Planar and vertical induction of anteroposterior pattern during the development of the amphibian central nervous system

Developmental Genes and Malformations in the Hypothalamus

The hypothalamus is a heterogeneous rostral forebrain region that regulates physiological processes essential for survival, energy metabolism, and reproduction, mainly mediated by the pituitary gland. In the updated prosomeric model, the hypothalamus represents the rostralmost forebrain, composed of two segmental regions (terminal and peduncular hypothalamus), which extend respectively into the non-evaginated preoptic telencephalon and the evaginated pallio-subpallial telencephalon. Complex genetic cascades of transcription factors and signaling molecules rule their development. Alterations of some of these molecular mechanisms acting during forebrain development are associated with more or less severe hypothalamic and pituitary dysfunctions, which may be associated with brain malformations such as holoprosencephaly or septo-optic dysplasia. Studies on transgenic mice with mutated genes encoding critical transcription factors implicated in hypothalamic-pituitary development are contributing to understanding the high clinical complexity of these pathologies. In this review article, we will analyze first the complex molecular genoarchitecture of the hypothalamus resulting from the activity of previous morphogenetic signaling centers and secondly some malformations related to alterations in genes implicated in the development of the hypothalamus.

Dual roles of Akirin2 protein during Xenopus neural development

To ensure correct spatial and temporal patterning, embryos must maintain pluripotent cell populations and control when cells undergo commitment. The newly identified nucleoprotein Akirin has been shown to modulate the innate immune response through epigenetic regulation and to play important roles in other physiological processes, but its role in neural development remains unknown. Here we show that Akirin2 is required for neural development in Xenopus and that knockdown of Akirin2 expands the expression of the neural progenitor marker Sox2 and inhibits expression of the differentiated neuronal marker N-tubulin. Akirin2 acts antagonistically to Geminin, thus regulating Sox2 expression, and maintains the neural precursor state by participating in the Brg1/Brm-associated factor (BAF) complex mediated by BAF53a. Additionally, Akirin2 also modulates N-tubulin expression by acting upstream of neuronal differentiation 1 (NeuroD) and in parallel with neurogenin-related 1 (Ngnr1) during terminal neuronal differentiation. Thus, our results reveal a novel model in which Akirin2 precisely coordinates and temporally controls Xenopus neural development.

Regulation of Hoxb4 induction after neurulation by somite signal and neural competence

BACKGROUND

While the body axis is largely patterned along the anterior-posterior (A-P) axis during gastrulation, the central nervous system (CNS) shows dynamic changes in the expression pattern of Hox genes during neurulation, suggesting that the CNS refines the A-P pattern continuously after neural tube formation. This study aims at clarifying the role of somites in up-regulating Hoxb4 expression to eventually establish its final pattern and how the neural tube develops a competence to respond to extrinsic signals.

RESULTS

We show that somites are required for the up-regulation of Hoxb4 in the neural tube at the level of somites 1 to 5, the anterior-most domain of expression. However, each somite immediately adjacent to the neural tube is not sufficient at each level; planar signaling is additionally required particularly at the anterior-most segments of the expression domain. We also show that the dorsal side of the neural tube has a greater susceptibility to expressing Hoxb4 than the ventral region, a feature associated with dorsalization of the neural tube by BMP signals. BMP4 is additionally able to up-regulate Hoxb4 ventrally, but the effect is restricted to the axial levels at which Hoxb4 is normally expressed, and only in the presence of retinoic acid (RA) or somites, suggesting a role for BMP in rendering the neural tube competent to express Hoxb4 in response to RA or somite signals.

CONCLUSION

In identifying the collaboration between somites and neural tube competence in the induction of Hoxb4, this study demonstrates interplay between A-P and dorsal-ventral (D-V) patterning systems, whereby a specific feature of D-V polarity may be a prerequisite for proper A-P patterning by Hox genes.

An obligatory caravanserai stop on the silk road to neural induction: Inhibition of BMP/GDF signaling

Work in Xenopus laevis produced the first molecular explanation for neural specification, the default model, where inactivation of the BMP pathway in ectodermal cells changes fates from epidermal to neural. This review covers the present status of our understanding of neural specification, with emphasis on Xenopus, but including relevant facts in other model systems. While recent experiments have increased the complexity of the molecular picture, they have also provided additional support for the default model and the central position of the BMP pathway. We conclude that synergy between accumulated knowledge and technical progress will maintain Xenopus at the forefront of research in neural development.

Convergent Wnt and FGF signaling at the gastrula stage induce the formation of the isthmic organizer

The development of the vertebrate brain depends on the formation of local organizing centres within the neural tube that express secreted signals that refine local neural progenitor identity. The isthmic organizer (IsO) forms at the isthmic constriction and is required for the growth and ordered development of mesencephalic and metencephalic structures. The formation of the IsO, which is characterized by the generation of a complex pattern of cells at the midbrain-hindbrain boundary, has been described in detail. However, when neural plate cells are initially instructed to form the IsO, the molecular nature of the inductive signals remain poorly defined. We now provide evidence that convergent Wnt and FGF signaling at the gastrula stage are required to generate the complex polarized pattern of cells characteristic of the IsO, and that Wnt and FGF signals in combination are sufficient to reconstruct, in naïve forebrain cells, an IsO-like structure that exhibits an organizing activity that mimics the endogenous IsO when transplanted into the diencephalon of chick embryos.

Neural and eye-specific defects associated with loss of the imitation switch (ISWI) chromatin remodeler in Xenopus laevis

Imitation Switch (ISWI) is a member of the SWI2/SNF2 superfamily of ATP-dependent chromatin remodelers, which regulate transcription and maintain chromatin structure by mobilizing nucleosomes using the energy of ATP. Four distinct ISWI complexes have been identified in Xenopus oocytes. The developmental role of Xenopus ISWI, however, has not previously been investigated in vivo. Here we report the tissue specificity, developmental expression, and requirement of ISWI for development of Xenopus embryos. Whole mount in situ hybridization shows ISWI localized in the lateral sides of the neural plate, brain, eye, and in later stages, the spinal cord. Injection of antisense ISWI RNA, morpholino oligonucleotides or dominant-negative ISWI mutant mRNA into fertilized eggs inhibits gastrulation and neural fold closure. Genes involved in neural patterning and development, such as BMP4 and Sonic hedgehog (Shh), are misregulated in the absence of functional ISWI, and ISWI binds to the BMP4 gene in vivo. Developmental and transcriptional defects caused by dominant-negative ISWI are rescued by co-injection of wild-type ISWI mRNA. Inhibition of ISWI function results in aberrant eye development and the formation of cataracts. These data suggest a critical role for ISWI chromatin remodeling complexes in neural development, including eye differentiation, in the Xenopus laevis embryo.

