Liver organogenesis promoted by endothelial cells prior to vascular function

Positive feedback loop between cancer stem cells and angiogenesis in hepatocellular carcinoma

Anti-angiogenesis-related therapies have become the standard care for patients with advanced hepatocellular carcinoma (HCC), as HCC is a highly vascularized solid tumor. Unfortunately, only modest and limited efficacies are observed. Emerging evidence have attributed to the limited efficacy to the presence of cancer stem cells (CSCs) in the tumor. CSCs predominantly drives angiogenesis via releasing proangiogenic factors and exosomes. They have the ability to resistant intratumoral hypoxia via autophagy or by directly forming the tubular structure to obtain blood. On the other hand, the vascular niche in tumor microenvironment also releases growth factors via juxtacrine and paracrine mechanisms to support the growth of CSCs and maintain its stemness features. This positive feedback loop between angiogenesis and CSCs exists in liver tumor microenvironment that is responsible for the development and poor prognosis of HCC. In this review, we summarize recent advances in our understanding of the crosstalks between angiogenesis and CSCs, and their interactions in liver tumor microenvironment and their purpose that an effective anti-angiogenic therapy should also target CSCs for HCC treatment.

Genetic lineage tracing identifies endocardial origin of liver vasculature

The hepatic vasculature is essential for liver development, homeostasis and regeneration, yet the developmental program of hepatic vessel formation and the embryonic origin of the liver vasculature remain unknown. Here we show in mouse that endocardial cells form a primitive vascular plexus surrounding the liver bud and subsequently contribute to a substantial portion of the liver vasculature. Using intersectional genetics, we demonstrate that the endocardium of the sinus venosus is a source for the hepatic plexus. Inhibition of endocardial angiogenesis results in reduced endocardial contribution to the liver vasculature and defects in liver organogenesis. We conclude that a substantial portion of liver vessels derives from the endocardium and shares a common developmental origin with coronary arteries.

Histology Atlas of the Developing Mouse Hepatobiliary Hemolymphatic Vascular System with Emphasis on Embryonic Days 11.5-18.5 and Early Postnatal Development

A critical event in embryo development is the proper formation of the vascular system, of which the hepatobiliary system plays a pivotal role. This has led researchers to use transgenic mice to identify the critical steps involved in developmental disorders associated with the hepatobiliary vascular system. Vascular development is dependent upon normal vasculogenesis, angiogenesis, and the transformation of vessels into their adult counterparts. Any alteration in vascular development has the potential to cause deformities or embryonic death. Numerous publications describe specific stages of vascular development relating to various organs, but a single resource detailing the stage-by-stage development of the vasculature pertaining to the hepatobiliary system has not been available. This comprehensive histology atlas provides hematoxylin & eosin and immunohistochemical-stained sections of the developing mouse blood and lymphatic vasculature with emphasis on the hepatobiliary system between embryonic days (E) 11.5-18.5 and the early postnatal period. Additionally, this atlas includes a 3-dimensional video representation of the E18.5 mouse venous vasculature. One of the most noteworthy findings of this atlas is the identification of the portal sinus within the mouse, which has been erroneously misinterpreted as the ductus venosus in previous publications. Although the primary purpose of this atlas is to identify normal hepatobiliary vascular development, potential embryonic abnormalities are also described.

Nanofibrous spongy microspheres for the delivery of hypoxia-primed human dental pulp stem cells to regenerate vascularized dental pulp

Dental pulp infection and necrosis are widespread diseases. Conventional endodontic treatments result in a devitalized and weakened tooth. In this work, we synthesized novel star-shaped polymer to self-assemble into unique nanofibrous spongy microspheres (NF-SMS), which were used to carry human dental pulp stem cells (hDPSCs) into the pulp cavity to regenerate living dental pulp tissues. It was found that NF-SMS significantly enhanced hDPSCs attachment, proliferation, odontogenic differentiation and angiogenesis, as compared to control cell carriers. Additionally, NF-SMS promoted vascular endothelial growth factor (VEGF) expression of hDPSCs in a 3D hypoxic culture. Hypoxia-primed hDPSCs/NF-SMS complexes were injected into the cleaned pulp cavities of rabbit molars for subcutaneous implantation in mice. After 4 weeks, the hypoxia group significantly enhanced angiogenesis inside the pulp chamber and promoted the formation of ondontoblast-like cells lining along the dentin-pulp interface, as compared to the control groups (hDPSCs alone group, NF-SMS alone group, and hDPSCs/NF-SMS group pre-cultured under normoxic conditions). Furthermore, in an in situ dental pulp repair model in rats, hypoxia-primed hDPSCs/NF-SMS were injected to fully fill the pulp cavity and regenerate pulp-like tissues with a rich vasculature and a histological structure similar to the native pulp.

STATEMENT OF SIGNIFICANCE

Vascularization is key to the regeneration of many vital tissues. However, it is challenging to create a suitable microenvironment for stem cells to regenerate vascularized tissue structure. This manuscript reports a novel star-shaped block copolymer that self-assembles into unique nanofibrous spongy microspheres, which as an injectable scaffold recapitulate the cell-cell and cell-matrix interactions in development. Using a clinically-relevant surgical procedure and a hypoxic treatment, the nanofibrous spongy microspheres were used to deliver stem cells and successfully regenerate dental pulp with a rich vasculature and a complex histologic structure similar to that of the native dental pulp. The novel microspheres can likely be used to regenerate many other vascularized tissues.

Fluid Flow Regulation of Revascularization and Cellular Organization in a Bioengineered Liver Platform

OBJECTIVE

Modeling of human liver development, especially cellular organization and the mechanisms underlying it, is fundamental for studying liver organogenesis and congenital diseases, yet there are no reliable models that mimic these processes ex vivo.

DESIGN

Using an organ engineering approach and relevant cell lines, we designed a perfusion system that delivers discrete mechanical forces inside an acellular liver extracellular matrix scaffold to study the effects of mechanical stimulation in hepatic tissue organization.

RESULTS

We observed a fluid flow rate-dependent response in cell distribution within the liver scaffold. Next, we determined the role of nitric oxide (NO) as a mediator of fluid flow effects on endothelial cells. We observed impairment of both neovascularization and liver tissue organization in the presence of selective inhibition of endothelial NO synthase. Similar results were observed in bioengineered livers grown under static conditions.

