Study of the murine allantois by allantoic explants

Origin and flow-mediated remodeling of the murine and human extraembryonic circulation systems

The transduction of mechanical stimuli produced by blood flow is an important regulator of vascular development. The vitelline and umbilico-placental circulations are extraembryonic vascular systems that are required for proper embryonic development in mammalian embryos. The morphogenesis of the extraembryonic vasculature and the cardiovascular system of the embryo are hemodynamically and molecularly connected. Here we provide an overview of the establishment of the murine and human vitelline and umbilico-placental vascular systems and how blood flow influences various steps in their development. A deeper comprehension of extraembryonic vessel development may aid the establishment of stem-cell based embryo models and provide novel insights to understanding pregnancy complications related to the umbilical cord and placenta.

Uncovering Phenotypes in Mutant Mice by Determining Embryo, Organ, Tissue, and Cell Developmental Potential

The death of an embryo during gestation does not necessarily preclude the study of the mutant embryo or the developmental potential of its individual cells, tissues, or organs. Whole-embryo in vitro culture prior to the time of death will allow real-time observation of living embryos and direct comparisons with controls. Organ anlage can be removed from embryos and cultured in vitro beyond the time of death of the whole embryo. In both whole embryos and organ anlage culture, fluorescent protein reporters may be used productively to follow cell types or specific gene expression changes. Some cells, such as hematopoietic cells, and organ anlage, may be suitable for transplantation to wild-type hosts for further analysis of their potential. Additionally, cell lines, including embryonic stem (ES) cells, trophoblast stem (TS) cells, extraembryonic endoderm (XEN) stem cells, and epiblast-derived stem cells (EpiSC), can be derived from mutant embryos to reveal the potential of the mutant cells outside the context of the whole organism. Mutant stem cells or even whole mutant embryos can be used to test potential in chimeras or in teratomas.

The mouse allantois: new insights at the embryonic-extraembryonic interface

During the early development of Placentalia, a distinctive projection emerges at the posterior embryonic-extraembryonic interface of the conceptus; its fingerlike shape presages maturation into the placental umbilical cord, whose major role is to shuttle fetal blood to and from the chorion for exchange with the mother during pregnancy. Until recently, the biology of the cord's vital vascular anlage, called the body stalk/allantois in humans and simply the allantois in rodents, has been largely unknown. Here, new insights into the development of the mouse allantois are featured, from its origin and mechanism of arterial patterning through its union with the chorion. Key to generating the allantois and its critical functions are the primitive streak and visceral endoderm, which together are sufficient to create the entire fetal-placental connection. Their newly discovered roles at the embryonic-extraembryonic interface challenge conventional wisdom, including the physical limits of the primitive streak, its function as sole purveyor of mesoderm in the mouse, potency of visceral endoderm, and the putative role of the allantois in the germ line. With this working model of allantois development, understanding a plethora of hitherto poorly understood orphan diseases in humans is now within reach. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

Mesenchymal Stem/Stromal Cells Derived from Human and Animal Perinatal Tissues-Origins, Characteristics, Signaling Pathways, and Clinical Trials

Mesenchymal stem/stromal cells (MSCs) are currently one of the most extensively researched fields due to their promising opportunity for use in regenerative medicine. There are many sources of MSCs, of which cells of perinatal origin appear to be an invaluable pool. Compared to embryonic stem cells, they are devoid of ethical conflicts because they are derived from tissues surrounding the fetus and can be safely recovered from medical waste after delivery. Additionally, perinatal MSCs exhibit better self-renewal and differentiation properties than those derived from adult tissues. It is important to consider the anatomy of perinatal tissues and the general description of MSCs, including their isolation, differentiation, and characterization of different types of perinatal MSCs from both animals and humans (placenta, umbilical cord, amniotic fluid). Ultimately, signaling pathways are essential to consider regarding the clinical applications of MSCs. It is important to consider the origin of these cells, referring to the anatomical structure of the organs of origin, when describing the general and specific characteristics of the different types of MSCs as well as the pathways involved in differentiation.

Inositol 1,4,5-trisphosphate receptors are essential for fetal-maternal connection and embryo viability

Inositol 1,4,5-trisphosphate receptors (IP3Rs) are a family of intracellular Ca2+ release channels located on the ER membrane, which in mammals consist of 3 different subtypes (IP3R1, IP3R2, and IP3R3) encoded by 3 genes, Itpr1, Itpr2, and Itpr3, respectively. Studies utilizing genetic knockout mouse models have demonstrated that IP3Rs are essential for embryonic survival in a redundant manner. Deletion of both IP3R1 and IP3R2 has been shown to cause cardiovascular defects and embryonic lethality. However, it remains unknown which cell types account for the cardiovascular defects in IP3R1 and IP3R2 double knockout (DKO) mice. In this study, we generated conditional IP3R1 and IP3R2 knockout mouse models with both genes deleted in specific cardiovascular cell lineages. Our results revealed that deletion of IP3R1 and IP3R2 in cardiomyocytes by TnT-Cre, in endothelial / hematopoietic cells by Tie2-Cre and Flk1-Cre, or in early precursors of the cardiovascular lineages by Mesp1-Cre, resulted in no phenotypes. This demonstrated that deletion of both IP3R genes in cardiovascular cell lineages cannot account for the cardiovascular defects and embryonic lethality observed in DKO mice. We then revisited and performed more detailed phenotypic analysis in DKO embryos, and found that DKO embryos developed cardiovascular defects including reduced size of aortas, enlarged cardiac chambers, as well as growth retardation at embryonic day (E) 9.5, but in varied degrees of severity. Interestingly, we also observed allantoic-placental defects including reduced sizes of umbilical vessels and reduced depth of placental labyrinth in DKO embryos, which could occur independently from other phenotypes in DKO embryos even without obvious growth retardation. Furthermore, deletion of both IP3R1 and IP3R2 by the epiblast-specific Meox2-Cre, which targets all the fetal tissues and extraembryonic mesoderm but not extraembryonic trophoblast cells, also resulted in embryonic lethality and similar allantoic-placental defects. Taken together, our results demonstrated that IP3R1 and IP3R2 play an essential and redundant role in maintaining the integrity of fetal-maternal connection and embryonic viability.

