Cell fate and morphogenetic movement in the late mouse primitive streak

Widespread but tissue-specific patterns of interferon-induced transmembrane protein 3 (IFITM3, FRAGILIS, MIL-1) in the mouse gastrula

Interferon-induced transmembrane protein 3 (IFITM3; FRAGILIS; MIL-1) is part of a larger family of important small interferon-induced transmembrane genes and proteins involved in early development, cell adhesion, and cell proliferation, and which also play a major role in response to bacterial and viral infections and, more recently, in pronounced malignancies. IFITM3, together with tissue-nonspecific alkaline phosphatase (TNAP), PRDM1, and STELLA, has been claimed to be a hallmark of segregated primordial germ cells (PGCs) (Saitou et al., 2002). However, whether IFITM3, like STELLA, is part of a broader stem/progenitor pool that builds the posterior region of the mouse conceptus (Mikedis and Downs, 2012) is obscure. To discover the whereabouts of IFITM3 during mouse gastrulation (~E6.5-9.0), systematic immunohistochemical analysis was carried out at closely spaced 2-4-h intervals. Results revealed diverse, yet consistent, profiles of IFITM3 localization throughout the gastrula. Within the putative PGC trajectory and surrounding posterior tissues, IFITM3 localized as a large cytoplasmic spot with or without staining in the plasma membrane. IFITM3, like STELLA, was also found in the ventral ectodermal ridge (VER), a posterior progenitor pool that builds the tailbud. The large cytoplasmic spot with plasma membrane staining was exclusive to the posterior region; the visceral yolk sac, non-posterior tissues, and epithelial tissues exhibited spots of IFITM3 without cell surface staining. Colocalization of the intracellular IFITM3 spot with the endoplasmic reticulum, Golgi apparatus, or endolysosomes was not observed. That relatively high levels of IFITM3 were found throughout the posterior primitive streak and its derivatives is consistent with evidence that IFITM3, like STELLA, is part of a larger stem/progenitor cell pool at the posterior end of the primitive streak that forms the base of the allantois and builds the fetal-umbilical connection, thus further obfuscating practical phenotypic distinctions between so-called PGCs and surrounding soma.

Sirenomelia: a review on embryogenic enviromental theories, novel three-dimensional ultrasound imaging and first trimester diagnosis in a case of mosaic 69,XXX/46,XX fetus

PURPOSE

Sirenomelia is caused by atrophy of the lower extremities that is commonly associated with gastrointestinal and urogenital malformations.

METHODS

Embryogenic environmental theories and systematic review of the literature are reported.

RESULTS

Genetic basis of the condition has been demonstrated in the animal model. In humans, association with de novo balanced translocation has only recently been documented.

CONCLUSIONS

A case of triploidy mosaic fetus with sirenomelia and posterior fossa anomaly diagnosed at first trimester using novel three-dimensional ultrasound imaging techniques is presented.

PDGF receptor alpha+ mesoderm contributes to endothelial and hematopoietic cells in mice

BACKGROUND

Early mesoderm can be classified into Flk-1+ or PDGF receptor alpha (PDGFRα)+ population, grossly representing lateral and paraxial mesoderm, respectively. It has been demonstrated that all endothelial (EC) and hematopoietic (HPC) cells are derived from Flk-1+ cells. Although PDGFRα+ cells give rise to ECs/HPCs in in vitro ES differentiation, whether PDGFRα+ population can become hemato-endothelial lineages has not been proved in mouse embryos.

RESULTS

Using PDGFRαMerCreMer mice, PDGFRα+ early mesoderm was shown to contribute to endothelial cells including hemogenic ECs, fetal liver B lymphocytes, and Lin-Kit+Sca-1+ (KSL) cells. Contribution of PDGFRα+ mesoderm into ECs and HPCs was limited until E8.5, indicating that PDGFRα+/Flk-1+ population that exists until E8.5 may be the source for hemato-endothelial lineages from PDGFRα+ population. The functional significance of PDGFRα+ mesoderm in vascular development and hematopoiesis was confirmed by genetic deletion of Etv2 or restoration of Runx1 in PDGFRα+ cells. Etv2 deletion and Runx1 restoration in PDGFRα+ cells resulted in abnormal vascular remodeling and rescue of fetal liver CD45+ and Lin-Kit+Sca-1+ (KSL) cells, respectively.

CONCLUSIONS

Endothelial and hematopoietic cells can be derived from PDGFRα+ early mesoderm in mice. PDGFRα+ mesoderm is functionally significant in vascular development and hematopoiesis from phenotype analysis of genetically modified embryos.

Foxa1 and Foxa2 are required for formation of the intervertebral discs

The intervertebral disc (IVD) is composed of 3 main structures, the collagenous annulus fibrosus (AF), which surrounds the gel-like nucleus pulposus (NP), and hyaline cartilage endplates, which are attached to the vertebral bodies. An IVD is located between each vertebral body. Degeneration of the IVD is thought to be a major cause of back pain, a potentially chronic condition for which there exist few effective treatments. The NP forms from the embryonic notochord. Foxa1 and Foxa2, transcription factors in the forkhead box family, are expressed early during notochord development. However, embryonic lethality and the absence of the notochord in Foxa2 null mice have precluded the study of potential roles these genes may play during IVD formation. Using a conditional Foxa2 allele in conjunction with a tamoxifen-inducible Cre allele (ShhcreER^T2), we removed Foxa2 from the notochord of E7.5 mice null for Foxa1. Foxa1^−/−;Foxa2^c/c;ShhcreER^T2 double mutant animals had a severely deformed nucleus pulposus, an increase in cell death in the tail, decreased hedgehog signaling, defects in the notochord sheath, and aberrant dorsal-ventral patterning of the neural tube. Embryos lacking only Foxa1 or Foxa2 from the notochord were indistinguishable from control animals, demonstrating a functional redundancy for these genes in IVD formation. In addition, we provide in vivo genetic evidence that Foxa genes are required for activation of Shh in the notochord.

