 1145-1147Crossref PubMed Scopus (12109) Google Scholar, 11Reubinoff B.E. Pera M.F. Fong C.Y. Trounson A. Bongso A. Embryonic stem cell lines from human blastocysts: somatic differentiation in vitro.Nat Biotechnol. 2000; 18: 399-404Crossref PubMed Scopus (2284) Google Scholar]) or liver stem cells [12Shafritz D.A. Dabeva M.D. Liver stem cells and model systems for liver repopulation.J Hepatol. 2002; 26: 552-564Abstract Full Text Full Text PDF Scopus (105) Google Scholar, 13Fausto N. Liver regeneration and repair: hepatocytes, progenitor cells, and stem cells.Hepatol. 2004; 39: 1477-1487Crossref PubMed Scopus (625) Google Scholar], are a potential source of hepatocytes for transplantation. A number of criteria must be fulfilled in order for stem cells to be therapeutically beneficial. Ideally, cells for liver therapies should expand extensively in vitro, differentiate into mature liver cells, have minimal immunogenicity and be able to reconstitute liver tissue when transplanted in vivo. Alternative approaches to using liver stem cells include the production of cells from differentiated cells or from embryonic or extra-hepatic stem cells (see below). While there is some evidence for the ability to induce the differentiation of embryonic stem (ES) cells and non-hepatic cells to cells with some hepatocyte properties, there is no clearly identifiable adult liver stem cell. There is however, evidence to suggest that candidate liver stem cells do exist and may be exploitable.There are a number of fundamental problems associated with the utilization of hepatic stem cells as therapeutic modalities. The first is one of identity. Despite many years of intensive research, the identity of liver stem cell(s) remains elusive. In vivo and in vitro studies have suggested that cells resembling liver stem cells are present in embryonic, foetal and adult liver. Whether these putative cells represent the same cell in a continuum or alternatively represent distinct entities remains to be established.The second problem concerns differentiation. One major challenge to using stem cells is the difficulty in controlling the process of differentiation to generate significant numbers of hepatocytes. Some progress has been made towards identifying the factors necessary for the induction and maintenance of hepatic differentiated properties. The last problem to be solved involves designing isolation protocols for liver progenitors and repopulation studies constitute an important goal in liver research. The present review will focus on the different types of liver stem cell, currently thought to be able to generate hepatocytes. We will examine the stem cell markers and comment briefly on the factors that induce differentiation. To conclude, we will describe results showing formation of hepatocytes from alternative sources. Many other excellent reviews have focused on the subject of hepatic stem cells and the reader is referred to these for further reading [12,13; reviews cited throughout, see also monograph Seminars in Liver Disease (2003) volume 23 issue 4).1.1 What is a stem cell?In the classical sense, a stem cell is defined as an undifferentiated cell that has continuous proliferative and has the ability to produce progeny (Fig. 1) [14Potten C.S. Loeffler M. Stem cells: attributes, cycles, spirals, pitfalls and uncertainties: lessons from the crypt.Development. 1990; 110: 1101-1120PubMed Google Scholar, 15Morrison S.J. Shah N.M. Anderson D.J. Regulatory mechanisms in stem cell biology.Cell. 1997; 88: 287-298Abstract Full Text Full Text PDF PubMed Scopus (883) Google Scholar]. Additional characteristics include the ability to repopulate a tissue or organ after transplantation and the potential to be serially transplanted (reviewed in Ref. [[12]Shafritz D.A. Dabeva M.D. Liver stem cells and model systems for liver repopulation.J Hepatol. 