Methods to label, image, and analyze the complex structural architectures of microvascular networks

Microvascular networks play key roles in oxygen transport and nutrient delivery to meet the varied and dynamic metabolic needs of different tissues throughout the body, and their spatial architectures of interconnected blood vessel segments are highly complex. Moreover, functional adaptations of the microcirculation enabled by structural adaptations in microvascular network architecture are required for development, wound healing, and often invoked in disease conditions, including the top eight causes of death in the Unites States. Effective characterization of microvascular network architectures is not only limited by the available techniques to visualize microvessels but also reliant on the available quantitative metrics that accurately delineate between spatial patterns in altered networks. In this review, we survey models used for studying the microvasculature, methods to label and image microvessels, and the metrics and software packages used to quantify microvascular networks. These programs have provided researchers with invaluable tools, yet we estimate that they have collectively attained low adoption rates, possibly due to limitations with basic validation, segmentation performance, and nonstandard sets of quantification metrics. To address these existing constraints, we discuss opportunities to improve effectiveness, rigor, and reproducibility of microvascular network quantification to better serve the current and future needs of microvascular research.

Consensus guidelines for the use and interpretation of angiogenesis assays

The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

Replacing nonmuscle myosin 2A with myosin 2C1 permits gastrulation but not placenta vascular development in mice

Three paralogues of nonmuscle myosin 2 (NM 2A, 2B, and 2C) are expressed in mammals, and the heavy chains are the products of three different genes (Myh9, Myh10, and Myh14, respectively). NM 2A and 2B are essential for mouse development, while 2C is not. Studies on NM 2C are limited and the in vivo function of this paralogue is not clear. Using homologous recombination, cDNA encoding nonmuscle myosin heavy chain 2C1 fused with GFP was introduced into the first coding exon of Myh9, replacing NM 2A expression with NM 2C1 expression in mice. In contrast to A^-/A^- embryos, which die by embryonic day (E) 6.5, A^C1*gfp/A^C1*gfp embryos survive through E8.5, demonstrating that NM 2C1 can support mouse development beyond gastrulation. At E9.5 and E10.5, however, A^C1*gfp/A^C1*gfp embryos are developmentally delayed, with abnormalities in placental vascular formation. The defect in vascular formation is confirmed in allantois explants from A^C1*gfp/A^C1*gfp embryos. Thus, NM 2C1 cannot support normal placental vascular formation. In addition, A^C1*gfp/A^C1*gfp mouse embryonic fibroblasts (MEFs) migrate rapidly but with impaired persistence and develop smaller, less mature focal adhesions than A⁺/A⁺ MEFs. This is attributed to enhanced NM 2C1 actomyosin stability and different NM 2C1 subcellular localization than in NM 2A.

Dissection and Explant Culture of Murine Allantois for the In Vitro Analysis of Allantoic Attachment

The placenta is essential for the growth and development of mammalian embryos. For this reason, numerous genetic alterations and likely also environmental insults that disturb placenta development or function can cause early pregnancy loss in mice and humans. Nevertheless, simple in vitro assays to screen for potential effects on placenta formation are lacking. Here, we focus on modeling the first and critical step in placenta formation, which consists of the attachment of the allantois to the chorion. We describe a method to rapidly assess the attachment of allantoic explants on immobilized α4β1 integrin, which serves as a chorio-mimetic substrate.This in vitro approach enables a qualitative evaluation of the attachment and spreading behavior of multiple allantois explants at different consecutive time points. The protocol may be used to investigate the effect of targeted mouse mutations, drugs, or various environmental factors that have been linked to pregnancy complications or fetal loss on allantois attachment ex vivo.

Analysis of extraembryonic mesodermal structure formation in the absence of morphological primitive streak

During mouse gastrulation, the primitive streak is formed on the posterior side of the embryo. Cells migrate out of the primitive streak to form the future mesoderm and endoderm. Fate mapping studies revealed a group of cell migrate through the proximal end of the primitive streak and give rise to the extraembryonic mesoderm tissues such as the yolk sac blood islands and allantois. However, it is not clear whether the formation of a morphological primitive streak is required for the development of these extraembryonic mesodermal tissues. Loss of the Cripto gene in mice dramatically reduces, but does not completely abolish, Nodal activity leading to the absence of a morphological primitive streak. However, embryonic erythrocytes are still formed and assembled into the blood islands. In addition, Cripto mutant embryos form allantoic buds. However, Drap1 mutant embryos have excessive Nodal activity in the epiblast cells before gastrulation and form an expanded primitive streak, but no yolk sac blood islands or allantoic bud formation. Lefty2 embryos also have elevated levels of Nodal activity in the primitive streak during gastrulation, and undergo normal blood island and allantois formation. We therefore speculate that low level of Nodal activity disrupts the formation of morphological primitive streak on the posterior side, but still allows the formation of primitive streak cells on the proximal side, which give rise to the extraembryonic mesodermal tissues formation. Excessive Nodal activity in the epiblast at pre-gastrulation stage, but not in the primitive streak cells during gastrulation, disrupts extraembryonic mesoderm development.

