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PubMed1993

Overlapping expression of Xwnt-3A and Xwnt-1 in neural tissue of Xenopus laevis embryos.

Xwnt-3A is a member of the Xenopus-Wnt gene family, a class of secreted, cysteine-rich proteins implicated in intercellular signaling during early development. Here we describe the full-length coding sequence of Xwnt-3A, as well as the spatial expression pattern of this Xwnt gene as determined by whole-mount in situ hybridization analysis. While Xwnt-3A shares considerable amino acid identity with both Wnt-3 (87%) and Wnt-3A (85%), its spatial expression pattern is most like that of Wnt-3A. Xwnt-3A, which is first detected at the neurula stage of development, is expressed exclusively along the dorsal midline of the developing brain and neural tube and along the dorsal surface of the otic vesicle. While the expression of Xwnt-1 extensively overlaps that of Xwnt-3A, Xwnt-1 is uniquely expressed along the midbrain/hindbrain boundary and is absent from the otic vesicle. The expression of Xwnt-3A in neural ectoderm is dependent upon neural induction as determined by experiments with recombined ectoderm and mesoderm tissue. These results suggest that Xwnt-3A may participate in patterning the central nervous system during early Xenopus development. Last, the ectopic expression of Xwnt-3A induces the formation of a secondary axis at the anterior end of the embryo.

Amino Acid Sequence↗
PubMed1997

Graded amounts of Xenopus dishevelled specify discrete anteroposterior cell fates in prospective ectoderm.

Signals emitted from the prospective dorsal marginal zone (the organizer) are thought to specify neuroectodermal cell fates along the anteroposterior (AP) axis, but the mechanisms underlying this signaling event remain to be elucidated. To assess the effect of Xenopus Dishevelled (Xdsh), a proposed component of the Wnt, Notch and Frizzled signal transduction pathways, on AP axis determination, it was supplied in varying doses to presumptive ectodermal cells. Two-fold increments in levels of microinjected Xdsh mRNA revealed a gradual shift in cell fates along the AP axis. Lower doses of Xdsh mRNA activated anterior neuroectodermal markers, XAG1 and Xotx2, whereas the higher doses induced more posterior neural tissue markers such as En2, Krox20 and HoxB9. At the highest dose of Xdsh mRNA, explants contained maximal amount of HoxB9 transcripts and developed notochord and somites. When compared with Xdsh, Xwnt8 mRNA also activated anterior neuroectodermal markers, but failed to elicit mesoderm formation. Analysis of explants overexpressing Xdsh at the gastrula stage revealed activation of several organizer-specific genes which have been implicated in determination of neural tissue (Xotx2, noggin, chordin and follistatin). Whereas Goosecoid, Xlim1 and Xwnt8 were not induced in these explants, another early marginal zone marker, Xbra, was activated at the highest level of Xdsh mRNA. These observations suggest that the effects of Xdsh on AP axis specification may be mediated by combinatorial action of several early patterning genes. Increasing levels of Xdsh mRNA activate posterior markers, whereas increasing amounts of the organizer stimulate the extent of anterior development (Stewart, R.M. and Gerhart, J.C. (1990) Development 109, 363-372). These findings argue against induction of the entire organizer by Xdsh in ectodermal cells and implicate signal transduction pathways involving Xdsh in AP axis determination. Thus, different levels of a single molecule, Xdsh, can specify distinct cell states along the AP axis.

Actins↗
PubMed1998

A role for SOX1 in neural determination.

In vertebrates, the delineation of the neural plate from a region of the primitive ectoderm is accompanied by the onset of specific gene expression which in turn promotes the formation of the nervous system. Here we show that SOX1, an HMG-box protein related to SRY, is one of the earliest transcription factors to be expressed in ectodermal cells committed to the neural fate: the onset of expression of SOX1 appears to coincide with the induction of neural ectoderm. We demonstrate a role for SOX1 in neural determination and differentiation using an inducible expression P19 cell system as an in vitro model of neurogenesis. Misexpression of SOX1 can substitute for the requirement of retinoic acid to impart neural fate to competent ectodermal P19 cells. Using a series of antigenic markers which identify early neural cell types in combination with BrdU labeling, we demonstrate a temporal and spatial correlation between the differentiation of cell types along the dorsoventral axis of the neural tube and the downregulation of SOX1 expression. SOX1, therefore, defines the dividing neural precursors of the embryonic central nervous system (CNS).

