In vitro transdifferentiation of adult hematopoietic stem cells: an alternative source of engraftable hepatocytes

Progress and Challenges in the Use of a Liver-on-a-Chip for Hepatotropic Infectious Diseases

The liver is a target organ of life-threatening pathogens and prominently contributes to the variation in drug responses and drug-induced liver injury among patients. Currently available drugs significantly decrease the morbidity and mortality of liver-dwelling pathogens worldwide; however, emerging clinical evidence reveals the importance of host factors in the design of safe and effective therapies for individuals, known as personalized medicine. Given the primary adherence of cells in conventional two-dimensional culture, the use of these one-size-fit-to-all models in preclinical drug development can lead to substantial failures in assessing therapeutic safety and efficacy. Advances in stem cell biology, bioengineering and material sciences allow us to develop a more physiologically relevant model that is capable of recapitulating the human liver. This report reviews the current use of liver-on-a-chip models of hepatotropic infectious diseases in the context of precision medicine including hepatitis virus and malaria parasites, assesses patient-specific responses to antiviral drugs, and designs personalized therapeutic treatments to address the need for a personalized liver-like model. Second, most organs-on-chips lack a monitoring system for cell functions in real time; thus, the review discusses recent advances and challenges in combining liver-on-a-chip technology with biosensors for assessing hepatocyte viability and functions. Prospectively, the biosensor-integrated liver-on-a-chip device would provide novel biological insights that could accelerate the development of novel therapeutic compounds.

Cellular Mechanisms of Liver Regeneration and Cell-Based Therapies of Liver Diseases

The emerging field of regenerative medicine offers innovative methods of cell therapy and tissue/organ engineering as a novel approach to liver disease treatment. The ultimate scientific foundation of both cell therapy of liver diseases and liver tissue and organ engineering is delivered by the in-depth studies of the cellular and molecular mechanisms of liver regeneration. The cellular mechanisms of the homeostatic and injury-induced liver regeneration are unique. Restoration of the mass of liver parenchyma is achieved by compensatory hypertrophy and hyperplasia of the differentiated parenchymal cells, hepatocytes, while expansion and differentiation of the resident stem/progenitor cells play a minor or negligible role. Participation of blood-borne cells of the bone marrow origin in liver parenchyma regeneration has been proven but does not exceed 1-2% of newly formed hepatocytes. Liver regeneration is activated spontaneously after injury and can be further stimulated by cell therapy with hepatocytes, hematopoietic stem cells, or mesenchymal stem cells. Further studies aimed at improving the outcomes of cell therapy of liver diseases are underway. In case of liver failure, transplantation of engineered liver can become the best option in the foreseeable future. Engineering of a transplantable liver or its major part is an enormous challenge, but rapid progress in induced pluripotency, tissue engineering, and bioprinting research shows that it may be doable.

Hepatocyte cell therapy in liver disease

Liver disease is a leading cause of morbidity and mortality. Liver transplantation remains the only proven treatment for end-stage liver failure but is limited by the availability of donor organs. Hepatocyte cell therapy, either with bioartificial liver devices or hepatocyte transplantation, may help address this by delaying or preventing liver transplantation. Early clinical studies have shown promising results, however in most cases, the benefit has been short lived and so further research into these therapies is required. Alternative sources of hepatocytes, including stem cell-derived hepatocytes, are being investigated as the isolation of primary human hepatocytes is limited by the same shortage of donor organs. This review summarises the current clinical experience of hepatocyte cell therapy together with an overview of possible alternative sources of hepatocytes. Current and future areas for research that might lead towards the realisation of the full potential of hepatocyte cell therapy are discussed.

Adult cell plasticity in vivo: de-differentiation and transdifferentiation are back in style

Biologists have long been intrigued by the possibility that cells can change their identity, a phenomenon known as cellular plasticity. The discovery that terminally differentiated cells can be experimentally coaxed to become pluripotent has invigorated the field, and recent studies have demonstrated that changes in cell identity are not limited to the laboratory. Specifically, certain adult cells retain the capacity to de-differentiate or transdifferentiate under physiological conditions, as part of an organ's normal injury response. Recent studies have highlighted the extent to which cell plasticity contributes to tissue homeostasis, findings that have implications for cell-based therapy.

