Foregut Endoderm Research Articles

Stem and progenitor cell systems in liver development and regeneration.

The liver comprises two stem/progenitor cell systems: fetal and adult liver stem/progenitor cells. Fetal hepatic progenitor cells, derived from foregut endoderm, differentiate into mature hepatocytes and cholangiocytes during liver development. Adult hepatic progenitor cells contribute to regeneration after severe and chronic liver injuries. However, the characteristics of these somatic hepatic stem/progenitor cells remain unknown. Culture systems that can be used to analyze these cells were recently established and hepatic stem/progenitor cell-specific surface markers including delta-like 1 homolog (DLK), cluster of differentiation (CD) 13, CD133, and LIV2 were identified. Cells purified using antibodies against these markers proliferate for an extended period and differentiate into mature cells both in vitro and in vivo. Methods to force the differentiation of human embryonic stem and induced pluripotent stem (iPS) cells into hepatic progenitor cells have been recently established. We demonstrated that the CD13(+) CD133(+) fraction of human iPS-derived cells contained numerous hepatic progenitor-like cells. These analyses of hepatic stem/progenitor cells derived from somatic tissues and pluripotent stem cells will contribute to the development of new therapies for severe liver diseases.

Gene Regulatory Networks Governing Lung Specification

The epithelial lining of the respiratory system originates from a small group of progenitor cells in the ventral foregut endoderm of the early embryo. Research in the last decade has revealed a number of paracrine signaling pathways that are critical for the development of these respiratory progenitors. In the post-genomic era the challenge now is to figure out at the genome wide level how these different signaling pathways and their downstream transcription factors interact in a complex "gene regulatory network" (GRN) to orchestrate early lung development. In this prospective, we review our growing understanding of the GRN governing lung specification. We discuss key gaps in our knowledge and describe emerging opportunities that will soon provide an unprecedented understanding of lung development and accelerate our ability to apply this knowledge to regenerative medicine.

Long noncoding RNAs are spatially correlated with transcription factors and regulate lung development.

Long noncoding RNAs (lncRNAs) are thought to play important roles in regulating gene transcription, but few have well-defined expression patterns or known biological functions during mammalian development. Using a conservative pipeline to identify lncRNAs that have important biological functions, we identified 363 lncRNAs in the lung and foregut endoderm. Importantly, we show that these lncRNAs are spatially correlated with transcription factors across the genome. In-depth expression analyses of lncRNAs with genomic loci adjacent to the critical transcription factors Nkx2.1, Gata6, Foxa2 (forkhead box a2), and Foxf1 mimic the expression patterns of their protein-coding neighbor. Loss-of-function analysis demonstrates that two lncRNAs, LL18/NANCI (Nkx2.1-associated noncoding intergenic RNA) and LL34, play distinct roles in endoderm development by controlling expression of critical developmental transcription factors and pathways, including retinoic acid signaling. In particular, we show that LL18/NANCI acts upstream of Nkx2.1 and downstream from Wnt signaling to regulate lung endoderm gene expression. These studies reveal that lncRNAs play an important role in foregut and lung endoderm development by regulating multiple aspects of gene transcription, often through regulation of transcription factor expression.

Endoderm-specific deletion of Tbx1 reveals an FGF-independent role for Tbx1 in pharyngeal apparatus morphogenesis.