CONCLUSION

Overall, we were able to unveil the potential central role of discrete mechanical stimulation through the NO pathway in the revascularization and cellular organization of a bioengineered liver. Last, we propose that this organ bioengineering platform can contribute significantly to the identification of physiological mechanisms of liver organogenesis and regeneration and improve our ability to bioengineer livers for transplantation.

Improved survival of porcine acute liver failure by a bioartificial liver device implanted with induced human functional hepatocytes

Acute liver failure (ALF) is a life-threatening illness. The extracorporeal cell-based bioartificial liver (BAL) system could bridge liver transplantation and facilitate liver regeneration for ALF patients by providing metabolic detoxification and synthetic functions. Previous BAL systems, based on hepatoma cells and non-human hepatocytes, achieved limited clinical advances, largely due to poor hepatic functions, cumbersome preparation or safety concerns of these cells. We previously generated human functional hepatocytes by lineage conversion (hiHeps). Here, by improving functional maturity of hiHeps and producing hiHeps at clinical scales (3 billion cells), we developed a hiHep-based BAL system (hiHep-BAL). In a porcine ALF model, hiHep-BAL treatment restored liver functions, corrected blood levels of ammonia and bilirubin, and prolonged survival. Importantly, human albumin and α-1-antitrypsin were detectable in hiHep-BAL-treated ALF pigs. Moreover, hiHep-BAL treatment led to attenuated liver damage, resolved inflammation and enhanced liver regeneration. Our findings indicate a promising clinical application of the hiHep-BAL system.

Design Principles for Engineering of Tissues from Human Pluripotent Stem Cells

Recent advances in human pluripotent stem cell (hPSC) technologies have enabled the engineering of human tissue constructs for developmental studies, disease modeling, and drug screening platforms. In vitro tissue formation can be generally described at three levels of cellular organization. Multicellular hPSC constructs are initially formed either with polymeric scaffold materials or simply via self-assembly, adhesive mechanisms. Heterotypic interactions within hPSC tissue constructs can be achieved by physically mixing independently differentiated cell populations or coaxed to simultaneously co-emerge from a common population of undifferentiated cells. Higher order tissue architecture can be engineered by imposing external spatial constraints, such as molds and scaffolds, or depend upon cell-driven organization that exploits endogenous innate developmental mechanisms. The multicellular, heterogeneous, and highly organized structure of hPSC constructs ultimately dictates the resulting form and function of in vitro engineered human tissue models.

From Endoderm to Liver Bud: Paradigms of Cell Type Specification and Tissue Morphogenesis

The early specification, rapid growth and morphogenesis, and conserved functions of the embryonic liver across diverse model organisms have made the system an experimentally facile paradigm for understanding basic regulatory mechanisms that govern cell differentiation and organogenesis. This essay highlights concepts that have emerged from studies of the discrete steps of foregut endoderm development into the liver bud, as well as from modeling the steps via embryonic stem cell differentiation. Such concepts include understanding the chromatin basis for the competence of progenitor cells to develop into specific lineages; the importance of combinatorial signaling from different sources to induce cell fates; the impact of inductive signaling on preexisting chromatin states; the ability of separately specified domains of cells to merge into a common tissue; and the marked cell biological dynamics, including interactions with the developing vasculature, which establish the initial morphogenesis and patterning of a tissue. The principles gleaned from these studies, focusing on the 2 days it takes for the endoderm to develop into a liver bud, should be instructive for many other organogenic systems and for manipulating tissues in regenerative contexts for biomedical purposes.

Hhex Is Necessary for the Hepatic Differentiation of Mouse ES Cells and Acts via Vegf Signaling

Elucidating the molecular mechanisms involved in the differentiation of stem cells to hepatic cells is critical for both understanding normal developmental processes as well as for optimizing the generation of functional hepatic cells for therapy. We performed in vitro differentiation of mouse embryonic stem cells (mESCs) with a null mutation in the homeobox gene Hhex and show that Hhex^-/- mESCs fail to differentiate from definitive endoderm (Sox17⁺/Foxa2⁺) to hepatic endoderm (Alb⁺/Dlk⁺). In addition, hepatic culture elicited a >7-fold increase in Vegfa mRNA expression in Hhex^-/- cells compared to Hhex^+/+ cells. Furthermore, we identified VEGFR2⁺/ALB⁺/CD34^- in early Hhex^+/+ hepatic cultures. These cells were absent in Hhex^-/- cultures. Finally, through manipulation of Hhex and Vegfa expression, gain and loss of expression experiments revealed that Hhex shares an inverse relationship with the activity of the Vegf signaling pathway in supporting hepatic differentiation. In summary, our results suggest that Hhex represses Vegf signaling during hepatic differentiation of mouse ESCs allowing for cell-type autonomous regulation of Vegfr2 activity independent of endothelial cells.

Endothelial Cords Promote Tumor Initial Growth prior to Vascular Function through a Paracrine Mechanism

The angiogenic switch is an important oncogenic step that determines whether microtumors remain dormant or progresses further. It has been generally perceived that the primary function of this tumorgenic event is to supply oxygen and nutrients through blood circulation. Using in vivo imaging of zebrafish and mouse tumor models, we showed that endothelial cords aggressively penetrated into microtumors and remained non-circulatory for several days before undergoing vascular blood perfusion. Unexpectedly, we found that initial tumor growth in both models was significantly reduced if endothelial cords were removed by blocking VEGF-VEGFR2 signaling or using a vascular deficient zebrafish mutant. It was further shown that soluble factors including IL-8, secreted by endothelial cells (ECs) were responsible for stimulating tumor cells proliferation. These findings establish that tumor angiogenesis play a much earlier and broader role in promoting tumor growth, which is independent of vascular circulation. Understanding this novel mechanism of angiogenic tumor progression offers new entry points for cancer therapeutics.

Vegfa signaling promotes zebrafish intestinal vasculature development through endothelial cell migration from the posterior cardinal vein

The mechanisms underlying organ vascularization are not well understood. The zebrafish intestinal vasculature forms early, is easily imaged using transgenic lines and in-situ hybridization, and develops in a stereotypical pattern thus making it an excellent model for investigating mechanisms of organ specific vascularization. Here, we demonstrate that the sub-intestinal vein (SIV) and supra-intestinal artery (SIA) form by a novel mechanism from angioblasts that migrate out of the posterior cardinal vein and coalesce to form the intestinal vasculature in an anterior to posterior wave with the SIA forming after the SIV. We show that vascular endothelial growth factor aa (vegfaa) is expressed in the endoderm at the site where intestinal vessels form and therefore likely provides a guidance signal. Vegfa/Vegfr2 signaling is required for early intestinal vasculature development with mutation in vegfaa or loss of Vegfr2 homologs causing nearly complete inhibition of the formation of the intestinal vasculature. Vegfc and Vegfr3 function, however, are dispensable for intestinal vascularization. Interestingly, ubiquitous overexpression of Vegfc resulted in an overgrowth of the SIV, suggesting that Vegfc is sufficient to induce SIV development. These results argue that Vegfa signaling directs endothelial cells to migrate out of existing vasculature and coalesce to form the intestinal vessels. It is likely that a similar mechanism is utilized during vascularization of other organs.