Identification of a Hematopoietic Cell Dedifferentiation-Inducing Factor

It has long been realized that hematopoietic cells may have the capacity to trans-differentiate into non-lymphohematopoietic cells under specific conditions. However, the mechanisms and the factors for hematopoietic cell trans-differentiation remain unknown. In an in vitro culture system, we found that using a conditioned medium from proliferating fibroblasts can induce a subset of hematopoietic cells to become adherent fibroblast-like cells (FLCs). FLCs are not fibroblasts nor other mesenchymal stromal cells, based on their expression of type-1 collagen, and other stromal cell marker genes. To identify the active factors in the conditioned medium, we cultured fibroblasts in a serum-free medium and collected it for further purification. Using the fractions from filter devices of different molecular weight cut-offs, and ammonium sulfate precipitation collected from the medium, we found the active fraction is a protein. We then purified this fraction by using fast protein liquid chromatography (FPLC) and identified it by mass spectrometer as macrophage colony-stimulating factor (M-CSF). The mechanisms of M-CSF-inducing trans-differentiation of hematopoietic cells seem to involve a tyrosine kinase signalling pathway and its known receptor. The FLCs express a number of stem cell markers including SSEA-1 and -3, OCT3/4, NANOG, and SOX2. Spontaneous and induced differentiation experiments confirmed that FLCs can be further differentiated into cell types of three germ layers. These data indicate that hematopoietic cells can be induced by M-CSF to dedifferentiate to multipotent stem cells. This study also provides a simple method to generate multipotent stem cells for clinical applications.

Murine Sca1(+)Lin(-) bone marrow contains an endodermal precursor population that differentiates into hepatocytes

The direct differentiation of hepatocytes from bone marrow cells remains controversial. Several mechanisms, including transdifferentiation and cell fusion, have been proposed for this phenomenon, although direct visualization of the process and the underlying mechanisms have not been reported. In this study, we established an efficient in vitro culture method for differentiation of functioning hepatocytes from murine lineage-negative bone marrow cells. These cells reduced liver damage and incorporated into hepatic parenchyma in two independent hepatic injury models. Our simple and efficient in vitro protocol for endodermal precursor cell survival and expansion enabled us to identify these cells as existing in Sca1⁺ subpopulations of lineage-negative bone marrow cells. The endodermal precursor cells followed a sequential developmental pathway that included endodermal cells and hepatocyte precursor cells, which indicates that lineage-negative bone marrow cells contain more diverse multipotent stem cells than considered previously. The presence of equivalent endodermal precursor populations in human bone marrow would facilitate the development of these cells into an effective treatment modality for chronic liver diseases.

Characteristics of liver cancer stem cells and clinical correlations

Liver cancer is an aggressive malignant disease with a poor prognosis. Patients with liver cancer are usually diagnosed at an advanced stage and thus miss the opportunity for surgical resection. Chemotherapy and radiofrequency ablation, which target tumor bulk, have exhibited limited therapeutic efficacy to date. Liver cancer stem cells (CSCs) are a small subset of undifferentiated cells existed in liver cancer, which are considered to be responsible for liver cancer initiation, metastasis, relapse and chemoresistance. Elucidating liver CSC characteristics and disclosing their regulatory mechanism might not only deepen our understanding of the pathogenesis of liver cancer but also facilitate the development of diagnostic, prognostic and therapeutic approaches to improve the clinical management of liver cancer. In this review, we will summarize the recent advances in liver CSC research in terms of the origin, identification, regulation and clinical correlation.

Autologous bone marrow transplantation in decompensated liver: Systematic review and meta-analysis

AIM: To evaluate the efficacy of autologous bone marrow mononuclear cell transplantation in decompensated liver disease.

METHODS: Medline, EMBASE, PubMed, Science Direct, and the Cochrane Library were searched for relevant studies. Retrospective case-control studies were included along with randomized clinical trials. Meta-analysis was performed in line with recommendations from the Cochrane Collaboration software review manager. Heterogeneity was assessed using a random-effects model.

RESULTS: Four randomized controlled trials and four retrospective studies were included. Cell transplantation increased serum albumin level by 1.96 g/L (95%CI: 0.74-3.17; P = 0.002], 2.55 g/L (95%CI: 0.32-4.79; P = 0.03), and 3.65 g/L (95%CI: 0.76-6.54; P = 0.01) after 1, 3, and 6 mo, respectively. Patients who had undergone cell transplantation also had a lower level of total bilirubin [mean difference (MD): -1.37 mg/dL; 95%CI: -2.68-(-0.06); P = 0.04] after 6 mo. This decreased after 1 year when compared to standard treatment (MD: -1.26; 95%CI: -2.48-(-0.03); P = 0.04]. A temporary decrease in alanine transaminase and aspartate transaminase were significant in the cell transplantation group. However, after 6 mo treatment, patients who had undergone cell transplantation had a slightly longer prothrombin time (MD: 5.66 s, 95%CI: 0.04-11.28; P = 0.05). Changes in the model for end-stage liver disease score and Child-Pugh score were not statistically significant.