Conditional BMP inhibition in Xenopus reveals stage-specific roles for BMPs in neural and neural crest induction

Bone morphogenetic protein (BMP) inhibition has been proposed as the primary determinant of neural cell fate in the developing Xenopus ectoderm. The evidence supporting this hypothesis comes from experiments in explanted "animal cap" ectoderm and in intact embryos using BMP antagonists that are unregulated and active well before gastrulation. While informative, these experiments cannot answer questions regarding the timing of signals and the behavior of cells in the more complex environment of the embryo. To examine the effects of BMP antagonism at defined times in intact embryos, we have generated a novel, two-component system for conditional BMP inhibition. We find that while blocking BMP signals induces ectopic neural tissue both in animal caps and in vivo, in intact embryos, it can only do so prior to late blastula stage (stage 9), well before the onset of gastrulation. Later inhibition does not induce neural identity, but does induce ectopic neural crest, suggesting that BMP antagonists play temporally distinct roles in establishing neural and neural crest identity. By combining BMP inhibition with fibroblast growth factor (FGF) activation, the neural inductive response in whole embryos is greatly enhanced and is no longer limited to pre-gastrula ectoderm. Thus, BMP inhibition during gastrulation is insufficient for neural induction in intact embryos, arguing against a BMP gradient as the sole determinant of ectodermal cell fate in the frog.

A developmental theory of the superior sagittal sinus(es) in craniopagus twins

INTRODUCTION

Based on the study of the vascular anatomy of craniopagus twins, and particularly that of Ladan and Laleh, a theory is hypothesised regarding the embryological formation of the sagittal sinuses.

THEORY AND DISCUSSION

The superior sagittal sinus develops in contact with the calvarium bone within the inter-periosteal leptomeningeal space following a signal from the midline of the telencephalic vesicle derived from a single prosencephalic one. The position of the falx cerebri and most likely in the deepest part, the corpus callosum, points to the position of this midline.

The Meis3 protein and retinoid signaling interact to pattern the Xenopus hindbrain

In Xenopus embryos, proper hindbrain formation requires activities of both XMeis3 protein and retinoic acid (RA) signaling. In this study, we show that XMeis3 protein and RA signaling differentially interact to regulate hindbrain patterning. The knockdown of XMeis3 protein prevented RA-caudalizing activity from inducing hindbrain marker expression in both explants and embryos. In contrast, inhibition of RA signaling differentially modulated XMeis3 activity. Target genes that are jointly activated by either RA or XMeis3 activities could not be efficiently induced by XMeis3 when RA signaling was inhibited. However, transcription of an XMeis3 target gene that is not an RA target gene was hyper-induced in the absence of retinoid signaling. Target genes jointly induced by RA or XMeis3 protein were synergistically activated in the presence of both activities, while RA treatment inhibits the ability of XMeis3 to activate transcription of neural genes that are not RA targets. HoxD1, an RA direct-target gene was also identified as an XMeis3 direct-target gene. HoxD1 protein acts downstream of XMeis3 to induce hindbrain marker gene transcription. To pattern the hindbrain, RA requires functional XMeis3 protein activity. XMeis3 protein appears crucial for initial hindbrain induction, whereas RA signaling defines the spatial limits of hindbrain gene expression by modifying XMeis3 protein activity.

XenopusMeis3 protein forms a hindbrain-inducing center by activating FGF/MAP kinase and PCP pathways

Knockdown studies in Xenopus demonstrated that the XMeis3gene is required for proper hindbrain formation. An explant assay was developed to distinguish between autonomous and inductive activities of XMeis3 protein. Animal cap explants caudalized by XMeis3 were recombined with explants neuralized by the BMP dominant-negative receptor protein. XMeis3-expressing cells induced convergent extension cell elongations in juxtaposed neuralized explants. Elongated explants expressed hindbrain and primary neuron markers, and anterior neural marker expression was extinguished. Cell elongation was dependent on FGF/MAP-kinase and Wnt-PCP activities. XMeis3 activates FGF/MAP-kinase signaling, which then modulates the PCP pathway. In this manner, XMeis3 protein establishes a hindbrain-inducing center that determines anteroposterior patterning in the brain.

Wise, a context-dependent activator and inhibitor of Wnt signalling

We have isolated a novel secreted molecule, Wise, by a functional screen for activities that alter the anteroposterior character of neuralised Xenopus animal caps. Wise encodes a secreted protein capable of inducing posterior neural markers at a distance. Phenotypes arising from ectopic expression or depletion of Wise resemble those obtained when Wnt signalling is altered. In animal cap assays, posterior neural markers can be induced by Wnt family members, and induction of these markers by Wise requires components of the canonical Wnt pathway. This indicates that in this context Wise activates the Wnt signalling cascade by mimicking some of the effects of Wnt ligands. Activation of the pathway was further confirmed by nuclear accumulation of beta-catenin driven by Wise. By contrast, in an assay for secondary axis induction, extracellularly Wise antagonises the axis-inducing ability of Wnt8. Thus, Wise can activate or inhibit Wnt signalling in a context-dependent manner. The Wise protein physically interacts with the Wnt co-receptor, lipoprotein receptor-related protein 6 (LRP6), and is able to compete with Wnt8 for binding to LRP6. These activities of Wise provide a new mechanism for integrating inputs through the Wnt coreceptor complex to modulate the balance of Wnt signalling.

Analysis of Wnt8 for neural posteriorizing factor by identifying Frizzled 8c and Frizzled 9 as functional receptors for Wnt8

The dorsal ectoderm of vertebrate gastrula is first specified into anterior fate by an activation signal and posteriorized by a graded transforming signal, leading to the formation of forebrain, midbrain, hindbrain and spinal cord along the anteroposterior (A-P) axis. Transplanted non-axial mesoderm rather than axial mesoderm has an ability to transform prospective anterior neural tissue into more posterior fates in zebrafish. Wnt8 is a secreted factor that is expressed in non-axial mesoderm. To investigate whether Wnt8 is the neural posteriorizing factor that acts upon neuroectoderm, we first assigned Frizzled 8c and Frizzled 9 to be functional receptors for Wnt8. We then, transplanted non-axial mesoderm into the embryos in which Wnt8 signaling is cell-autonomously blocked by the dominant-negative form of Wnt8 receptors. Non-axial mesodermal transplants in embryos in which Wnt8 signaling is cell-autonomously blocked induced the posterior neural markers as efficiently as in wild-type embryos, suggesting that Wnt8 signaling is not required in neuroectoderm for posteriorization by non-axial mesoderm. Furthermore, Wnt8 signaling, detected by nuclear localization of beta-catenin, was not activated in the posterior neuroectoderm but confined in marginal non-axial mesoderm. Finally, ubiquitous over-expression of Wnt8 does not expand neural ectoderm of posterior character in the absence of mesoderm or Nodal-dependent co-factors. We thus conclude that other factors from non-axial mesoderm may be required for patterning neuroectoderm along the A-P axis.