2002; 26: 552-564Abstract Full Text Full Text PDF Scopus (105) Google Scholar]). Stem cells normally do not proliferate rapidly. The offspring of stem cells are referred to as progenitor cells. Progenitor cells may be further classified as early and late. The early progenitors can possess characteristics similar to stem cells and may be multipotent. In contrast, the late progenitors appear to have progressed along the differentiation pathway and may only produce one or two cell types only. Progenitor cells are not self-renewing or capable of serial transplantation. The regulatory mechanisms that underlie hepatic stem cell differentiation might be recapitulated during development and regeneration.1.2 Liver developmentDuring development, many organs such as the liver, lung, pancreas and intestine, bud from the foregut endoderm in response to signals from adjacent germ layers. The liver and ventral pancreas arise from neighbouring regions of the ventral foregut endoderm [16Grapin-Botton A. Melton D.A. Endoderm development: From patterning to organogenesis.Trends Genet. 2000; 16: 124-130Abstract Full Text Full Text PDF PubMed Scopus (228) Google Scholar, 17Wells J.M. Melton D.A. Vertebrate endoderm development.Ann Rev Cell Dev Biol. 1999; 15: 393-410Crossref PubMed Scopus (419) Google Scholar] and FGF signalling has been shown to direct the ventral pancreas to express genes for liver [[18]Deutsch G. Jung J.N. Zheng M.H. Lora J. Zaret K.S. A bipotential precursor population for pancreas and liver within the embryonic endoderm development. 2001; 128: 871-881Google Scholar]. In the mouse, the first morphological sign of liver development is around 8.5 days post-coitum, when the ventral floor of the foregut endoderm thickens to form the liver diverticulum [[19]Zaret K.S. Molecular genetics of early liver development.Ann Rev Physiol. 1996; 58: 231-251Crossref PubMed Scopus (96) Google Scholar]. It is now known that hepatic specification occurs in response to fibroblast growth factors and bone morphogenetic proteins secreted from the mesenchyme of the septum transversum, respectively [20Rossi J.M. Dunn N.R. Hogan B.L.M. Zaret K.S. Distinct mesodermal signals, including BMPs from the septum transversum mesenchyme, are required in combination for hepatogenesis from the endoderm.Genes and Dev. 2001; 15: 1998-2009Crossref PubMed Scopus (509) Google Scholar, 21Jung J.N. Zheng M.H. Goldfarb M. Zaret K.S. Initiation of mammalian liver development from endoderm by fibroblast growth factors.Science. 1999; 284: 1998-2003Crossref PubMed Scopus (599) Google Scholar]. Hepatoblasts, are considered to be bipotential committed cells and the precursors of foetal hepatocytes and biliary epithelial cells. Hepatoblasts migrate from the endoderm to form a tissue bud and then proliferate into the septum transversum. The hepatoblasts express alpha-fetoprotein and albumin [22Shiojiri N. The origin of intrahepatic bile duct cells in the mouse.J Embryol Exp Morphol. 1984; 79: 25-39PubMed Google Scholar, 23Lemaigre F.P. Development of the biliary tract.Mech Dev. 2003; 120: 81-87Crossref PubMed Scopus (148) Google Scholar] and later start to express dual markers for both the hepatocyte (AFP) and biliary (cytokeratins 7 and 19) lineages.1.3 Liver regenerationThe ability to regenerate is a unique characteristic of the liver and offers an excellent model system to investigate the role of hepatocyte proliferation and stem cells as a source of renewal. Following surgical removal of 70% of the liver, there is a compensatory response and the remaining liver restores the liver mass and total parenchymal cell number within 14 days in the rat [24Michalopoulos G.K. DeFrances M.C. Liver regeneration.Science. 1997; 276: 60-66Crossref PubMed Scopus (2864) Google Scholar, 25Fausto N. Liver regeneration: From laboratory to clinic.Liver Transplantation. 2001; 7: 835-844Crossref PubMed Scopus (123) Google Scholar]. Replication of hepatocytes following liver regeneration has been well documented [24Michalopoulos G.K. DeFrances M.C. Liver regeneration.Science. 1997; 276: 60-66Crossref PubMed Scopus (2864) Google Scholar, 25Fausto N. Liver regeneration: From laboratory to clinic.Liver Transplantation. 2001; 7: 835-844Crossref PubMed Scopus (123) Google Scholar, 26Bucher N.L.R. Farmer S. Liver regeneration after partial hepatectomy: genes and metabolism.in: Strain A.J. Diehl A.M. Liver growth and repair. Chapman and Hall, London1998: 3-27Crossref Google Scholar] and differentiation of oval cells can contribute to the hepatocyte population usually when hepatocyte proliferation is impeded or hepatocyte destruction is severe [[13]Fausto N. Liver regeneration and repair: hepatocytes, progenitor cells, and stem cells.Hepatol. 