CONCLUSION: Autologous bone marrow transplantation showed some benefits in patients with decompensated liver disease. However, further studies are still needed to verify its role in clinical treatment for end-stage liver disease.

The multiple functional roles of mesenchymal stem cells in participating in treating liver diseases

Mesenchymal stem cells (MSCs) are a group of stem cells derived from the mesodermal mesenchyme. MSCs can be obtained from a variety of tissues, including bone marrow, umbilical cord tissue, umbilical cord blood, peripheral blood and adipose tissue. Under certain conditions, MSCs can differentiate into many cell types both in vitro and in vivo, including hepatocytes. To date, four main strategies have been developed to induce the transdifferentiation of MSCs into hepatocytes: addition of chemical compounds and cytokines, genetic modification, adjustment of the micro-environment and alteration of the physical parameters used for culturing MSCs. Although the phenomenon of transdifferentiation of MSCs into hepatocytes has been described, the detailed mechanism is far from clear. Generally, the mechanism is a cascade reaction whereby stimulating factors activate cellular signalling pathways, which in turn promote the production of transcription factors, leading to hepatic gene expression. Because MSCs can give rise to hepatocytes, they are promising to be used as a new treatment for liver dysfunction or as a bridge to liver transplantation. Numerous studies have confirmed the therapeutic effects of MSCs on hepatic fibrosis, cirrhosis and other liver diseases, which may be related to the differentiation of MSCs into functional hepatocytes. In addition to transdifferentiation into hepatocytes, when MSCs are used to treat liver disease, they may also inhibit hepatocellular apoptosis and secrete various bioactive molecules to promote liver regeneration. In this review, the capacity and molecular mechanism of MSC transdifferentiation, and the therapeutic effects of MSCs on liver diseases are thoroughly discussed.

Role of liver stem cells in hepatocarcinogenesis

Liver cancer is an aggressive disease with a high mortality rate. Management of liver cancer is strongly dependent on the tumor stage and underlying liver disease. Unfortunately, most cases are discovered when the cancer is already advanced, missing the opportunity for surgical resection. Thus, an improved understanding of the mechanisms responsible for liver cancer initiation and progression will facilitate the detection of more reliable tumor markers and the development of new small molecules for targeted therapy of liver cancer. Recently, there is increasing evidence for the “cancer stem cell hypothesis”, which postulates that liver cancer originates from the malignant transformation of liver stem/progenitor cells (liver cancer stem cells). This cancer stem cell model has important significance for understanding the basic biology of liver cancer and has profound importance for the development of new strategies for cancer prevention and treatment. In this review, we highlight recent advances in the role of liver stem cells in hepatocarcinogenesis. Our review of the literature shows that identification of the cellular origin and the signaling pathways involved is challenging issues in liver cancer with pivotal implications in therapeutic perspectives. Although the dedifferentiation of mature hepatocytes/cholangiocytes in hepatocarcinogenesis cannot be excluded, neoplastic transformation of a stem cell subpopulation more easily explains hepatocarcinogenesis. Elimination of liver cancer stem cells in liver cancer could result in the degeneration of downstream cells, which makes them potential targets for liver cancer therapies. Therefore, liver stem cells could represent a new target for therapeutic approaches to liver cancer in the near future.

Concise review: bone marrow autotransplants for liver disease?

There are increasing reports of using bone marrow-derived stem cells to treat advanced liver disease. We consider several critical issues that underlie this approach. For example, are there multipotent stem cell populations in human adult bone marrow? Can they develop into liver cells or supporting cell types? What are stromal stem/progenitor cells, and can they promote tissue repair without replacing hepatocytes? Does reversal of end-stage liver disease require new hepatocytes, a new liver microenvironment, both, neither or something else? Although many of these questions are unanswered, we consider the conceptual and experimental bases underlying these issues and critically analyze results of clinical trials of stem cell therapy of end-stage liver disease.