An expression pattern screen for genes involved in the induction of the posterior nervous system of zebrafish

The posterior nervous system, including the hindbrain and the spinal cord, has been shown to be formed by the transformation of neural plate of anterior character by signals derived from non-axial mesoderm. Although secreted factors, such as fibroblast growth factors (FGFs), Wnts, retinoic acid (RA) and Nodal, have been proposed to be the posteriorizing factors, the mechanism how neural tissue of posterior character is induced and subsequently specified along the anteroposterior axis remains elusive. To identify intercellular signaling molecules responsible for posteriorization of the neural plate as well as to find molecules induced intracellularly by the posteriorizing signal in the caudal neural plate, we screened by in situ hybridization for genes specifically expressed in posterior tissues, including the posterior neural plate and non-axial mesoderm when posteriorization of the neural plate takes place. From a subtracted library differentiating anterior versus posterior neural plate, 420 cDNA clones were tested, out of which 76 cDNA fragments showed expression restricted to the posterior tissue. These clones turned out to represent 32 different genes, including one novel secreted factor and one transmembrane protein. Seven genes were induced by non-axial mesodermal implants and bFGF beads, suggesting that these are among the early-response genes of the posteriorizing signal. Thus, our approach employing cDNA subtraction and subsequent expression pattern screening allows us to clone candidate genes involved in a novel signaling pathway contributing to the formation of the posterior nervous system.

Regional specification of the head and trunk-tail organizers of a urodele (Cynops pyrrhogaster) embryo is patterned during gastrulation

The dorsal marginal zone (DMZ) of an amphibian early gastrula is thought to consist of at least two distinct domains: the future head and trunk-tail organizers. We studied the mechanism by which the organizing activities of the lower half of the DMZ (LDMZ) of the urodelean (Cynops pyrrhogaster) embryo are changed. The uninvoluted LDMZ induces the notochord and then organizes the trunk-tail structures, whereas after cultivation in vitro or suramin treatment, the same LDMZ loses the notochord-inducing ability and organizes the head structures. A cell-lineage experiment indicated that the change in the organizing activity of the LDMZ was reflected in the transformation of the inductive ability: from notochord-inducing to neural-inducing activity. Using RT-PCR, we showed that the LDMZ expressed gsc, lim-1, chordin, and noggin, but not the mesoderm marker bra. In the sandwich assay, the LDMZ induced bra expression in the animal cap ectoderm, but the inductive activity was inhibited by cultivation or suramin treatment. The present study indicates that the change in the organizing activity of the LDMZ from trunk-tail to head is coupled with the loss of notochord-inducing activity. Based on these results, we suggest that this change is essential for the specification of the head and trunk-tail organizers during gastrulation.

XMeis3 protein activity is required for proper hindbrain patterning in Xenopus laevis embryos

Meis-family homeobox proteins have been shown to regulate cell fate specification in vertebrate and invertebrate embryos. Ectopic expression of RNA encoding the Xenopus Meis3 (XMeis3) protein caused anterior neural truncations with a concomitant expansion of hindbrain and spinal cord markers in Xenopus embryos. In naïve animal cap explants, XMeis3 activated expression of posterior neural markers in the absence of pan-neural markers. Supporting its role as a neural caudalizer, XMeis3 is expressed in the hindbrain and spinal cord. We show that XMeis3 acts like a transcriptional activator, and its caudalizing effects can be mimicked by injecting RNA encoding a VP16-XMeis3 fusion protein. To address the role of endogenous XMeis3 protein in neural patterning, XMeis3 activity was antagonized by injecting RNA encoding an Engrailed-XMeis3 antimorph fusion protein or XMeis3 antisense morpholino oligonucleotides. In these embryos, anterior neural structures were expanded and posterior neural tissues from the midbrain-hindbrain junction through the hindbrain were perturbed. In neuralized animal cap explants, XMeis3-antimorph protein modified caudalization by basic fibroblast growth factor and Wnt3a. XMeis3-antimorph protein did not inhibit caudalization per se, but re-directed posterior neural marker expression to more anterior levels; it reduced expression of spinal cord and hindbrain markers, yet increased expression of the more rostral En2 marker. These results provide evidence that XMeis3 protein in the hindbrain is required to modify anterior neural-inducing activity, thus, enabling the transformation of these cells to posterior fates.

Embryonic regionalization of the neocortex

Understanding the development of the vertebrate brain and in particular that of the neocortex, where high brain functions reside, remains one of the most difficult and exciting tasks in biology. In this review, we discuss recent experimental evidence as well as different possibilities for the intrinsic regionalization of the embryonic dorsal telencephalon, which may be related to the formation of distinct functional areas in the adult neocortex.

Hoxd10 induction and regionalization in the developing lumbosacral spinal cord

We have used Hoxd10 expression as a primary marker of the lumbosacral region to examine the early programming of regional characteristics within the posterior spinal cord of the chick embryo. Hoxd10 is uniquely expressed at a high level in the lumbosacral cord, from the earliest stages of motor column formation through stages of motoneuron axon outgrowth. To define the time period when this gene pattern is determined, we assessed Hoxd10 expression after transposition of lumbosacral and thoracic segments at early neural tube stages. We present evidence that there is an early prepattern for Hoxd10 expression in the lumbosacral neural tube; a prepattern that is established at or before stages of neural tube closure. Cells within more posterior lumbosacral segments have a greater ability to develop high level Hoxd10 expression than the most anterior lumbosacral segments or thoracic segments. During subsequent neural tube stages, this prepattern is amplified and stabilized by environmental signals such that all lumbosacral segments acquire the ability to develop high levels of Hoxd10, independent of their axial environment. Results from experiments in which posterior neural segments and/or paraxial mesoderm segments were placed at different axial levels suggest that signals setting Hoxd10 expression form a decreasing posterior-to-anterior gradient. Our experiments do not, however, implicate adjacent paraxial mesoderm as the only source of graded signals. We suggest, instead, that signals from more posterior embryonic regions influence Hoxd10 expression after the early establishment of a regional prepattern. Concurrent analyses of patterns of LIM proteins and motor column organization after experimental surgeries suggest that the programming of these characteristics follows similar rules.