Proper BMP Signaling Levels Are Essential for 3D Assembly of Hepatic Cords from Hepatoblasts and Mesenchymal Cells

BACKGROUND

Because the molecular mechanisms of morphogenesis of the hepatic cord and sinus are unclear, we investigated the involvement of bone morphogenetic protein (BMP4) in hepatic sinusoid morphogenesis.

METHODS

We used embryonic chicken livers, which develop rapidly, as our model, and investigated expression of BMP-related genes. BMP4 activity was manipulated by overexpressing BMP4 and its antagonist, noggin.

RESULTS

During hepatic cord morphogenesis, BMP4 and its receptors are expressed in both peri-sinusoidal cells and hepatoblasts as the sinusoids form, whereas noggin is expressed transiently in peri-sinusoidal cells at early stages. Suppression of BMP activity with noggin overexpression disrupted normal hepatic sinusoid structure, leading to liver congestion, failure of fibronectin deposition, and markedly reduced numbers of peri-sinusoidal cells. However, overexpression of BMP did not change sinusoidal morphology but increased endothelial cell number. Noggin overexpression resulted in disrupted cord organization, and dilated sinusoidal space, eventually leading to increased apoptosis and failed hepatocyte differentiation.

CONCLUSIONS

Our results show that proper BMP signaling mediates peri-sinusoidal cell-hepatoblast interactions during development; this is essential for hepatic cord organization among hepatoblasts, endothelium, and presumptive hepatic stellate cells.

Infusion of endothelial progenitor cells ameliorates liver injury in mice after haematopoietic stem cell transplantation

BACKGROUND & AIMS

Injury to liver sinusoidal endothelial cells (LSECs) is thought to be the initial factor for Hepatic veno-occlusive disease, a severe complication after haematopoietic stem cell transplantation (HSCT). Endothelial progenitor cells (EPCs) have the capacity to differentiate into endothelial cells and play a critical role in vasculogenesis, tissue regeneration and repair. Whether EPCs infusion ameliorates LSECs injury remains unclear. The aim of this study was to evaluate the effects of EPCs on liver injury in mice after HSCT.

METHODS

Mice received HSCT without or with EPCs infusion (HSCT + EPCs). Untreated mice were used as control. Liver and whole blood were collected post HSCT and used for the analysis of pathology of liver sinusoidal endothelial cells (LSECs) and hepatocytes, liver ultrastructure, function, level of IL-6, TNF-α and platelet activation.

RESULTS

Severe LSECs injury, hepatocyte damage, abnormal liver function was observed in HSCT group. In addition, increased P-selectin expression and secretion of IL-6, TNF-α was also found. However, all the above changes were alleviated in HSCT + EPCs at all the time points and normalized at the endpoint. Meanwhile, EPCs-induced repair of LSECs and hepatocytes was totally inhibited by the addition of anti-VE-cadherin antibody.

CONCLUSIONS

EPCs infusion ameliorated the damage to LSECs and hepatocytes as well as reduced secretion of IL-6, TNF-α and inhibited platelet activation after HSCT, leading to improved liver function, suggesting EPCs might be a new therapeutic strategy in the prophylaxis of liver injury after HSCT.

Engraftment of human induced pluripotent stem cell-derived hepatocytes in immunocompetent mice via 3D co-aggregation and encapsulation

Cellular therapies for liver diseases and in vitro models for drug testing both require functional human hepatocytes (Hum-H), which have unfortunately been limited due to the paucity of donor liver tissues. Human pluripotent stem cells (hPSCs) represent a promising and potentially unlimited cell source to derive Hum-H. However, the hepatic functions of these hPSC-derived cells to date are not fully comparable to adult Hum-H and are more similar to fetal ones. In addition, it has been challenging to obtain functional hepatic engraftment of these cells with prior studies having been done in immunocompromised animals. In this report, we demonstrated successful engraftment of human induced pluripotent stem cell (iPSC)-derived hepatocyte-like cells (iPS-H) in immunocompetent mice by pre-engineering 3D cell co-aggregates with stromal cells (SCs) followed by encapsulation in recently developed biocompatible hydrogel capsules. Notably, upon transplantation, human albumin and α1-antitrypsin (A1AT) in mouse sera secreted by encapsulated iPS-H/SCs aggregates reached a level comparable to the primary Hum-H/SCs control. Further immunohistochemistry of human albumin in retrieved cell aggregates confirmed the survival and function of iPS-H. This proof-of-concept study provides a simple yet robust approach to improve the engraftment of iPS-H, and may be applicable to many stem cell-based therapies.

Stem cells: An insight into the therapeutic aspects from medical and dental perspectives

The recent advancements in the field of stem cell (SC) biology have increased the hope of achieving the definitive treatments for the diseases which are now considered incurable such as diabetes, Parkinson's disease and other chronic long standing conditions. To achieve this possibility, it is necessary to understand the basic concepts of SC biology to utilize in various advanced techniques of regenerative medicine including tissue engineering and gene therapy. This article highlights the types of SCs available and their therapeutic capacity in regenerative medical and dental fields.

Venous-derived angioblasts generate organ-specific vessels during zebrafish embryonic development

Formation and remodeling of vascular beds are complex processes orchestrated by multiple signaling pathways. Although it is well accepted that vessels of a particular organ display specific features that enable them to fulfill distinct functions, the embryonic origins of tissue-specific vessels and the molecular mechanisms regulating their formation are poorly understood. The subintestinal plexus of the zebrafish embryo comprises vessels that vascularize the gut, liver and pancreas and, as such, represents an ideal model in which to investigate the early steps of organ-specific vessel formation. Here, we show that both arterial and venous components of the subintestinal plexus originate from a pool of specialized angioblasts residing in the floor of the posterior cardinal vein (PCV). Using live imaging of zebrafish embryos, in combination with photoconvertable transgenic reporters, we demonstrate that these angioblasts undergo two phases of migration and differentiation. Initially, a subintestinal vein forms and expands ventrally through a Bone Morphogenetic Protein-dependent step of collective migration. Concomitantly, a Vascular Endothelial Growth Factor-dependent shift in the directionality of migration, coupled to the upregulation of arterial markers, is observed, which culminates with the generation of the supraintestinal artery. Together, our results establish the zebrafish subintestinal plexus as an advantageous model for the study of organ-specific vessel development and provide new insights into the molecular mechanisms controlling its formation. More broadly, our findings suggest that PCV-specialized angioblasts contribute not only to the formation of the early trunk vasculature, but also to the establishment of late-forming, tissue-specific vascular beds.

