A bipotential precursor population for pancreas and liver within the embryonic endoderm

Screening for novel pancreatic genes from in vitro-induced pancreas in Xenopus

The processes of development and differentiation of the pancreas, an endoderm-derived vital organ that consists of both endocrine and exocrine cells, are highly conserved across most vertebrates. Recently, an in vitro system has been reported to induce embryonic pancreas using multipotent Xenopus ectodermal cells treated with activin and retinoic acid. In this study, this system was first modified to eliminate the mesoderm-derived pronephros. It was found that pronephros, which appeared with the use of low concentrations of activin, was eliminated at higher concentrations (400 ng/mL), while pancreas developed at a high frequency. Using this modified system, subtractive hybridization screening for novel pancreatic genes was done to better understand the molecular mechanisms of pancreas formation. Four novel genes were identified and characterized that were also found to be specifically expressed in the developing pancreas: carboxyl ester lipase, pancreatic elastase2, placental protein11 and protein disulfide isomerase A2 precursor. This in vitro pancreas-induction system may provide a useful model for analysis of the molecular mechanisms that function during pancreas development.

A novel mitochondrial Ca2+-dependent solute carrier in the liver identified by mRNA differential display

Pancreatic AR42J cells have the feature of pluripotency of the precursor cells of the gut endoderm. Dexamethasone converts them to exocrine cells or liver cells. Using mRNA differential display techniques, we have identified a novel Ca2+-dependent member of the mitochondrial solute carrier superfamily, which is expressed during the course of differentiation, and have designated it MCSC. The corresponding cDNA comprises an open reading frame of 1407 base pairs encoding a polypeptide of 469 amino acids. The carboxyl-terminal-half of MCSC has high similarity with other mitochondrial carriers, and the amino-terminal-half has three canonical elongation factor-hand motifs and has calcium binding capacity. The deduced amino acid sequence revealed 79.1% homology to the rabbit peroxisomal Ca2+-dependent member of the mitochondrial superfamily, but the subcellular localization of the protein was exclusively mitochondrial, not peroxisomal. Northern blot and Western blot analyses revealed its predominant expression in the liver and the skeletal muscle. In the liver, the expression level of MCSC was higher in the adult stage than in the fetal stage, and MCSC was highly up-regulated in dexamethasone-treated AR42J cells before the expression of albumin. Taken together, MCSC may play an important role in regulating the function of hepatocytes rather than in differentiation in vivo.

The potential of stem cells

Although stem cells have held the fascination of scientists for years, the attention of the general public has recently been captured by the derivation of human embryonic stem cells. In this review we describe the historical experiments leading up to the isolation of human embryonic stem cells and discuss recent advances in our understanding of both embryonic and somatic stem cells. Select examples are used to illustrate the potential of stem cells, both in the sense of their ability to differentiate into specific cell types and in the sense of their power to treat various diseases and conditions. Also discussed are recent studies describing current progress toward the treatment of Parkinson disease, spinal cord injuries, diabetes, and cardiac disease.

TARGET AUDIENCE

Obstetricians & Gynecologists, Family Physicians

LEARNING OBJECTIVES

After completion of this article, the reader will be able to describe the various types of stem cells, outline potential clinical uses of stem cells, and summarize the somatic cell transdifferentiation debate.

Cell-autonomous and signal-dependent expression of liver and intestine marker genes in pluripotent precursor cells from Xenopus embryos