Signaling by FGF4 and FGF8 is required for axial elongation of the mouse embryo

Fibroblast growth factor (FGF) signaling has been shown to play critical roles in vertebrate segmentation and elongation of the embryonic axis. Neither the exact roles of FGF signaling, nor the identity of the FGF ligands involved in these processes, has been conclusively determined. Fgf8 is required for cell migration away from the primitive streak when gastrulation initiates, but previous studies have shown that drastically reducing the level of FGF8 later in gastrulation has no apparent effect on somitogenesis or elongation of the embryo. In this study, we demonstrate that loss of both Fgf8 and Fgf4 expression during late gastrulation resulted in a dramatic skeletal phenotype. Thoracic vertebrae and ribs had abnormal morphology, lumbar and sacral vertebrae were malformed or completely absent, and no tail vertebrae were present. The expression of Wnt3a in the tail and the amount of nascent mesoderm expressing Brachyury were both severely reduced. Expression of genes in the NOTCH signaling pathway involved in segmentation was significantly affected, and somite formation ceased after the production of about 15-20 somites. Defects seen in the mutants appear to result from a failure to produce sufficient paraxial mesoderm, rather than a failure of mesoderm precursors to migrate away from the primitive streak. Although the epiblast prematurely decreases in size, we did not detect evidence of a change in the proliferation rate of cells in the tail region or excessive apoptosis of epiblast or mesoderm cells. We propose that FGF4 and FGF8 are required to maintain a population of progenitor cells in the epiblast that generates mesoderm and contributes to the stem cell population that is incorporated in the tailbud and required for axial elongation of the mouse embryo after gastrulation.

Scoliosis and segmentation defects of the vertebrae

The vertebral column derives from somites, which are transient paired segments of mesoderm that surround the neural tube in the early embryo. Somites are formed by a genetic mechanism that is regulated by cyclical expression of genes in the Notch, Wnt, and fibroblast growth factor (FGF) signaling pathways. These oscillators together with signaling gradients within the presomitic mesoderm help to set somitic boundaries and rostral-caudal polarity that are essential for the precise patterning of the vertebral column. Disruption of this mechanism has been identified as the cause of severe segmentation defects of the vertebrae in humans. These segmentation defects are part of a spectrum of spinal disorders affecting the skeletal elements and musculature of the spine, resulting in curvatures such as scoliosis, kyphosis, and lordosis. While the etiology of most disorders with spinal curvatures is still unknown, genetic and developmental studies of somitogenesis and patterning of the axial skeleton and musculature are yielding insights into the causes of these diseases.

STELLA-positive subregions of the primitive streak contribute to posterior tissues of the mouse gastrula

The developmental relationship between the posterior embryonic and extraembryonic regions of the mammalian gastrula is poorly understood. Although many different cell types are deployed within this region, only the primordial germ cells (PGCs) have been closely studied. Recent evidence has suggested that the allantois, within which the PGCs temporarily take up residence, contains a pool of cells, called the Allantoic Core Domain (ACD), critical for allantoic elongation to the chorion. Here, we have asked whether the STELLA-positive cells found within this region, thought to be specified PGCs, are actually part of the ACD and to what extent they, and other ACD cells, contribute to the allantois and fetal tissues. To address these hypotheses, STELLA was immunolocalized to the mouse gastrula between Early Streak (ES) and 12-somite pair (-s) stages (~6.75-9.0 days post coitum, dpc) in histological sections. STELLA was found in both the nucleus and cytoplasm in a variety of cell types, both within and outside of the putative PGC trajectory. Fate-mapping the headfold-stage (~7.75-8.0 dpc) posterior region, by which time PGCs are thought to be segregated into a distinct lineage, revealed that the STELLA-positive proximal ACD and intraembryonic posterior primitive streak (IPS) contributed to a wide range of somatic tissues that encompassed derivatives of the three primary germ layers. This contribution included STELLA-positive cells localizing to tissues both within and outside of the putative PGC trajectory. Thus, while STELLA may identify a subpopulation of cells destined for the PGC lineage, our findings reveal that it may be part of a broader niche that encompasses the ACD and through which the STELLA population may contribute cells to a wide variety of posterior tissues of the mouse gastrula.

The hypoblast (visceral endoderm): an evo-devo perspective

When amniotes appeared during evolution, embryos freed themselves from intracellular nutrition; development slowed, the mid-blastula transition was lost and maternal components became less important for polarity. Extra-embryonic tissues emerged to provide nutrition and other innovations. One such tissue, the hypoblast (visceral endoderm in mouse), acquired a role in fixing the body plan: it controls epiblast cell movements leading to primitive streak formation, generating bilateral symmetry. It also transiently induces expression of pre-neural markers in the epiblast, which also contributes to delay streak formation. After gastrulation, the hypoblast might protect prospective forebrain cells from caudalizing signals. These functions separate mesendodermal and neuroectodermal domains by protecting cells against being caught up in the movements of gastrulation.

Concerted involvement of Cdx/Hox genes and Wnt signaling in morphogenesis of the caudal neural tube and cloacal derivatives from the posterior growth zone

Decrease in Cdx dosage in an allelic series of mouse Cdx mutants leads to progressively more severe posterior vertebral defects. These defects are corrected by posterior gain of function of the Wnt effector Lef1. Precocious expression of Hox paralogous 13 genes also induces vertebral axis truncation by antagonizing Cdx function. We report here that the phenotypic similarity also applies to patterning of the caudal neural tube and uro-rectal tracts in Cdx and Wnt3a mutants, and in embryos precociously expressing Hox13 genes. Cdx2 inactivation after placentation leads to posterior defects, including incomplete uro-rectal septation. Compound mutants carrying one active Cdx2 allele in the Cdx4-null background (Cdx2/4), transgenic embryos precociously expressing Hox13 genes and a novel Wnt3a hypomorph mutant all manifest a comparable phenotype with similar uro-rectal defects. Phenotype and transcriptome analysis in early Cdx mutants, genetic rescue experiments and gene expression studies lead us to propose that Cdx transcription factors act via Wnt signaling during the laying down of uro-rectal mesoderm, and that they are operative in an early phase of these events, at the site of tissue progenitors in the posterior growth zone of the embryo. Cdx and Wnt mutations and premature Hox13 expression also cause similar neural dysmorphology, including ectopic neural structures that sometimes lead to neural tube splitting at caudal axial levels. These findings involve the Cdx genes, canonical Wnt signaling and the temporal control of posterior Hox gene expression in posterior morphogenesis in the different embryonic germ layers. They shed a new light on the etiology of the caudal dysplasia or caudal regression range of human congenital defects.