The T-box transcription factor Tbx1, is essential for the normal development of multiple organ systems in the embryo. One of the most striking phenotypes in Tbx1-/- embryos is the failure of the caudal pharyngeal pouches to evaginate from the foregut endoderm. Despite considerable interest in the role of Tbx1 in development, the mechanisms whereby Tbx1 controls caudal pouch formation have remained elusive. In particular, the question as to how Tbx1 expression in the pharyngeal endoderm regulates pharyngeal pouch morphogenesis in the mouse embryo is not known. To address this question, we produced mouse embryos in which Tbx1 was specifically deleted from the pharyngeal endoderm and, as expected, embryos failed to form caudal pharyngeal pouches. To determine the molecular mechanism, we examined expression of Fgf3 and Fgf8 ligands and downstream effectors. Although Fgf8 expression is greatly reduced in Tbx1-deficient endoderm, FGF signaling levels are unaffected. Furthermore, pouch morphogenesis is only partially perturbed by the loss of both Fgf3 and Fgf8 from the endoderm, indicating that neither are required for pouch formation. Tbx1 deletion from the pharyngeal endoderm is sufficient to cause caudal pharyngeal arch segmentation defects by FGF-independent effectors that remain to be identified.

Ectopic Ptf1a Expression in Murine ESCs Potentiates Endocrine Differentiation and Models Pancreas Development In Vitro

Besides its role in exocrine differentiation, pancreas-specific transcription factor 1a (PTF1a) is required for pancreas specification from the foregut endoderm and ultimately for endocrine cell formation. Examining the early role of PTF1a in pancreas development has been challenging due to limiting amounts of embryonic tissue material for study. Embryonic stem cells (ESCs) which can be differentiated in vitro, and without limit to the amount of experimental material, can serve as a model system to study these early developmental events. To this end, we derived and characterized a mouse ESC line with tetracycline-inducible expression of PTF1a (tet-Ptf1a mESCs). We found that transient ectopic expression of PTF1a initiated the pancreatic program in differentiating ESCs causing cells to activate PDX1 expression in bud-like structures resembling pancreatic primordia in vivo. These bud-like structures also expressed progenitor markers characteristic of a developing pancreatic epithelium. The epithelium differentiated to generate a wave of NGN3+ endocrine progenitors, and further formed cells of all three pancreatic lineages. Notably, the insulin+ cells in the cultures were monohormonal, and expressed PDX1 and NKX6.1. PTF1a-induced cultures differentiated into significantly more endocrine and exocrine cells and the ratio of endocrine-to-exocrine cell differentiation could be regulated by retinoic acid (RA) and nicotinamide (Nic) signaling. Moreover, induced cultures treated with RA and Nic exhibited a modest glucose response. Thus, this tet-Ptf1a ESC-based in vitro system is a valuable new tool for interrogating the role of PTF1a in pancreas development and in directing differentiation of ESCs to endocrine cells.

Pulmonary endoderm, second heart field and the morphogenesis of distal outflow tract in mouse embryonic heart

The second heart field (SHF), foregut endoderm and sonic hedgehog (SHH) signaling pathway are all reported to associate with normal morphogenesis and septation of outflow tract (OFT). However, the morphological relationships of the development of foregut endoderm and expression of SHH signaling pathway members with the development of surrounding SHF and OFT are seldom described. In this study, serial sections of mouse embryos from ED9 to ED13 (midgestation) were stained with a series of marker antibodies for specifically highlighting SHF (Isl-1), endoderm (Foxa2), basement membrane (Laminin), myocardium (MHC) and smooth muscle (α-SMA) respectively, or SHH receptors antibodies including patched1 (Ptc1), patched2 (Ptc2) and smoothened, to observe the spatiotemporal relationship between them and their contributions to OFT morphogenesis. Our results demonstrated that the development of an Isl-1 positive field in the splanchnic mesoderm ventral to foregut, a subset of SHF, is closely coupled with pulmonary endoderm or tracheal groove, the Isl-1 positive cells surrounding pulmonary endoderm are distributed in a special cone-shaped pattern and take part in the formation of the lateral walls of the intrapericardial aorta and pulmonary trunk and the transient aortic-pulmonary septum, and Ptc1 and Ptc2 are exclusively expressed in pulmonary endoderm during this Isl-l positive field development, suggesting special roles played in inducing the Isl-l positive field formation by pulmonary endoderm. It is indicated that pulmonary endoderm plays a role in the development and specification of SHF in midgestation, and that pulmonary endoderm-associated Isl-l positive field is involved in patterning the morphogenesis and septation of the intrapericardial arterial trunks.