Early regulatory events in respect to the embryonic development of the vertebrate liver are only poorly defined. A better understanding of the gene network that mediates the formation of hepatocytes from pluripotent embryonic precursor cells may help to establish in vitro protocols for hepatocyte differentiation. Here, we describe our first attempts to make use of early embryonic explants from the amphibian Xenopus laevis in order to address these questions. We have identified several novel embryonic liver and intestine marker genes in a random expression pattern screen with cDNA libraries derived from the embryonic liver anlage and from the adult liver of Xenopus laevis. Based on their embryonic expression characteristics, these genes, together with the previously known ones, can be categorized into four different groups: the liver specific group (LS), the liver and intestine group A (LIA), the liver and intestine group B (LIB), and the intestine specific group (IS). Dissociation of endodermal explants isolated from early neurula stage embryos reveals that all genes in the LIB and IS groups are expressed in a cell-autonomous manner. In contrast, expression of genes in the LS and LIA groups requires cell-cell interactions. The regular temporal expression profile of genes in all four groups is mimicked in ectodermal explants from early embryos, reprogrammed by co-injection of VegT and beta-catenin mRNAs. FGF signaling is found to be required for the induction of liver specific marker (LS group) gene expression in the same system.

Unique and conserved aspects of gut development in zebrafish

Although the development of the digestive system of humans and vertebrate model organisms has been well characterized, relatively little is known about how the zebrafish digestive system forms. We define developmental milestones during organogenesis of the zebrafish digestive tract, liver, and pancreas and identify important differences in the way the digestive endoderm of zebrafish and amniotes is organized. Such differences account for the finding that the zebrafish digestive system is assembled from individual organ anlagen, whereas the digestive anlagen of amniotes arise from a primitive gut tube. Despite differences of organ morphogenesis, conserved molecular programs regulate pharynx, esophagus, liver, and pancreas development in teleosts and mammals. Specifically, we show that zebrafish faust/gata-5 is a functional ortholog of gata-4, a gene that is essential for the formation of the mammalian and avian foregut. Further, extraembryonic gata activity is required for this function in zebrafish as has been shown in other vertebrates. We also show that a loss-of-function mutation that perturbs sonic hedgehog causes defects in the development of the esophagus that parallel those associated with targeted disruption of this gene in mammals. Perturbation of sonic hedgehog also affects zebrafish liver and pancreas development, and these effects occur in a reciprocal fashion, as has been described during mammalian liver and ventral pancreas development. Together, these data define aspects of digestive system development necessary for the characterization of zebrafish mutants. Given the similarities of teleost and mammalian digestive physiology and anatomy, these findings have implications for developmental and evolutionary studies as well as research of human diseases, such as diabetes, liver cirrhosis, and cancer.

Organogenesis: making pancreas from liver

Making endocrine pancreas cells at will is one of the major goals of cellular based therapies for diabetes. The experimentally induced conversion of hepatocytes into pancreatic cells, using a modified version of the transcription factor Pdx-1, may provide an alternative to stem cell approaches.

Experimental conversion of liver to pancreas

BACKGROUND

The liver and the pancreas arise from adjacent regions of endoderm in embryonic development. Pdx1 is a key transcription factor that is essential for the development of the pancreas and is not expressed in the liver. The aim of this study was to determine whether a gene overexpression protocol based on Pdx1 would be able to cause conversion of liver to pancreas.

RESULTS

We show that a modified form of Pdx1, carrying the VP16 transcriptional activation domain, can cause conversion of liver to pancreas, both in vivo and in vitro. Transgenic Xenopus tadpoles carrying the construct TTR-Xlhbox8-VP16:Elas-GFP were prepared. Xlhbox8 is the Xenopus homolog of Pdx1, the TTR (transthyretin) promoter directs expression to the liver, and the GFP is under the control of an elastase promoter and provides a real-time visible marker of pancreatic differentiation. In the transgenic tadpoles, part or all of the liver is converted to pancreas, containing both exocrine and endocrine cells, while liver differentiation products are lost from the regions converted to pancreas. The timing of events is such that the liver is differentiating by the time Xlhbox8-VP16 is expressed, so we consider this a transdifferentiation event rather than a reprogramming of embryonic development. Furthermore, this same construct will bring about transdifferentiation of human hepatocytes in culture, with formation of both exocrine and endocrine cells.