Tissue-specific roles of Axin2 in the inhibition and activation of Wnt signaling in the mouse embryo

Axin proteins are key negative regulators of the canonical Wnt signal transduction pathway. Although Axin2 null mice are viable, we identified an unusual ENU-induced recessive allele of Axin2, canp, that causes midgestation lethality in homozygotes. We show that the Axin2(canp) mutation is a V26D substitution in an invariant N-terminal sequence motif and that the Axin2(canp) protein is more stable than wild type. As predicted for an increased level of a negative regulator, the Axin2(canp) mutation leads to decreased Wnt signaling in most tissues, and this can account for most of the morphological phenotypes of Axin2(canp) mutants. In contrast, there is a paradoxical increase in canonical Wnt activity in the late primitive streak of all Axin2(canp) mutant embryos that is associated with the formation of an ectopic tail in some mutants. Treatment of wild-type embryos with an inhibitor of Tankyrase that stabilizes Axin proteins also causes inhibition of Wnt signaling in anterior regions of the embryo and a gain of Wnt signaling in the primitive streak. The results indicate that although increased stability of Axin2 leads to a loss of canonical Wnt signaling in most tissues, stabilized Axin2 enhances Wnt pathway activity in a specific progenitor population in the late primitive streak.

A clinical and experimental overview of sirenomelia: insight into the mechanisms of congenital limb malformations

Sirenomelia, also known as sirenomelia sequence, is a severe malformation of the lower body characterized by fusion of the legs and a variable combination of visceral abnormalities. The causes of this malformation remain unknown, although the discovery that it can have a genetic basis in mice represents an important step towards the understanding of its pathogenesis. Sirenomelia occurs in mice lacking Cyp26a1, an enzyme that degrades retinoic acid (RA), and in mice that develop with reduced bone morphogenetic protein (Bmp) signaling in the caudal embryonic region. The phenotypes of these mutant mice suggest that sirenomelia in humans is associated with an excess of RA signaling and a deficit in Bmp signaling in the caudal body. Clinical studies of sirenomelia have given rise to two main pathogenic hypotheses. The first hypothesis, based on the aberrant abdominal and umbilical vascular pattern of affected individuals, postulates a primary vascular defect that leaves the caudal part of the embryo hypoperfused. The second hypothesis, based on the overall malformation of the caudal body, postulates a primary defect in the generation of the mesoderm. This review gathers experimental and clinical information on sirenomelia together with the necessary background to understand how deviations from normal development of the caudal part of the embryo might lead to this multisystemic malformation.

Members of the TEAD family of transcription factors regulate the expression of Myf5 in ventral somitic compartments

The transcriptional regulation of the Mrf4/Myf5 locus depends on a multitude of enhancers that, in equilibria with transcription balancing sequences and the promoters, regulate the expression of the two genes throughout embryonic development and in the adult. Transcription in a particular set of muscle progenitors can be driven by the combined outputs of several enhancers that are not able to recapitulate the entire expression pattern in isolation, or by the action of a single enhancer the activity of which in isolation is equivalent to that within the context of the locus. We identified a new enhancer element of this second class, ECR111, which is highly conserved in all vertebrate species and is necessary and sufficient to drive Myf5 expression in ventro-caudal and ventro-rostral somitic compartments in the mouse embryo. EMSA analyses and data obtained from binding-site mutations in transgenic embryos show that a binding site for a TEA Domain (TEAD) transcription factor is essential for the function of this new enhancer, while ChIP assays show that at least two members of the family of transcription factors bind to it in vivo.

Regional differences in the expression of laminin isoforms during mouse neural tube development

Many significant human birth defects originate around the time of neural tube closure or early during post-closure nervous system development. For example, failure of the neural tube to close generates anencephaly and spina bifida, faulty cell cycle progression is implicated in primary microcephaly, while defective migration of neuroblasts can lead to neuronal migration disorders such as lissencephaly. At the stage of neural tube closure, basement membranes are becoming organised around the neuroepithelium, and beneath the adjacent non-neural surface ectoderm. While there is circumstantial evidence to implicate basement membrane dynamics in neural tube and surface ectodermal development, we have an incomplete understanding of the molecular composition of basement membranes at this stage. In the present study, we examined the developing basement membranes of the mouse embryo at mid-gestation (embryonic day 9.5), with particular reference to laminin composition. We performed in situ hybridization to detect the mRNAs of all eleven individual laminin chains, and immunohistochemistry to identify which laminin chains are present in the basement membranes. From this information, we inferred the likely laminin variants and their tissues of origin: that is, whether a given basement membrane laminin is contributed by epithelium, mesenchyme, or both. Our findings reveal major differences in basement composition along the body axis, with the rostral neural tube (at mandibular arch and heart levels) exhibiting many distinct laminin variants, while the lumbar level where the neural tube is just closing shows a much simpler laminin profile. Moreover, there appears to be a marked difference in the extent to which the mesenchyme contributes laminin variants to the basement membrane, with potential contribution of several laminins rostrally, but no contribution caudally. This information paves the way towards a mechanistic analysis of basement membrane laminin function during early neural tube development in mammals.

Protogenin mediates cell adhesion for ingression and re-epithelialization of paraxial mesodermal cells

In the avian embryo, precursor cells of the paraxial mesoderm that reside in the epiblast ingress through the primitive streak and migrate bilaterally in an anterolateral direction. Herein, we report on the roles of Protogenin (PRTG), an immunoglobulin superfamily protein expressed on the surface of the ingressing and migrating cells that give rise to the paraxial mesoderm, in paraxial mesoderm development. An aggregation assay using L-cells showed that PRTG mediates homophilic cell adhesion. Overexpression of PRTG in the presumptive paraxial mesoderm delayed mesodermal cell migration due to augmented adhesiveness. In contrast, siRNA knockdown of PRTG impaired successive ingression of epiblast cells and disorganized the epithelial structure of the somites. These results suggest that PRTG mediates cell adhesion to regulate continuous ingression of cells giving rise to the paraxial mesodermal lineage, as well as tissue integrity.