Cdc42 is required for cytoskeletal support of endothelial cell adhesion during blood vessel formation in mice

The Rho family of small GTPases has been shown to be required in endothelial cells (ECs) during blood vessel formation. However, the underlying cellular events controlled by different GTPases remain unclear. Here, we assess the cellular mechanisms by which Cdc42 regulates mammalian vascular morphogenesis and maintenance. In vivo deletion of Cdc42 in embryonic ECs (Cdc42(Tie2KO)) results in blocked lumen formation and endothelial tearing, leading to lethality of mutant embryos by E9-10 due to failed blood circulation. Similarly, inducible deletion of Cdc42 (Cdc42(Cad5KO)) at mid-gestation blocks angiogenic tubulogenesis. By contrast, deletion of Cdc42 in postnatal retinal vessels leads to aberrant vascular remodeling and sprouting, as well as markedly reduced filopodia formation. We find that Cdc42 is essential for organization of EC adhesion, as its loss results in disorganized cell-cell junctions and reduced focal adhesions. Endothelial polarity is also rapidly lost upon Cdc42 deletion, as seen by failed localization of apical podocalyxin (PODXL) and basal actin. We link observed failures to a defect in F-actin organization, both in vitro and in vivo, which secondarily impairs EC adhesion and polarity. We also identify Cdc42 effectors Pak2/4 and N-WASP, as well as the actomyosin machinery, to be crucial for EC actin organization. This work supports the notion of Cdc42 as a central regulator of the cellular machinery in ECs that drives blood vessel formation.

Candidate gene approach identifies multiple genes and signaling pathways downstream of Tbx4 in the developing allantois

Loss of Tbx4 results in absence of chorio-allantoic fusion and failure of formation of the primary vascular plexus of the allantois leading to embryonic death at E10.5. We reviewed the literature for genes implicated in chorio-allantoic fusion, cavitation and vascular plexus formation, processes affected in Tbx4 mutant allantoises. Using this candidate gene approach, we identified a number of genes downstream of Tbx4 in the allantois including extracellular matrix molecules Vcan, Has2, and Itgα5, transcription factors Snai1 and Twist, and signaling molecules Bmp2, Bmp7, Notch2, Jag1 and Wnt2. In addition, we show that the canonical Wnt signaling pathway contributes to the vessel-forming potential of the allantois. Ex vivo, the Tbx4 mutant phenotype can be rescued using agonists of the Wnt signaling pathway and, in wildtype allantoises, an inhibitor of the canonical Wnt signaling pathway disrupts vascular plexus formation. In vivo, Tbx4 and Wnt2 double heterozygous placentas show decreased vasculature suggesting interactions between Tbx4 and the canonical Wnt signaling pathway in the process of allantois-derived blood vessel formation.

The murine allantois: a model system for the study of blood vessel formation

The allantois is the embryonic precursor of the umbilical cord in mammals and is one of several embryonic regions, including the yolk sac and dorsal aorta, that undergoes vasculogenesis, the de novo formation of blood vessels. Despite its importance in establishing the chorioallantoic placenta and umbilical circulation, the allantois frequently is overlooked in embryologic studies. Nonetheless, recent studies demonstrate that vasculogenesis, vascular remodeling, and angiogenesis are essential allantois functions in the establishment of the chorioallantoic placenta. Here, we review blood vessel formation in the murine allantois, highlighting the expression of genes and involvement of pathways common to vasculogenesis or angiogenesis in other parts of the embryo. We discuss experimental techniques available for manipulation of the allantois that are unavailable for yolk sac or dorsal aorta, and review how this system has been used as a model system to discover new genes and mechanisms involved in vessel formation. Finally, we discuss the potential of the allantois as a model system to provide insights into disease and therapeutics.

A computational tool for quantitative analysis of vascular networks

Angiogenesis is the generation of mature vascular networks from pre-existing vessels. Angiogenesis is crucial during the organism' development, for wound healing and for the female reproductive cycle. Several murine experimental systems are well suited for studying developmental and pathological angiogenesis. They include the embryonic hindbrain, the post-natal retina and allantois explants. In these systems vascular networks are visualised by appropriate staining procedures followed by microscopical analysis. Nevertheless, quantitative assessment of angiogenesis is hampered by the lack of readily available, standardized metrics and software analysis tools. Non-automated protocols are being used widely and they are, in general, time--and labour intensive, prone to human error and do not permit computation of complex spatial metrics. We have developed a light-weight, user friendly software, AngioTool, which allows for quick, hands-off and reproducible quantification of vascular networks in microscopic images. AngioTool computes several morphological and spatial parameters including the area covered by a vascular network, the number of vessels, vessel length, vascular density and lacunarity. In addition, AngioTool calculates the so-called "branching index" (branch points/unit area), providing a measurement of the sprouting activity of a specimen of interest. We have validated AngioTool using images of embryonic murine hindbrains, post-natal retinas and allantois explants. AngioTool is open source and can be downloaded free of charge.

How blood vessel networks are made and measured

Tissue and organ viability depends on the proper systemic distribution of cells, nutrients, and oxygen through blood vessel networks. These networks arise in part via angiogenic sprouting. Vessel sprouting involves the precise coordination of several endothelial cell processes including cell-cell communication, cell migration, and proliferation. In this review, we discuss zebrafish and mammalian models of blood vessel sprouting and the quantification methods used to assess vessel sprouting and network formation in these models. We also review the mechanisms involved in angiogenic sprouting, and we propose that the process consists of distinct stages. Sprout initiation involves endothelial cell interactions with neighboring cells and the environment to establish a specialized tip cell responsible for leading the emerging sprout. Furthermore, local sprout guidance cues that spatially regulate this outward migration are discussed. We also examine subsequent events, such as sprout fusion and lumenization, that lead to maturation of a nascent sprout into a patent blood vessel.

PI3K signaling through the dual GTPase-activating protein ARAP3 is essential for developmental angiogenesis

One function of phosphoinositide 3-kinase α (PI3Kα), which generates the lipid second messenger phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P(3)], is its regulation of angiogenesis in the developing embryo and in pathological situations. ARAP3 is a PtdIns(3,4,5)P(3)- and Rap-activated guanosine triphosphatase (GTPase)-activating protein (GAP) for the small GTPases RhoA and Arf6. Here, we show that deleting Arap3 in the mouse caused embryonic death in mid-gestation due to an endothelial cell-autonomous defect in sprouting angiogenesis. Explants taken at a developmental stage at which no defect was yet present reproduced this phenotype ex vivo, demonstrating that the defect was not secondary to hypoxia, placental defects, or organ failure. In addition, knock-in mice expressing an ARAP3 point mutant that cannot be activated by PtdIns(3,4,5)P(3) had angiogenesis defects similar to those of Arap3(-/-) embryos. Our work delineates a previously unknown signaling pathway that controls angiogenesis immediately downstream of PI3Kα through ARAP3 to the Rho and Arf family of small GTPases.