CONCLUSIONS

We consider that the conversion of liver to pancreas could be the basis of a new type of therapy for insulin-dependent diabetes. Although expression of the transgene is transient, once the ectopic pancreas is established, it persists thereafter.

Hedgehog signaling in pancreas development

Hedgehog proteins are secreted molecules that bind to their cell surface receptors to elicit concentration dependent responses essential for numerous tissue patterning and cell differentiation events during embryogenesis. However, during early stages of pancreas organogenesis, hedgehog signaling has been shown to inhibit tissue morphogenesis and cell differentiation. By contrast, recent cell culture studies indicate that an active hedgehog pathway might be required for maintenance of adult endocrine cell functions. This review describes our current understanding of the requirement of hedgehog signaling during pancreas morphogenesis and cell differentiation and discusses how individual hedgehog genes might act at various stages to ensure proper pancreas development and organ function.

Role of Sonic hedgehog in the development of the trachea and oesophagus

BACKGROUND/PURPOSE

The secreted glycoprotein, Sonic hedgehog (Shh) plays an important patterning role in the development of many organ systems. The authors aimed to study the temporal and spatial pattern of expression of Shh and its receptor Ptc1 during the development of the anterior foregut and to test the hypothesis that the Shh expression pattern is disturbed during the development of oesophageal atresia (OA) and tracheo-oesophageal fistula (TOF) in Adriamycin-treated mouse embryos.

METHODS

Saline and Adriamycin-treated (4 mg/kg) CBA/Ca embryos were harvested between embryonic days (E) 10.5 and 12.5, and Shh and Ptc1 expression was studied by whole-mount and section in situ hybridisation using digoxygenin-labelled riboprobes.

RESULTS

At E10.5, saline-treated embryos had an undivided foregut in which the ventrally placed prospective tracheal epithelium was positive for Shh, whereas the dorsal part was negative. At E11.5, this pattern had reversed with the separated trachea becoming negative and the oesophagus gaining expression of Shh. Ptc1 was expressed in the mesoderm adjacent to Shh expressing endoderm at both stages. Affected Adriamycin-treated embryos had an undivided foregut at E11.5, the epithelium of which showed diffuse Shh staining that lacked the dorso-ventral patterning seen in controls.

CONCLUSIONS

The reversal in the dorso-ventral pattern of Shh expression during the narrow embryologic window in which tracheo-oesophageal separation is initiated suggests that Shh may play an important role in this process. Transient disturbance of this pattern may underlie the abnormal organogenesis in the Adriamycin model.

Pancreatic-hepatic switches in vivo

The existence of a common endodermal progenitor cell to both pancreas and liver is suggested by anatomy of the development of pancreatic and liver buds in embryogenesis. Here we review the large body of evidence that pancreas to liver and liver to pancreas cell differentiation can also occur in adult life. The published data are consistent with the hypothesis that endodermal progenitor cells, capable of giving rise to multiple hepatic and pancreatic cell types continue to persist. These cells may represent a stem cell reservoir with potential in cell therapy applications in the future.

Role for VPAC2 receptor-mediated signals in pancreas development

Mature pancreatic cells develop from progenitors that proliferate and differentiate into endocrine and exocrine cells. This development is thought to be controlled by secreted soluble factors acting on their target cells after binding to membrane receptors. Here, we analyzed the impact on embryonic pancreatic development of ligands that bind to protein G-coupled receptors and increase cAMP accumulation. We found that embryonic pancreatic epithelial cells were sensitive to vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide. These factors generate signals after binding to the VPAC2 receptor, which is expressed by immature pancreatic epithelial cells between embryonic days 12 and 16. Finally, in vitro, VIP exposure increased the survival and proliferation of immature pancreatic cells, leading to an increase in the number of endocrine cells that will develop.