Otx genes in brain morphogenesis

Most of the gene candidates for the control of developmental programmes that underlie brain morphogenesis in vertebrates are the homologues of Drosophila genes coding for signalling molecules or transcription factors. Among these, the orthodenticle group includes the Drosophila orthodenticle (otd) and the vertebrate Otx1 and Otx2 genes, which are mostly involved in fundamental processes of anterior neural patterning. These genes encode transcription factors that recognise specific target sequences through the DNA binding properties of the homeodomain. In Drosophila, mutations of otd cause the loss of the anteriormost head neuromere where the gene is transcribed, suggesting that it may act as a segmentation "gap" gene. In mouse embryos, the expression patterns of Otx1 and Otx2 have shown a remarkable similarity with the Drosophila counterpart. This suggested that they could be part of a conserved control system operating in the brain and different from that coded by the HOX complexes controlling the hindbrain and spinal cord. To verify this hypothesis a series of mouse models have been generated in which the functions of the murine genes were: (i) fully inactivated, (ii) replaced with each others, (iii) replaced with the Drosophila otd gene. Otx1-/- mutants suffer from epilepsy and are affected by neurological, hormonal, and sense organ defects. Otx2-/- mice are embryonically lethal, they show gastrulation impairments and fail in specifying anterior neural plate. Analysis of the Otx1-/-; Otx2+/- double mutants has shown that a minimal threshold level of the proteins they encode is required for the correct positioning of the midbrain-hindbrain boundary (MHB). In vivo otd/Otx reciprocal gene replacement experiments have provided evidence of a general functional equivalence among otd, Otx1 and Otx2 in fly and mouse. Altogether these data highlight a crucial role for the Otx genes in specification, regionalization and terminal differentiation of rostral central nervous system (CNS) and lead to hypothesize that modification of their regulatory control may have influenced morphogenesis and evolution of the brain.

Neural patterning in the vertebrate embryo

The embryonic central nervous system (CNS) is patterned along its antero-posterior, dorsal-ventral, and left-right axes. Along the dorsal-ventral axis, cell fate determination occurs during and following neural tube closure and involves the action of two opposing signaling pathways: SHH ventrally from the notochord and BMP/GDF dorsally from the boundary of neural and nonneural ectoderm and later from the roof plate. In addition, Wnt and retinoic acid signaling have been shown to act in dorsal-ventral patterning; however, their roles are understood in less detail. Along the antero-posterior axis, signals divide the neural tube into four major divisions: forebrain, midbrain, hindbrain, and spinal cord, and these differences can be detected soon after the formation of the neural plate. The FGF, Wnt, and retinoic acid signaling pathways have been implicated in the caudalization of neural tissue. Boundaries of Hox gene expression are observed along the anteroposterior axis and have been suggested to be involved in establishing different identities in the hindbrain and spinal cord. Complex gene expression patterns in the brain suggest the development of neuromeres dividing the brain into different regions that are elaborated further during development. Patterning along the left-right axis occurs concurrently with antero-posterior and dorsal-ventral patterning during gastrulation. A leading candidate for initiating asymmetry is activin, which acts through Nodal and Lefty before any morphological differences are observed. The big challenge will be understanding how these diverse signaling pathways interact both temporally and spatially to generate the complex adult nervous system.

The midbrain--hindbrain boundary organizer

Cell fate in the cephalic neural primordium is controlled by an organizer located at the midbrain-hindbrain boundary. Studies in chick, mouse and zebrafish converge to show that mutually repressive interactions between homeodomain transcription factors of the Otx and Gbx class position this organizer in the neural primordium. Once positioned, independent signaling pathways converge in their activity to drive organizer function. Fibroblast growth factors secreted from the organizer are necessary for, and sufficient to mimic, organizer activity in patterning the midbrain and anterior hindbrain, and are tightly controlled by feedback inhibition.

Evolution of vertebrate forebrain development: how many different mechanisms?

Over the past 50 years and more, many models have been proposed to explain how the nervous system is initially induced and how it becomes subdivided into gross regions such as forebrain, midbrain, hindbrain and spinal cord. Among these models is the 2-signal model of Nieuwkoop & Nigtevecht (1954), who suggested that an initial signal ('activation') from the organiser both neuralises and specifies the forebrain, while later signals ('transformation') from the same region progressively caudalise portions of this initial territory. An opposing idea emerged from the work of Otto Mangold (1933) and other members of the Spemann laboratory: 2 or more distinct organisers, emitting different signals, were proposed to be responsible for inducing the head, trunk and tail regions. Since then, evidence has accumulated that supports one or the other model, but it has been very difficult to distinguish between them. Recently, a considerable body of work from mouse embryos has been interpreted as favouring the latter model, and as suggesting that a 'head organiser', required for the induction of the forebrain, is spatially separate from the classic organiser (Hensen's node). An extraembryonic tissue, the 'anterior visceral endoderm' (AVE), was proposed to be the source of forebrain-inducing signals. It is difficult to find tissues that are directly equivalent embryologically or functionally to the AVE in other vertebrates, which led some (e.g. Kessel, 1998) to propose that mammals have evolved a new way of patterning the head. We will present evidence from the chick embryo showing that the hypoblast is embryologically and functionally equivalent to the mouse AVE. Like the latter, the hypoblast also plays a role in head development. However, it does not act like a true organiser. It induces pre-neural and pre-forebrain markers, but only transiently. Further development of neural and forebrain phenotypes requires additional signals not provided by the hypoblast. In addition, the hypoblast plays a role in directing cell movements in the adjacent epiblast. These movements distance the future forebrain territory from the developing organiser (Hensen's node), and we suggest that this is a mechanism to protect the forebrain from caudalising signals from the node. These mechanisms are consistent with all the findings obtained from the mouse to date. We conclude that the mechanisms responsible for setting up the forebrain and more caudal regions of the nervous system are probably similar among different classes of higher vertebrates. Moreover, while reconciling the two main models, our findings provide stronger support for Nieuwkoop's ideas than for the concept of multiple organisers, each inducing a distinct region of the CNS.