BMP-9 induces proliferation of multiple types of endothelial cells in vitro and in vivo

Members of the bone morphogenetic protein (BMP) family have been implicated in the development and maintenance of vascular systems. Whereas members of the BMP-2/4 and osteogenic protein-1 groups signal via activin receptor-like kinase (ALK)-2, ALK-3 and ALK-6, BMP-9 and BMP-10 have been reported to bind to ALK-1 in endothelial cells. However, the roles of BMP-9–ALK-1 signaling in the regulation of endothelial cells have not yet been fully elucidated. Here, using various systems, we examined the effects of BMP-9 on the proliferation of endothelial cells. Vascular-tube formation from ex vivo allantoic explants of mouse embryos was promoted by BMP-9. BMP-9, as well as BMP-4 and BMP-6, also induced the proliferation of in-vitro-cultured mouse embryonic-stem-cell-derived endothelial cells (MESECs) by inducing the expression of vascular endothelial growth factor receptor 2 and Tie2, a receptor for angiopoietin-1. A decrease in ALK-1 expression or expression of constitutively active ALK-1 in MESECs abrogated and mimicked the effects of BMP-9 on the proliferation of MESECs, respectively, suggesting that BMP-9 promotes the proliferation of these cells via ALK-1. Furthermore, in vivo angiogenesis was promoted by BMP-9 in a Matrigel plug assay and in a BxPC3 xenograft model of human pancreatic cancer. Consistent with these in vivo findings, BMP-9 enhanced the proliferation of in-vitro-cultured normal endothelial cells from dermal tissues of adult mice and of tumor-associated endothelial cells isolated from tumor xenografts in host mice. These findings suggest that BMP-9 signaling activates the endothelium tested in the present study via ALK-1.

Collagen type IV and Perlecan exhibit dynamic localization in the Allantoic Core Domain, a putative stem cell niche in the murine allantois

A body of evidence suggests that the murine allantois contains a stem cell niche, the Allantoic Core Domain (ACD), that may contribute to a variety of allantoic and embryonic cell types. Given that extracellular matrix (ECM) regulates cell fate and function in niches, the allantois was systematically examined for Collagen type IV (ColIV) and Perlecan, both of which are associated with stem cell proliferation and differentiation. Not only was localization of ColIV and Perlecan more widespread during gastrulation than previously reported, but protein localization profiles were particularly robust and dynamic within the allantois and associated visceral endoderm as the ACD formed and matured. We propose that these data provide further evidence that the ACD is a stem cell niche whose activity is synchronized with associated visceral endoderm, possibly via ECM proteins.

The Allantoic Core Domain: new insights into development of the murine allantois and its relation to the primitive streak

The whereabouts and properties of the posterior end of the primitive streak have not been identified in any species. In the mouse, the streak's posterior terminus is assumed to be confined to the embryonic compartment, and to give rise to the allantois, which links the embryo to its mother during pregnancy. In this study, we have refined our understanding of the biology of the murine posterior primitive streak and its relation to the allantois. Through a combination of immunostaining and morphology, we demonstrate that the primitive streak spans the posterior extraembryonic and embryonic regions at the onset of the neural plate stage ( approximately 7.0 days postcoitum, dpc). Several hours later, the allantoic bud emerges from the extraembryonic component of the primitive streak (XPS). Then, possibly in collaboration with overlying allantois-associated extraembryonic visceral endoderm, the XPS establishes a germinal center within the allantois, named here the Allantoic Core Domain (ACD). Microsurgical removal of the ACD beyond headfold (HF) stages resulted in the formation of allantoic regenerates that lacked the ACD and failed to elongate; nevertheless, vasculogenesis and vascular patterning proceeded. In situ and transplantation fate mapping demonstrated that, from HF stages onward, the ACD's progenitor pool contributed to the allantois exclusive of the proximal flanks. By contrast, the posterior intraembryonic primitive streak (IPS) provided the flanks. Grafting the ACD into T(C)/T(C) hosts, whose allantoises are significantly foreshortened, restored allantoic elongation. These results revealed that the ACD is essential for allantoic elongation, but the cues required for vascularization lie outside of it. On the basis of these and previous findings, we conclude that the posterior primitive streak of the mouse conceptus is far more complex than was previously believed. Our results provide new directives for addressing the origin and development of the umbilical cord, and establish a novel paradigm for investigating the fetal/placental relationship.

Epiblast-specific Snai1 deletion results in embryonic lethality due to multiple vascular defects

BACKGROUND

Members of the Snail gene family, which encode zinc finger proteins that function as transcriptional repressors, play essential roles during embryonic development in vertebrates. Mouse embryos with conditional deletion of the Snail1 (Snai1) gene in the epiblast, but not in most extraembryonic membranes, exhibit defects in left-right asymmetry specification and migration of mesoderm cells through the posterior primitive streak. Here we describe phenotypic defects that result in death of the mutant embryos by 9.5 days of gestation.

FINDINGS

Endothelial cells differentiated in epiblast-specific Snai1-deficient embryos, but formation of an interconnected vascular network was abnormal. To determine whether the observed vascular defects were dependent on disruption of blood flow, we analyzed vascular remodeling in cultured allantois explants from the mutant embryos. Similar vascular defects were observed in the mutant allantois explants.

CONCLUSION

These studies demonstrate that lethality in the Snai1-conditional mutant embryos is caused by multiple defects in the cardiovascular system.