Direct lineage tracing reveals the ontogeny of pancreatic cell fates during mouse embryogenesis

Lineage tracing follows the progeny of labeled cells through development. This technique identifies precursors of mature cell types in vivo and describes the cell fate restriction steps they undergo in temporal order. In the mouse pancreas, direct cell lineage tracing reveals that Pdx1- expressing progenitors in the early embryo give rise to all pancreatic cells. The progenitors for the mature pancreatic ducts separate from the endocrine/exocrine tissues before E12.5. Expression of Ngn3 and pancreatic polypeptide marks endocrine cell lineages during early embryogenesis, and these cells behave as transient progenitors rather than stem cells. In adults, Ngn3 is expressed within the endocrine islets, and the NGN3+ cells seem to contribute to pancreatic islet renewal. These results indicate the stage at which each progenitor population is restricted to a particular fate and provide markers for isolating progenitors to study their growth, differentiation, and the genes necessary for their development.

Differentiation of mouse embryonic stem cells into pancreatic and hepatic cells

Here, we present efficient strategies to differentiate ES cells either into pancreatic or into hepatic cell types. We recommend a strategy to select nestin+ cells, an early progenitor cell type with high developmental plasticity, followed by differentiation induction with specific growth and extracellular matrix factors into pancreatic and hepatic cell types. Cells differentiating via nestin+ cells into the pancreatic and hepatic lineage expressed tissue-specific genes. Proteins characteristic for mature endocrine pancreatic or hepatic cells were synthesized and released. Further, a histotypic "spinner" culture system was introduced to generate mature insulin- and albumin-producing cells at high efficiency.

Transdifferentiation of pancreas to liver

Transdifferentiation is the name used to describe the direct conversion of one differentiated cell type into another. Cells which have the potential to interconvert by transdifferentiation generally arise from adjacent regions in the developing embryo. For example, the liver and pancreas arise from the same region of the endoderm. The transdifferentiation of pancreas to liver (and vice versa) has been observed in animal experiments and in certain human pathologies. Understanding transdifferentiation is important to developmental biologists because it will help elucidate the cellular and molecular differences that distinguish neighbouring regions of the embryo. While the in vivo models for the transdifferentiation of liver to pancreas have been valuable, it is more difficult to extrapolate from these studies to individual changes at the cellular or molecular levels. The recent development of two in vitro systems (AR42J cells and embryonic pancreatic cultures) for the transdifferentiation of pancreas to liver has shown that an environmental change in the form of an exogenous glucocorticoid can cause the conversion of pancreatic exocrine cells into hepatocytes. The AR42J cell system has been used to elucidate the cell lineage and the molecular basis of transdifferentiation of pancreas to liver.

Mechanisms controlling early development of the liver

Over the last decade significant advances have been made in our understanding of the molecular mechanisms that control early aspects of mammalian liver development. Studies using tissue explant cultures and molecular biology techniques as well as the analysis of transgenic and knockout mice have identified signaling molecules and transcription factors that are necessary for the onset of hepatogenesis. This review presents an overview of these studies and discusses the role of individual factors during hepatic development.

Pediatric liver tumors and hepatic ontogenesis: common and distinctive pathways

Several types of pediatric liver tumors exhibit structural features apparently reflecting processes which normally occur during hepatic ontogenesis: some hepatoblastomas mimic distinct phases of hepatogenesis, including the formation of mesenchymal structures closely associated with immature epithelia, and there are tumors almost exclusively consisting of complex mesenchymal patterns. Current classifications of hepatoblastomas refer to the identification of more or less mature (differentiated) single or mixed components seen in histologic preparations. These do not, however, attempt to integrate ontogenic pathways, in contrast for example, to nephroblastoma and associated lesions, where such a view has proved to be highly fruitful. Based on the fact that an enormous amount of knowledge has recently been accumulated regarding hepatic ontogenesis, time may have come to look at these tumors with a new eye. In what follows, we aim at trying to analyze distinct features of pediatric hepatic tumors (except vascular tumors) within the background of ontogenesis. Some key steps of hepatogenesis and the regulatory factors involved may, in the future, deliver an armamentarium to search for novel molecular mechanisms involved in tumorigenic pathways.