Animals↗
PubMed1989

The effect of valproic acid on 65Zn distribution in the pregnant rat.

The effect of valproic acid on the distribution of gavaged 65Zn in maternal and embryonic tissue of Sprague-Dawley rats was examined 24 h after gavaging of the drug on d 13 of pregnancy. Valproic acid treatment resulted in a significantly higher retention of 65Zn in maternal liver and lower amounts in uterus, placenta and embryos than in controls. Compared to controls, gel chromatography of maternal liver from valproic acid-treated dams showed higher 65Zn counts associated with a protein peak of molecular weight of 6,500, the approximate molecular weight of the Zn-binding protein metallothionein. These results support the idea that the teratogenicity of valproic acid is in part due to an induction of embryonic Zn deficiency secondary to a drug-induced sequestering of Zn into maternal liver that results in a decrease in maternal plasma Zn and subsequent reduction in embryonic Zn uptake.

Abnormalities, Drug-Induced↗
PubMed1993

The community effect, dorsalization and mesoderm induction.

The community effect is an interaction among muscle progenitor cells of amphibian gastrula, and is necessary for the initiation of muscle-specific gene expression. Dorsalization provides a signal that can convert ventral mesoderm cells to a muscle fate. Neither process involves mesoderm-inducing molecules. We suggest that the developmental significance of the community effect is to generate homogeneous but clearly demarcated groups of cells from progenitor cells arranged in a continuous gradient of developmental potential.

Animals↗
PubMed1995

Myogenic specification of somites is mediated by diffusible factors.

Specification of myogenesis in early chicken somites and segmental plate was studied using transfilter explant cultures to determine if myogenic specification by axial structures is mediated by cell-cell contact. Formation of muscle fibers that express myosin heavy chain was assessed in somites transfilter from neural tube and notochord. Either the neural tube or the notochord from early chick embryos (ED 2) induces myogenesis in unspecified somites (Hamburger and Hamilton (HH) stages 11-14) when these tissues are separated by a 0.2- or a 0.05-micron pore filter. The ventral neural tube is found to be a strong inducer of myogenesis, while the dorsal neural tube is found to have low inducing activity. The reduced myogenic inducing activity of the dorsal neural tube is associated with an activity that inhibits myogenic differentiation in specified somites, somites containing cells already committed to myogenesis. Only 6-8 hr of transfilter exposure to neural tube is required to initiate somite myogenesis that is sustained in the absence of neural tube. Other tissues such as specified somites, heart, or ectoderm from the same aged embryo do not induce myogenesis in unspecified somites. Removal of the prospective floor plate of the caudal neural plate at HH stage 8 or 9 does not eliminate the inducing activity of the neurectoderm on unspecified somites (HH stage 11-14). Recombination of somites with neural tube or spinal cord from progressively older embryos (ED 4-20) showed myogenesis inducing activity at all ages, though the activity waned as development proceeded. Paraxial mesoderm need not segment into somites to respond to the inducing activity of the neural tube. We conclude that induction of myogenesis in somites does not require cell-cell contact between either the neural tube or the notochord and therefore induction is mediated by diffusible factor(s); that the inducing activity is localized principally to the notochord and ventral half of the neural tube, but not necessarily to the floor plate; that the myogenic response is specific to neural tube and notochord; and that the dorsal neural tube weakly induces myogenesis and contains an activity that inhibits myogenic cells from differentiating.

Animals↗
PubMed2000

Coordinated expression of Hoxb genes and signaling molecules during development of the chick respiratory tract.