VEGF-mediated fusion in the generation of uniluminal vascular spheroids

Embryonic mouse allantoic tissue (E8.5) was cultured in hanging drops to generate a three-dimensional vascular micro-tissue. The resulting tissue spheroids had an inner network of small diameter vessels expressing platelet endothelial cell adhesion molecule-1 (PECAM-1) and an outer layer of cells expressing SMalphaA, SM22-alpha, and SM-MHC. In a subsequent phase of culture, the fusion-promoting activity of vascular endothelial growth factor (VEGF) was used to transform the inner network of small diameter endothelial tubes into a contiguous layer of cells expressing PECAM-1, CD34, and VE-cadherin that circumscribed a central lumen-like cavity. The blood vessel-like character of the VEGF-treated spheroids was further demonstrated by their physiologically relevant vasodilatory and contractile responses, including contraction induced by KCl and relaxation stimulated by high-density lipoproteins and acetylcholine-induced nitric oxide production.

Systematic localization of Oct-3/4 to the gastrulating mouse conceptus suggests manifold roles in mammalian development

Oct-3/4 was localized to the mouse conceptus between the onset of gastrulation and 16-somite pairs (-s; approximately 6.5-9.25 days postcoitum, dpc). Results revealed Oct-3/4 in a continuum of morphologically distinct epiblast-derived embryonic and extraembryonic tissues. In the allantois, distal-to-proximal diminution in the Oct-3/4 domain over time and co-localization with Flk-1 in angioblasts accorded with a role in vascular differentiation and the presence of a stem cell reservoir. In addition, visceral endoderm exhibited a dynamic salt-and-pepper distribution, which, combined with previous results of fate mapping and gene expression, suggested that Oct-3/4 is involved in the genesis of definitive endoderm. By 8-s, Oct-3/4 was globally down regulated in all but putative primordial germ cells (PGCs) and some allantoic cell clusters. Taken together, Oct-3/4's expression profile suggests unexpected and potentially far more versatile roles in development than have been previously appreciated.

VE-cadherin is a critical endothelial regulator of TGF-beta signalling

VE-cadherin is an endothelial-specific transmembrane protein concentrated at cell-to-cell adherens junctions. Besides promoting cell adhesion and controlling vascular permeability, VE-cadherin transfers intracellular signals that contribute to vascular stabilization. However, the molecular mechanism by which VE-cadherin regulates vascular homoeostasis is still poorly understood. Here, we report that VE-cadherin expression and junctional clustering are required for optimal transforming growth factor-beta (TGF-beta) signalling in endothelial cells (ECs). TGF-beta antiproliferative and antimigratory responses are increased in the presence of VE-cadherin. ECs lacking VE-cadherin are less responsive to TGF-beta/ALK1- and TGF-beta/ALK5-induced Smad phosphorylation and target gene transcription. VE-cadherin coimmunoprecipitates with all the components of the TGF-beta receptor complex, TbetaRII, ALK1, ALK5 and endoglin. Clustered VE-cadherin recruits TbetaRII and may promote TGF-beta signalling by enhancing TbetaRII/TbetaRI assembly into an active receptor complex. Taken together, our data indicate that VE-cadherin is a positive and EC-specific regulator of TGF-beta signalling. This suggests that reduction or inactivation of VE-cadherin may contribute to progression of diseases where TGF-beta signalling is impaired.

Vascular sprout formation entails tissue deformations and VE-cadherin-dependent cell-autonomous motility

Embryonic and fetal vascular sprouts form within constantly expanding tissues. Nevertheless, most biological assays of vascular spouting are conducted in a static mechanical milieu. Here we study embryonic mouse allantoides, which normally give raise to an umbilical artery and vein. However, when placed in culture, allantoides assemble a primary vascular network. Unlike other in vitro assays, allantoic primordial vascular cells are situated on the upper surface of a cellular layer that is engaged in robust spreading motion. Time-lapse imaging allows quantification of primordial vascular cell motility as well as the underlying mesothelial tissue motion. Specifically, we calculate endothelial cell-autonomous motion by subtracting the tissue-level mesothelial motion from the total endothelial cell displacements. Formation of new vascular polygons is hindered by administration of function-blocking VE-cadherin antibodies. Time-lapse recordings reveal that (1) cells at the base of sprouts normally move distally "over" existing sprout cells to form new tip-cells; and (2) loss of VE-cadherin activity prevents this motile behavior. Thus, endothelial cell-cell-adhesion-based motility is required for the advancement of vascular sprouts within a moving tissue environment. To the best of our knowledge, this is the first study that couples endogenous tissue dynamics to assembly of vascular networks in a mammalian system.

The murine allantois: emerging paradigms in development of the mammalian umbilical cord and its relation to the fetus

The fertilized egg of the mammal gives rise to the embryo and its extraembryonic structures, all of which develop in intimate relation with each other. Yet, whilst the past several decades have witnessed a vast number of studies on the embryonic component of the conceptus, study of the extraembryonic tissues and their relation to the fetus have been largely ignored. The allantois, precursor tissue of the mature umbilical cord, is a universal feature of all placental mammals that establishes the vital vascular bridge between the fetus and its mother. The allantois differentiates into the umbilical blood vessels, which become secured onto the chorionic component of the placenta at one end and onto the fetus at the other. In this way, fetal blood is channeled through the umbilical cord for exchange with the mother. Despite the importance of this vascular bridge, little is known about how it is made. The aim of this review is to address current understanding of the biology of the allantois in the mouse and genetic control of its features and functions, and to highlight new paradigms concerning the developmental relationship between the fetus and its umbilical cord.

The allantois and chorion, when isolated before circulation or chorio-allantoic fusion, have hematopoietic potential

The chorio-allantoic placenta forms through the fusion of the allantois (progenitor tissue of the umbilical cord), with the chorionic plate. The murine placenta contains high levels of hematopoietic stem cells, and is therefore a stem cell niche. However, it is not known whether the placenta is a site of hematopoietic cell emergence, or whether hematopoietic cells originate from other sites in the conceptus and then colonize the placenta. Here, we show that the allantois and chorion, isolated prior to the establishment of circulation, have the potential to give rise to myeloid and definitive erythroid cells following explant culture. We further show that the hematopoietic potential of the allantois and chorion does not require their union, indicating that it is an intrinsic property of these tissues. These results suggest that the placenta is not only a niche for, but also a source of, hematopoietic cells.