Generation of multiciliated cells in functional airway epithelium from human iPSC (1094.5)

Despite a significant improvement in the availability of therapeutic options to treat lung diseases, pulmonary disease still remains a major cause of morbidity and mortality around the world. Currently there are limited opportunities to study human lung disease either in vivo and in vitro. Using induced pluripotent stem cells (iPSC) we have generated a reproducible differentiation protocol to make mature post‐mitotic multiciliated cells in a functional airway epithelium.iPSC were generated from human skin biopsies and differentiated via FOXA2+SOX17+ definitive endoderm (>90% efficiency) to FOXA2+NKx2.1+ anterior foregut endoderm, FOXA2+NKx2.1+SOX2+ (~50% efficiency) pulmonary endoderm and then matured in an air liquid interface. Robust multiciliogenesis occurred when Notch signaling was inhibited and was confirmed by; i) the assembly of multiple pericentrin stained centrioles at the apical surface, ii) expression of transcription factor FOXJ1 and iii) presence of multiple acetylated tubulin labeled cilia projections in individual cells. The presence of NKx2.1+CC10+ Clara cells, MUC5A/C+ goblet cells and FOXA2+p63+ basal cells was also confirmed showing we are generating a complete polarized epithelial cell layer comprised of all relevant cell types. Functional cAMP activated and CFTRinh‐172 sensitive CFTR currents were recorded in isolated epithelial cells by whole cell patch clamp technique. Furthermore, we have corrected the deltaF508 mutation in the CFTR gene (>80% of all cases of CF) using a combination of CRISPR‐Cas9 endonuclease‐mediated genome editing and piggyBac transposase technologies, in the CF patient‐derived iPSC.The generation of mature multiciliated cells in a human iPSC differentiated respiratory epithelium and the ability to correct disease causing mutations provides a significant advancement toward modeling a number of human respiratory diseases in vitro.Grant Funding Source: Supported in part by CIRM and the Berger Foundation

BMP antagonism by Noggin is required in presumptive notochord cells for mammalian foregut morphogenesis

Esophageal atresia with tracheoesophageal fistula (EA/TEF) is a serious human birth defect, in which the esophagus ends before reaching the stomach, and is aberrantly connected with the trachea. Several mouse models of EA/TEF have recently demonstrated that proper dorsal/ventral (D/V) patterning of the primitive anterior foregut endoderm is essential for correct compartmentalization of the trachea and esophagus. Here we elucidate the pathogenic mechanisms underlying the EA/TEF that occurs in mice lacking the BMP antagonist Noggin, which display correct dorsal/ventral patterning. To clarify the mechanism of this malformation, we use spatiotemporal manipulation of Noggin and BMP receptor 1A conditional alleles during foregut development. Surprisingly, we find that the expression of Noggin in the compartmentalizing endoderm is not required to generate distinct tracheal and esophageal tubes. Instead, we show that Noggin and BMP signaling attenuation are required in the early notochord to correctly resolve notochord cells from the dorsal foregut endoderm, which in turn, appears to be a prerequisite for foregut compartmentalization. Collectively, our findings support an emerging model for a mechanism underlying EA/TEF in which impaired notochord resolution from the early endoderm causes the foregut to be hypo-cellular just prior to the critical period of compartmentalization. Our further characterizations suggest that Noggin may regulate a cell rearrangement process that involves reciprocal E-cadherin and Zeb1 expression in the resolving notochord cells.