Cdx mutant axial progenitor cells are rescued by grafting to a wild type environment

Cdx transcription factors are required for axial extension. Cdx genes are expressed in the posterior growth zone, a region that supplies new cells for axial elongation. Cdx2(+/-)Cdx4(-/-) (Cdx2/4) mutant embryos show abnormalities in axis elongation from E8.5, culminating in axial truncation at E10.5. These data raised the possibility that the long-term axial progenitors of Cdx mutants are intrinsically impaired in their ability to contribute to posterior growth. We investigated whether we could identify cell-autonomous defects of the axial progenitor cells by grafting mutant cells into a wild type growth zone environment. We compared the contribution of GFP labeled mutant and wild type progenitors grafted to unlabeled wild type recipients subsequently cultured over the period during which Cdx2/4 defects emerge. Descendants of grafted cells were scored for their contribution to differentiated tissues in the elongating axis and to the posterior growth zone. No difference between the contribution of descendants from wild type and mutant grafted progenitors was detected, indicating that rescue of the Cdx mutant progenitors by the wild type recipient growth zone is provided non-cell autonomously. Recently, we showed that premature axial termination of Cdx mutants can be partly rescued by stimulating canonical Wnt signaling in the posterior growth zone. Taken together with the data shown here, this suggests that Cdx genes function to maintain a signaling-dependent niche for the posterior axial progenitors.

Bmpr1a is required for proper migration of the AVE through regulation of Dkk1 expression in the pre-streak mouse embryo

Here, we report a novel mechanism regulating migration of the anterior visceral endoderm (AVE) by BMP signaling through BMPRIA. In Bmpr1a-deficient (Bmpr-null) embryos, the AVE does not migrate at all. In embryos with an epiblast-specific deletion of Bmpr1a (Bmpr1a(null/flox); Sox2Cre embryos), the AVE cells migrate randomly from the distal end of embryos, resulting in an expansion of the AVE. Dkk1, which is normally expressed in the anterior proximal visceral endoderm (PxVE), is downregulated in Bmpr-null embryos, whereas it is circumferentially expressed in Bmpr1a(null/flox); Sox2Cre embryos at E5.75-6.5. These results demonstrate an association of the position of Dkk1 expressing cells with direction of the migration of AVE. In Bmpr1a(null/flox); Sox2Cre embryos, a drastic decrease of WNT signaling is observed at E6.0. Addition of WNT3A to the culture of Bmpr1a(null/flox); Sox2Cre embryos at E5.5 restores expression patterns of Dkk1 and Cer1. These data indicate that BMP signaling in the epiblast induces Wnt3 and Wnt3a expression to maintain WNT signaling in the VE, resulting in downregulation of Dkk1 to establish the anterior expression domain. Thus, our results suggest that BMP signaling regulates the expression patterns of Dkk1 for anterior migration of the AVE.

Notch signalling regulates the contribution of progenitor cells from the chick Hensen's node to the floor plate and notochord

Hensen's node of the chick embryo contains multipotent self-renewing progenitor cells that can contribute to either the floor plate or the notochord. Floor plate cells are a population of epithelial cells that lie at the ventral midline of the developing neural tube, whereas the notochord is a rod of axial mesoderm that lies directly beneath the floor plate. These two tissues serve as a source of a potent signalling morphogen, sonic hedgehog (Shh), which patterns the dorsoventral axis of the neural tube. We show, through both gain- and loss-of-function approaches, that Notch signalling promotes the contribution of chick axial progenitor cells to the floor plate and inhibits contribution to the notochord. Thus, we propose that Notch regulates the allocation of appropriate numbers of progenitor cells from Hensen's node of the chick embryo to the notochord and the floor plate.

The chick somitogenesis oscillator is arrested before all paraxial mesoderm is segmented into somites

BACKGROUND

Somitogenesis is the earliest sign of segmentation in the developing vertebrate embryo. This process starts very early, soon after gastrulation has initiated and proceeds in an anterior-to-posterior direction during body axis elongation. It is widely accepted that somitogenesis is controlled by a molecular oscillator with the same periodicity as somite formation. This periodic mechanism is repeated a specific number of times until the embryo acquires a defined specie-specific final number of somites at the end of the process of axis elongation. This final number of somites varies widely between vertebrate species. How termination of the process of somitogenesis is determined is still unknown.

RESULTS

Here we show that during development there is an imbalance between the speed of somite formation and growth of the presomitic mesoderm (PSM)/tail bud. This decrease in the PSM size of the chick embryo is not due to an acceleration of the speed of somite formation because it remains constant until the last stages of somitogenesis, when it slows down. When the chick embryo reaches its final number of somites at stage HH 24-25 there is still some remaining unsegmented PSM in which expression of components of the somitogenesis oscillator is no longer dynamic. Finally, we identify a change in expression of retinoic acid regulating factors in the tail bud at late stages of somitogenesis, such that in the chick embryo there is a pronounced onset of Raldh2 expression while in the mouse embryo the expression of the RA inhibitor Cyp26A1 is downregulated.

CONCLUSIONS

Our results show that the chick somitogenesis oscillator is arrested before all paraxial mesoderm is segmented into somites. In addition, endogenous retinoic acid is probably also involved in the termination of the process of segmentation, and in tail growth in general.

Hox genes and segmentation of the hindbrain and axial skeleton

Segmentation is an important process that is frequently used during development to segregate groups of cells with distinct features. Segmental compartments provide a mechanism for generating and organizing regional properties along an embryonic axis and within tissues. In vertebrates the development of two major systems, the hindbrain and the paraxial mesoderm, displays overt signs of compartmentalization and depends on the process of segmentation for their functional organization. The hindbrain plays a key role in regulating head development, and it is a complex coordination center for motor activity, breathing rhythms, and many unconscious functions. The paraxial mesoderm generates somites, which give rise to the axial skeleton. The cellular processes of segmentation in these two systems depend on ordered patterns of Hox gene expression as a mechanism for generating a combinatorial code that specifies unique identities of the segments and their derivatives. In this review, we compare and contrast the signaling inputs and transcriptional mechanisms by which Hox gene regulatory networks are established during segmentation in these two different systems.

Differential axial requirements for lunatic fringe and Hes7 transcription during mouse somitogenesis

Vertebrate segmentation is regulated by the “segmentation clock”, which drives cyclic expression of several genes in the caudal presomitic mesoderm (PSM). One such gene is Lunatic fringe (Lfng), which encodes a modifier of Notch signalling, and which is also expressed in a stripe at the cranial end of the PSM, adjacent to the newly forming somite border. We have investigated the functional requirements for these modes of Lfng expression during somitogenesis by generating mice in which Lfng is expressed in the cranial stripe but strongly reduced in the caudal PSM, and find that requirements for Lfng activity alter during axial growth. Formation of cervical, thoracic and lumbar somites/vertebrae, but not sacral and adjacent tail somites/vertebrae, depends on caudal, cyclic Lfng expression. Indeed, the sacral region segments normally in the complete absence of Lfng and shows a reduced requirement for another oscillating gene, Hes7, indicating that the architecture of the clock alters as segmentation progresses. We present evidence that Lfng controls dorsal-ventral axis specification in the tail, and also suggest that Lfng controls the expression or activity of a long-range signal that regulates axial extension.