Highlighted article: A specialized pool of angioblasts is the origin of the zebrafish subintestinal plexus, a structure that gives rise to the organ-specific vessels of the gut, liver and pancreas.

Relevance of the plasma membrane calcium-ATPase in the homeostasis of calcium in the fetal liver

During the early stages of development, the embryo depends on the placenta as provider of oxygen and calcium, among other essential compounds. Although fetal liver accomplishes a well-known haematopoietic function, its contribution to calcium homeostasis upon development is poorly understood. The homeostasis of cell calcium contributes to diverse signaling pathways across developmental stages of most tissues and the calcium-ATPase located at the plasma membrane (PMCA) helps pumping excess calcium into the extracellular space. To date, the understanding of the equilibrium shift between PMCA isoforms during liver development is still missing. This review focuses on the characterization of the hepatic PMCA along the early stages of development, followed by a description of modern approaches to study calcium homeostasis involving several types of pluripotent cells. The application of interdisciplinary techniques to improve our understanding of liver development and the role calcium homeostasis plays in the definition of pathogenesis is also discussed.

"The sinusoid" in the liver: lessons learned from the original definition by Charles Sedgwick Minot (1900)

The hepatic sinusoid with its associated sinusoidal cells is a multifunctional cell-complex in the liver. Despite recent advances in research on the hepatic sinusoid, no investigator has played a more basic role in its characterization than Charles Sedgwick Minot (1852-1914), a pioneer who distinguished the sinusoid from the blood-capillary as early as 1900. According to Minot, sinusoids are typically larger in diameter than capillaries, particularly at the early embryonic stage. They closely approach the parenchymal tissue, are formed passively by the adjacent parenchymal tissue, and are on rare occasion surrounded with connective tissue. Sinusoids (sinus-like) are small blood-channels formed by subdivision of the lumen of large blood vessels (sinuses) by the invasion of developing parenchymal cell-cords. Although some of Minot's definitions may no longer be accepted, he described some fundamental and interesting characteristics of sinusoids, to which we have not paid much attention. Here, we have attempted to illustrate lessons we have learned from Minot's view point of sinusoids at this occasion of centenary of his death.

Orchestrating liver development

The liver is a central regulator of metabolism, and liver failure thus constitutes a major health burden. Understanding how this complex organ develops during embryogenesis will yield insights into how liver regeneration can be promoted and how functional liver replacement tissue can be engineered. Recent studies of animal models have identified key signaling pathways and complex tissue interactions that progressively generate liver progenitor cells, differentiated lineages and functional tissues. In addition, progress in understanding how these cells interact, and how transcriptional and signaling programs precisely coordinate liver development, has begun to elucidate the molecular mechanisms underlying this complexity. Here, we review the lineage relationships, signaling pathways and transcriptional programs that orchestrate hepatogenesis.

3-D reconstruction and multiple marker analysis of mouse proepicardial endothelial cell population

BACKGROUND

The proepicardium (PE), a transient embryonic structure crucial for the development of the epicardium and heart, contains its own population of endothelial cells (ECs). The aim of our study was to determine the pattern, anatomical orientation and phenotypic marker expression of the endothelial cell network within the PE.

RESULTS

Immunohistochemical findings revealed that proepicardial ECs express both early and late EC-specific markers such as CD31, Flk-1, Lyve-1 and Tie-2 but not SCL/Tal1, vWF, Dll4 or Notch1. Proepicardial ECs are present in the vicinity of the sinus venosus (SV) and form a continuous network of vascular sprouts/tubules connected with the SV endothelium, with Ter-119-positive erythroblasts in the vascular lumina.

CONCLUSIONS

On the basis of our results, we postulate the existence of a continuous network of ECs in the PE, exhibiting connection and/or patency with the SV and forming vessels/tubules/strands. Marker expression suggests that ECs are immature and undifferentiated, which was also confirmed with a transmission electron microscopy (TEM) analysis. Our results deliver new data for a better understanding of the nature of proepicardial ECs.

Regulation of lung development and regeneration by the vascular system

Blood vessels have been described a long time ago as passive circuits providing sufficient blood supply to ensure proper distribution of oxygen and nutrition. Blood vessels are mainly formed during embryonic development and in the early postnatal period. In the adult, blood vessels are quiescent, but can be activated and subsequently induced under pathophysiological conditions, such as ischemia and tumor growth. Surprisingly, recent data have suggested an active function for blood vessels, named angiocrine signaling, releasing trophogens which regulate organ development and organ regeneration including in the pancreas, lung, tumor cells, liver and bone. Lung development is driven by hypoxia as well as an intense endothelial-epithelial interaction, and important mechanisms contributing to these processes have recently been identified. This review aims to summarize recent developments and concepts about embryonic pulmonary vascular development and lung regeneration. We discuss hypoxia-inducible factor HIF-2α and vascular endothelial growth factor VEGF as important mediators in lung development and focus on endothelial-epithelial interactions and angiocrine signaling mechanisms.

Endothelium-derived essential signals involved in pancreas organogenesis

Endothelial cells (ECs) are essential for pancreas differentiation, endocrine specification, and endocrine function. They are also involved in the physiopathology of type 1 and type 2 diabetes. During embryogenesis, aortic ECs provide specific factors that maintain the expression of key genes for pancreas development such as pancreatic and duodenal homeobox-1. Other unknown factors are also important for pancreatic endocrine specification and formation of insulin-producing beta cells. Endocrine precursors proliferate interspersed with ductal cells and exocrine precursors and, at some point of development, these endocrine precursors migrate to pancreatic mesenchyme and start forming the islets of Langerhans. By the end of the gestation and close to birth, these islets contain immature beta cells with the capacity to express vascular endothelial growth factor and therefore to recruit ECs from the surrounding microenvironment. ECs in turn produce factors that are essential to maintain insulin secretion in pancreatic beta cells. Once assembled, a cross talk between endocrine cells and ECs maintain the integrity of islets toward an adequate function during the whole life of the adult individual. This review will focus in the EC role in the differentiation and maturation of pancreatic beta cells during embryogenesis as well as the current knowledge about the involvement of endothelium to derive pancreatic beta cells in vitro from mouse or human pluripotent stem cells.