Separation of neural induction and neurulation in Xenopus

Cellular interactions with laminin are important for numerous morphogenetic events. In Xenopus, the first of these is neurulation. The integrin alpha6 subunit mediates an attachment of the cells of the neural plate to the underlying basal lamina. A disruption of this interaction results in embryos that fail to neurulate (T. E. Lallier et al., 1996, Development 122, 2539-2554). Here we provide evidence supporting the specificity of this phenomenon and characterize developmental events as either disrupted or unaffected by a perturbation of alpha6 integrin expression. First, reduction of alpha6 integrin expression does not halt mitotic division throughout the embryo, indicating that the neural defects observed are not simply a global perturbation of all developmental processes. Second, a gene associated with dorsal mesoderm formation, brachyury, is expressed normally in alpha6 integrin-perturbed embryos. Third, the expression of BMP4, noggin, chordin, and follistatin, all of which are critical for neural induction, are at near normal levels. In addition, several genes expressed shortly after neural induction (N-CAM, nrp1, and Xanf1) are not perturbed in nonneurulating embryos. Interestingly, expression of one neural-specific gene (synaptobrevin), which is normally detectable late in neurulation, is abolished in these alpha6 integrin-perturbed embryos. Furthermore, the spatial expression of several transcripts is expanded in alpha6 integrin-perturbed embryos (orthodenticle and engrailed). Taken together, these data indicate that while alpha6 integrin-mediated interactions with laminin are required for neurulation, they are not required for the initial processes of neural induction. However, these cell-extracellular matrix interactions appear to be important in later inductive events and rostrocaudal patterning of the neural tube.

Expression pattern of an axolotl floor plate-specific fork head gene reflects early developmental differences between frogs and salamanders

Gastrulation is one of the most important stages of animal development and, as such, tends to be remarkably conserved. Therefore it is interesting to see that the two amphibian species, Xenopus laevis (frog) and Ambystoma mexicanum (axolotl), are different in the arrangement of cell types just before and during gastrulation. In Xenopus, the cells that will form dorsal mesoderm are located deep in the dorsal marginal zone, while in the axolotl, these are on the surface of the embryo. In this study we investigated whether homologous genes known to be involved in the formation of dorsal structures show a different pattern of expression in these two species. For this purpose, we isolated a fork head gene (AxFKH 1) from the axolotl, which is likely to be the homologue of the Xenopus fork head gene, XFKH 1 (Pintallavis, XFD-1). We find that AxFKH 1 and XFH 1 have a similar pattern of expression, but there are some important differences. In early gastrulae, transcripts are detected in the organizer region of both species. In late gastrulae, the transcripts in Xenopus are located in both the superficial and deep layers, but they are only found in the superficial layer of axolotl embryos. During neurulation, XFKH 1 is expressed in notochord and neural floor plate, whereas AxFKH 1 is expressed in the neural floor plate only. We propose that the differences in expression pattern of these two genes are due to a difference in formation of dorsal structures between these two species. Furthermore, the expression pattern of these two genes early in gastrulation is consistent with the idea that at least some of the neural floor plate cells are already determined at this time.

Genetic and molecular roles of Otx homeodomain proteins in head development

Insights into the molecular mechanisms underlying neural development in vertebrates come from the cloning and the functional analysis of genes which are involved in the molecular pathways leading to neural induction, tissue specification and regionalization of the brain. Among them, transcription factors belonging to the orthodenticle family (Otx1, Otx2) play an important role during early and later events required for proper brain development. To better understand their functions, several mouse mutants have been generated by homologous recombination. Their analysis clearly indicates that Otx1 is involved in corticogenesis, sense organ development and pituitary functions, while Otx2 is necessary earlier in development, for the correct anterior neural plate specification and organisation of the primitive streak. A molecular mechanism depending on a precise threshold of OTX proteins is necessary for the correct positioning of the isthmic region and for anterior brain patterning. Finally, vertebrate Otx genes share functional equivalence with the Drosophila homologue otd, indicating that the genetic mechanisms underlying pattern formation in insect and mammalian brain development are evolutionarily conserved.

The early expression control of Xepsin by nonaxial and planar posteriorizing signals in Xenopus epidermis

The control mechanism of the anteroposterior axis specification in Xenopus epidermis was studied by comparing the expression of a novel anterior marker, Xepsin, with that of a panepidermal marker, type I keratin. Xepsin mRNA, which encodes a novel Xenopus serine protease, is transcribed zygotically with the expression peak in neurula stages. In normal development, its expression is limited to the anterior and anterior-dorsal portions within epidermis during neurula and tailbud stages, respectively. In UV-irradiated ventralized embryos (dorsoanterior index, DAI 0 and 1), an expression boundary for Xepsin is apparently formed within the epidermis. In contrast, Xepsin expression was observed throughout the epidermis in LiCl-treated dorsalized embryos (DAI 10), as seen from an expression pattern indistinguishable from that of type I keratin. These data suggest that posteriorizing signals which suppress the transcription of Xepsin are present in nonaxial regions and absent in the anterior dorsal mesoderm. That posteriorizing signals were present in nonaxial regions was also supported by a conjugation experiment in which Xepsin expression was suppressed in ectodermal explants conjugated with lateral or ventral marginal zone. Moreover, the partly suppressed expression of Xepsin in the epidermal region of exogastrulae indicates that the signals may travel horizontally within the plane of the epidermis. We also present data showing that both treatment with retinoic acid and the overexpression of a constitutively active form of a retinoic acid receptor caused the suppression of Xepsin mRNA transcription, suggesting that anterior-posterior patterning in the central nervous system and in the epidermis may share common endogenous factors, i.e. , retinoids, in the Xenopus embryo.

A role for xGCNF in midbrain-hindbrain patterning in Xenopus laevis

Cells in the presumptive neural ectoderm of Xenopus are committed to neural fate through a process called neural induction, which may involve proteins that antagonize BMP signaling pathways. To identify genes that are induced by the BMP antagonists and that may be involved in subsequent neural patterning, we used a suppression PCR-based subtraction screen. Here we investigate the prospective activities and functions of one of the genes, a nuclear orphan receptor previously described as xGCNF. In animal cap assays, xGCNF synergizes with ectopic chordin to induce the midbrain-hindbrain marker engrailed-2 (En-2). In Keller explants, which rely on endogenous factors for neural induction, similar increases in En-2 are observed. Expression in embryos of a dominant interfering form of xGCNF reduces the expression of endogenous En-2 and Krox-20. These gain-of-function and prospective loss-of-function experiments, taken with the observation that xGCNF is expressed in the early neural plate and is elevated in the prospective midbrain-hindbrain region, which subsequently expresses En-2, suggest that xGCNF may play a role in regulating En-2 and thus midbrain-hindbrain identity.