Cdx2 regulation of posterior development through non-Hox targets

The homeodomain transcription factors Cdx1, Cdx2 and Cdx4 play essential roles in anteroposterior vertebral patterning through regulation of Hox gene expression. Cdx2 is also expressed in the trophectoderm commencing at E3.5 and plays an essential role in implantation, thus precluding assessment of the cognate-null phenotype at later stages. Cdx2 homozygous null embryos generated by tetraploid aggregation exhibit an axial truncation indicative of a role for Cdx2 in elaborating the posterior embryo through unknown mechanisms. To better understand such roles, we developed a conditional Cdx2 floxed allele in mice and effected temporal inactivation at post-implantation stages using a tamoxifen-inducible Cre. This approach yielded embryos that were devoid of detectable Cdx2 protein and exhibited the axial truncation phenotype predicted from previous studies. This phenotype was associated with attenuated expression of genes encoding several key players in axial elongation, including Fgf8, T, Wnt3a and Cyp26a1, and we present data suggesting that T, Wnt3a and Cyp26a1 are direct Cdx2 targets. We propose a model wherein Cdx2 functions as an integrator of caudalizing information by coordinating axial elongation and somite patterning through Hox-independent and -dependent pathways, respectively.

Wnt signaling maintains the notochord fate for progenitor cells and supports the posterior extension of the notochord

The notochord develops from notochord progenitor cells (NPCs) and functions as a major signaling center to regulate trunk and tail development. NPCs are initially specified in the node by Wnt and Nodal signals at the gastrula stage. However, the underlying mechanism that maintains the NPCs throughout embryogenesis to contribute to the posterior extension of the notochord remains unclear. Here, we demonstrate that Wnt signaling in the NPCs is essential for posterior extension of the notochord. Genetic labeling revealed that the Noto-expressing cells in the ventral node contribute the NPCs that reside in the tail bud. Robust Wnt signaling in the NPCs was observed during posterior notochord extension. Genetic attenuation of the Wnt signal via notochord-specific beta-catenin gene ablation resulted in posterior truncation of the notochord. In the NPCs of such mutant embryos, the expression of notochord-specific genes was down-regulated, and an endodermal marker, E-cadherin, was observed. No significant alteration of cell proliferation or apoptosis of the NPCs was detected. Taken together, our data indicate that the NPCs are derived from Noto-positive node cells, and are not fully committed to a notochordal fate. Sustained Wnt signaling is required to maintain the NPCs' notochordal fate.

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.

New horizons at the caudal embryos: coordinated urogenital/reproductive organ formation by growth factor signaling

The cloaca/urogenital sinus and its adjacent region differentiate into the urogenital/reproductive organs. Caudal regression syndrome (CRS; including mermaid syndrome), a type of severe cloacal malformation displays hindlimb fusion and urogenital organ defects, thus suggesting that such defects are caused by several morphogenetic alterations during early development. The attenuation of bone morphogenetic protein (Bmp) signaling at the posterior primitive streak of embryos leads to the caudal dysmorphogenesis including the cloaca and fusion of both hindlimbs. Genetic tissue lineage studies indicate the presence of coordinated organogenesis. Hedgehog (HH)-responding cells derived from peri-cloacal mesenchyme (PCM) contribute to the urogenital/reproductive organs. These findings indicate the existence of developmental programs for the coordinated organogenesis of urogenital/reproductive tissues based on growth factor function and crosstalk.

Stem cells, signals and vertebrate body axis extension

The progressive generation of chick and mouse axial tissues - the spinal cord, skeleton and musculature of the body - has long been proposed to depend on the activity of multipotent stem cells. Here, we evaluate evidence for the existence and multipotency of axial stem cells. We show that although the data strongly support their existence, there is little definitive information about their multipotency or extent of contribution to the axis. We also review the location and molecular characteristics of these putative stem cells, along with their evolutionary conservation in vertebrates and the signalling mechanisms that regulate and arrest axis extension.

Hematopoiesis from Human Embryonic Stem Cells: Overcoming the Immune Barrier in Stem Cell Therapies

The multipotency and proliferative capacity of human embryonic stem cells (hESCs) make them a promising source of stem cells for transplant therapies and of vital importance given the shortage in organ donation. Recent studies suggest some immune privilege associated with hESC-derived tissues. However, the adaptability of the immune system makes it unlikely that fully differentiated tissues will permanently evade immune rejection. One promising solution is to induce a state of immune tolerance to a hESC line using tolerogenic hematopoietic cells derived from it. This could provide acceptance of other differentiated tissues from the same line. However, this approach will require efficient multilineage hematopoiesis from hESCs. This review proposes that more efficient differentiation of hESCs to the tolerogenic cell types required is most likely to occur through applying knowledge gained of the ontogeny of complex regulatory signals used by the embryo for definitive hematopoietic development in vivo. Stepwise formation of mesoderm, induction of definitive hematopoietic stem cells, and the application of factors key to their self-renewal may improve in vitro production both quantitatively and qualitatively.

The role of integrin binding sites in fibronectin matrix assembly in vivo

The extracellular matrix (ECM) glycoprotein fibronectin (FN) requires the help of cells to assemble into a functional fibrillar matrix, which then orchestrates the assembly of other ECM proteins and promotes cell adhesion, migration and signalling. Fibrillogenesis is initiated and governed by cell surface integrins that bind to specific sites in the FN molecule. Recent studies identified novel integrin binding sites in FN that can also participate in FN fibril formation and in morphogenetic events during development.

The migration of paraxial and lateral plate mesoderm cells emerging from the late primitive streak is controlled by different Wnt signals

BACKGROUND

Co-ordinated cell movement is a fundamental feature of developing embryos. Massive cell movements occur during vertebrate gastrulation and during the subsequent extension of the embryonic body axis. These are controlled by cell-cell signalling and a number of pathways have been implicated. Here we use long-term video microscopy in chicken embryos to visualize the migration routes and movement behaviour of mesoderm progenitor cells as they emerge from the primitive streak (PS) between HH stages 7 and 10.