Progenitor Cells in Liver Development, Regeneration and Repair

During organogenesis, the liver develops from the foregut endoderm and grows into the adjacent septum transversum resulting in the formation of the liver bud. Growth factors released from the septum transversum and the cardiac mesenchyme induce endodermal differentiation and proliferation, thus, forming the primordial liver and extrahepatic biliary structures. Endodermal precursor cells within the liver bud comprise bi-potential liver progenitor cells called hepatoblasts, which differentiates into hepatocytes and cholangiocytes. While the postnatal liver has remarkable capacity to regenerate via the proliferation of mature hepatocytes, this compensatory mechanism may be overwhelmed during states of chronic injury. Under these conditions, resident stem/progenitor cells proliferate to replace lost liver parenchyma. Significant progress has been made recently in elucidating the role of various signaling pathways and progenitor cells in liver development and regeneration. In this review, we summarize our recent understanding of progenitor cells in liver development, regeneration and repair.

Hepatobiliary and Pancreatic Neoplasms in Patients With McCune-Albright Syndrome

McCune-Albright syndrome (MAS), which includes polycystic fibrous dysplasia, precocious puberty, and café au lait spots, is a rare disorder caused by somatic activating mutations of the GNAS gene. GNAS mutations have also been implicated in various sporadic tumors, including hepatobiliary and pancreatic neoplasms. The aim of this study was to assess the prevalence of hepatobiliary and pancreatic neoplasms in patients with McCune-Albright syndrome. Nineteen patients diagnosed between 1995 and 2012 with MAS in a tertiary referral center for rare growth disorders were screened with dedicated gadolinium-enhanced magnetic resonance imaging for hepatobiliary and pancreatic neoplasms between June 2011 and December 2012. Six (32%) of the 19 screened patients were found to have hepatic, pancreatic, or biliary lesions, excluding liver hemangiomas, liver cysts, and focal nodular hyperplasia. This includes pancreatic ductal lesions observed in 4 patients, including numerous branch-duct intraductal papillary mucinous neoplasms in 3 patients. Biliary lesions were observed in 1 patient, with a large choledochal cyst also involving the left biliary branch. Finally, multiple inflammatory/telangiectatic hepatic adenomas were observed in 2 patients, including 1 with proven somatic GNAS mutation. We describe the first observation of syndromic intraductal papillary mucinous neoplasms and the new association between MAS and pancreatic neoplasms, namely intraductal papillary mucinous neoplasms of the pancreas but also rare hepatobiliary neoplasms including liver adenomas and choledochal cysts. These findings strongly suggest that somatic activating GNAS mutations, possibly through cAMP pathway disorders, are involved in the tumorigenesis of hepatobiliary and pancreatic tissues originating from the foregut endoderm and have led us to use a routine screening by dedicated magnetic resonance imaging including both pancreatobiliary and liver sequences in patients with MAS.

Analysis of SOX2-Expressing Cell Populations Derived from Human Pluripotent Stem Cells

SummarySOX2 is involved in several cell and developmental processes, including maintenance of embryonic stem cells, differentiation of neural progenitor cells, and patterning of gut endoderm. To study its role in a human system, we generated a human embryonic stem cell (hESC) line harboring a reporter gene encoding GFP in the SOX2 locus. This SOX2 reporter line faithfully recapitulates expression of the SOX2 gene in undifferentiated human pluripotent stem cells (hPSCs), neural progenitor cells (NPCs), and anterior foregut endoderm (AFE). In undifferentiated hESCs, GFP expression corresponds to those cells with highest levels of expression of genes associated with the pluripotent state. In NPCs, expression of GFP can be employed to isolate cells expressing markers associated with NPC multipotency. In AFE, we used transcriptome-wide expression analysis to identify cell surface markers with elevated expression in this population, thereby facilitating isolation and purification of this hPSC-derived cell population.