To elucidate the molecular mechanism for regulating the region-specific morphogenesis of the chicken respiratory tract, we analyzed the spatiotemporal expression patterns of the Hoxb genes, Bmp-2, Bmp-4, Wnt-5a, and Wnt-11 in the developing respiratory tract. We found region-specific expression of these genes in the mesenchymal layer of the respiratory tract. Before bronchial branching proceeds, Hoxb genes show nested expression patterns around the ventral-distal tip of the lung bud. As morphogenesis proceeds, these expression domains correspond to the morphological subdivisions of the chick respiratory tract. Hoxb-5 and Hoxb-6 expression domains demarcate the trachea, bronchial tree, and air sacs. Particularly the expression domains of Hoxb-6 to -9 correspond to the morphological subdivisions of the air sacs along the proximodistal axis. Bmp-4 and Bmp-2 are expressed throughout the entire pulmonary mesenchyme and its dorsal half, respectively. Wnt-5a and Wnt-11 are expressed in the tracheal mesenchyme. Interestingly, the expression domain of Bmp-2 is complementary to the Hoxb-6 domain. The respiratory mesenchyme influences the process of epithelial branching during morphogenesis. By tissue recombination experiments, we found that the dorsal and the ventral pulmonary mesenchyme, demarcated by Hoxb-6 expression, have different inductive capacities toward the tracheal epithelium. These observations suggest the possibility that Hoxb genes are involved in the system specifying regional differences in morphogenesis and cytodifferentiation of respiratory tract. In addition, it is possible that BMPs and WNTs mediate region-specific epithelial-mesenchymal interaction in this system.

Animals↗
PubMed2004

Mouse FGF15 is the ortholog of human and chick FGF19, but is not uniquely required for otic induction.

The inner ear develops from an ectodermal placode that is specified by inductive signals from the adjacent neurectoderm and underlying mesoderm. In chick, fibroblast growth factor (Fgf)-19 is expressed in mesoderm underlying the presumptive otic placode, and human FGF19 induces expression of otic markers in a tissue explant containing neural plate and surface ectoderm. We show here that mouse Fgf15 is the sequence homolog of chick and human Fgf19/FGF19. In addition, we show that FGF15, like FGF19, is sufficient to induce expression of otic markers in a chick explant assay, suggesting that these FGFs are orthologs. Mouse embryos lacking Fgf15, however, do not have otic abnormalities at E9.5-E10.5, suggesting that Fgf15 is not uniquely required for otic induction or early patterning of the otocyst. To compare FGF15 and FGF19 signaling components and assess where signals potentially redundant with FGF15 might function, we determined the expression patterns of Fgf15 and Fgf19. Unlike Fgf19, Fgf15 is not expressed in mesoderm underlying the presumptive otic placode, but is expressed in the adjacent neurectoderm. Fgfr4, which encodes the likely receptor for both FGF19 and FGF15, is expressed in the neurectoderm of both species, and is also expressed in the mesoderm only in chick. These results suggest the hypotheses that during otic induction, FGF19 signals in either an autocrine fashion to the mesoderm or a paracrine fashion to the neurectoderm, whereas FGF15 signals in an autocrine fashion to the neurectoderm. Thus, the FGFs that signal to the neurectoderm are the best potential candidates for redundancy with FGF15 during mouse otic development.

Animals↗
PubMed2000

Mesenchymal/epithelial induction mediates olfactory pathway formation.

In the olfactory pathway, as in the limbs, branchial arches, and heart, mesenchymal/epithelial induction, mediated by retinoic acid (RA), FGF8, sonic hedgehog (shh), and the BMPs, defines patterning, morphogenesis, and differentiation. Neuronal differentiation in the olfactory epithelium and directed growth of axons in the nascent olfactory nerve depend critically upon this inductive interaction. When RA, FGF8, shh, or BMP signaling is disrupted, distinct aspects of olfactory pathway patterning and differentiation are compromised. Thus, a cellular and molecular mechanism that facilitates musculoskeletal and vascular development elsewhere in the embryo has been adapted to guide the differentiation of the olfactory pathway in the developing forebrain.

Animals↗
PubMed1996

A truncated FGF receptor blocks neural induction by endogenous Xenopus inducers.

We have examined the role of fibroblast growth factor (FGF) signalling in neural induction. The approach takes advantage of the fact that both noggin and the dominant negative mutant activin receptor (delta1XAR1) directly induce neural tissues in the absence of dorsal mesoderm. A truncated FGF receptor (XFD) is co-expressed with noggin or delta1XAR1 in both whole embryos and isolated animal caps. We demonstrate that inhibition of FGF signalling prevents neural induction by both factors. Furthermore, neural induction by organizers (the dorsal lip of blastopore and Hensen's node) is also blocked by inhibiting FGF signalling in ectoderm. It has been proposed that the specification of anterior neuroectoderm, including the cement gland, occurs in a sequential manner as gastrulation proceeds. We show that the specification of the most anterior neuroectoderm by noggin may occur before gastrulation and does not require FGF signalling, since both the cement gland marker XCG-1 and the anterior neural marker Otx-2 are normally expressed in ectodermal explants co-injected with noggin and XFD RNAs, but the cement gland cells are poorly differentiated. In contrast, the expression of both genes induced by CSKA.noggin, which is expressed after the mid-blastula transition, is strongly inhibited by the presence of XFD. Therefore the noggin-mediated neural induction that takes place at gastrula stages is abolished in the absence of FGF signalling. Since inhibition of FGF signalling blocks the neuralizing effect of different neural inducers that function through independent mechanisms, we propose that FGF receptor-related-signalling is required for the response to inducing signals of ectodermal cells from gastrula.