Outcomes of autologous bone marrow mononuclear cell transplantation in decompensated liver cirrhosis

AIM: To determine the long-term efficacy of autologous bone marrow mononuclear cells (BM-MNCs) transplantation in terms of improving liver function and reducing complications in patients with decompensated cirrhosis.

METHODS: A total of 47 inpatients with decompensated liver cirrhosis were enrolled in this trial, including 32 patients undergoing a single BM-MNCs transplantation plus routine medical treatment, and 15 patients receiving medical treatment only as controls. Forty-three of 47 patients were infected with hepatitis B virus. Bone marrow of 80-100 mL was obtained from each patient and the BM-MNCs suspension was transfused into the liver via the hepatic artery. The efficacy of BM-MNCs transplantation was monitored during a 24-mo follow-up period.

RESULTS: Liver function parameters in the two groups were observed at 1 mo after BM-MNCs transfusion. Prealbumin level was 118.3 ± 25.3 mg/L vs 101.4 ± 28.7 mg/L (P = 0.047); albumin level was 33.5 ± 3.6 g/L vs 30.3 ± 2.2 g/L (P = 0.002); total bilirubin 36.9 ± 9.7 mmol/L vs 45.6 ± 19.9 mmol/L (P = 0.048); prothrombin time 14.4 ± 2.3 s vs 15.9 ± 2.8 s (P = 0.046); prothrombin activity 84.3% ± 14.3% vs 74.4% ± 17.8% (P = 0.046); fibrinogen 2.28 ± 0.53 g/L vs 1.89 ± 0.44 g/L (P = 0.017); and platelet count 74.5 ± 15.7 × 10⁹/L vs 63.3 ± 15.7 × 10⁹/L (P = 0.027) in the treatment group and control group, respectively. Differences were statistically significant. The efficacy of BM-MNCs transplantation lasted 3-12 mo as compared with the control group. Serious complications such as hepatic encephalopathy and spontaneous bacterial peritonitis were also significantly reduced in BM-MNCs transfused patients compared with the controls. However, these improvements disappeared 24 mo after transplantation.

CONCLUSION: BM-MNCs transplantation is safe and effective in patients with decompensated cirrhosis. It also decreases the incidence of serious complications.

Ovarian adult stem cells: hope or pitfall?

For many years, ovarian biology has been based on the dogma that oocytes reserve in female mammals included a finite number, established before or at birth and it is determined by the number and quality of primordial follicles developed during the neonatal period. The restricted supply of oocytes in adult female mammals has been disputed in recent years by supporters of postnatal neo-oogenesis. Recent experimental data showed that ovarian surface epithelium and cortical tissue from both mouse and human were proved to contain very low proportion of cells able to propagate themselves, but also to generate immature oocytes in vitro or in vivo, when transplanted into immunodeficient mice ovaries. By mentioning several landmarks of ovarian stem cell reserve and addressing the exciting perspective of translation into clinical practice as treatment for infertility pathologies, the purpose of this article is to review the knowledge about adult mammalian ovarian stem cells, a topic that, since the first approach quickly attracted the attention of both the scientific media and patients.

Perspective on liver regeneration by bone marrow-derived stem cells-a scientific realization or a paradox

Bone marrow (BM)-derived stem cells are reported to have cellular plasticity, which provoked many investigators to use of these cells in the regeneration of nonhematopoietic tissues. However, adult stem cell plasticity contradicts our classic understanding on progressive restriction of the developmental potential of a cell type. Many alternate mechanisms have been proposed to explain this phenomenon; the working hypotheses for elucidating the cellular plasticity of BM-derived stem cells are on the basis of direct differentiation and/or fusion between donor and recipient cells. This review dissects the different outcomes of the investigations on liver regeneration, which were performed with the use of BM-derived stem cells in experimental animals, and reveals some critical factors to explain cellular plasticity. It has been hypothesized that the competent BM-derived stem/progenitor cells, under the influence of liver-regenerating cues, can directly differentiate into hepatic cells. This differentiation takes place as a result of genetic reprogramming, which may be possible in the chemically induced acute liver injury model or at the stage of fetal liver development. Cellular plasticity emerges as an important phenomenon in cell-based therapies for the treatment of many liver diseases in which tissue regeneration is necessary.