Development of the Human Pancreas From Foregut to Endocrine Commitment

Knowledge of human pancreas development underpins our interpretation and exploitation of human pluripotent stem cell (PSC) differentiation toward a β-cell fate. However, almost no information exists on the early events of human pancreatic specification in the distal foregut, bud formation, and early development. Here, we have studied the expression profiles of key lineage-specific markers to understand differentiation and morphogenetic events during human pancreas development. The notochord was adjacent to the dorsal foregut endoderm during the fourth week of development before pancreatic duodenal homeobox-1 detection. In contrast to the published data from mouse embryos, during human pancreas development, we detected only a single-phase of Neurogenin 3 (NEUROG3) expression and endocrine differentiation from approximately 8 weeks, before which Nirenberg and Kim homeobox 2.2 (NKX2.2) was not observed in the pancreatic progenitor cell population. In addition to revealing a number of disparities in timing between human and mouse development, these data, directly assembled from human tissue, allow combinations of transcription factors to define sequential stages and differentiating pancreatic cell types. The data are anticipated to provide a useful reference point for stem cell researchers looking to differentiate human PSCs in vitro toward the pancreatic β-cell so as to model human development or enable drug discovery and potential cell therapy.

Coordination of heart and lung co-development by a multipotent cardiopulmonary progenitor

SUMMARYCo-development of the cardiovascular and pulmonary systems is a recent evolutionary adaption to terrestrial life that couples cardiac output with the gas exchange function of the lung 1. In this report, we show that the pulmonary vasculature develops even in the absence of lung development. We have identified a population of multi-potent cardiopulmonary mesoderm progenitors (CPPs) within the posterior pole of the heart that are marked by the expression of Wnt2/Gli1/Isl1. We show that CPPs arise from cardiac progenitors prior to lung development. Lineage tracing and clonal analysis demonstrates that CPPs generate the mesoderm lineages within the cardiac inflow tract and lung including cardiomyocytes, pulmonary vascular and airway smooth muscle, proximal vascular endothelium, and pericyte-like cells. CPPs are regulated by hedgehog expression from the foregut endoderm, which is required for connection of the pulmonary vasculature to the heart. Together, these studies identify a novel population of multipotent cardiopulmonary progenitors that coordinates heart and lung co-development that is required for adaptation to terrestrial existence.

Generation of organized anterior foregut epithelia from pluripotent stem cells using small molecules

Anterior foregut endoderm (AFE) gives rise to therapeutically relevant cell types in tissues such as the esophagus, salivary glands, lung, thymus, parathyroid and thyroid. Despite its importance, reports describing the generation of AFE from pluripotent stem cells (PSCs) by directed differentiation have mainly focused on the Nkx2.1+ lung and thyroid lineages. Here, we describe a novel protocol to derive a subdomain of AFE, identified by expression of Pax9, from PSCs using small molecules and defined media conditions. We generated a reporter PSC line for isolation and characterization of Pax9+ AFE cells, which when transplanted in vivo, can form several distinct complex AFE-derived epithelia, including mucosal glands and stratified squamous epithelium. Finally, we show that the directed differentiation protocol can be used to generate AFE from human PSCs. Thus, this work both broadens the range of PSC-derived AFE tissues and creates a platform enabling the study of AFE disorders.

Specification of hepatopancreas progenitors in zebrafish by hnf1ba and wnt2bb.

Although the liver and ventral pancreas are thought to arise from a common multipotent progenitor pool, it is unclear whether these progenitors of the hepatopancreas system are specified by a common genetic mechanism. Efforts to determine the role of Hnf1b and Wnt signaling in this crucial process have been confounded by a combination of factors, including a narrow time frame for hepatopancreas specification, functional redundancy among Wnt ligands, and pleiotropic defects caused by either severe loss of Wnt signaling or Hnf1b function. Using a novel hypomorphic hnf1ba zebrafish mutant that exhibits pancreas hypoplasia, as observed in HNF1B monogenic diabetes, we show that hnf1ba plays essential roles in regulating β-cell number and pancreas specification, distinct from its function in regulating pancreas size and liver specification, respectively. By combining Hnf1ba partial loss of function with conditional loss of Wnt signaling, we uncover a crucial developmental window when these pathways synergize to specify the entire ventrally derived hepatopancreas progenitor population. Furthermore, our in vivo genetic studies demonstrate that hnf1ba generates a permissive domain for Wnt signaling activity in the foregut endoderm. Collectively, our findings provide a new model for HNF1B function, yield insight into pancreas and β-cell development, and suggest a new mechanism for hepatopancreatic specification.