RESULTS

We observed distinct cell movement behaviours along the length of the streak and determined that this is position dependent with cells responding to environmental cues. The behaviour of cells was altered by exposing embryos or primitive streak explants to cell pellets expressing Wnt3a and Wnt5a, without affecting cell fates, thus implicating these ligands in the regulation of cell movement behaviour. Interestingly younger embryos were not responsive, suggesting that Wnt3a and Wnt5a are specifically involved in the generation of posterior mesoderm, consistent with existing mouse and zebrafish mutants. To investigate which downstream components are involved mutant forms of dishevelled (dsh) and prickle1 (pk1) were electroporated into the primitive streak. These had differential effects on the behaviour of mesoderm progenitors emerging from anterior or posterior regions of the streak, suggesting that multiple Wnt pathways are involved in controlling cell migration during extension of the body axis in amniote embryos.

CONCLUSION

We suggest that the distinct behaviours of paraxial and lateral mesoderm precursors are regulated by the opposing actions of Wnt5a and Wnt3a as they leave the primitive streak in neurula stage embryos. Our data suggests that Wnt5a acts via prickle to cause migration of cells from the posterior streak. In the anterior streak, this is antagonised by Wnt3a to generate non-migratory medial mesoderm.

Regionalisation of the endoderm progenitors and morphogenesis of the gut portals of the mouse embryo

This fate-mapping study reveals that the progenitors of all major parts of the embryonic gut are already present in endoderm of the early-head-fold to early-somite stage (1-9 somites) mouse embryo. The anterior endoderm contributes primarily to the anterior intestinal portal of the early-organogenesis stage (16-19 somites) embryo. Endoderm cells around and lateral to the node are allocated to the open "midgut" region of the embryonic gut. The posterior (post-nodal) endoderm contributes not only to the posterior intestinal portal but also the open "midgut". Descendants of the posterior endoderm span a length of the gut from the level of the 3rd-5th somites to the posterior end of the embryonic gut. The formation of the anterior and posterior intestinal portals is accompanied by similar repertoires of morphogenetic tissue movement. We also discovered that cells on contralateral sides of the anterior endoderm are distributed asymmetrically to the dorsal and ventral sides of the anterior intestinal portal, heralding the acquisition of laterality by the embryonic foregut.

Cessation of gastrulation is mediated by suppression of epithelial-mesenchymal transition at the ventral ectodermal ridge

In the gastrula stage embryo, the epiblast migrates toward the primitive streak and ingresses through the primitive groove. Subsequently, the ingressing epiblast cells undergo epithelial-mesenchymal transition (EMT) and differentiate into the definitive endoderm and mesoderm during gastrulation. However, the developmental mechanisms at the end of gastrulation have not yet been elucidated. Histological and genetic analyses of the ventral ectodermal ridge (VER), a derivative of the primitive streak, were performed using chick and mouse embryos. The analyses showed a continued cell movement resembling gastrulation associated with EMT during the early tailbud stage of both embryos. Such gastrulation-like cell movement was gradually attenuated by the absence of EMT during tail development. The kinetics of the expression pattern of noggin (Nog) and basal membrane degradation adjacent to the chick and the mouse VER indicated a correlation between the temporal and/or spatial expression of Nog and the presence of EMT in the VER. Furthermore, Nog overexpression suppressed EMT and arrested ingressive cell movement in the chick VER. Mice mutant in noggin displayed dysregulation of EMT with continued ingressive cell movement. These indicate that the inhibition of Bmp signaling by temporal and/or spatial Nog expression suppresses EMT and leads to the cessation of the ingressive cell movement from the VER at the end of gastrulation.

The DVE changes distal epiblast fate from definitive endoderm to neurectoderm by antagonizing nodal signaling

To assess the function of the distal visceral endoderm (DVE) of embryonic day 5.5 (E5.5) embryos, we established a system to directly ablate the DVE and observe the consequences after culture. When the DVE was successfully ablated, such embryos (DVE-ablated embryos) showed deregulated expression of Nodal and Wnt3 and ectopically formed the primitive streak at the proximal portion of the embryo. The DVE and anterior visceral endoderm (AVE) are implicated in the development of neurectoderm. We found that the distal epiblast of E5.5 embryo rotates anteriorly by the beginning of gastrulation. These cells remained to be anteriorly located during gastrulation and contributed to the ectoderm in the anterior side of the embryo. This indicates that the distal epiblast of E5.5 embryo becomes neurectoderm in normal development. In DVE-ablated embryos, the distal epiblast did not show any movement during culture and was abnormally fated to early definitive endoderm lineage. The data suggest that down-regulation of Nodal signaling in the distal epiblast of E5.5 embryo may be an initial step of neural development.

Two distinct sources for a population of maturing axial progenitors

In mammals, the primitive streak region and its descendant, the tail bud,are the source of nascent mesoderm and spinal cord throughout axial elongation. A localised population of long-term axial progenitors has been identified in a region of the tail bud, the chordoneural hinge, but the localisation of such progenitors at earlier stages is so far untested. By studying gene expression, we have shown that a specific topological arrangement of domains persists from the streak to the tail bud, and includes an area (the node-streak border) in which ectoderm that expresses primitive streak markers overlies the prospective notochord. This arrangement persists in the chordoneural hinge. Homotopic grafts show that, as in other vertebrates, cells in the streak and node predominantly produce mesoderm,whereas those in the node-streak border and lateral to the streak additionally produce neurectoderm. Node-streak border descendants populate not only neurectoderm, somites and notochord throughout the axis, but also the chordoneural hinge. Ectoderm lateral to the embryonic day (E)8.5 streak is later recruited to the midline, where it produces somites and chordoneural hinge cells, the position of which overlaps that of border-derived cells. Therefore, the E8.5 axial progenitors that will make the tail comprise cells from two distinct sources: the border and lateral ectoderm. Furthermore,heterotopic grafts of cells from outside the border to this region also populate the chordoneural hinge. Expression of several streak- and tail bud-specific genes declines well before elongation ends, even though this late population can be successfully transplanted into earlier embryos. Therefore,at least some aspects of progenitor status are conferred by the environment and are not an intrinsic property of the cells.