Hematopoietic stem cell development, niches, and signaling pathways

Hematopoietic stem cells (HSCs) play a key role in hematopoietic system that functions mainly in homeostasis and immune response. HSCs transplantation has been applied for the treatment of several diseases. However, HSCs persist in the small quantity within the body, mostly in the quiescent state. Understanding the basic knowledge of HSCs is useful for stem cell biology research and therapeutic medicine development. Thus, this paper emphasizes on HSC origin, source, development, the niche, and signaling pathways which support HSC maintenance and balance between self-renewal and proliferation which will be useful for the advancement of HSC expansion and transplantation in the future.

Hepatic differentiation from human mesenchymal stem cells on a novel nanofiber scaffold

The emerging fields of tissue engineering and biomaterials have begun to provide potential treatment options for liver failure. The goal of the present study is to investigate the ability of a poly L-lactic acid (PLLA) nanofiber scaffold to support and enhance hepatic differentiation of human bone marrow-derived mesenchymal stem cells (hMSCs). A scaffold composed of poly L-lactic acid and collagen was fabricated by the electrospinning technique. After characterizing isolated hMSCs, they were seeded onto PLLA nanofiber scaffolds and induced to differentiate into a hepatocyte lineage. The mRNA levels and protein expression of several important hepatic genes were determined using RT-PCR, immunocytochemistry and ELISA. Flow cytometry revealed that the isolated bone marrow-derived stem cells were positive for hMSC-specific markers CD73, CD44, CD105 and CD166 and negative for hematopoietic markers CD34 and CD45. The differentiation of these stem cells into adipocytes and osteoblasts demonstrated their multipotency. Scanning electron microscopy showed adherence of cells in the nanofiber scaffold during differentiation towards hepatocytes. Our results showed that expression levels of liver-specific markers such as albumin, α-fetoprotein, and cytokeratins 8 and 18 were higher in differentiated cells on the nanofibers than when cultured on plates. Importantly, liver functioning serum proteins, albumin and α-1 antitrypsin were secreted into the culture medium at higher levels by the differentiated cells on the nanofibers than on the plates, demonstrating that our nanofibrous scaffolds promoted and enhanced hepatic differentiation under our culture conditions. Our results show that the engineered PLLA nanofibrous scaffold is a conducive matrix for the differentiation of MSCs into functional hepatocyte-like cells. This represents the first step for the use of this nanofibrous scaffold for culture and differentiation of stem cells that may be employed for tissue engineering and cell-based therapy applications.

Hepatocyte nuclear factor-4alpha induces transdifferentiation of hematopoietic cells into hepatocytes

Hematopoietic stem cells can directly transdifferentiate into hepatocytes because of cellular plasticity, but the molecular basis of transdifferentiation is not known. Here, we show the molecular basis using lineage-depleted oncostatin M receptor beta-expressing (Lin(-)OSMRbeta(+)) mouse bone marrow cells in a hepatic differentiation culture system. Differentiation of the cells was marked by the expression of albumin. Hepatocyte nuclear factor (HNF)-4alpha was expressed and translocated into the nuclei of the differentiating cells. Suppression of its activation in OSM-neutralized culture medium inhibited cellular differentiation. Ectopic expression of full-length HNF4alpha in 32D myeloid cells resulted in decreased myeloid colony-forming potential and increased expression of hepatocyte-specific genes and proteins. Nevertheless, the neohepatocytes produced in culture expressed active P450 enzyme. The obligatory role of HNF4alpha in hepatic differentiation was confirmed by transfecting Lin(-)OSMRbeta(+) cells with dominant negative HNF4alpha in the differentiation culture because its expression inhibited the transcription of the albumin and tyrosine aminotransferase genes. The loss and gain of functional activities strongly suggested that HNF4alpha plays a central role in the transdifferentiation process. For the first time, this report demonstrates the mechanism of transdifferentiation of hematopoietic cells into hepatocytes, in which HNF4alpha serves as a molecular switch.