Critical role for NF-kappaB-induced JunB in VEGF regulation and tumor angiogenesis

Regulation of vascular endothelial growth factor (VEGF) expression is a complex process involving a plethora of transcriptional regulators. The AP-1 transcription factor is considered as facilitator of hypoxia-induced VEGF expression through interaction with hypoxia-inducible factor (HIF) which plays a major role in mediating the cellular hypoxia response. As yet, both the decisive AP-1 subunit leading to VEGF induction and the molecular mechanism by which this subunit is activated have not been deciphered. Here, we demonstrate that the AP-1 subunit junB is a target gene of hypoxia-induced signaling via NF-kappaB. Loss of JunB in various cell types results in severely impaired hypoxia-induced VEGF expression, although HIF is present and becomes stabilized. Thus, we identify JunB as a critical independent regulator of VEGF transcription and provide a mechanistic explanation for the inherent vascular phenotypes seen in JunB-deficient embryos, ex vivo allantois explants and in vitro differentiated embryoid bodies. In support of these findings, tumor angiogenesis was impaired in junB(-/-) teratocarcinomas because of severely impaired paracrine-acting VEGF and the subsequent inability to efficiently recruit host-derived vessels.

Brachyury is required for elongation and vasculogenesis in the murine allantois

Mouse conceptuses homozygous for mutations in brachyury (T) exhibit a short, misshapen allantois that fails to fuse with the chorion. Ultimately, mutant embryos die during mid-gestation. In the 60 years since this discovery, the role of T in allantoic development has remained obscure. T protein was recently identified in several new sites during mouse gastrulation, including the core of the allantois, where its function is not known. Here, using molecular, genetic and classical techniques of embryology, we have investigated the role of T in allantoic development. Conceptuses homozygous for the T(Curtailed) (T(C)) mutation (T(C)/T(C)) exhibited allantoic dysmorphogenesis shortly after the allantoic bud formed. Diminution in allantoic cell number and proliferation was followed by cell death within the core. Fetal liver kinase (Flk1)-positive angioblasts were significantly decreased in T(C)/T(C) allantoises and did not coalesce into endothelial tubules, possibly as a result of the absence of platelet endothelial cell adhesion molecule 1 (Pecam1), whose spatiotemporal relationship to Flk1 suggested a role in patterning the umbilical vasculature. Remarkably, microsurgical perturbation of the wild-type allantoic core phenocopied the T(C)/T(C) vascularization defect, providing further support that an intact core is essential for vascularization. Last, abnormalities were observed in the T(C)/T(C) heart and yolk sac, recently reported sites of T localization. Our findings reveal that T is required to maintain the allantoic core, which is essential for allantoic elongation and vascular patterning. In addition, morphological defects in other extraembryonic and embryonic vascular organs suggest a global role for T in vascularization of the conceptus.

Hematopoietic potential of the pre-fusion allantois

We previously showed that the fetal component of the placenta has a vigorous hematopoietic activity. Whether this organ is an environmental niche where hematopoietic stem cells (HSC) proliferate and become committed to various lineages, or whether it is also a site for HSC emergence, was left open. This issue can be addressed only if the components that will give rise to the placenta are tested prior to vascularization. The fetal part of the placenta forms through the fusion of the allantois and the chorionic plate around the stage of 7 somite pairs. The allantois, a mesodermal rudiment that provides fetal blood vessels to the placenta, was retrieved before fusion. We found in this rudiment expression of CD41, a known marker of early embryonic hematopoietic progenitors. c-Kit encoding a progenitor specific receptor was also expressed. Significantly, as early as the 1-2 somite stage, the allantois yielded erythroid, myeloid and multipotent clonogenic progenitors, when pre-cultured in toto prior to seeding in a semisolid medium. These results provide evidence that the allantois has hematopoietic potential per se. Whether this potential also involves the ability to produce HSC is still to be determined.

Localization of Brachyury (T) in embryonic and extraembryonic tissues during mouse gastrulation

T-box gene family members have important roles during murine embryogenesis, gastrulation, and organogenesis. Although relatively little is known about how T-box genes are regulated, published gene expression studies have revealed dynamic and specific patterns in both embryonic and extraembryonic tissues of the mouse conceptus. Mutant alleles of the T-box gene Brachyury (T) have identified roles in formation of mesoderm and its derivatives, such as somites and the allantois. However, given the cell autonomous nature of T gene activity and conflicting results of gene expression studies, it has been difficult to attribute a primary function to T in normal allantoic development. We report localization of T protein by sectional immunohistochemistry in both embryonic and extraembryonic tissues during mouse gastrulation, emphasizing T localization within the allantois. T was detected in all previously reported sites within the conceptus, including the primitive streak and its derivatives, nascent embryonic mesoderm, the node and notochord, as well as notochord-associated endoderm and posterior neurectoderm. In addition, we have clarified T within the allantois, where it was first detected in the proximal midline of the late allantoic bud (approximately 7.5 days postcoitum, dpc) and persisted within an expanded midline domain until 6-somite pairs (s; approximately 8.5 dpc). Lastly, we have discovered several novel T sites, including the developing heart, visceral endoderm, extraembryonic ectoderm, and its derivative, chorionic ectoderm. Together, these data provide a unified picture of T in the mammalian conceptus, and demonstrate T's presence in unrelated cell types and tissues in highly dynamic spatiotemporal patterns in both embryonic and extraembryonic tissues.