B cells develop in the zebrafish pancreas

The zebrafish, with its transparent free-living embryo, is a useful organism for investigating early stages in lymphopoiesis. Previously, we showed that T cells differentiate in the thymus by day 4, but no sites for B cell differentiation were seen until 3 weeks. We report here that on day 4, we detect rearrangements of genes encoding B cell receptors in DNA extracted from whole fish. Also by day 4, rag1 transcripts are seen in the pancreas, an organ not previously associated with lymphopoiesis; by day 10, Igmu transcripts are detected here. Thus, in zebrafish, the pancreas assumes the role of both the liver in fetal mice and the spleen in neonatal mice.

Prox1 is an early specific marker for the developing liver and pancreas in the mammalian foregut endoderm

Although important progress has been made recently in the elucidation of the molecular mechanisms that regulate differentiation and morphogenesis of endoderm-derived tissues such as pancreas and liver, less is known about the preliminary steps of early regional specification. Recent evidence supports the proposal that the early endoderm contains a bipotential precursor cell type for pancreas and liver. We have also previously shown that the activity of the homeobox gene Prox1 controls hepatocyte migration during liver morphogenesis. Using detailed comparative analysis of whole embryos and reverse transcriptase polymerase chain reaction of dissected embryonic endoderm, we have now determined that in the early endoderm, Prox1 expression is restricted to regions giving rise to the mammalian pancreas and liver. This finding indicates that Prox1 is one of the earliest specific markers of this commonly fated region of the mammalian endoderm.

Lineage commitment and cellular differentiation in exocrine pancreas

Exocrine pancreatic cell types comprise greater than 90% of parenchymal cell mass in the adult pancreas. However, the factors regulating differentiation of acinar and ductal epithelial cells remain incompletely characterized. Like pancreatic islet cells, acinar and ductal cells arise from pluripotent precursors within embryonic pancreatic epithelium. Recent studies have suggested that a common pool of pluripotent stem cells is responsible for generating both endocrine and exocrine cell types, and that specific signaling pathways regulate a critical balance between endocrine and exocrine lineage commitment.

Retinoic acid signaling is required for a critical early step in zebrafish pancreatic development

The mechanisms that subdivide the endoderm into the discrete primordia that give rise to organs such as the pancreas and liver are not well understood. However, it is known that retinoic acid (RA) signaling is critical for regionalization of the vertebrate embryo: when RA signaling is either prevented or augmented, anteroposterior (AP) patterning of the CNS and mesoderm is altered and major developmental defects occur. We have investigated the role of RA signaling in regionalization of the zebrafish endoderm. Using a mutant that prevents RA synthesis and an antagonist of the RA receptors, we show that specification of both the pancreas and liver requires RA signaling. By contrast, RA signaling is not required for the formation of the endodermal germ layer or for differentiation of other endodermal organs. Timed antagonist and RA treatments show that the RA-dependent step in pancreatic specification occurs at the end of gastrulation, significantly earlier than the expression of known markers of pancreatic progenitors. In addition to being required for pancreatic specification, RA has the capacity to transfate anterior endoderm to a pancreatic fate.

Development and diseases of the pancreas

The pancreas is a vital gland of exocrine and endocrine function. It is the target of two main affections: diabetes and pancreatic cancer. We describe the tissue interactions, signaling pathways and intracellular targets that are involved in the emergence of the pancreas primordium and its proliferation, morphogenesis and differentiation. It appears that several genes of developmental relevance have an adult function and are involved in pancreas affections. Embryological experimentation in animals contributed to provide candidate genes for human disease and holds promise for future treatments.