2004; 39: 1477-1487Crossref PubMed Scopus (625) Google Scholar]. It has long been assumed that hepatocytes are quiescent cells with limited proliferative capacity. This is no longer the case and there is recent evidence to suggest the replicative capacity of hepatocytes is not limited to 1–2 divisions as was predicted based on the number of cells required to repopulate the liver. A recent striking addition to the growing list of observations was the serial transplantation experiments performed by Grompe and colleagues [[27]Overturf K. Al-Dhalimy M. Finegold M. Grompe M. The repopulation potential of hepatocyte populations differing in size and prior mitotic expansion.Am J Pathol. 1999; 155: 2135-2143Abstract Full Text Full Text PDF PubMed Scopus (158) Google Scholar]. Using the FAH-deficient mice (see below), the authors demonstrated that the hepatocytes could replicate 70 or more times. During the serial transplantation of hepatocytes there was no evidence for repopulation being dependent on a population of hepatic stem cells [[27]Overturf K. Al-Dhalimy M. Finegold M. Grompe M. The repopulation potential of hepatocyte populations differing in size and prior mitotic expansion.Am J Pathol. 1999; 155: 2135-2143Abstract Full Text Full Text PDF PubMed Scopus (158) Google Scholar].1.4 Stem cell markersA current problem facing hepatologists and liver researchers is the lack of specific markers for hepatic stem cells. This problem reflects the normal characteristic of a stem cell, namely, the absence of markers of differentiation. Until now, the usual accepted method for identifying stem cells have been analysis of their replication potential. Despite this limitation, there are some characteristics of the oval cell progenitors that are known. By analysing some markers expressed on oval cells, it is thought that there are populations of proliferating cells that are different in their capacity to differentiate and stage of differentiation. In addition, some cells may function as hepatocyte progenitors and others are difficult to distinguish from biliart epithelial cells (BEC). Oval cells and BEC epitopes that react with other proteins including cytokeratins 7, 8, 18 and 19 [[13]Fausto N. Liver regeneration and repair: hepatocytes, progenitor cells, and stem cells.Hepatol. 2004; 39: 1477-1487Crossref PubMed Scopus (625) Google Scholar]. In addition to expressing markers of the hepatocyte and bile duct lineage, OV-6, a monoclonal antibody raised against cells isolated from carcinogen-treated rat liver, remains one of the best available and most commonly used markers. That said it is important to note that OV6 also reacts with normal biliary epithelial cells and in both rat [[28]Dunsford H.A. Sell S. Production of monoclonal antibodies to preneoplastic liver cell populations induced by chemical carcinogens in rats and to transplantable Morris hepatomas.Cancer Res. 1989; 49: 4887-4893PubMed Google Scholar] and human liver [[29]Roskams T. De Vos R. Van Eyken P. Myazaki H. Van Damme B. Desmet V.J. Hepatic OV-6 expression in human liver disease and rat experiments: evidence for hepatic progenitor cells in man.J Hepatol. 1998; 29: 455-463Abstract Full Text PDF PubMed Scopus (256) Google Scholar]. The epitope recognised by OV-6 on rat liver is shared with cytokeratins 14 and 19 [[30]Bisgaard H.C. Parmelee D.C. Dunsford H.A. Sechi S. Thorgeisson S.S. Keratin 14 protein in cultured non-parenchymal reat hepatic epithelial cells: characterization of keratin 14 and keratin 19 as antigens for the commonly used mouse monoclonal antibody OV-6.Mol Carcinogen. 1993; 7: 60-66Crossref PubMed Scopus (63) Google Scholar]. Cytokeratins are cytoskeletal intermediate filaments that provide structural support in the cytoplasm of higher eukaryotes and function to sustain the cells against mechanical and non-mechanical stresses. The antigen recognised by OV-6 in human liver has not yet been determined but the immunostaining pattern differs from that of CK19 [31Crosby H.A. Hubscher S. Fabris L. Joplin R. Sell S. Kelly D.A. et al.Immunolocalisation of putative human liver progenitor cells in liver of patients with end-stage primary biliary cirrhosis and sclerosing cholangitis using the monoclonal antibody OV-6.Am J Pathol. 