The prechordal region lacks neural inducing ability, but can confer anterior character to more posterior neuroepithelium

Organizing activities of axial mesoderm

For almost a century, developmental biologists have appreciated that the ability of the embryonic organizer to induce and pattern the body plan is intertwined with its differentiation into axial mesoderm. Despite this, we still have a relatively poor understanding of the contribution of axial mesoderm to induction and patterning of different body regions, and the manner in which axial mesoderm-derived information is interpreted in tissues of changing competence. Here, with a particular focus on the nervous system, we review the evidence that axial mesoderm notochord and prechordal mesoderm/mesendoderm act as organizers, discuss how their influence extends through the different axes of the developing organism, and describe how the ability of axial mesoderm to direct morphogenesis impacts on its role as a local organizer.

The organizer and neural induction in birds and mammals

In avian and mammalian embryos the "organizer" property associated with neural induction of competent ectoderm into a neural plate and its subsequent patterning into rostro-caudal domains resides at the tip of the primitive streak before neurulation begins, and before a morphological Hensen's node is discernible. The same region and its later derivatives (like the notochord) also have the ability to "dorsalize" the adjacent mesoderm, for example by converting lateral plate mesoderm into paraxial (pre-somitic) mesoderm. Both neural induction and dorsalization of the mesoderm involve inhibition of BMP, and the former also requires other signals. This review surveys the key experiments done to elucidate the functions of the organizer and the mechanisms of neural induction in amniotes. We conclude that the mechanisms of neural induction in amniotes and anamniotes are likely to be largely the same; apparent differences are likely to be due to differences in experimental approaches dictated by embryo topology and other practical constraints. We also discuss the relationships between "neural induction" assessed by grafts of the organizer and normal neural plate development, as well as how neural induction relates to the generation of neuronal cells from embryonic and other stem cells in vitro.

The Phylotypic Brain of Vertebrates, from Neural Tube Closure to Brain Diversification

BACKGROUND

The phylotypic or intermediate stages are thought to be the most evolutionary conserved stages throughout embryonic development. The contrast with divergent early and later stages derived from the concept of the evo-devo hourglass model. Nonetheless, this developmental constraint has been studied as a whole embryo process, not at organ level. In this review, we explore brain development to assess the existence of an equivalent brain developmental hourglass. In the specific case of vertebrates, we propose to split the brain developmental stages into: (1) Early: Neurulation, when the neural tube arises after gastrulation. (2) Intermediate: Brain patterning and segmentation, when the neuromere identities are established. (3) Late: Neurogenesis and maturation, the stages when the neurons acquire their functionality. Moreover, we extend this analysis to other chordates brain development to unravel the evolutionary origin of this evo-devo constraint.

SUMMARY

Based on the existing literature, we hypothesise that a major conservation of the phylotypic brain might be due to the pleiotropy of the inductive regulatory networks, which are predominantly expressed at this stage. In turn, earlier stages such as neurulation are rather mechanical processes, whose regulatory networks seem to adapt to environment or maternal geometries. The later stages are also controlled by inductive regulatory networks, but their effector genes are mostly tissue-specific and functional, allowing diverse developmental programs to generate current brain diversity. Nonetheless, all stages of the hourglass are highly interconnected: divergent neurulation must have a vertebrate shared end product to reproduce the vertebrate phylotypic brain, and the boundaries and transcription factor code established during the highly conserved patterning will set the bauplan for the specialised and diversified adult brain.

KEY MESSAGES

The vertebrate brain is conserved at phylotypic stages, but the highly conserved mechanisms that occur during these brain mid-development stages (Inducing Regulatory Networks) are also present during other stages. Oppositely, other processes as cell interactions and functional neuronal genes are more diverse and majoritarian in early and late stages of development, respectively. These phenomena create an hourglass of transcriptomic diversity during embryonic development and evolution, with a really conserved bottleneck that set the bauplan for the adult brain around the phylotypic stage.