Convergent inductive signals specify midbrain, hindbrain, and spinal cord identity in gastrula stage chick embryos

In the chick embryo, neural cells acquire midbrain, hindbrain, and spinal cord character over a approximately 6 hr period during gastrulation. The convergent actions of four signals appear to specify caudal neural character. Fibroblast growth factors (FGFs) and a paraxial mesoderm-caudalizing (PMC) activity are involved, but neither signal is sufficient to induce any single region. FGFs act indirectly by inducing mesoderm that expresses PMC and retinoid activity and also directly on prospective neural cells, in combination with PMC activity and a rostralizing signal, to induce midbrain character. Hindbrain character emerges from cells that possess the potential to acquire midbrain character upon exposure to higher levels of PMC activity. Induction of spinal cord character appears to involve PMC and retinoid activities.

Targeted disruption of Fgf8 causes failure of cell migration in the gastrulating mouse embryo

Fgf8 and Fgf4 encode FGF family members that are coexpressed in the primitive streak of the gastrulating mouse embryo. We have analyzed the phenotype of Fgf8(-/-) embryos and discovered that they fail to express Fgf4 in the streak. In the absence of both FGF8 and FGF4, epiblast cells move into the streak and undergo an epithelial-to-mesenchymal transition, but most cells then fail to move away from the streak. As a consequence, no embryonic mesoderm- or endoderm-derived tissues develop, although extraembryonic tissues form. Patterning of the prospective neuroectoderm is greatly perturbed in the mutant embryos. Anterior neuroectoderm markers are widely expressed, at least in part because the anterior visceral endoderm, which provides signals that regulate their expression, is not displaced proximally in the absence of definitive endoderm. Posterior neuroectoderm markers are not expressed, presumably because there is neither mesendoderm underlying the prospective neuroectoderm nor a morphologically normal node to provide the inductive signals necessary for their expression. This study identifies Fgf8 as a gene essential for gastrulation and shows that signaling via FGF8 and/or FGF4 is required for cell migration away from the primitive streak.

Otx genes and the genetic control of brain morphogenesis

Understanding the genetic mechanisms that control brain patterning in vertebrates represents a major challenge for developmental neurobiology. The cloning of genes likely to be involved in the organization of the brain and an analysis of their roles have revealed insights into the molecular pathways leading to neural induction, tissue specification, and regionalization of the brain. Among these genes, both Otx1 and Otx2, two murine homologs of the Drosophila orthodenticle (otd) gene, contribute to several steps in brain morphogenesis. Recent findings have demonstrated that Otx2 plays a major role in gastrulation and in the early specification of the anterior neural plate while Otx1 is mainly involved in corticogenesis, and Otx1 and Otx2 genes cooperate in such a way that a minimal level of OTX proteins are required for proper regionalization and subsequent patterning of the developing brain. Finally, experiments have shown functional equivalence between Drosophila otd and vertebrate Otx genes, suggesting a surprising conservation of function required in brain development throughout evolution.

A Meis family protein caudalizes neural cell fates in Xenopus

A homologue of the Drosophila homothorax (hth) gene, Xenopus Meis3 (XMeis3), was cloned from Xenopus laevis. XMeis3 is expressed in a single stripe of cells in the early neural plate stage. By late neurula, the gene is expressed predominantly in rhombomeres two, three and four, and in the anterior spinal cord. Ectopic expression of RNA encoding XMeis3 protein causes anterior neural truncations with a concomitant expansion of hindbrain and spinal cord. Ectopic XMeis3 expression inhibits anterior neural induction in neuralized animal cap ectoderm explants without perturbing induction of pan-neural markers. In naive animal cap ectoderm, ectopic XMeis3 expression activates transcription of the posteriorly expressed neural markers, but not pan-neural markers. These results suggest that caudalizing proteins, such as XMeis3, can alter A-P patterning in the nervous system in the absence of neural induction. Regionally expressed proteins like XMeis3 could be required to overcome anterior signals and to specify posterior cell fates along the A-P axis.

Visceral endoderm-restricted translation of Otx1 mediates recovery of Otx2 requirements for specification of anterior neural plate and normal gastrulation

Otx1 and Otx2, two murine homologs of the Drosophila orthodenticle (otd) gene, contribute to brain morphogenesis. In particular Otx1 null mice are viable and show spontaneous epileptic seizures and abnormalities affecting the dorsal telencephalic cortex. Otx2 null mice die early in development and fail in specification of the rostral neuroectoderm and proper gastrulation. In order to determine whether Otx1(−/−)and Otx2(−/−) highly divergent phenotypes reflect differences in temporal expression or biochemical activity of OTX1 and OTX2 proteins, the Otx2-coding sequence was replaced by a human Otx1 full-coding cDNA. Homozygous mutant embryos recovered anterior neural plate and proper gastrulation but failed to maintain forebrain-midbrain identities, displaying a headless phenotype from 9 days post coitum (d.p.c.) onwards. Unexpectedly, in spite of the RNA distribution in both visceral endoderm (VE) and epiblast, the hOTX1 protein was synthesized only in the VE. This VE-restricted translation was sufficient to recover Otx2 requirements for specification of the anterior neural plate and proper organization of the primitive streak, thus providing evidence that the difference between Otx1 and Otx2 null mice phenotypes originates from their divergent expression patterns. Moreover, our data lead us to hypothesize that the differential post-transcriptional control existing between VE and epiblast cells may potentially contribute to fundamental regulatory mechanisms required for head specification.

Otx1 and Otx2 in the development and evolution of the mammalian brain

In the last decade, a number of genes related to the induction, specification and regionalization of the brain were isolated and their functional properties currently are being dissected. Among these, Otx1 and Otx2 play a pivotal role in several processes of brain morphogenesis. Findings from several groups now confirm the importance of Otx2 in the early specification of neuroectoderm destined to become fore-midbrain, the existence of an Otx gene dosage-dependent mechanism in patterning the developing brain, and the involvement of Otx1 in corticogenesis. Some of these properties appear particularly fascinating when considered in evolutionary terms and highlight the central role of Otx genes in the establishment of the genetic program defining the complexity of a vertebrate brain. This review deals with the major aspects related to the roles played by Otx1 and Otx2 in the development and evolution of the mammalian brain.