Heterotypic control of basement membrane dynamics during branching morphogenesis

Many mammalian organs undergo branching morphogenesis to create highly arborized structures with maximized surface area for specialized organ function. Cooperative cell-cell and cell-matrix adhesions that sculpt the emerging tissue architecture are guided by dynamic basement membranes. Properties of the basement membrane are reciprocally controlled by the interacting epithelial and mesenchymal cell populations. Here we discuss how basement membrane remodeling is required for branching morphogenesis to regulate cell-matrix and cell-cell adhesions that are required for cell patterning during morphogenesis and how basement membrane impacts morphogenesis by stimulation of cell patterning, force generation, and mechanotransduction. We suggest that in addition to creating mature epithelial architecture, remodeling of the epithelial basement membrane during branching morphogenesis is also essential to promote maturation of the stromal mesenchyme to create mature organ structure. Recapitulation of developmental cell-matrix and cell-cell interactions are of critical importance in tissue engineering and regeneration strategies that seek to restore organ function.

Neural crest cell-derived VEGF promotes embryonic jaw extension

Craniofacial development is a complex morphogenic event that relies on highly orchestrated interactions between multiple cell types. Since the first description of Meckel’s cartilage in the lower jaw more than 180 years ago, we have come to realize that expansion of this specialized structure underpins correct mandible development. Here we demonstrate that an intricate association between neural crest cells and blood vessels plays an important role in promoting chondrocyte proliferation and expansion of Meckel’s cartilage as a prerequisite of correct mandibular morphogenesis. These findings provide direct insight into the origins and potential treatments of highly prevalent disorders affecting the mandible.

Jaw morphogenesis depends on the growth of Meckel’s cartilage during embryogenesis. However, the cell types and signals that promote chondrocyte proliferation for Meckel’s cartilage growth are poorly defined. Here we show that neural crest cells (NCCs) and their derivatives provide an essential source of the vascular endothelial growth factor (VEGF) to enhance jaw vascularization and stabilize the major mandibular artery. We further show in two independent mouse models that blood vessels promote Meckel’s cartilage extension. Coculture experiments of arterial tissue with NCCs or chondrocytes demonstrated that NCC-derived VEGF promotes blood vessel growth and that blood vessels secrete factors to instruct chondrocyte proliferation. Computed tomography and X-ray scans of patients with hemifacial microsomia also showed that jaw hypoplasia correlates with mandibular artery dysgenesis. We conclude that cranial NCCs and their derivatives provide an essential source of VEGF to support blood vessel growth in the developing jaw, which in turn is essential for normal chondrocyte proliferation, and therefore jaw extension.

Vascularized and Complex Organ Buds from Diverse Tissues via Mesenchymal Cell-Driven Condensation

Transplantation of in-vitro-generated organ buds is a promising approach toward regenerating functional and vascularized organs. Though it has been recently shown in the context of liver models, demonstrating the applicability of this approach to other systems by delineating the molecular mechanisms guiding organ bud formation is critical. Here, we demonstrate a generalized method for organ bud formation from diverse tissues by combining pluripotent stem cell-derived tissue-specific progenitors or relevant tissue samples with endothelial cells and mesenchymal stem cells (MSCs). The MSCs initiated condensation within these heterotypic cell mixtures, which was dependent upon substrate matrix stiffness. Defining optimal mechanical properties promoted formation of 3D, transplantable organ buds from tissues including kidney, pancreas, intestine, heart, lung, and brain. Transplanted pancreatic and renal buds were rapidly vascularized and self-organized into functional, tissue-specific structures. These findings provide a general platform for harnessing mechanical properties to generate vascularized, complex organ buds with broad applications for regenerative medicine.

Metastasis prevention by targeting the dormant niche

Despite considerable advancements that shattered previously held dogmas about the metastatic cascade, the evolution of therapies to treat metastatic disease has not kept up. In this Opinion article, I argue that, rather than waiting for metastases to emerge before initiating treatment, it would be more effective to target metastatic seeds before they sprout. Specifically, I advocate directing therapies towards the niches that harbour dormant disseminated tumour cells to sensitize them to cytotoxic agents. Treatment sensitization, achieved by disrupting reservoirs of leukaemic stem cells and latent HIV, argues that this approach, although unconventional, could succeed in improving patient survival by delaying or even preventing metastasis.

The contribution of specific cell subpopulations to submandibular salivary gland branching morphogenesis

Branching morphogenesis is the developmental program responsible for generating a large surface to volume ratio in many secretory and absorptive organs. To accomplish branching morphogenesis, spatiotemporal regulation of specific cell subpopulations is required. Here, we review recent studies that define the contributions of distinct cell subpopulations to specific cellular processes during branching morphogenesis in the mammalian submandibular salivary gland, including the initiation of the gland, the coordination of cleft formation, and the contribution of stem/progenitor cells to morphogenesis. In conclusion, we provide an overview of technological advances that have opened opportunities to further probe the contributions of specific cell subpopulations and to define the integration of events required for branching morphogenesis.

Branching of lung epithelium in vitro occurs in the absence of endothelial cells

BACKGROUND

Early lung morphogenesis is driven by tissue interactions. Signals from the lung mesenchyme drive epithelial morphogenesis, but which individual mesenchymal cell types are influencing early epithelial branching and differentiation remains unclear. It has been shown that endothelial cells are involved in epithelial repair and regeneration in the adult lung, and they may also play a role in driving early lung epithelial branching. These data, in combination with evidence that endothelial cells influence early morphogenetic events in the liver and pancreas, led us to hypothesize that endothelial cells are necessary for early lung epithelial branching.

RESULTS

We blocked vascular endothelial growth factor (VEGF) signaling in embryonic day (E) 12.5 lung explants with three different VEGF receptor inhibitors (SU5416, Ki8751, and KRN633) and found that in all cases the epithelium was able to branch despite the loss of endothelial cells. Furthermore, we found that distal lung mesenchyme depleted of endothelial cells retained its ability to induce terminal branching when recombined with isolated distal lung epithelium (LgE). Additionally, isolated E12.5 primary mouse lung endothelial cells, or human lung microvascular endothelial cells (HMVEC-L), were not able to induce branching when recombined with LgE.