Pancreatic organogenesis--developmental mechanisms and implications for therapy

The pancreas is a mixed exocrine and endocrine gland that controls many homeostatic functions. The exocrine pancreas produces and secretes digestive enzymes, whereas the endocrine compartment consists of four distinct hormone-producing cell types. Studies that further our knowledge of the basic mechanisms that underlie the formation of the pancreas will be crucial for understanding the development and homeostasis of this organ and of the mechanisms that cause diabetes. This information is also pivotal for any attempt to generate functional insulin-producing beta-cells that are suitable for transplantation.

Adult stem cell plasticity

Observations made in the last few years support the existence of pathways, in adult humans and rodents, that allow adult stem cells to be surprisingly flexible in their differentiation repertoires. Termed plasticity, this property allows adult stem cells, assumed, until now, to be committed to generating a fixed range of progeny, to switch, when they have been relocated, to make other specialized sets of cells appropriate to their new niche. Reprogramming of some adult stem cells can occur in vivo; the stem cells normally resident in bone marrow appear particularly flexible and are able to contribute usefully to multiple recipient organs. This process produces cells with specialized structural and metabolic adaptations commensurate with their new locations. In a few examples, the degree of support is sufficient to assist or even rescue recipient mice from genetic defects. Some studies provide evidence for the expansion of the reprogrammed cells locally, but in most it remains possible that cells arrive and redifferentiate, but are no longer stem cells. Nevertheless, the fact that appropriately differentiated cells are delivered deep within organs simply by injection of bone marrow cells should make us think differently about the way that organs regenerate and repair. Migratory pathways for stem cells in adult organisms may exist that could be exploited to effect repairs using an individual's own stem cells, perhaps after gene therapy. Logical extensions of this concept are that a transplanted organ would become affected by the genetic susceptibilities of the recipient, alleles that re-express themselves via marrow-derived stem cells, and that plasticity after bone marrow transplantation would also transfer different phenotypes, affecting important parameters such as susceptibility to long-term complications of diabetes, or the ability to metabolize drugs in the liver. This article reviews some of the evidence for stem cell plasticity in rodents and man.

Regulatory phases of early liver development: paradigms of organogenesis

Genetic analysis, embryonic tissue explantation and in vivo chromatin studies have together identified the distinct regulatory steps that are necessary for the development of endoderm into a bud of liver tissue and, subsequently, into an organ. In this review, I discuss the acquisition of competence to express liver-specific genes by the endoderm, the control of early hepatic growth, the coordination of hepatic and vascular development and the cell differentiation that is necessary to generate a functioning liver. The regulatory mechanisms that underlie these phases are common to the development of many organ systems and might be recapitulated or disrupted during stem-cell differentiation and adult tissue pathogenesis.

Pluripotent stem cells--model of embryonic development, tool for gene targeting, and basis of cell therapy

Embryonic stem (ES) cells are pluripotent cell lines with the capacity of self-renewal and a broad differentiation plasticity. They are derived from pre-implantation embryos and can be propagated as a homogeneous, uncommitted cell population for an almost unlimited period of time without losing their pluripotency and their stable karyotype. Murine ES cells are able to reintegrate fully into embryogenesis when returned into an early embryo, even after extensive genetic manipulation. In the resulting chimeric offspring produced by blastocyst injection or morula aggregation, ES cell descendants are represented among all cell types, including functional gametes. Therefore, mouse ES cells represent an important tool for genetic engineering, in particular via homologous recombination, to introduce gene knock-outs and other precise genomic modifications into the mouse germ line. Because of these properties ES cell technology is of high interest for other model organisms and for livestock species like cattle and pigs. However, in spite of tremendous research activities, no proven ES cells colonizing the germ line have yet been established for vertebrate species other than the mouse (Evans and Kaufman, 1981; Martin, 1981) and chicken (Pain et al., 1996). The in vitro differentiation capacity of ES cells provides unique opportunities for experimental analysis of gene regulation and function during cell commitment and differentiation in early embryogenesis. Recently, pluripotent stem cells were established from human embryos (Thomson et al., 1998) and early fetuses (Shamblott et al., 1998), opening new scenarios both for research in human developmental biology and for medical applications, i.e. cell replacement strategies. At about the same time, research activities focused on characteristics and differentiation potential of somatic stem cells, unravelling an unexpected plasticity of these cell types. Somatic stem cells are found in differentiated tissues and can renew themselves in addition to generating the specialized cell types of the tissue from which they originate. Additional to discoveries of somatic stem cells in tissues that were previously not thought to contain these kinds of cells, they also appear to be capable of developing into cell types of other tissues, but have a reduced differentiation potential as compared to embryo-derived stem cells. Therefore, somatic stem cells are referred to as multipotent rather than pluripotent. This review summarizes characteristics of pluripotent stem cells in the mouse and in selected livestock species, explains their use for genetic engineering and basic research on embryonic development, and evaluates their potential for cell therapy as compared to somatic stem cells.