The RGD motif in fibronectin is essential for development but dispensable for fibril assembly

Fibronectin (FN) is secreted as a disulfide-bonded FN dimer. Each subunit contains three types of repeating modules: FN-I, FN-II, and FN-III. The interactions of α5β1 or αv integrins with the RGD motif of FN-III repeat 10 (FN-III₁₀) are considered an essential step in the assembly of FN fibrils. To test this hypothesis in vivo, we replaced the RGD motif with the inactive RGE in mice. FN-RGE homozygous embryos die at embryonic day 10 with shortened posterior trunk, absent tail bud–derived somites, and severe vascular defects resembling the phenotype of α5 integrin–deficient mice. Surprisingly, the absence of a functional RGD motif in FN did not compromise assembly of an FN matrix in mutant embryos or on mutant cells. Matrix assembly assays and solid-phase binding assays reveal that αvβ3 integrin assembles FN-RGE by binding an isoDGR motif in FN-I₅, which is generated by the nonenzymatic rearrangement of asparagines (N) into an iso-aspartate (iso-D). Our findings demonstrate that FN contains a novel motif for integrin binding and fibril formation whose activity is controlled by amino acid modification.

Tailbud-derived mesenchyme promotes urinary tract segmentation via BMP4 signaling

Urinary tract morphogenesis requires the sub-division of the ureteric bud (UB) into the intra-renal collecting system and ureter, two tissues with unique structural and functional properties. In this report we investigate the cellular and molecular mechanisms that mediate their differentiation. Fate mapping experiments in the developing chick indicate that the UB is surrounded by two distinct mesenchymal populations: nephrogenic mesenchyme derived from the intermediate mesoderm and tailbud-derived mesoderm, which is selectively associated with the domain of the UB that differentiates into the ureter. Functional experiments utilizing murine metanephric kidney explants show that BMP4, a paracrine factor secreted by tailbud-derived mesenchyme, is required for ureter morphogenesis. Conversely, ectopic BMP4 signaling is sufficient to induce ureter morphogenesis in domains of the UB normally fated to differentiate into the intra-renal collecting system. Collectively, these results indicate that the border between the kidney and ureter forms where mesenchymal tissues originating in two different areas of the early embryo meet. These data raise the possibility that the susceptibility of this junction to congenital defects in humans, such as ureteral-pelvic obstructions, may be related to the complex morphogenetic movements that are required to integrate cells from these different lineages into a single functional structure.

BMP signaling in the epiblast is required for proper recruitment of the prospective paraxial mesoderm and development of the somites

Bmpr1a encodes the BMP type IA receptor for bone morphogenetic proteins (BMPs), including 2 and 4. Here, we use mosaic inactivation of Bmpr1a in the epiblast of the mouse embryo (Bmpr-MORE embryos) to assess functions of this gene in mesoderm development. Unlike Bmpr1a-null embryos, which fail to gastrulate, Bmpr-MORE embryos initiate gastrulation, but the recruitment of prospective paraxial mesoderm cells to the primitive streak is delayed. This delay causes a more proximal distribution of cells with paraxial mesoderm character within the primitive streak, resulting in a lateral expansion of somitic mesoderm to form multiple columns. Inhibition of FGF signaling restores the normal timing of recruitment of prospective paraxial mesoderm and partially rescues the development of somites. This suggests that BMP and FGF signaling function antagonistically during paraxial mesoderm development.

Notch signaling regulates midline cell specification and proliferation in zebrafish

Notochord and floor plate cells are sources of molecules that pattern tissues near the midline, including the spinal cord. Hypochord cells are also found at the midline of anamniote embryos and are important for aorta development. Delta-Notch signaling regulates midline patterning in the dorsal organizer by inhibiting notochord formation and promoting hypochord and possibly floor plate development, but the precise mechanisms by which this regulation occurs are unknown. We demonstrate here that floor plate and hypochord cells arise from distinct regions of the zebrafish shield. Blocking Notch signaling during gastrulation entirely prevented hypochord specification but only reduced the number of floor plate cells that developed compared to control embryos. In contrast, elevation of Notch signaling at the beginning of gastrulation caused expansion of hypochord at the expense of notochord, but floor plate was not affected. A cell proliferation assay revealed that Notch signaling maintains dividing floor plate progenitors. Together, our results indicate that Notch signaling regulates allocation of appropriate numbers of different midline cells by different mechanisms.

Early steps in neural development

We studied early neurulation events in vitro by transplanting quail Hensen's node, central prenodal regions (before the nodus as such develops), or upper layer parts of it on the not yet definitively committed upper layer of chicken anti-sickle regions (of unincubated blastoderms), eventually associated with central blastoderm fragments. We could demonstrate by this quail-chicken chimera technique that after the appearance of a pronounced thickening of the chicken upper layer by the early inductive effect of neighboring endophyll, a floor plate forms by insertion of Hensen's node-derived quail cells into the median part of the groove. This favors, at an early stage, the floor plate "allocation" model that postulates a common origin for notochord and median floor plate cells from the vertebrate's secondary major organizer (Hensen's node in this case). A comparison is made with results obtained after transplantation of similar Hensen's nodes in isolated chicken endophyll walls or with previously obtained results after the use of the grafting procedure in the endophyll walls of whole chicken blastoderms.

Temporal/spatial expression and efflux activity of ABC transporter, P-glycoprotein/Abcb1 isoforms and Bcrp/Abcg2 during early murine development

ABC transporters pump out from cells a large number of endo- and xenobiotics including signal molecules and toxins; they are molecular markers of stem/progenitor cells as well. Here, we present the study of temporal/spatial patterns of Abcb1 isoforms and Abcg2 transporter expression and efflux activity in pre- and early postimplantation murine embryos. We found in 2-cell embryos abcb1a, abcb1b and abcg2 mRNAs which were believed to be maternally inherited. The expression of abcb1b and abcg2 genes was found in blastocysts and in 7 days postcoitum (dpc) embryos, while in 9dpc embryos beside of abcb1b/abcg2, the abcb1a gene was expressed. The abcb2 mRNA was detectable neither in pre- nor in postimplantation embryos. Moreover, we analysed temporal/spatial patterns of rhodamine 123/Hoechst 33342 efflux, which mirrors the ABC transporter phenotype, from individual cells of pre- and postimplantation murine embryos. The blastomeres of 2-, 4- and 8-cell embryos had efflux-inactive phenotype. Single, efflux-active cells emerged first in the morulae and their number increased in blastocyst inner cell mass. In 6 and 7 dpc embryos, all embryonic cells hold the efflux-active phenotype. Proximal embryonic endoderm of 6-8 dpc embryos contained two sub-domains: one consisted of efflux-active cells and another one of efflux-inactive cells reflecting polarity of an embryo. Between 7 and 8 dpc, at the onset of organogenesis, the vehement surge of efflux-inactive embryonic cells occurred, and their number increased in 9 dpc embryos, which consequently contained few efflux-active cells.