Activin Receptors↗
PubMed1997

Coordinate actions of BMPs, Wnts, Shh and noggin mediate patterning of the dorsal somite.

Shortly after their formation, somites of vertebrate embryos differentiate along the dorsoventral axis into sclerotome, myotome and dermomyotome. The dermomyotome is then patterned along its mediolateral axis into medial, central and lateral compartments, which contain progenitors of epaxial muscle, dermis and hypaxial muscle, respectively. Here, we used Wnt-11 as a molecular marker for the medial compartment of dermomyotome (the 'medial lip') to demonstrate that BMP in the dorsal neural tube indirectly induces formation of the medial lip by up-regulating Wnt-1 and Wnt-3a (but not Wnt-4) expression in the neural tube. Noggin in the dorsal somite may inhibit the direct action of BMP on this tissue. Wnt-11 induction is antagonized by Sonic Hedgehog, secreted by the notochord and the floor plate. Together, our results show that the coordinated actions of the dorsal neural tube (via BMP and Wnts), the ventral neural tube/notochord (via Shh) and the somite itself (via noggin) mediates patterning of the dorsal compartment of the somite.

Animals↗
PubMed1998

The snail repressor establishes a muscle/notochord boundary in the Ciona embryo.

Previous studies have identified a minimal 434 bp enhancer from the promoter region of the Ciona Brachyury gene (Ci-Bra), which is sufficient to direct a notochord-specific pattern of gene expression. Here we present evidence that a Ciona homolog of snail (Ci-sna) encodes a repressor of the Ci-Bra enhancer in the tail muscles. DNA-binding assays identified four Ci-Sna-binding sites in the Ci-Bra enhancer, and mutations in these sites cause otherwise normal Ci-Bra/lacZ transgenes to be misexpressed in ectopic tissues, particularly the tail muscles. Selective misexpression of Ci-sna using a heterologous promoter results in the repression of Ci-Bra/lacZ transgenes in the notochord. Moreover, the conversion of the Ci-Sna repressor into an activator results in the ectopic induction of Ci-Bra/lacZ transgenes in the muscles, and also causes an intermixing of notochord and muscle cells during tail morphogenesis. These results suggest that Ci-Sna functions as a boundary repressor, which subdivides the mesoderm into separate notochord and tail muscle lineages.

Animals↗
PubMed1995

Transformation of cardiac endothelium into cushion mesenchyme is dependent on ES/130: temporal, spatial, and functional studies in the early chick embryo.

ES/130 is a novel 130-kDa protein that has been linked previously to the transformation of endocardial endothelium into cushion mesenchyme. In the present study we report the localization of protein and mRNA for ES/130 in stages 7-plus through 20 chick embryos and present functional data related to a potential mechanism for ES/130. The temporal and spatial regulation of ES/130 expression suggests that this epithelial-to-mesenchymal transformation is a result of homogenetic induction. Functional studies indicate that myocardially derived ES/130 elicits expression of this protein by target AV endothelial cells, which is linked to a signal transduction cascade. The localization of ES/130 to other sites of inductive interactions (e.g., limb bud ectoderm, gut, and notochord) implies that this protein may have a more widespread importance to embryogenesis beyond its involvement in cardiac cushion tissue formation.

Animals↗
PubMed1992

DVR-4 (bone morphogenetic protein-4) as a posterior-ventralizing factor in Xenopus mesoderm induction.