CONCLUSIONS

Our observations support the conclusion that endothelial cells are not required for early lung branching.

Vascular patterning regulates interdigital cell death by a ROS-mediated mechanism

Blood vessels serve as key regulators of organogenesis by providing oxygen, nutrients and molecular signals. During limb development, programmed cell death (PCD) contributes to separation of the digits. Interestingly, prior to the onset of PCD, the autopod vasculature undergoes extensive patterning that results in high interdigital vascularity. Here, we show that in mice, the limb vasculature positively regulates interdigital PCD. In vivo, reduction in interdigital vessel number inhibited PCD, resulting in syndactyly, whereas an increment in vessel number and distribution resulted in elevation and expansion of PCD. Production of reactive oxygen species (ROS), toxic compounds that have been implicated in PCD, also depended on interdigital vascular patterning. Finally, ex vivo incubation of limbs in gradually decreasing oxygen levels led to a correlated reduction in both ROS production and interdigital PCD. The results support a role for oxygen in these processes and provide a mechanistic explanation for the counterintuitive positive role of the vasculature in PCD. In conclusion, we suggest a new role for vascular patterning during limb development in regulating interdigital PCD by ROS production. More broadly, we propose a double safety mechanism that restricts PCD to interdigital areas, as the genetic program of PCD provides the first layer and vascular patterning serves as the second.

The development of the vascular system: a historical overview

Development of the vascular system involves a complex sequence of inductive and differentiating signals leading to vasculogenesis and/or angiogenesis. Dissecting and exploring this process in its multifaceted morphological and molecular aspects has represented a basic contribution and a fascinating adventure in the history of biology. Vasculogenesis, that is de novo formation of vascular channels, initiates early during embryo development and prevails at the beginning of embryo patterning and organ formation. Angiogenesis, the process of shaping new vessels from preexisting blood vessels, mainly operates during postnatal life. In this historical introduction, we try to retrace the early steps of scientific speculation on vascular development and to recapitulate the principal paths leading to our present appreciation of blood vessel formation.

KDR identifies a conserved human and murine hepatic progenitor and instructs early liver development

Understanding the fetal hepatic niche is essential for optimizing the generation of functional hepatocyte-like cells (hepatic cells) from human embryonic stem cells (hESCs). Here, we show that KDR (VEGFR2/FLK-1), previously assumed to be mostly restricted to mesodermal lineages, marks a hESC-derived hepatic progenitor. hESC-derived endoderm cells do not express KDR but, when cultured in media supporting hepatic differentiation, generate KDR+ hepatic progenitors and KDR- hepatic cells. KDR+ progenitors require active KDR signaling both to instruct their own differentiation into hepatic cells and to non-cell-autonomously support the functional maturation of cocultured KDR- hepatic cells. Analysis of human fetal livers suggests that similar progenitors are present in human livers. Lineage tracing in mice provides in vivo evidence of a KDR+ hepatic progenitor for fetal hepatoblasts, adult hepatocytes, and adult cholangiocytes. Altogether, our findings reveal that KDR is a conserved marker for endoderm-derived hepatic progenitors and a functional receptor instructing early liver development.

Regulation of tissue morphogenesis by endothelial cell-derived signals

Endothelial cells (ECs) form an extensive network of blood vessels that has numerous essential functions in the vertebrate body. In addition to their well-established role as a versatile transport network, blood vessels can induce organ formation or direct growth and differentiation processes by providing signals in a paracrine (angiocrine) fashion. Tissue repair also requires the local restoration of vasculature. ECs are emerging as important signaling centers that coordinate regeneration and help to prevent deregulated, disease-promoting processes. Vascular cells are also part of stem cell niches and have key roles in hematopoiesis, bone formation, and neurogenesis. Here, we review these newly identified roles of ECs in the regulation of organ morphogenesis, maintenance, and regeneration.

Vascular patterning sets the stage for macro and micro hepatic architecture

Background The liver is a complex organ with a variety of tissue components that require a precise architecture for optimal function of metabolic and detoxification processes. As a result of the delicate orchestration required between the various hepatic tissues, it is not surprising that impairment of hepatic function can be caused by a variety of factors leading to chronic liver disease. Results Despite the growing rate of chronic liver disease, there are currently few effective treatment options besides orthotopic liver transplantation. Better therapeutic options reside in the potential for genetic and cellular therapies that promote progenitor cell activation aiding de novo epithelial and vascular regeneration, cell replacement, or population of bioartificial hepatic devices. In order to explore this area of new therapeutic potential, it is crucial to understand the factors that promote hepatic function through regulating cell identities and tissue architecture. Conclusions In this commentary, we review the signals regulating liver cell fates during development and regeneration and highlight the importance of patterning the hepatic vascular systems to set the groundwork for the macro and micro hepatic architecture of the epithelium.

Hepatic differentiation of human embryonic stem cells as microscaled multilayered colonies leading to enhanced homogeneity and maturation

Directed differentiation of human embryonic stem cells (hESCs) towards hepatocyte-like cells on planar tissue culture plates has been extensively investigated with great promise to provide alternative cell sources for drug metabolism/toxicity testing. Recently, hepatic differentiation of hESCs in 3D configuration with better mimicry of embryonic liver development represents incremental efforts to improve the differentiation efficiency and cellular maturation. However, most of the present 3D differentiation configurations involved interruptive operations during the multi-staged differentiation process, which might impose unwanted influence on cellular differentiation. Most of the current researches resulted in generation of hepatocytes with high expression of AFP, which is minimally expressed in primary hepatocytes. Here, off-the-shelf micro-stencil arrays are developed to generate adherent multilayered colonies composed of hESCs-derived cells. Uninterrupted cellular differentiation and proliferation is achieved to recapitulate the continuous and multi-stage liver development. Compared with conventional 2D format, the micro-scaled multilayered colonies with uniform and defined sizes constrained within the microwells are composed of more homogenous and mature hepatocyte-like cells with significantly lowered AFP expression and elevated hepatic functions. The multilayered colonies as novel 3D configuration for hepatic differentiation of hESCs represent a significant step toward efficient generation of functional hepatocytes for regenerative medicine and drug discovery.