Transducing positional information to the Hox genes: critical interaction of cdx gene products with position-sensitive regulatory elements

Studies of pattern formation in the vertebrate central nervous system indicate that anteroposterior positional information is generated in the embryo by signalling gradients of an as yet unknown nature. We searched for transcription factors that transduce this information to the Hox genes. Based on the assumption that the activity levels of such factors might vary with position along the anteroposterior axis, we devised an in vivo assay to detect responsiveness of cis-acting sequences to such differentially active factors. We used this assay to analyze a Hoxb8 regulatory element, and detected the most pronounced response in a short stretch of DNA containing a cluster of potential CDX binding sites. We show that differentially expressed DNA binding proteins are present in gastrulating embryos that bind to these sites in vitro, that cdx gene products are among these, and that binding site mutations that abolish binding of these proteins completely destroy the ability of the regulatory element to drive regionally restricted expression in the embryo. Finally, we show that ectopic expression of cdx gene products anteriorizes expression of reporter transgenes driven by this regulatory element, as well as that of the endogenous Hoxb8 gene, in a manner that is consistent with them being essential transducers of positional information. These data suggest that, in contrast to Drosophila Caudal, vertebrate cdx gene products transduce positional information directly to the Hox genes, acting through CDX binding sites in their enhancers. This may represent the ancestral mode of action of caudal homologues, which are involved in anteroposterior patterning in organisms with widely divergent body plans and modes of development.

Vax1 is a novel homeobox-containing gene expressed in the developing anterior ventral forebrain

The vertebrate forebrain is formed at the rostral end of the neural plate under the regulation of local and specific signals emanating from both the endomesoderm and neuroectoderm. The development of the rostral and ventral forebrain in particular was difficult to study, mainly because no specific markers are available to date. Here, we report the identification of Vax1, a novel homeobox-containing gene identified in mouse, Xenopus and human. It is closely related to members of the Not and Emx gene families, all of which are required for the formation of structures where they are expressed. In mouse and Xenopus, Vax1 expression first occurs in the rostral neural plate, in the medial anterior neural ridge and adjacent ectoderm. Later, at midgestation in the mouse and tadpole stage in Xenopus, the expression remains confined in the derivatives of this territory which differentiate into rostromedial olfactory placode, optic nerve and disc, and anterior ventral forebrain. Together, these observations suggest that Vax1 could have an early evolutionary origin and could participate in the specification and formation of the rostral and ventral forebrain in vertebrates. Comparison of the limits of the expression territory of Vax1 with that of Dlx1, Pax6 and Emx1 indicates that the corticostriatal ridge is a complex structure with distinct identifiable genetic compartments. Besides, the study of Vax1 expression in Pax6-deficient homozygous brains indicates that its regulation is independent of Pax6, although the expression patterns of these two genes appear complementary in wild-type animals. Vax1 chromosomal location is mapped at the distal end of the mouse chromosome 19, linked with that of Emx2. These two genes may have arisen by tandem duplication. The Vax1 gene is thus an interesting new tool to study the rostral ventral forebrain patterning, morphogenesis and evolution as well as the terminal differentiation of the forebrain in mouse and Xenopus.

Initiation of rhombomeric Hoxb4 expression requires induction by somites and a retinoid pathway

Anteroposterior (AP) patterning in the vertebrate hindbrain is dependent upon the establishment of segmental domains of Hox expression. We investigated the mechanism that governs the early expression of Hoxb4 and found that transient signaling from the paraxial mesoderm induces expression in the hindbrain. Induction involves a retinoid pathway requiring retinoic acid receptor (RAR) function within the neural plate. Characterization of a prerhombomeric enhancer from Hoxb4 reveals that a retinoic acid (RA) response element is an essential component of the early neural response to somite (s) signaling and can interpret positional information for setting the anterior boundary of expression. These data suggest a mechanism whereby, during normal hindbrain development, Hoxb4 expression is initiated by extrinsic signals and is subsequently maintained by Hox feedback circuits. This mechanism also accounts for the ectopic response of Hoxb4 in rhombomere (r) transpositions and after exposure to retinoids.

Specification of the hindbrain fate in the zebrafish

We determine the timing of neural commitment by hindbrain tissue in the zebrafish using microsurgical transplantation. When transplanted at shield stage to the ventral side of the embryo, presumptive hindbrain cells are not committed, as they can adapt to their environment and give rise to epidermis. In contrast, when transplanted at 80% epiboly, hindbrain cells retain their neural fate and express neural-specific antigens. Moreover, they are able to maintain regional fate, as is evident by the expression of the hindbrain-specific marker, Krox20. In addition, we observe that committed hindbrain tissues are able to induce presumptive ventral epidermis to form neural crest derivatives, otic vesicles, and neural tissues. We propose that hindbrain progenitors have acquired regional identity as a group at 80% epiboly even before making vertical contact with axial mesoderm. These results suggest that planar induction may constitute a significant component in the zebrafish neural patterning pathway.

Regionalization of the prosencephalic neural plate

Recent embryological studies are beginning to establish that the underlying organization of the forebrain may be reduced to relatively simple elements that are common to all vertebrates. We begin this chapter by reviewing studies that describe the similarities in prospective fate and molecular organization of the developing neural plate in fish, frogs, chickens, and mice. The chapter next addresses mechanisms that regulate regional specification in the anterior central nervous system. There is now evidence that the axial mesendoderm anterior to the notochord (the prechordal plate) has a central role in induction of the floor and basal plate primordia (hypothalamus) of the forebrain. Patterning of the anterolateral neural plate (telencephalon) may be regulated by FGF8 produced in the anterior neural ridge. Thus, the synthesis of information from fate mapping and experimental embryological and genetic studies is illuminating the mechanisms that generate the different components of the forebrain.

The Xenopus Emx genes identify presumptive dorsal telencephalon and are induced by head organizer signals

We have isolated and studied the expression pattern of Xemx1 and Xemx2 genes in Xenopus laevis. Xemx genes are the homologues of mouse Emx genes, related to Drosophila empty spiracles. They are expressed in selected regions of the developing brain, particularly in the telencephalon, and, outside the brain, in the otic vesicles, olfactory placodes, visceral arches and the developing excretory system. We also report on experiments concerning the tissue and molecular signals responsible for their activation in competent ectoderm. Xemx genes are activated in ectoderm conjugated with head organizer tissue, but not with tail organizer tissue. Furthermore, they are not activated in animal cap either by noggin or by Xnr3, thus suggesting that a different inducer or the integration of several signals may be responsible for their activation.

