Foxa2 regulates polarity and epithelialization in the endoderm germ layer of the mouse embryo

Transitions between epithelial and mesenchymal states and the morphogenesis of the early mouse embryo

Multicellular organisms arise from the generation of different cell types and the organization of cells into tissues and organs. Cells of metazoa display two main phenotypes, the ancestral epithelial state and the recent mesenchymal derivative. Epithelial cells are usually stationary and reside in two-dimensional sheets. By contrast mesenchymal cells are loosely packed and can move to new positions, thereby providing a vehicle for cell rearrangement, dispersal and novel cell-cell interactions. Transitions between epithelial and mesenchymal states drive key morphogenetic events in the early vertebrate embryo, including gastrulation, germ layer formation and somitogenesis. The cell behaviors and molecular mechanisms promoting transitions between these two states in the early mouse embryo are discussed in this review.

Information processing at the foxa node of the sea urchin endomesoderm specification network

The foxa regulatory gene is of central importance for endoderm specification across Bilateria, and this gene lies at an essential node of the well-characterized sea urchin endomesoderm gene regulatory network (GRN). Here we experimentally dissect the cis-regulatory system that controls the complex pattern of foxa expression in these embryos. Four separate cis-regulatory modules (CRMs) cooperate to control foxa expression in different spatial domains of the endomesoderm, and at different times. A detailed mutational analysis revealed the inputs to each of these cis-regulatory modules. The complex and dynamic expression of foxa is regulated by a combination of repressors, a permissive switch, and multiple activators. A mathematical kinetic model was applied to study the dynamic response of foxa cis-regulatory modules to transient inputs. This study shed light on the mesoderm-endoderm fate decision and provides a functional explanation, in terms of the genomic regulatory code, for the spatial and temporal expression of a key developmental control gene.

FGF signalling as a mediator of lineage transitions--evidence from embryonic stem cell differentiation

The fibroblast growth factor (FGF) signalling pathway is one of the most ubiquitous in biology. It has diverse roles in development, differentiation and cancer. Embryonic stem (ES) cells are in vitro cell lines capable of differentiating into all the lineages of the conceptus. As such they have the capacity to differentiate into derivatives of all three germ layers and to some extent the extra-embryonic lineages as well. Given the prominent role of FGF signalling in early embryonic development, we explore the role of this pathway in early ES cell differentiation towards the major lineages of the embryo. As early embryonic differentiation is intricately choreographed at the level of morphogenetic movement, adherent ES cell culture affords a unique opportunity to study the basic steps in early lineage specification in the absence of ever shifting complex in vivo microenvironments. Thus recent experiments in ES cell differentiation are able to pinpoint specific FGF dependent lineage transitions that are difficult to resolve in vivo. Here we review the role of FGF signalling in early development alongside the ES cell data and suggest that FGF dependent signalling via phospho-Erk activation maybe a major mediator of transitions in lineage specification.

Gene expression profiling of the hedgehog signaling pathway in human meningiomas

The Hedgehog (Hh) signaling pathway has an important role during embryogenesis and in adult life, regulating proliferation, angiogenesis, matrix remodeling and stem-cell renewal. Deregulation of the Hh pathway is involved in tumor development, since mutations in several components of this pathway were found in patients with basal cell carcinoma, medulloblastoma and other tumors; however, the role of Hh in meningiomas has not been studied yet. Meningiomas represent 30% of primary cranial tumors, are mostly benign and prevail in the second half of life. Novel therapies for meningiomas such as targeted molecular agents could use Hh pathway components. To provide information concerning molecular alterations, by use of real-time RT-PCR, we studied expression at the mRNA level of 32 Hh pathway and target genes in 36 meningioma specimens of different grades. mRNA levels of 16 genes, involved mainly in Hh pathway activation and cell proliferation, increased in meningiomas in comparison with normal tissue, whereas those of 7 genes, mainly related to Hh pathway repression, decreased. The most significant changes occurred in signal transduction (SMO) and GLI-transcription factor genes, and the target FOXM1 mRNA attained the highest values; their over-expression was found in aggressive and in benign tumors. Some proliferation-related genes (SPP1, IGF2) were overexpressed in higher meningioma grades. A correlation in expression between genes with a similar function was also found. Our results show a marked activation of the Hh pathway in meningiomas, which may be important for their biological and clinical characterization and would be useful for gene therapy.

Formation of the murine endoderm: lessons from the mouse, frog, fish, and chick

The mammalian definitive endoderm arises as a simple epithelial sheet. This sheet of cells will eventually produce the innermost tube that comprises the entire digestive tract from the esophagus to the colon as well as the epithelial component of the digestive and respiratory organs including the thymus, thyroid, lung, liver, gallbladder, and pancreas. Thus a wide array of tissue types are derived from the early endodermal sheet, and understanding the morphological and molecular mechanisms used to produce this tissue is integral to understanding the development of all these organs. The goal of this chapter is to summarize what is known about the morphological and molecular mechanisms used to produce this embryonic germ layer. Although this chapter mainly focuses on the mechanisms used to generate the murine endoderm, supportive or suggestive data from other species, including chick, frog (Xenopus laevis), and the Zebrafish (Danio rerio) are also examined.

Imaging mouse embryonic development

For the past three decades, methods for culturing mouse embryos ex vivo have been optimized in order to improve embryo viability and physiology throughout critical stages of embryogenesis. Combining advances made in the production of transgenic animals and in the development of different varieties of fluorescent proteins (FPs), time-lapse imaging is becoming more and more popular in the analysis of dynamic events during mouse development. Targeting FPs to specific cell types or subcellular compartments has enabled researchers to study cell proliferation, apoptosis, migration, and changes in cell morphology in living mouse embryos in real time. Here we provide a guide for time-lapse imaging of early stages of mouse embryo development.

Redefining the progression of lineage segregations during mammalian embryogenesis by clonal analysis

Clonal lineage information is fundamental in revealing cell fate choices. Using genetic single-cell labeling in utero, we investigated lineage segregations during anteroposterior axis formation in mouse. We show that while endoderm and surface ectoderm segregate during gastrulation, neural ectoderm and mesoderm share a common progenitor persisting through all stages of axis elongation. These data challenge the paradigm that the three germ layers, formed by gastrulation, constitute the primary branchpoints in differentiation of the pluripotent epiblast toward tissue-specific precursors. Bipotent neuromesodermal progenitors show self-renewing characteristics and may represent the cellular substrate coupling sustained axial elongation and coordinated differentiation of these tissues. These findings have important implications for the interpretation of the phenotypic defects of several mouse mutants and the directed differentiation of embryonic stem (ES) cells in vitro.

Vertebrate endoderm development and organ formation

The endoderm germ layer contributes to the respiratory and gastrointestinal tracts and to all of their associated organs. Over the past decade, studies in vertebrate model organisms, including frog, fish, chick, and mouse, have greatly enhanced our understanding of the molecular basis of endoderm organ development. We review this progress with a focus on early stages of endoderm organogenesis including endoderm formation, gut tube morphogenesis and patterning, and organ specification. Lastly, we discuss how developmental mechanisms that regulate endoderm organogenesis are used to direct differentiation of embryonic stem cells into specific adult cell types, which function to alleviate disease symptoms in animal models.