Is There a Prechordal Region and an Acroterminal Domain in Amphioxus?

This essay re-examines the singular case of the supposedly unique rostrally elongated notochord described classically in amphioxus. We start from our previous observations in hpf 21 larvae [Albuixech-Crespo et al.: PLoS Biol. 2017;15(4):e2001573] indicating that the brain vesicle has rostrally a rather standard hypothalamic molecular configuration. This correlates with the notochord across a possible rostromedian acroterminal hypothalamic domain. The notochord shows some molecular differences that specifically characterize its pre-acroterminal extension beyond its normal rostral end under the mamillary region. We explored an alternative interpretation that the putative extension of this notochord actually represents a variant form of the prechordal plate in amphioxus, some of whose cells would adopt the notochordal typology, but would lack notochordal patterning properties, and might have some (but not all) prechordal ones instead. We survey in detail the classic and recent literature on gastrulation, prechordal plate, and notochord formation in amphioxus, compare the observed patterns with those of some other vertebrates of interest, and re-examine the literature on differential gene expression patterns in this rostralmost area of the head. We noted that previous literature failed to identify the amphioxus prechordal primordia at appropriate stages. Under this interpretation, a consistent picture can be drawn for cephalochordates, tunicates, and vertebrates. Moreover, there is little evidence for an intrinsic capacity of the early notochord to grow rostralwards (it normally elongates caudalwards). Altogether, we conclude that the hypothesis of a prechordal nature of the elongated amphioxus notochord is consistent with the evidence presented.

Live imaging of avian epiblast and anterior mesendoderm grafting reveals the complexity of cell dynamics during early brain development

Despite previous intensive investigations on epiblast cell migration in avian embryos during primitive streak development before stage (st.) 4, this migration at later stages of brain development has remained uninvestigated. By live imaging of epiblast cells sparsely labeled with green fluorescence protein, we investigated anterior epiblast cell migration to form individual brain portions. Anterior epiblast cells from a broad area migrated collectively towards the head axis during st. 5-7 at a rate of 70-110 µm/h, changing directions from diagonal to parallel and forming the brain portions and abutting head ectoderm. This analysis revised the previously published head portion precursor map in anterior epiblasts at st. 4/5. Grafting outside the brain precursor region of mCherry-expressing nodes producing anterior mesendoderm (AME) or isolated AME tissues elicited new cell migration towards ectopic AME tissues. These locally convergent cells developed into secondary brains with portions that depended on the ectopic AME position in the anterior epiblast. Thus, anterior epiblast cells are bipotent for brain/head ectoderm development with given brain portion specificities. A brain portion potential map is proposed, also accounting for previous observations.

Summary: The first high-resolution live imaging of anterior epiblast cells at the brain-forming stages in avian embryos is reported, revealing their long-distance migration and interaction with the anterior mesendoderm to form brain tissues.

Neuropeptides: developmental signals in placode progenitor formation

Few families of signaling factors have been implicated in the control of development. Here, we identify the neuropeptides nociceptin and somatostatin, a neurotransmitter and neuroendocrine hormone, as a class of developmental signals in both chick and zebrafish. We show that signals from the anterior mesendoderm are required for the formation of anterior placode progenitors, with one of the signals being somatostatin. Somatostatin controls ectodermal expression of nociceptin, and both peptides regulate Pax6 in lens and olfactory progenitors. Consequently, loss of somatostatin and nociceptin signaling leads to severe reduction of lens formation. Our findings not only uncover these neuropeptides as developmental signals but also identify a long-sought-after mechanism that initiates Pax6 in placode progenitors and may explain the ancient evolutionary origin of neuropeptides, predating a complex nervous system.