Development of structures and transport functions in the mouse placenta

The placenta is essential for sustaining the growth of the fetus during gestation, and defects in its function result in fetal growth restriction or, if more severe, fetal death. Several molecular pathways have been identified that are essential for development of the placenta, and mouse mutants offer new insights into the cell biology of placental development and physiology of nutrient transport.

Experimental transplantation study for possible transformation of bone marrow cells in the mouse placenta

The aim of the present study is to establish a mouse model of the transplantation of bone marrow cells into the placenta in mid-gestation. The mononuclear fraction of bone marrow cells was isolated by Ficoll gradient centrifugation from the femur bones of C57BL/6 green fluorescent protein (GFP) gene transgenic (Tg) mice. After intraperitoneal injection of pentobarbital sodium, the abdominal cavities of pregnant non-Tg (C57BL/6 or ICR) mice were opened at 9.5 days postcoitum (dpc). The mononuclear fraction of bone marrow cells from Tg mice (3-5 x 10(5)cells/3 microl) was directly injected into the placental portion of the pregnant uterus, at a depth of approximately 3 mm, using a 31-gauge injector. The placenta was sampled at 14.5 dpc. Confocal laser scanning microscopic analysis of the serial sections of the sampled placenta (150-250 sections/placenta) was carried out to detect GFP-positive cells and to assess immunostaining for cytokeratin, CD34, p57(Kip2) and prolactin. Most pregnant mice survived until sampling of the placenta at 14.5-18.5 dpc (88.9% for C57BL6 and 100% for ICR). The survival rate of fetuses from mice in which the placenta was transplanted with GFP-positive bone marrow cells was approximately 50%. A small population (0.154%) of injected bone marrow cells was retained in the placental tissue. Immunohistochemically, cytokeratin, CD34 and p57(Kip2) were positively stained in 0.062%, 4.5% and 2.1% of GFP-positive cells, respectively, while prolactin was not positive in any of the cells examined. GFP-positive bone marrow cells were successfully transplanted to the murine placenta. Future investigations of the specific antigens in bone marrow cells retained in the placenta may enable a better understanding of the local regulation of placental development.

Sphingosine-1-phosphate signaling promotes critical migratory events in vasculogenesis

Here we have investigated the role of sphingosine-1-phosphate (S1P) signaling in the process of vasculogenesis in the mouse embryo. At stages preceding the formation of blood vessels (7.5-8 dpc) in the embryo proper, yolk sac, and allantois, the S1P receptor S1P(2) is expressed in conjunction with S1P(1) and/or S1P(3). Additionally, sphingosine kinase-2 (SK2), an enzyme that catalyzes the formation of S1P, is expressed in these tissues throughout periods of vasculogenesis. Using the cultured mouse allantois explant model of blood vessel formation, we found that vasculogenesis was dependent on S1P signaling. We showed that S1P could replace the ability of serum to promote vasculogenesis in cultured allantois explants. Instead of small poorly reticulated clusters of rounded endothelial cells that formed under serum-free conditions, S1P promoted the formation of elongated endothelial cells that arranged into expansive branched networks of capillary-like vessels. These effects could not be reproduced by vascular endothelial growth factor or basic fibroblast growth factor administration. The ability of S1P to promote blood vessel formation was not due to effects on cell survival or on changes in numbers of endothelial cells (Flk1(+)/PECAM(+)), angioblasts (Flk1(+)/PECAM(-)), or undifferentiated mesodermal cells (Flk1(-)/PECAM(-)). The S1P effect on blood vessel formation was attributed to it promoting migratory activities of angioblasts and early endothelial cells required for the expansion of vascular networks. Together, our findings suggest that migratory events critical to the de novo formation of blood vessels are under the influence of S1P, possibly synthesized via the action of SK2, with signaling mediated by S1P receptors that include S1P(1), S1P(2), and S1P(3).

VE-cadherin expression and clustering maintain low levels of survivin in endothelial cells

Survivin is strongly expressed in embryonic organs and in tumor cells but is low or absent in differentiated normal tissues. Resting endothelium expresses low levels of survivin but can up-regulate its synthesis on activation to proliferate. The mechanisms responsible for survivin down-regulation in resting conditions are still unknown. We report here that confluence and vascular endothelial-cadherin (VE-cadherin) expression induce contact inhibition of cell growth and survivin down-regulation in the endothelium. Using beta-catenin null and positive isogenic endothelial cell lines we found that the effect requires beta-catenin expression and its association to VE-cadherin cytoplasmic tail. Furthermore, in allantois organ cultures, survivin expression is up-regulated in areas of growing vessels where VE-cadherin is partially dismantled from junctions or in VE-cadherin -/- specimens. Overall, these data indicate that VE-cadherin and beta-catenin may negatively regulate survivin synthesis in endothelial cells. Consistently, in epidermal and pancreatic cell lines or ovarian tumors, epithelial-cadherin (E-cadherin) and survivin expression is inversely related, suggesting a non-cell-specific role of cadherins in reducing survivin synthesis.

Investigation into a role for the primitive streak in development of the murine allantois