1998; 152: 771-779PubMed Google Scholar, 32Crosby H.A. Hubscher S. Joplin R. Kelly D.A. Strain A.J. Immunolocalisation of OV-6, a putative stem cell marker in human fetal and diseased paediatric liver.Hepatology. 1998; 28: 980-985Crossref PubMed Scopus (103) Google Scholar].2. Candidate liver stem cellsIt is now clear that there are several different candidate liver stem cells as well as potential sources of liver cells for transplantation. According to the definition, a stem cell must be self-renewing and be able to generate progeny. Whether the hepatic stem cells are pluripotent or multipotent is not clear.2.1 Foetal liver stem cellsDuring development, hepatoblasts give rise to the two epithelial cell populations of the liver, hepatocytes and biliary epithelial cells, and are, therefore, classified as bi-potential progenitor cells. They have also been used extensively in cell transplantation experiments and shown to successfully repopulate both cell types liver in animals [[33]Sandu J.S. Petkov P.M. Dabevaa M.D. Shafritz D.A. Stem cell properties and repopulation of the rat liver by fetal liver epithelial progenitor cells.Am J Pathol. 2001; 159: 1323-1334Abstract Full Text Full Text PDF PubMed Scopus (147) Google Scholar]. More recently human hepatoblasts have been demonstrated to undergo extensive replication in vitro and differentiation into hepatocytes again following transplantation [[34]Malhi H. Irani A.N. Gagandeep S. Gupta S. Isolation of human progenitor liver epithelial cells with extensive replicationcaoacity and differentiation into mature hepatocytes.J Cell Sci. 2002; 115: 2679-2688PubMed Google Scholar]. These exciting studies indicate, providing sufficient human foetal liver tissue becomes available, and that this may ultimately offer the best source of mature hepatocytes for clinical studies. Alternatively, embryonic liver stem cell lines have been developed from several species including mouse [[35]Strick-Marchand H. Weiss M.C. Inducible differentiation and morphogenesis of bipotential liver cell lines from wild type mouse embryos.Hepatology. 2002; 36: 794-804PubMed Google Scholar], and primate [[36]Allain J-E. Dagher I. Mahlieu-Caputo D. Loux N. Andreolotti M. Westerman K. et al.Immortalisatioon of a bipotent epithelial liver stem cell.Proc Natl Acad Sci. 2002; 99: 3639-3644Crossref PubMed Scopus (63) Google Scholar] with impressive expansion and differentiation potential. Since these lines develop using the so-called 'plate and wait' technique [[37]Strick-Marchand H. Weiss M.C. Embryonic liver cell lines and permanent lines as models for hepatocyte and bile duct cell differentiation.Mech Dev. 2003; 120: 89-98Crossref PubMed Scopus (29) Google Scholar], their cellular origin within the liver remains obscure. Nevertheless they offer a viable alternative to primary hepatoblasts. A similar human cell line has not yet been described.2.2 Adult liver stem cellsThe existence of adult stem cells in the liver is still controversial [[38]Sell S. Heterogeneity and plasticity of hepatocyte lineage cells.Hepatology. 2001; 33: 738-750Crossref PubMed Scopus (378) Google Scholar]. Following liver injury from agents that induce carcinogenesis there is proliferation of small cells just at the periductular junction also called the canal of Hering [[39]Theise N.D. Saxena R. Portmann B.C. Thung S.N. Yee H. Chirboga L. et al.The canals of Hering and heptic stem cells in humans.Hepatology. 1999; 30: 1425-1433Crossref PubMed Scopus (604) Google Scholar]. They have oval nuclei, scant cytoplasmand have been termed oval cells [[13]Fausto N. Liver regeneration and repair: hepatocytes, progenitor cells, and stem cells.Hepatol. 2004; 39: 1477-1487Crossref PubMed Scopus (625) Google Scholar]. Oval cells are examples of hepatic progenitor cells i.e. they are the progeny of the liver stem cell and are bipotential i.e. they have the ability to differentiate into hepatocytes or biliary epithelial cells.Intriguingly, oval cells have also been identified in the ductal system of the copper-deficient rat [40Rao M.S. Subbarao V. Reddy J.K. Induction of hepatocytes in the pancreas of copper-depleted rats following copper repletion.Cell Differ. 