Developmental mechanisms directing early anterior forebrain specification in vertebrates

Research from the last 15 years has provided a working model for how the anterior forebrain is induced and specified during the early stages of embryogenesis. This model relies on three basic processes: (1) induction of the neural plate from naive ectoderm requires the inhibition of BMP/TGFβ signaling; (2) induced neural tissue initially acquires an anterior identity (i.e., anterior forebrain); (3) maintenance and expansion of the anterior forebrain depends on the antagonism of posteriorizing signals that would otherwise transform this tissue into posterior neural fates. In this review, we present a historical perspective examining some of the significant experiments that have helped to delineate this molecular model. In addition, we discuss the function of the relevant tissues that act prior to and during gastrulation to ensure proper anterior forebrain formation. Finally, we elaborate data, mainly obtained from the analyses of mouse mutants, supporting a role for transcriptional repressors in the regulation of cell competence within the anterior forebrain. The aim of this review is to provide the reader with a general overview of the signals as well as the signaling centers that control the development of the anterior neural plate.

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.

Rostral paraxial mesoderm regulates refinement of the eye field through the bone morphogenetic protein (BMP) pathway

The eye field is initially a large single domain at the anterior end of the neural plate and is the first indication of optic potential in the vertebrate embryo. During the course of development, this domain is subject to interactions that shape and refine the organogenic field. The action of the prechordal mesoderm in bisecting this single region into two bilateral domains has been well described, however the role of signalling interactions in the further restriction and refinement of this domain has not been previously characterised. Here we describe a role for the rostral cephalic paraxial mesoderm in limiting the extent of the eye field. The anterior transposition of this mesoderm or its ablation disrupted normal development of the eye. Importantly, perturbation of optic vesicle development occurred in the absence of any detectable changes in the pattern of neighbouring regions of the neural tube. Furthermore, negative regulation of eye development is a property unique to the rostral paraxial mesoderm. The rostral paraxial mesoderm expresses members of the bone morphogenetic protein (BMP) family of signalling molecules and manipulation of endogenous BMP signalling resulted in abnormalities of the early optic primordia.

Tweaking the hinge and caps: testing a model of the organization of jaws

Historically, examinations of gnathostome skulls have indicated that for essentially the entirety of their existence, jaws have been characterized by a high degree of fidelity to an initial basic structural design that will then go on to manifest an amazing array of end-point phenotypes. These two traits-bauplan fidelity and elaboration of design-are inter-connected and striking, and beg a number of questions, including: Are all jaws made in the same manner and if not how not? To begin to tackle such questions, we herein operationally define jaws as two appositional, hinged cranial units for which polarity and potential modularity are characteristics, and then address what is necessary for them to form, including delineating both the sources of cells and tissues that will formally yield the jaws as well as what informs their ontogeny (e.g., sources of positional information and factors directing the interpretation of developmental cues). Following on this, we briefly describe a predictive, testable model of jaw development (the "Hinge and Caps" model) and present evidence that the Satb2+cell population in the developing jaw primordia of mice defines a developmentally and evolutionarily significant jaw module such as would be predicted by the model.

Xenopus hairy2b specifies anterior prechordal mesoderm identity within Spemann's organizer

Spemann's organizer is a region of the gastrula stage embryo that contains future anterior endodermal and dorsal mesodermal tissues. During gastrulation, the dorsal mesoderm is divided into the prechordal mesoderm and the chordamesoderm. However, little is known regarding how this division is established. We analyzed the role of the anterior prechordal mesoderm-specific gene Xhairy2b in the regionalization of the organizer. We found that mesoderm-inducing transforming growth factor-beta signaling induced Xhairy2b expression. On the other hand, the ectopic expression of Xhairy2b induced the expression of organizer-specific genes and resulted in the formation of a secondary dorsal axis lacking head and notochord structures. We also showed that Xhairy2b down-regulated the expression of ventral mesodermal, anterior endodermal, and chordamesodermal genes. In Xhairy2b-depleted embryos, defects in the specification of anterior prechordal mesoderm identity were observed as the border between the prechordal mesoderm and the chordamesoderm was anteriorly shifted. These results suggest that Xhairy2b establishes the identity of the anterior prechordal mesoderm within Spemann's organizer by inhibiting the formation of neighboring tissues.