Insulin-secreting cell regeneration: a dream or reality?

189 ISSN 1758-1907 10.2217/DMT.13.12 © 2013 Future Medicine Ltd Diabetes Manage. (2013) 3(3), 189–191 Autoimmune destruction of insulinproducing b-cells leads to Type 1 diabetes (T1D). The global challenge to find a cure for T1D centers on transplanting or regenerating insulin-producing cells, while at the same time preventing disease recurrence. This quest for cure dominates prevention in the public’s mind, although in reality there can be no cure of an autoimmune disease without prevention. The impetus comes from the demands of daily insulin injections to treat T1D and the inability of this approach to attain physiologic control of blood glucose, in many cases leading to longer-term complications secondary to hyperglycemia. Although transplantation of the pancreas or purif ied islets allows near-normal maintenance of blood glucose, it comes at a high technical and financial cost and is severely limited by the shortage of organ donors. In addition, long-term insulinindependence appears to be the exception rather than the rule [1]. Alternative sources of b-cells, primarily embryonic or adult stem/progenitor cells, have, therefore, been pursued [2–5]. Turning the dream of regenerating b-cells into a reality requires an understanding of normal b-cell ontogeny and how to mimic and evaluate it. The b-cell regeneration progress has to match the known molecular identity of a developing pancreas. Over the last 30 years, pancreatic endocrine cell lineages have been mapped through the role of specialized growth factors and sequential activation/inhibition of signaling pathways. Thus, early specification of dorsal and ventral pancreatic buds from foregut endoderm requires Hedgehog and FGF signaling [6]. Growth and branching morphogenesis involve FGF, Notch, Wnt, TGF-b and EGF pathways [7]. These pathways are activated by interactions with surrounding tissues, causing expression of specialized transcription factors. Sets of transcription factors form transcriptome signatures responsible for tissue transformation. Although incomplete, these groups are used to identify whether cultivated cells reached late-def initive endoderm, pancreatic progenitor or immature b-cell stages of development. In addition, the b-cell regeneration progress has to match known b-cell function. Cellular response to incretins, such as glucose and exenatide, is a valuable feature. It displays insulin production, reveals presence of secretory networks and detects dose-dependent insulin

Different thresholds of Wnt-Frizzled 7 signaling coordinate proliferation, morphogenesis and fate of endoderm progenitor cells

Wnt signaling has multiple dynamic roles during development of the gastrointestinal and respiratory systems. Differential Wnt signaling is thought to be a critical step in Xenopus endoderm patterning such that during late gastrula and early somite stages of embryogenesis, Wnt activity must be suppressed in the anterior to allow the specification of foregut progenitors. However, the foregut endoderm also expresses the Wnt-receptor Frizzled 7 (Fzd7) as well as several Wnt ligands suggesting that the current model may be too simple. In this study, we show that Fzd7 is required to transduce a low level of Wnt signaling that is essential to maintain foregut progenitors. Foregut-specific Fzd7-depletion from the Xenopus foregut resulted in liver and pancreas agenesis. Fzd7-depleted embryos failed to maintain the foregut progenitor marker hhex and exhibited decreased proliferation; in addition the foregut cells were enlarged with a randomized orientation. We show that in the foregut Fzd7 signals via both the Wnt/β-catenin and Wnt/JNK pathways and that different thresholds of Wnt-Fzd7 activity coordinate progenitor cell fate, proliferation and morphogenesis.