Despite its importance as the source of one of three major vascular systems in the mammalian conceptus, little is known about the murine allantois, which will become the umbilical cord of the chorio-allantoic placenta. During gastrulation, the allantois grows into the exocoelomic cavity as a mesodermal extension of the posterior primitive streak. On the basis of morphology, gene expression and/or function, three cell types have been identified in the allantois: an outer layer of mesothelial cells, whose distal portion will become transformed into chorio-adhesive cells, and endothelial cells within the core. Formation of endothelium and chorio-adhesive cells begins in the distal region of the allantois, farthest from the streak. Over time, endothelium spreads to the proximal allantoic region, whilst the distal outer layer of presumptive mesothelium gradually acquires vascular cell adhesion molecule (VCAM1) and mediates chorio-allantoic union. Intriguingly, the VCAM1 domain does not extend into the proximal allantoic region. How these three allantoic cell types are established is not known, although contact with the chorion has been discounted. In this study, we have investigated how the allantois differentiates, with the goal of discriminating between extrinsic mechanisms involving the primitive streak and an intrinsic role for the allantois itself. Exploiting previous observations that the streak contributes mesoderm to the allantois throughout the latter's early development, microsurgery was used to remove allantoises at ten developmental stages. Subsequent whole embryo culture of operated conceptuses resulted in the formation of regenerated allantoises at all time points. Aside from being generally shorter than normal, none of the regenerates exhibited abnormal differentiation or inappropriate cell relationships. Rather, all of them resembled intact allantoises by morphological, molecular and functional criteria. Moreover, fate mapping adjacent yolk sac and amniotic mesoderm revealed that these tissues and their associated bone morphogenetic protein 4 (BMP4) did not contribute to restoration of allantoic outgrowth and differentiation during allantoic regeneration. Thus, on the basis of these observations, we conclude that specification of allantoic endothelium, mesothelium and chorio-adhesive cells does not occur by a streak-related mechanism during the time that proximal epiblast travels through it and is transformed into allantoic mesoderm. Rather, all three cell-types are established by mechanisms intrinsic to the allantois, and possibly include roles for cell age and cell position. However, although chorio-adhesive cells were not specified within the streak, we discovered that the streak nonetheless plays a role in establishing VCAM1's expression domain, which typically began and was thereafter maintained at a defined distance from the primitive streak. When allantoises were removed from contact with the streak, normally VCAM1-negative proximal allantoic regions acquired VCAM1. These results suggested that the streak suppresses formation of chorio-adhesive cells in allantoic mesoderm closest to it. Together with previous results, findings presented here suggest a model of differentiation of allantoic mesoderm that invokes intrinsic and extrinsic mechanisms, all of which appear to be activated once the allantoic bud has formed.

Multiple developmental roles of Ahnak are suggested by localization to sites of placentation and neural plate fusion in the mouse conceptus

Ahnak is a gigantic (700 kD) phosphoprotein with a unique structure whose expression and cellular localization are dynamically regulated during cell cycle progression. Here, we report that Ahnak is localized to sites of major morphogenesis during mouse placentation and neurulation. Ahnak was found in: (i) derivatives of trophectoderm, including chorionic ectoderm prior to and during union with the ectoplacental cone, presumptive syncytiotrophoblast cells in the chorionic labyrinth, and giant cells at the trophoblast-uterine interface; (ii) the allantois prior to, during, and after union with the chorion; and (iii) the tips of the neural plate during formation of the neural tube. On the basis of these observations, we suggest that Ahnak may play heretofore unrecognized roles in tissue union during normal mouse development.

Early placental ontogeny in the mouse

Fundamental to placental morphogenesis is union between the allantois and the chorion, two tissues initially separated in the conceptus. Results of previous studies in the mouse have suggested that chorio-allantoic union is driven by the developmental maturity of the allantois and involves molecular interactions between Vascular Cell Adhesion Molecule (VCAM-1) in the allantois and alpha4-integrin in the chorion. Little more is known about the cellular and/or molecular control of this important morphogenetic event in any species.Gross, histological, microsurgical and immunohistochemical analyses in the mouse conceptus revealed that placental ontogeny took place in three major steps. The first, chorio-allantoic contact, was not enduring and was mediated by the allantois' mesothelial surface and the mesodermal component of the chorion. Modest amounts of VCAM-1 were found in distal allantoic mesothelium, whilst levels of alpha4-integrin were high throughout chorionic mesoderm. The second step, chorio-allantoic fusion, was more enduring. During this time, the distal allantoic region contained maximal levels of VCAM-1, and all allantoises had expanded far enough to reach the posterior chorion from where they spread toward a central chorionic depression. The last step, breakdown of chorio-allantoic fusing surfaces, was dependent upon chorio-allantoic fusion and resulted in the intimate juxtaposition of allantoic endothelium and chorionic ectoderm, possibly as a result of VCAM-1-mediated interactions. The umbilical connection was thereafter fixed at its perimeter to the chorionic surface by large amounts of VCAM-1 in disto-lateral allantoic mesothelium and alpha4-integrin in the remaining peripheral mesodermal component of the chorion.Thus, chorio-allantoic union is highly regulated, taking place in multiple steps. It is dependent upon the developmental maturity of distal allantoic mesothelium and involves the mesodermal component of the chorion. Breakdown of fusing surfaces enables penetration of the allantoic vasculature into the chorion. These findings provide a secure developmental foundation in which to elucidate the genetic control of early placentation.

Bone morphogenetic protein 4 in the extraembryonic mesoderm is required for allantois development and the localization and survival of primordial germ cells in the mouse

Evidence suggests that the specification of primordial germ cells (PGCs) in the mammalian embryo does not depend on maternal determinants. Rather, previous genetic analysis in the mouse has shown that bone morphogenetic protein 4 (Bmp4) is required for the formation of both PGCs and allantois. Bmp4 is expressed in both the trophoblast-derived extraembryonic ectoderm (ExE) and in the epiblast-derived extraembryonic mesoderm (ExM), in which the PGCs, allantois primordium, and angioblasts are first detected. We have shown that Bmp4 made in the ExE functions to induce precursors of PGCs and allantois in the adjacent epiblast, resulting in complete lack of both cell types in homozygous null mutants. However, the function of Bmp4 in the ExM is totally unknown. To address this question, we generated tetraploid (4N) chimeras by aggregating Bmp4 null ES cells with wild-type tetraploid embryos. In this combination, wild-type tetraploid cells contribute to the extraembryonic trophoblast and primitive endoderm lineages but are excluded from the epiblast and its derivatives, including the ExM. Our results clearly demonstrate that Bmp4 made in the ExM does not affect the establishment of either PGC or allantois lineages, but is required for PGC localization and survival and for the differentiation of the allantois. These findings suggest that Bmp4 expressed in epiblast-derived tissues plays vital roles in reproduction by regulating both the development of the germ line and the vascular connection between the embryo and the placenta.