The oxidizing enzyme CYP26a1 tightly regulates the availability of retinoic acid in the gastrulating mouse embryo to ensure proper head development and vasculogenesis

Retinoic acid (RA) has been implicated as one of the signals providing a posterior character to the developing vertebrate central nervous system. Embryonic RA first appears in the posterior region of the gastrulating embryo up to the node level, where it may signal within the adjacent epiblast and/or newly induced neural plate to induce a hindbrain and spinal cord fate. Conversely, rostral head development requires forebrain-inducing signals produced by the anterior visceral endoderm and/or prechordal mesoderm, and there is evidence that RA receptors must be in an unliganded state to ensure proper head development. As RA is a diffusible lipophilic molecule, some mechanism(s) must therefore have evolved to prevent activation of RA targets in anterior regions of the embryo. This might result from RA catabolism mediated by the CYP26A1 oxidizing enzyme, which is transiently expressed in anteriormost embryonic tissues; however, previous analysis of Cyp26a1(-/-) mouse mutants did not clearly support this hypothesis. Here we show that Cyp26a1(-/-) null mutants undergo head truncations when exposed to maternally-derived RA, at doses that do not affect wild-type head development. These anomalies are linked to a widespread ectopic RA signaling activity in rostral head tissues of CYP26A1-deficient embryos. Thus, CYP26A1 is required in the anterior region of the gastrulating mouse embryo to prevent teratological effects that may result from RA signaling. We also report a novel role of CYP26A1 during early development of the intra- and extra-embryonic vascular networks.

21st century neontology and the comparative development of the vertebrate skull

Classic neontology (comparative embryology and anatomy), through the application of the concept of homology, has demonstrated that the development of the gnathostome (jawed vertebrate) skull is characterized both by a fidelity to the gnathostome bauplan and the exquisite elaboration of final structural design. Just as homology is an old concept amended for modern purposes, so are many of the questions regarding the development of the skull. With due deference to Geoffroy-St. Hilaire, Cuvier, Owen, Lankester et al., we are still asking: How are bauplan fidelity and elaboration of design maintained, coordinated, and modified to generate the amazing diversity seen in cranial morphologies? What establishes and maintains pattern in the skull? Are there universal developmental mechanisms underlying gnathostome autapomorphic structural traits? Can we detect and identify the etiologies of heterotopic (change in the topology of a developmental event), heterochronic (change in the timing of a developmental event), and heterofacient (change in the active capacetence, or the elaboration of capacity, of a developmental event) changes in craniofacial development within and between taxa? To address whether jaws are all made in a like manner (and if not, then how not), one needs a starting point for the sake of comparison. To this end, we present here a "hinge and caps" model that places the articulation, and subsequently the polarity and modularity, of the upper and lower jaws in the context of cranial neural crest competence to respond to positionally located epithelial signals. This model expands on an evolving model of polarity within the mandibular arch and seeks to explain a developmental patterning system that apparently keeps gnathostome jaws in functional registration yet tractable to potential changes in functional demands over time. It relies upon a system for the establishment of positional information where pattern and placement of the "hinge" is driven by factors common to the junction of the maxillary and mandibular branches of the first arch and of the "caps" by the signals emanating from the distal-most first arch midline and the lamboidal junction (where the maxillary branch meets the frontonasal processes). In this particular model, the functional registration of jaws is achieved by the integration of "hinge" and "caps" signaling, with the "caps" sharing at some critical level a developmental history that potentiates their own coordination. We examine the evidential foundation for this model in mice, examine the robustness with which it can be applied to other taxa, and examine potential proximate sources of the signaling centers. Lastly, as developmental biologists have long held that the anterior-most mesendoderm (anterior archenteron roof or prechordal plate) is in some way integral to the normal formation of the head, including the cranial skeletal midlines, we review evidence that the seminal patterning influences on the early anterior ectoderm extend well beyond the neural plate and are just as important to establishing pattern within the cephalic ectoderm, in particular for the "caps" that will yield medial signaling centers known to coordinate jaw development.