The therapeutic effect of bone marrow-derived liver cells in the phenotypic correction of murine hemophilia A

The transdifferentiation of bone marrow cells (BMCs) into hepatocytes has created enormous interest in applying this process to the development of cellular medicine for degenerative and genetic diseases. Because the liver is the primary site of factor VIII (FVIII) synthesis, we hypothesized that the partial replacement of mutated liver cells by healthy cells in hemophilia A mice could manage the severity of the bleeding disorder. We perturbed the host liver with acetaminophen to facilitate the engraftment and hepatic differentiation of lineage-depleted enhanced green fluorescent protein-expressing BMCs. Immunohistochemistry experiments with the liver tissue showed that the donor-derived cells expressed the markers of both hepatocytes (albumin and cytokeratin-18) and endothelial cells (von Willebrand factor). The results of fluorescent in situ hybridization and immunocytochemistry experiments suggested that differentiation was direct in this model. The BMC-recipient mice expressed FVIII protein and survived in a tail clip challenge experiment. Furthermore, a coagulation assay confirmed that the plasma FVIII activity was maintained at 20.4% (+/- 3.6%) of normal pooled plasma activity for more than a year without forming its inhibitor. Overall, this report demonstrated that BMCs rescued the bleeding phenotype in hemophilia A mice, suggesting a potential therapy for this and other related disorders.

Differentiation and transplantation of human embryonic stem cell-derived hepatocytes

BACKGROUND & AIMS

The ability to obtain unlimited numbers of human hepatocytes would improve the development of cell-based therapies for liver diseases, facilitate the study of liver biology, and improve the early stages of drug discovery. Embryonic stem cells are pluripotent, potentially can differentiate into any cell type, and therefore could be developed as a source of human hepatocytes.

METHODS

To generate human hepatocytes, human embryonic stem cells were differentiated by sequential culture in fibroblast growth factor 2 and human activin-A, hepatocyte growth factor, and dexamethasone. Functional hepatocytes were isolated by sorting for surface asialoglycoprotein-receptor expression. Characterization was performed by real-time polymerase chain reaction, immunohistochemistry, immunoblot, functional assays, and transplantation.

RESULTS

Embryonic stem cell-derived hepatocytes expressed liver-specific genes, but not genes representing other lineages, secreted functional human liver-specific proteins similar to those of primary human hepatocytes, and showed human hepatocyte cytochrome P450 metabolic activity. Serum from rodents given injections of embryonic stem cell-derived hepatocytes contained significant amounts of human albumin and alpha1-antitrypsin. Colonies of cytokeratin-18 and human albumin-expressing cells were present in the livers of recipient animals.

CONCLUSIONS

Human embryonic stem cells can be differentiated into cells with many characteristics of primary human hepatocytes. Hepatocyte-like cells can be enriched and recovered based on asialoglycoprotein-receptor expression and potentially could be used in drug discovery research and developed as therapeutics.

Feasibility investigation of allogeneic endometrial regenerative cells

Endometrial Regenerative Cells (ERC) are a population of mesenchymal-like stem cells having pluripotent differentiation activity and ability to induce neoangiogenesis. In vitro and animal studies suggest ERC are immune privileged and in certain situations actively suppress ongoing immune responses. In this paper we describe the production of clinical grade ERC and initial safety experiences in 4 patients with multiple sclerosis treated intravenously and intrathecally. The case with the longest follow up, of more than one year, revealed no immunological reactions or treatment associated adverse effects. These preliminary data suggest feasibility of clinical ERC administration and support further studies with this novel stem cell type.

Hematopoietic progenitors from early murine fetal liver possess hepatic differentiation potential

Bipotential hepatoblasts differentiate into hepatocytes and cholangiocytes during liver development. It is believed that hepatoblasts originate from endodermal tissue. Here, we provide evidence for the presence of hepatic progenitor cells in the hematopoietic compartment at an early stage of liver development. Flow cytometric analysis showed that at early stages of liver development, approximately 13% of CD45(+) cells express Delta-like protein-1, a marker of hepatoblasts. Furthermore, reverse transcriptase-PCR data suggest that many hepatic genes are expressed in these cells. Cell culture experiments confirmed the hepatic differentiation potential of these cells with the loss of the CD45 marker. We observed that both hematopoietic activity in Delta-like protein-1(+) cells and hepatic activity in CD45(+) cells were high at embryonic day 10.5 and declined thereafter. Clonal analysis revealed that the hematopoietic fraction of fetal liver cells at embryonic day 10.5 gave rise to both hepatic and hematopoietic colonies. The above results suggest a common source of these two functionally distinct cell lineages. In utero transplantation experiments confirmed these results, as green fluorescent protein-expressing CD45(+) cells at the same stage of development yielded functional hepatocytes and hematopoietic reconstitution. Since these cells were unable to differentiate into cytokeratin-19-expressing cholangiocytes, we distinguished them from hepatoblasts. This preliminary study provides hope to correct many liver diseases during prenatal development via transplantation of fetal liver hematopoietic cells.