Endothelial cells provide a notch-dependent pro-tumoral niche for enhancing breast cancer survival, stemness and pro-metastatic properties

Treating metastasis has been challenging due to tumors complexity and heterogeneity. This complexity is partly related to the crosstalk between tumor and its microenvironment. Endothelial cells -the building blocks of tumor vasculature- have been shown to have additional roles in cancer progression than angiogenesis and supplying oxygen and nutrients. Here, we show an alternative role for endothelial cells in supporting breast cancer growth and spreading independent of their vascular functions. Using endothelial cells and breast cancer cell lines MDA-MB231 and MCF-7, we developed co-culture systems to study the influence of tumor endothelium on breast tumor development by both in vitro and in vivo approaches. Our results demonstrated that endothelial cells conferred survival advantage to tumor cells under complete starvation and enriched the CD44^HighCD24^Low/- stem cell population in tumor cells. Moreover, endothelial cells enhanced the pro-metastatic potential of breast cancer cells. The in vitro and in vivo results concordantly confirmed a role for endothelial Jagged1 to promote breast tumor through notch activation. Here, we propose a role for endothelial cells in enhancing breast cancer progression, stemness, and pro-metastatic traits through a perfusion-independent manner. Our findings may be beneficial in developing novel therapeutic approaches.

New vessel formation in the context of cardiomyocyte regeneration--the role and importance of an adequate perfusing vasculature

The history of revascularization for cardiac ischemia dates back to the early 1960's when the first coronary artery bypass graft procedures were performed in humans. With this 50 year history of providing a new vasculature to ischemic and hibernating myocardium, a profound depth of experience has been amassed in clinical cardiovascular medicine as to what does, and does not work in the context of cardiac revascularization, alleviating ischemia and adequacy of myocardial perfusion. These issues are of central relevance to contemporary cell-based cardiac regenerative approaches. While the cardiovascular cell therapy field is surging forward on many exciting fronts, several well accepted clinical axioms related to the cardiac arterial supply appear to be almost overlooked by some of our current basic conceptual and experimental cell therapy paradigms. We present here information drawn from five decades of the clinical revascularization experience, review relevant new data on vascular formation via cell therapy, and put forward the case that for optimal cell-based cardiac regeneration due attention must be paid to providing an adequate vascular supply.

Hypoxia and fibrosis in chronic kidney disease: crossing at pericytes

Chronic kidney disease (CKD) is placing an increasing burden on patients and societies because no decisive therapy has been established. Tubulointerstitial lesions accompanied by fibrosis, inflammatory cells, and capillary rarefaction not only characterize, but also aggravate renal dysfunction in CKD. In this setting, renal cells, particularly tubular cells, suffer from hypoxia caused by the imbalance of blood perfusion and oxygen demand despite their adaptive responses represented by upregulation of hypoxia-inducible factors (HIFs). Fibrosis is a pathological state characterized by excess extracellular matrix (ECM) deposition, which is also a hallmark and causative factor of many chronic diseases including CKD. Recent studies have suggested that the dominant origin of ECM-producing myofibroblasts (MFs) may be pericytes, which are indispensable cells for maintaining proper capillary functions, as they wrap capillaries and stabilize them through a fine-tuned interplay with endothelial cells. During fibrosis, pericytes are activated and detach from capillaries before conversion into MFs, which compromises capillaries and worsens hypoxia. We also discuss how hypoxia and HIFs affect fibrogenesis. Given that hypoxia is caused by insufficient angiogenesis and that fibrosis results from pericyte loss, restoration of pericytes should be an intriguing target for overcoming both hypoxia and fibrosis. We propose the deactivation of MFs to recover lost pericytes as a promising therapy for CKD.

von Hippel-Lindau gene disruption in mouse pancreatic progenitors and its consequences on endocrine differentiation in vivo: importance of HIF1-α and VEGF-A upregulation

AIM/HYPOTHESIS

Different studies have linked hypoxia to embryonic development. Specifically, when embryonic pancreases are cultured ex vivo under hypoxic conditions (3% O2), beta cell development is impaired. Different cellular signalling pathways are involved in adaptation to hypoxia, including the ubiquitous hypoxia-inducible-factor 1-α (HIF1-α) pathway. We aimed to analyse the effects of HIF1-α stabilisation on fetal pancreas development in vivo.

METHODS

We deleted the Vhl gene, which encodes von Hippel-Lindau protein (pVHL), a factor necessary for HIF1-α degradation, by crossing Vhl-floxed mice with Sox9-Cre mice.

RESULTS

HIF1-α was stabilised in pancreatic progenitor cells in which the HIF pathway was induced. The number of neurogenin-3 (NGN3)-expressing cells was reduced and consequently endocrine development was altered in Vhl knockout pancreases. HIF1-α stabilisation induced Vegfa upregulation, leading to increased vascularisation. To investigate the impact of increased vascularisation on NGN3 expression, we used a bioassay in which Vhl mutant pancreases were cultured with or without vascular endothelial growth factor (VEGF) receptor 2 (VEGF-R2) inhibitors (e.g. Ki8751). Ex vivo analysis showed that Vhl knockout pancreases developed fewer NGN3-positive cells compared with controls. Interestingly, this effect was blocked when vascularisation was inhibited in the presence of VEGF-R2 inhibitors.

CONCLUSIONS/INTERPRETATION

Our data demonstrate that HIF1-α negatively controls beta cell differentiation in vivo by regulating NGN3 expression, and that this effect is mediated by signals from blood vessels.

Engineering liver

Interest in "engineering liver" arises from multiple communities: therapeutic replacement; mechanistic models of human processes; and drug safety and efficacy studies. An explosion of micro- and nanofabrication, biomaterials, microfluidic, and other technologies potentially affords unprecedented opportunity to create microphysiological models of the human liver, but engineering design principles for how to deploy these tools effectively toward specific applications, including how to define the essential constraints of any given application (available sources of cells, acceptable cost, and user-friendliness), are still emerging. Arguably less appreciated is the parallel growth in computational systems biology approaches toward these same problems-particularly in parsing complex disease processes from clinical material, building models of response networks, and in how to interpret the growing compendium of data on drug efficacy and toxicology in patient populations. Here, we provide insight into how the complementary paths of engineering liver-experimental and computational-are beginning to interplay toward greater illumination of human disease states and technologies for drug development.