Cell death in developing human spinal cord

Cell death in the developing human spinal cord was investigated in 5-12 week human conceptuses using immunohistochemical and TUNEL methods. Expression of pro-apoptotic (Fas-receptor, caspase-3) and anti-apoptotic (bcl-2) markers and marker for internucleosomal fragmentation (TUNEL) were analysed in the cranial and caudal parts of the human spinal cord. In early developmental stages (5-6 weeks) of the cranial spinal cord, bcl-2 positive cells were seen in the ventricular zone and in the roof plate, while in the caudal part they were seen surrounding the central lumen. Subsequently, bcl-2 expression appeared in the basal plates of the grey matter and in the spinal ganglia, and from the seventh week on they also appeared in the intermediate horn of the grey matter. In the fetal period, bcl-2 expression appeared in the dorsal horns of the grey matter (9 weeks) but ceased in the ventricular zone (12 weeks) . In the trunk region, TUNEL-positive cells were found in ventricular and mantle zones along the whole length of the spinal cord. Caspase-3 positive cells and Fas-receptor positive cells appeared only in the grey matter of the cranial segments (head and trunk) of the spinal cord, but they were missing in the caudal parts. Caspase-3 dependant pathway, probably activated by Fas-receptor, seems to operate only in the cranial part of the human spinal cord. In the caudal (sacrococcygeal and tail) parts, cells seem to die by caspase-3 independent pathway. The interplay of pro-apoptotic and anti-apoptotic factors may be associated with cranial spinal cord morphogenesis, adjustment of cells number and selective survival of neurons, while in the caudal regions these factors cause massive cell death associated with regression of the caudal spinal cord.

her9 promotes floor plate development in zebrafish

Notochord, floor plate, and in anamniotes hypochord, are vertebrate embryonic midline structures that are the sources of molecules that pattern the nervous system, somites, and dorsal aorta. Midline precursor cells arise from the dorsal organizer during gastrulation, and Notch signaling is an important regulator of midline cell fate specification. To understand fully how Notch signaling regulates midline development, we investigated the role of potential Notch target genes. We show here that midline precursors express her9, a member of the hairy/Enhancer of split gene family. Although her9 inhibits notochord development and promotes floor plate specification, her9 expression in floor plate cells appears not to require Notch signaling. We show that, instead, her9 is a downstream effector of Nodal signaling for floor plate specification.

The Notch-target gene hairy2a impedes the involution of notochordal cells by promoting floor plate fates in Xenopus embryos

We have previously shown that the early Xenopus organiser contains cells equally potent to give rise to notochord or floor plate, and that Notch signalling triggers a binary decision, favouring the floor plate fate at the expense of the notochord. Now, we present evidence that Delta1 is the ligand that triggers the binary switch, which is executed through the Notch-mediated activation of hairy2a in the surrounding cells within the organiser, impeding their involution through the blastopore and promoting their incorporation into the hairy2a+ notoplate precursors (future floor-plate cells) in the dorsal non-involuting marginal zone.

Morphological Features of Tail Bud Development in Truncate Mouse Mutants

A key malformation in the homozygous truncate mouse mutants is a partial lack of the notochord in the embryo tail. In order to analyze if tail bud development was affected by the truncate (tc) mutation, serial semithin sections of tails of the homozygous mutant embryos were compared to the wild-type controls. In the wild-type embryos morphologically uniform mesenchyme of the tail bud was continuous via the medullary cord to the secondary neural tube, and via the tail cord to the notochord and the gut. In truncate embryos the tail cord was not continuous to the notochord, but to the additional lumen of the tail gut resulting in tail gut duplication. Toward the base of the tail two tail guts subsequently fused together or the additional one disappeared. If present in the tip of the tail, the notochord in truncate embryos ended near the ventral border of the secondary neural tube. The rest of the tail notochord was fragmented and the posterior ends of the fragments were frequently adjacent or even continuous to the neural tube. We suggest that the improper regionalization of the tail bud, where notochord is associated with the neural tube rather than with the tail gut, is related to the subsequent segmental lack of the notochord in truncate mutants.

Hedgehog signaling is required for commitment but not initial induction of slow muscle precursors

In zebrafish, skeletal muscle precursors can adopt at least three distinct fates: fast, non-pioneer slow, or pioneer slow muscle fibers. Slow muscle fibers develop from adaxial cells and depend on Hedgehog signaling. We analyzed when precursors become committed to their fates and the step(s) along their differentiation pathway affected by Hedgehog. Unexpectedly, we find that embryos deficient in Hedgehog signaling still contain postmitotic adaxial cells that differentiate into fast muscle fibers instead of slow. We show that by the onset of gastrulation, slow and fast muscle precursors are already spatially segregated but uncommitted to their fates until much later, in the segmental plate when slow precursors become independent of Hedgehog. In contrast, pioneer and non-pioneer slow muscle precursors share a common lineage from the onset of gastrulation. Our results demonstrate that slow muscle precursors form independently of Hedgehog signaling and further provide direct evidence for a multipotent muscle precursor population whose commitment to the slow fate depends on Hedgehog at a late stage of development when postmitotic adaxial cells differentiate into slow muscle fibers.

The forkhead genes, Foxc1 and Foxc2, regulate paraxial versus intermediate mesoderm cell fate

During vertebrate embryogenesis, the newly formed mesoderm is allocated to the paraxial, intermediate, and lateral domains, each giving rise to different cell and tissue types. Here, we provide evidence that the forkhead genes, Foxc1 and Foxc2, play a role in the specification of mesoderm to paraxial versus intermediate fates. Mouse embryos lacking both Foxc1 and Foxc2 show expansion of intermediate mesoderm markers into the paraxial domain, lateralization of somite patterning, and ectopic and disorganized mesonephric tubules. In gain of function studies in the chick embryo, Foxc1 and Foxc2 negatively regulate intermediate mesoderm formation. By contrast, their misexpression in the prospective intermediate mesoderm appears to drive cells to acquire paraxial fate, as revealed by expression of the somite markers Pax7 and Paraxis. Taken together, the data indicate that Foxc1 and Foxc2 regulate the establishment of paraxial versus intermediate mesoderm cell fates in the vertebrate embryo.