Neurogenin3 Cooperates with Foxa2 to Autoactivate Its Own Expression

The transcription factor Neurogenin3 functions as a master regulator of endocrine pancreas formation, and its deficiency leads to the development of diabetes in humans and mice. In the embryonic pancreas, Neurogenin3 is transiently expressed at high levels for a narrow time window to initiate endocrine differentiation in scattered progenitor cells. The mechanisms controlling these rapid and robust changes in Neurogenin3 expression are poorly understood. In this study, we characterize a Neurogenin3 positive autoregulatory loop whereby this factor may rapidly induce its own levels. We show that Neurogenin3 binds to a conserved upstream fragment of its own gene, inducing deposition of active chromatin marks and the activation of Neurog3 transcription. Additionally, we show that the broadly expressed endodermal forkhead factors Foxa1 and Foxa2 can cooperate synergistically to amplify Neurogenin3 autoregulation in vitro. However, only Foxa2 colocalizes with Neurogenin3 in pancreatic progenitors, thus indicating a primary role for this factor in regulating Neurogenin3 expression in vivo. Furthermore, in addition to decreasing Neurog3 autoregulation, inhibition of Foxa2 by RNA interference attenuates Neurogenin3-dependent activation of the endocrine developmental program in cultured duct mPAC cells. Hence, these data uncover the potential functional cooperation between the endocrine lineage-determining factor Neurogenin3 and the widespread endoderm progenitor factor Foxa2 in the implementation of the endocrine developmental program in the pancreas.

Cellular and molecular basis of liver development.

Liver is a prime organ responsible for synthesis, metabolism, and detoxification. The organ is endodermal in origin and its development is regulated by temporal, complex, and finely balanced cellular and molecular interactions that dictate its origin, growth, and maturation. We discuss the relevance of endoderm patterning, which truly is the first step toward mapping of domains that will give rise to specific organs. Once foregut patterning is completed, certain cells within the foregut endoderm gain competence in the form of expression of certain transcription factors that allow them to respond to certain inductive signals. Hepatic specification is then a result of such inductive signals, which often emanate from the surrounding mesenchyme. During hepatic specification bipotential hepatic stem cells or hepatoblasts become apparent and undergo expansion, which results in a visible liver primordium during the stage of hepatic morphogenesis. Hepatoblasts next differentiate into either hepatocytes or cholangiocytes. The expansion and differentiation is regulated by cellular and molecular interactions between hepatoblasts and mesenchymal cells including sinusoidal endothelial cells, stellate cells, and also innate hematopoietic elements. Further maturation of hepatocytes and cholangiocytes continues during late hepatic development as a function of various growth factors. At this time, liver gains architectural novelty in the form of zonality and at cellular level acquires polarity. A comprehensive elucidation of such finely tuned developmental cues have been the basis of transdifferentiation of various types of stem cells to hepatocyte-like cells for purposes of understanding health and disease and for therapeutic applications.

Wnt-Dependent Epithelial Transitions Drive Pharyngeal Pouch Formation

The pharyngeal pouches, which form by budding of the foregut endoderm, are essential for segmentation of the vertebrate face. To date, the cellular mechanism and segmental nature of such budding have remained elusive. Here, we find that Wnt11r and Wnt4a from the head mesoderm and ectoderm, respectively, play distinct roles in the segmental formation of pouches in zebrafish. Time-lapse microscopy, combined with mutant and tissue-specific transgenic experiments, reveal requirements of Wnt signaling in two phases of endodermal epithelial transitions. Initially, Wnt11r and Rac1 destabilize the endodermal epithelium to promote the lateral movement of pouch-forming cells. Next, Wnt4a and Cdc42 signaling induce the rearrangement of maturing pouch cells into bilayers through junctional localization of the Alcama immunoglobulin-domain protein, which functions to restabilize adherens junctions. We propose that this dynamic control of epithelial morphology by Wnt signaling may be a common theme for the budding of organ anlagen from the endoderm.