Transgenic expression of FGF8 and FGF10 induces transdifferentiation of pancreatic islet cells into hepatocytes and exocrine cells

FGF signaling is essential for normal development of pancreatic islets. To examine the effects of overexpressed FGF8 and FGF10 on pancreatic development, we generated FGF8- and FGF10-transgenic mice (Tg mice) under the control of the glucagon promoter. In FGF8-Tg mice, hepatocyte-like cells were observed in the periphery of pancreatic islets, but areas of alpha and beta cells did not decrease, whereas in FGF10-Tg mice, pancreatic ductal and acinar cells were found in islets, concomitantly with disturbed beta-cell differentiation. These results suggest that FGF8 and FGF10 play important roles in development of hepatocytes and exocrine cells, respectively, and explain the absence of FGF8 expression in normal islets and pancreatic hypoplasia in FGF10-deficient mice.

Regulation of pancreatic cell differentiation and morphogenesis

Organogenesis requires tissue interactions to initiate the cascade of inductive and repressive signals necessary for normal organ development. Tissue interactions initiate the pancreatic lineage within the primitive foregut endodermal epithelium and continue to direct the morphogenesis and differentiation of the endocrine, exocrine and ductal portions of the pancreas. An understanding of the mechanisms controlling pancreatic growth would enable the development of alternative therapies for diseases such as type 1 diabetes.

Clonal identification and characterization of self-renewing pluripotent stem cells in the developing liver

Using flow cytometry and single cell-based assays, we prospectively identified hepatic stem cells with multilineage differentiation potential and self-renewing capability. These cells could be clonally propagated in culture where they continuously produced hepatocytes and cholangiocytes as descendants while maintaining primitive stem cells. When cells that expanded in vitro were transplanted into recipient animals, they morphologically and functionally differentiated into hepatocytes and cholangiocytes with reconstitution of hepatocyte and bile duct structures. Furthermore, these cells differentiated into pancreatic ductal and acinar cells or intestinal epithelial cells when transplanted into pancreas or duodenal wall. These data indicate that self-renewing pluripotent stem cells persist in the developing mouse liver and that such cells can be induced to become cells of other organs of endodermal origin under appropriate microenvironment. Manipulation of hepatic stem cells may provide new insight into therapies for diseases of the digestive system.