Establishment of mesodermal tissues in the amphibian body involves a series of inductive interactions probably elicited by a variety of peptide growth factors. Results reported here suggest that mesodermal patterning involves an array of signalling molecules including DVR-4, a TGF-beta-like molecule. We show that ectopic expression of DVR-4 causes embryos to develop with an overall posterior and/or ventral character, and that DVR-4 induces ventral types of mesoderm in animal cap explants. Moreover, DVR-4 overrides the dorsalizing effects of activin. DVR-4 is therefore the first molecule reported both to induce posteroventral mesoderm and to counteract dorsalizing signals such as activin. Possible interactions between these molecules resulting in establishment of the embryonic body plan are discussed.

Activins↗
PubMed1998

Determination of the zebrafish forebrain: induction and patterning.

We report an analysis of forebrain determination and patterning in the zebrafish Danio rerio. In order to study these events, we isolated zebrafish homologs of two neural markers, odd-paired-like (opl), which encodes a zinc finger protein, and fkh5, which encodes a forkhead domain protein. At mid-gastrula, expression of these genes defines a very early pattern in the presumptive neurectoderm, with opl later expressed in the telencephalon, and fkh5 in the diencephalon and more posterior neurectoderm. Using in vitro explant assays, we show that forebrain induction has occurred even earlier, by the onset of gastrulation (shield stage). Signaling from the early gastrula shield, previously shown to be an organizing center, is sufficient for activation of opl expression in vitro. In order to determine whether the organizer is required for opl regulation, we removed from late blastula stage embryos either the presumptive prechordal plate, marked by goosecoid (gsc) expression, or the entire organizer, marked by chordin (chd) expression. opl was correctly expressed after removal of the presumptive prechordal plate and consistently, opl was correctly expressed in one-eyed pinhead (oep) mutant embryos, where the prechordal plate fails to form. However, after removal of the entire organizer, no opl expression was observed, indicating that this region is crucial for forebrain induction. We further show that continued organizer function is required for forebrain induction, since beads of BMP4, which promotes ventral fates, also prevented opl expression when implanted during gastrulation. Our data show that forebrain specification begins early during gastrulation, and that a wide area of dorsal mesendoderm is required for its patterning.

Amino Acid Sequence↗
PubMed1980

Tooth induction in chick epithelium: expression of quiescent genes for enamel synthesis.

Intraocular grafts of chick epithelium combined with mouse molar mesenchyme produced a variety of dental structures including perfectly formed crowns with differentiated ameloblasts depositing enamel matrix. The results suggest that the loss of teeth in Aves did not result from a loss of genetic coding for enamel synthesis in the oral epithelium but from an alteration in the tissue interactions requisite for odontogenesis.

Amelogenesis↗
PubMed1998

FGF and Shh signals control dopaminergic and serotonergic cell fate in the anterior neural plate.

During development, distinct classes of neurons are specified in precise locations along the dorso-ventral and anterior-posterior axes of the neural tube. We provide evidence that intersections of Shh, which is expressed along the ventral neural tube, and FGF8, which is locally produced at the mid/hindbrain boundary and in the rostral forebrain, create induction sites for dopaminergic neurons in the midbrain and forebrain. The same intersection, when preceded by a third signal, FGF4, which is expressed in the primitive streak, defines an inductive center for hindbrain 5-HT neurons. These findings illustrate that cell patterning in the neural plate is a multistep process in which early inducers, which initially divide the neural plate into crude compartments, are replaced by multiple local organizing centers, which specify distinct neuronal cell types within these compartments.

Animals↗
PubMed1995

Neural induction and regionalisation by different subpopulations of cells in Hensen's node.

Cell lineage analysis has revealed that the amniote organizer, Hensen's node, is subdivided into distinct regions, each containing a characteristic subpopulation of cells with defined fates. Here, we address the question of whether the inducing and regionalising ability of Hensen's node is associated with a specific subpopulation. Quail explants from Hensen's node are grafted into an extraembryonic site in a host chick embryo allowing host- and donor-derived cells to be distinguished. Cell-type- and region-specific markers are used to assess the fates of the mesodermal and neural cells that develop. We find that neural inducing ability is localised in the epiblast layer and the mesendoderm (deep portion) of the medial sector of the node. The deep portion of the posterolateral part of the node does not have neural inducing ability. Neural induction also correlates with the presence of particular prospective cell types in our grafts: chordamesoderm (notochord/head process), definitive (gut) endoderm or neural tissue. However, only grafts that include the epiblast layer of the node induce neural tissue expressing a complete range of anteroposterior characteristics, although prospective prechordal plate cells may also play a role in specification of the forebrain.

Animals↗