1986; 18: 109-117Crossref PubMed Scopus (92) Google Scholar, 41Rao M.S. Reddy J.K. Hepatic transdifferentiation in the pancreas.Semin Cell Bio. 1995; 6: 151-156Crossref PubMed Scopus (52) Google Scholar], presumably, reflecting the close developmental relationship between the two organs. It has been shown that when rats are subjected to a copper deficient diet, the pancreatic acini degenerate while the ducts and the endocrine tissue persist. On re-feeding with a normal diet the acini regenerate and are accompanied by the appearance of foci of hepatocytes. The appearance of hepatocytes in the pancreas led to the idea that there were hepatocyte stem cells within the pancreas. The cells generating the hepatocytes were thought to be pancreatic oval cells [[41]Rao M.S. Reddy J.K. Hepatic transdifferentiation in the pancreas.Semin Cell Bio. 1995; 6: 151-156Crossref PubMed Scopus (52) Google Scholar]. Indeed, when pancreatic oval cells from copper-deficient rats were transplanted into rats, they started to express proteins characteristic of hepatocytes e.g. using a dipeptidyl dipeptidase IV genetic marker system to follow the fate of transplanted cells, the transdifferentiated cells started to express liver proteins [[42]Dabeva M.D. et al.Differentiation of pancreatic epithelial progenitor cells into hepatocytes.Proc Natl Acad Sci USA. 1997; 94: 7356-7361Crossref PubMed Scopus (176) Google Scholar].2.3 Where do hepatic stem cells reside?All stem cells have the unique ability to self-renew and to differentiate into progeny. Within the liver, stem cells are thought to reside in a niche or microenvironment. The niche is composed of cells, extracellular matrix and soluble factors released by the niche cells that help to maintain the characteristics of the stem cells. Two current views are held regarding the localization of the liver stem cell. The first is that liver stem cells are located in the canals of Hering [39Theise N.D. Saxena R. Portmann B.C. Thung S.N. Yee H. Chirboga L. et al.The canals of Hering and heptic stem cells in humans.Hepatology. 1999; 30: 1425-1433Crossref PubMed Scopus (604) Google Scholar, 43Paku S. Schnur J. Nagy P. Thorgeirsson S.S. Origin and structural evolution of the early proliferating oval cells in rat liver.Am J Pathol. 2001; 158: 1313-1323Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar]. The second is that liver stem cells are found in the periductular/intraportal zone of the liver [38Sell S. Heterogeneity and plasticity of hepatocyte lineage cells.Hepatology. 2001; 33: 738-750Crossref PubMed Scopus (378) Google Scholar, 44Crosby H.A. Nijjar S.S. de Ville de Goyet J. Kelly D.A. Strain A.J. Progenitor cells of the bilary epithelial lineage.Sem Cell Dev Biol. 2002; 13: 397-403Crossref PubMed Scopus (48) Google Scholar].The cells of the canal of Hering are those terminal bile ductular cells of the smallest branches of the biliary tree which are immediately adjacent to the periportal hepatocytes and hence have also been describe as transitional cells (Theise et al., 1999). In rodent models of liver injury or treatment with heptocarcinogens, the oval cells which are so prominent are thought to arise from this anatomical location [[45]Grisham J. Thorgeirsson S.S. Liver stem cells.in: Potten C. Stem cells. Academic press, London1997: 233-282Crossref Google Scholar]. Over several decades, there is a substantial literature describing the isolation, culture differentiation and transplantation back into experimental animals (reviewed in Ref. [[46]Fausto N. Campbell J.S. The role of hepatocytes and oval cells in liver regeneration and re-population.Mech Dev. 2003; 120: 117-130Crossref PubMed Scopus (555) Google Scholar]). Despite the fact that isolation, purification and culture of rat oval cells was first described over 20 years ago [[47]Yaswen P. Hayner N.T. Fausto N. Isolation of oval cells by centrifugal elutriation and comparison with other cell types during hepatocarcinogensis.Cancer Res. 1984; 44: 324-331PubMed Google Scholar] their precursor, the cells of the Canals of Hering have not been characterised due to a lack of specific markers. The same problem prevails in the human liver, where oval cells arise during the pathogenesis of chronic liver disease [[48]Desmet V. Roskams T. Van Eyken P. Ductular reaction in the liver.Path Res Pract. 