In-vitro differentiation of pancreatic beta-cells

Stem cell biology is a new field that holds promise for in-vitro mass production of pancreatic beta-cells, which are responsible for insulin synthesis, storage, and release. Lack or defect of insulin produces diabetes mellitus, a devastating disease suffered by 150 million people in the world. Transplantation of insulin-producing cells could be a cure for type 1 and some cases of type 2 diabetes, however this procedure is limited by the scarcity of material. Obtaining pancreatic beta-cells from embryonic stem cells would overcome this problem. We have derived insulin-producing cells from mouse embryonic stem cells by a 3-step in-vitro differentiation method consisting of directed differentiation, cell-lineage selection, and maturation. These insulin-producing cells normalize blood glucose when transplanted into streptozotocin-diabetic mice. Strategies to increase islet precursor cells from embryonic stem cells include the expression of relevant transcription factors (Pdx1, Ngn3, Isl-1, etc), together with the use of extracellular factors. Once a high enough proportion of islet precursors has been obtained there is a need for cell-lineage selection in order to purify the desired cell population. For this purpose, we designed a cell-trapping method based on a chimeric gene that fuses the human insulin gene regulatory region with the structural gene that confers resistance to neomycin. When incorporated into embryonic stem cells, this fusion gene will generate neomycin resistance in those cells that initiate the synthesis of insulin. Not only embryonic, but also adult stem cells are potential sources for insulin-containing cells. Duct cells from the adult pancreas are committed to differentiate into the four islet cell types; other possibilities may include nestin-positive cells from islets and adult pluripotent stem cells from other origins. Whilst the former are committed to be islet cells but have a reduced capacity to expand, the latter are more pluripotent and more expandable, but a longer pathway separates them from the insulin-producing stage. The aim of this review is to discuss the different strategies that may be followed to in-vitro differentiate pancreatic beta-cells from stem cells.

Hepatocyte differentiation: from the endoderm and beyond

Hepatocytes differentiate from the endoderm during embryonic development. Recent studies show, however, that hepatocytes can also be derived from rare cells that reside in the pancreas, bone marrow, and brain. Indeed, the latest discoveries indicate that embryonic hepatocytes normally arise by diversion of an endodermal cell population that would otherwise default to a pancreatic fate. Convergent FGF and BMP signals from distinct mesodermal cell types control this transition. Molecular signals that govern the differentiation of hepatocytes from non-endodermal cells and the role of such cells in normal liver physiology remain to be discovered.

Hedgehog signaling pathway is essential for pancreas specification in the zebrafish embryo

Recent studies have implicated the signaling factor Sonic hedgehog (Shh) as a negative regulator of pancreatic development, but as a positive regulator of pancreas function in amniotes [1-4]. Here, using genetic analysis, we show that specification of the pancreas in the teleost embryo requires the activity of Hh proteins. Zebrafish embryos compromised in Hh signaling exhibit disruption in the expression of the pancreas-specifying homeobox gene pdx-1 and concomitantly show almost complete absence of the endocrine pancreas. Reciprocally, ubiquitous activation of the Hh pathway in wild-type embryos causes ectopic induction of endodermal pdx-1 expression and the differentiation of supernumerary endocrine cells. Our results suggest that Hh proteins influence pancreas specification via inductive interactions from the axial midline rather than through their localized expression in the endodermal cells themselves.

Distinct mesodermal signals, including BMPs from the septum transversum mesenchyme, are required in combination for hepatogenesis from the endoderm

Mesodermal signaling is critical for patterning the embryonic endoderm into different tissue domains. Classical tissue transplant experiments in the chick and recent studies in the mouse indicated that interactions with the cardiogenic mesoderm are necessary and sufficient to induce the liver in the ventral foregut endoderm. Using molecular markers and functional assays, we now show that septum transversum mesenchyme cells, a distinct mesoderm cell type, are closely apposed to the ventral endoderm and contribute to hepatic induction. Specifically, using a mouse Bmp4 null mutation and an inhibitor of BMPs, we find that BMP signaling from the septum transversum mesenchyme is necessary to induce liver genes in the endoderm and to exclude a pancreatic fate. BMPs apparently function, in part, by affecting the levels of the GATA4 transcription factor, and work in parallel to FGF signaling from the cardiac mesoderm. BMP signaling also appears critical for morphogenetic growth of the hepatic endoderm into a liver bud. Thus, the endodermal domain for the liver is specified by simultaneous signaling from distinct mesodermal sources.