Leptin enhances endothelial cell differentiation and angiogenesis in murine embryonic stem cells

The metabolic regulation of leptin and its angiogenic effects have been well characterized in adult mammals. However, the role of leptin in the differentiation of embryonic stem cells (ESCs) to endothelial cells (ECs) has not been characterized. We hypothesized that leptin enhances the generation of ECs derived from ESCs and, in this way, promotes angiogenesis in embryonic vessels. To address this hypothesis, we utilized an in vitro model consisting of murine ESCs-derived embryoid bodies (EBs). Vascular density, EC and angiogenesis markers as well as phosphorylation levels of signal transducer and activator of transcription 3 (pSTAT3) were investigated in leptin-treated EBs and in untreated EBs as controls. ESC-derived ECs were isolated by magnetic sorting based on the expression of platelet endothelial cell adhesion molecule (PECAM-1/CD31). Significant upregulation of EC and angiogenic markers as well as higher vessel density were found in leptin-treated EBs compared to controls. CD31 positive enriched cells derived from leptin-treated EBs had improved proliferation and survival rate and showed higher levels of pSTAT3. These results suggested that leptin promotes EC differentiation and angiogenesis in mouse EBs and that janus tyrosine kinase (JAK)/STAT pathway can play a role in this biological process. Leptin-mediated EC differentiation and angiogenesis in ESCs can be a useful application towards regenerative medicine and tissue engineering.

A Molecular atlas of Xenopus respiratory system development

BACKGROUND

Respiratory system development is regulated by a complex series of endoderm-mesoderm interactions that are not fully understood. Recently Xenopus has emerged as an alternative model to investigate early respiratory system development, but the extent to which the morphogenesis and molecular pathways involved are conserved between Xenopus and mammals has not been systematically documented.

RESULTS

In this study, we provide a histological and molecular atlas of Xenopus respiratory system development, focusing on Nkx2.1+ respiratory cell fate specification in the developing foregut. We document the expression patterns of Wnt/β-catenin, fibroblast growth factor (FGF), and bone morphogenetic protein (BMP) signaling components in the foregut and show that the molecular mechanisms of respiratory lineage induction are remarkably conserved between Xenopus and mice. Finally, using several functional experiments we refine the epistatic relationships among FGF, Wnt, and BMP signaling in early Xenopus respiratory system development.

CONCLUSIONS

We demonstrate that Xenopus trachea and lung development, before metamorphosis, is comparable at the cellular and molecular levels to embryonic stages of mouse respiratory system development between embryonic days 8.5 and 10.5. This molecular atlas provides a fundamental starting point for further studies using Xenopus as a model to define the conserved genetic programs controlling early respiratory system development.

Transient vascularization of transplanted human adult-derived progenitors promotes self-organizing cartilage

Millions of patients worldwide are affected by craniofacial deformations caused by congenital defects or trauma. Current surgical interventions have limited therapeutic outcomes; therefore, methods that would allow cartilage restoration are of great interest. A number of studies on embryonic limb development have shown that chondrogenesis is initiated by cellular condensation, during which mesenchymal progenitors aggregate and form 3D structures. Here, we demonstrated efficient regeneration of avascular elastic cartilage from in vitro-grown mesenchymal condensation, which recapitulated the early stages of chondrogenesis, including transient vascularization. After transplantation of vascularized condensed progenitors into immunodeficient mice, we used an intravital imaging approach to follow cartilage maturation. We determined that endothelial cells are present inside rudimentary cartilage (mesenchymal condensation) prior to cartilage maturation. Recreation of endothelial interactions in culture enabled a recently identified population of adult elastic cartilage progenitors to generate mesenchymal condensation in a self-driven manner, without requiring the support of exogenous inductive factors or scaffold materials. Moreover, the culture-grown 3D condensed adult-derived progenitors were amenable to storage via simple freezing methods and efficiently reconstructed 3D elastic cartilage upon transplantation. Together, our results indicate that transplantation of endothelialized and condensed progenitors represents a promising approach to realizing a regenerative medicine treatment for craniofacial deformations.

Potential and Challenges of Induced Pluripotent Stem Cells in Liver Diseases Treatment

Tens of millions of patients are affected by liver disease worldwide. Many of these patients can benefit from cell therapy involving living metabolically active cells, either by treatment of their liver disease, or by prevention of their disease phenotype. Cell therapies, including hepatocyte transplantation and bioartificial liver (BAL) devices, have been proposed as therapeutic alternatives to the shortage of transplantable livers. Both BAL and hepatocyte transplantation are cellular therapies that avoid use of a whole liver. Hepatocytes are also widely used in drug screening and liver disease modelling. However, the demand for human hepatocytes, heavily outweighs their availability by conventional means. Induced pluripotent stem cells (iPSCs) technology brings together the potential benefits of embryonic stem cells (ESCs) (i.e., self-renewal, pluripotency) and addresses the major ethical and scientific concerns of ESCs: embryo destruction and immune-incompatibility. It has been shown that hepatocyte-like cells (HLCs) can be generated from iPSCs. Furthermore, human iPSCs (hiPSCs) can provide an unlimited source of human hepatocytes and hold great promise for applications in regenerative medicine, drug screening and liver diseases modelling. Despite steady progress, there are still several major obstacles that need to be overcome before iPSCs will reach the bedside. This review will focus on the current state of efforts to derive hiPSCs for potential use in modelling and treatment of liver disease.

Immunomagnetic exclusion of PECAM-1-positive endothelial cells in fetal mouse liver cell cultures causes impaired growth and gene expression of hepatoblasts and stellate cells

Previous studies using mice having defective VEGF signaling have demonstrated that vascular development is indispensable for early hepatic organogenesis. However, not only whether its action lasts during later hepatic development, but also what molecules are involved in that action remains to be determined. The present study was undertaken to examine the effects of primitive sinusoidal endothelial cells on hepatic growth and maturation in primary culture of fetal mouse liver cells, and to determine their molecular mechanisms. When endothelial cells were excluded from E12.5 liver cell cultures by using PECAM-1-antibody-coated magnetic beads, the growth of hepatoblasts and stellate cells was conspicuously reduced and hepatic maturation was also suppressed. Conditioned medium prepared from fetal liver cell cultures containing almost all hepatic cell types stimulated the growth and gene expression of hepatoblasts and stellate cells similarly to the cultures in the presence of endothelial cells. HGF mRNA expression was downregulated in endothelial cellfree cultures of fetal liver cells, and the addition of HGF to the culture medium rescued the cells from the effects of endothelial cell depletion. These data suggest that humoral factors, including HGF, which are produced by endothelial cells or stellate cells, are involved in fetal hepatocyte growth and maturation.