1995; 191: 513-524Crossref PubMed Scopus (168) Google Scholar]. A variety or markers including OV6 [29Roskams T. De Vos R. Van Eyken P. Myazaki H. Van Damme B. Desmet V.J. Hepatic OV-6 expression in human liver disease and rat experiments: evidence for hepatic progenitor cells in man.J Hepatol. 1998; 29: 455-463Abstract Full Text PDF PubMed Scopus (256) Google Scholar, 31Crosby H.A. Hubscher S. Fabris L. Joplin R. Sell S. Kelly D.A. et al.Immunolocalisation of putative human liver progenitor cells in liver of patients with end-stage primary biliary cirrhosis and sclerosing cholangitis using the monoclonal antibody OV-6.Am J Pathol. 1998; 152: 771-779PubMed Google Scholar, 32Crosby H.A. Hubscher S. Joplin R. Kelly D.A. Strain A.J. Immunolocalisation of OV-6, a putative stem cell marker in human fetal and diseased paediatric liver.Hepatology. 1998; 28: 980-985Crossref PubMed Scopus (103) Google Scholar, 49Lowes K.N. Brennan B.A. Yoeh G.C. Olynyk J.K. Oval cell numbers in human chronic liver diseases are directly related to disease severity.Am J Pathol. 1999; 154: 537-541Abstract Full Text Full Text PDF PubMed Scopus (398) Google Scholar, 50Roskams T. van den Oord J.J. De Vos R. Desmet V.J. Neuroendocrine features of reactive bile ducts in cholestatic liver disease.Am J Pathol. 1990; 137: 1019-1025PubMed Google Scholar, 51Fabris L. Strazzabosco M. Crosby H.A. Hubscher S.G. Kelly D.A. Neuberger J.M. et al.Characterization and isolation of ductular cells co-expressing neural cell adhesion molecule and Bcl-2 from primary cholangiopathies and ductal plate malformations.Am J Pathol. 2000; 156: 1599-1612Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar] have been used to identify human oval cells, but none of these are specific cell and, therefore, are of limited use in defining a stem cell population which may be of clinical use.Recently, one of our laboratories (AS) has characterised a new antibody, GCTM5, which during development appears to differentiate between extraembryonic cells and embryonic endoderm. Intriguingly, expression in the 8 week old human foetus is restricted to hepatoblasts [[52]Stamp L. Crosby H.A. Hawes S. Strain A.J. Pera M.F. A novel cell surface marker expressed expressed on human embryonic hepatoblasts and a subpopulation of hepatic biliary cells.Stem Cells. 2005; 23: 103-112Crossref PubMed Scopus (12) Google Scholar]. In adult tissue it recognises a discrete sub-population of human oval cells in diseased tissue and also discrete cells in normal human liver (Fig. 1). Interestingly it also appears to recognise a sub-population of ductal cells in the adult pancreas. Further work has shown it to recognise a cells surface marker. Immunisolated cells demonstrate some degree of bipotentiality yielding cells with either hepatocyte of biliary features. This antibody is the first which may help distinguish the cells of the Canals of Hering from those of mature bilary phenotype and if so could prove invaluable in furthering the exploitation of such cells.Periductular cells and cells present in the portal tracts purported to have hepatic stem cell features are likely to be extrahepatic (see below). Using the hematopoietic stem cell markers c-kit and CD34, we identified cells in the portal tracts which appeared to give rise to cells of biliary phenotype [[53]Baumann U. Crosby H.A. Ramani P. Kelly D.A. Strain A.J. Expression of stem cell factor receptor c-kit in normal and diseased paediatric liver.Hepatology. 2002; 30: 112-117Crossref Scopus (186) Google Scholar]. These cells can be isolated with a high degree of purity and specificity from human both normal and diseased liver tissue can be expanded in culture and differentiate into bilary cells [[54]Crosby H.A. Kelly D.A. Strain A.J. Human hepatic stem-like cells isolated using c-kit or CD34 can differentiate into biliary epithelium.Gastroenterology. 2001; 120: 534-544Abstract Full Text Full Text PDF PubMed Scopus (213) Google Scholar]. Their true pasticity in vitro is in question however, since in vitro they do not readily give rise to cells with hepat
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