Neural induction and patterning by fibroblast growth factor, notochord and somite tissue in Xenopus

Two natural neural inducing sources have been used, the notochord and the somites together with the growth factor bFGF, to investigate the anterior/posterior patterning of neural tissue in an animal cap explant model in Xenopus laevis. Notochord and somite tissue from stages 12.5/13 and 16, respectively, were manually isolated, and combined heterochronically with responding animal cap ectoderm aged to gastrula stages. Somite recombinants were also constructed with animal caps injected with noggin mRNA. The responses of the ectoderm were analyzed by reverse transcription polymerase chain reaction (RT-PCR) detection of marker gene expression, and in some cases by in situ hybridization. The requirement for FGF receptor function was analyzed using the dominant negative FGF receptor (XFD). The experiments showed that bFGF is capable of direct neural induction in caps aged to stage 10.5. It was also shown that notochords are capable of inducing anterior neural tissue in gastrula stage animal cap ectoderm, and this induction is sensitive to XFD in the responding tissue. Injection of noggin mRNA results in the induction of anterior neural differentiation, and it was demonstrated that this induction was insensitive to the expression of XFD in the responding tissue. It was also shown that somite tissue recombined with gastrula stage animal cap ectoderm, can induce both anterior and posterior nervous tissue and can also posteriorize noggin-induced anterior neural tissue when combined with noggin-injected animal cap ectoderm. This response is partially sensitive to XFD expression. The results shed light on the role of competence of animal cap ectoderm and the signals from postgastrulation axial and paraxial mesoderm in the patterning of the amphibian nervous system.

Xiro, a Xenopus homolog of the Drosophila Iroquois complex genes, controls development at the neural plate

The Drosophila homeoproteins Ara and Caup are members of a combination of factors (prepattern) that control the highly localized expression of the proneural genes achaete and scute. We have identified two Xenopus homologs of ara and caup, Xiro1 and Xiro2. Similarly to their Drosophila counterparts, they control the expression of proneural genes and, probably as a consequence, the size of the neural plate. Moreover, Xiro1 and Xiro2 are themselves controlled by noggin and retinoic acid and, similarly to ara and caup, they are overexpressed by expression in Xenopus embryos of the Drosophila cubitus interruptus gene. These and other findings suggest the conservation of at least part of the genetic cascade that regulates proneural genes, and the existence in vertebrates of a prepattern of factors important to control the differentiation of the neural plate.

The Spemann organizer of Xenopus is patterned along its anteroposterior axis at the earliest gastrula stage

The Spemann organizer is largely responsible for organizing and patterning the anteroposterior axis during the development of amphibians. In this report, we examine the degree of anteroposterior pattern in the earliest gastrula organizer of Xenopus using a combination of embryological and molecular techniques. When we divide the earliest gastrula organizer, a region measuring 20 cells high by 25 cells wide, into stereotyped anterior (vegetal) and posterior (animal) halves, each half not only has a distinct fate and state of specification, but also induces a unique set of region-specific neural genes. When wrapped in animal cap ectoderm, the anterior half induces only anterior-specific genes (XAG-1 and otxA), while the posterior half induces anterior (otxA and reduced levels of XAG-1) and posterior (Hox B9) neural genes, revealing early localization of neural posteriorizing activity to posterior mesendoderm. This is the earliest demonstration of regionalized neural induction by the Xenopus organizer. Additionally, based on the expression of gsc, Xbra, and Xnot, we show that the organizer is patterned both at the early gastrula stage and prior to the appearance of bottle cells.

Cooperation between Otx1 and Otx2 genes in developmental patterning of rostral brain

Otx1 and Otx2 genes are mouse cognates of a Drosophila head gap gene orthodenticle. The homozygous mutants have previously indicated that Otx2 is essential to development of structures anterior to rhombomere 3, probably reflecting its expression around the early primitive streak stage. Otx2 mutation also exhibits craniofacial defects by haplo-insufficiency. Affected structures correspond to the most anterior and most posterior parts of the Otx2 expression where Otx1 is not, or is only weakly, expressed at the time of brain regionalization. No apparent defects are found in early brain development by the Otx1 mutation, suggesting that the Otx1 and Otx2 functions overlap in the regions where both are expressed. To demonstrate this, the Otx1/Otx2 double heterozygous phenotype was examined in this study. Analyses with molecular markers at 9.5 days post coitus suggested the failure in development of mesencephalon and caudal diencephalon with the expansion of anterior metencephalon. Genes expressed in isthmus exhibited a characteristic lateral stripe normally, although rostrally shifted, except that Fgf8 expression was expanded dorsally. The defects were apparent at the 6-somite stage, but not at the 3-somite stage. Broad Fgf8 expression at the 3-somite stage took place normally, but it did not concentrate into a spot corresponding to future isthmus. The double heterozygous phenotype implicates a previously unsuspected mechanism for development of the mes/metencephalic territory; at the 3- to 6-somite stage Otx1 cooperates with Otx2 to establish the mes/diencephalic domain, allowing for the correct development of isthmus/ rhombomere 1.

Xenopus hindbrain patterning requires retinoid signaling

We have asked how posterior neural tissue is patterned in Xenopus by assaying the involvement of endogenous retinoic acid (RA) in this process and by using the labial Hox gene, HoxD1, as a posterior marker. Although RA is able to inhibit anterior gene expression and activate expression of more posterior genes, the normal role of retinoids in anteroposterior (A/P) patterning is unclear. HoxD1 is an early posterior neurectodermal marker, expressed from midgastrula with a later anterior expression limit in the future hindbrain. We previously showed that HoxD1 was induced as an immediate early response to retinoic acid in naive ectoderm (animal caps). Here, we use a truncated RARalpha2.2 receptor (RARDelta) to dominantly interfere with retinoid signaling. In embryos injected with RARDelta expression of HoxD1 is eliminated. Conjugates of ectoderm and dorsolateral mesoderm show that retinoid receptors are required in the ectoderm for HoxD1 induction. Further, expression of Krox-20 in r3 and r5 of the presumptive hindbrain is compressed into a single stripe that suggests elimination of r5. RARalpha2.2 expression almost precisely overlaps that of HoxD1, suggesting that this receptor may normally activate HoxD1. Expression of neither more anterior genes including cement gland, forebrain, and midbrain markers nor a more posterior spinal cord marker is affected by RARDelta. These data suggest that the posterior hindbrain is the region of the nervous system most sensitive to retinoid loss. Finally, we compare the ability of RA and fibroblast growth factor (FGF) to posteriorize isolated anterior neurectoderm and show that both factors can act directly on this substrate. RA acts in a more anterior domain than does FGF; however, neither factor is equivalent to the natural posteriorizing capacity of the posterior mesoderm. We propose that endogenous retinoid and FGF signals pattern largely nonoverlapping regions along the A/P axis and that posterior neural patterning requires multiple inducers.