Holoprosencephaly: clinical, anatomic, and molecular dimensions

Holoprosencephaly is addressed under the following headings: alobar, semilobar, and lobar holoprosencephaly; arrhinencephaly; agenesis of the corpus callosum; pituitary abnormalities; hindbrain abnormalities; syntelencephaly; aprosencephaly/atelencephaly; neural tube defects; facial anomalies; median cleft lip; minor facial anomalies; single maxillary central incisor; holoprosencephaly-like phenotype; epidemiology; genetic causes of holoprosencephaly; teratogenic causes of holoprosencephaly; SHH mutations; ZIC2 mutations; SIX3 mutations; TGIF mutations; PTCH mutations; GLI2 mutations; FAST1 mutations; TDGF1 mutations; and DHCR7 mutations.

A role for the hypoblast (AVE) in the initiation of neural induction, independent of its ability to position the primitive streak

The mouse anterior visceral endoderm (AVE) has been implicated in embryonic polarity: it helps to position the primitive streak and some have suggested that it might act as a "head organizer", inducing forebrain directly. Here we explore the role of the hypoblast (the chick equivalent of the AVE) in the early steps of neural induction and patterning. We report that the hypoblast can induce a set of very early markers that are later expressed in the nervous system and in the forebrain, but only transiently. Different combinations of signals are responsible for different aspects of this early transient induction: FGF initiates expression of Sox3 and ERNI, retinoic acid can induce Cyp26A1 and only a combination of low levels of FGF8 together with Wnt- and BMP-antagonists can induce Otx2. BMP- and Wnt-antagonists and retinoic acid, in different combinations, can maintain the otherwise transient induction of these markers. However, neither the hypoblast nor any of these factors or combinations thereof can induce the definitive neural marker Sox2 or the formation of a mature neural plate or a forebrain, suggesting that the hypoblast is not a head organizer and that other signals remain to be identified. Interestingly, FGF and retinoids, generally considered as caudalizing factors, are shown here to play a role in the induction of a transient "pre-neural/pre-forebrain" state.

Morphological and molecular analysis of the early developing chick requires an expanded series of primitive streak stages

A detailed analysis of the gastrulating chick embryo was performed using three methods : time-lapse videotaping of embryos in culture, histological semithin sections, and in situ hybridization with 10 mRNA signals expressed during gastrulation. The results suggest that the gene expression pattern of Goosecoid, Hex, Crescent, and Bmp7 may be involved in the axial establishment of the temporal and spatial arrangement of cells forming the prechordal plate endoderm, and that Chordin, cNot1, Noggin, and Brachyury are precocious markers of cells coming from Hensen's node, which contribute to the rostralmost tip of the notochord, its arrowhead, the head process, and, later, the elongating notochord. These results explain several earlier descriptions based only on morphological analyses of the axial mesodermal structures characteristic of the gastrulation stages. The data, carefully observed and compared with the whole-mount observation in time-lapse video, show that the changes in cell populations, movements, and cell differentiation occur step-by-step over a precise temporal range, which requires the establishment of a subdivision of the stages usually employed. Knowledge of new aspects of avian gastrulation, including gene expression patterns, immunocytochemical analyses, and the great number of recent experiments based on microinjections or transplants of groups of cells to analyze processes of induction or regulation, need the support of a precisely defined scheme of primitive streak stages (PS-stages), and a correlation of these stages with other approaches to provide a finer resolution of the staging steps, and thus to facilitate a better understanding of the initial gastrulation period.

The acquisition of neural fate in the chick

Neural development in the chick embryo is now understood in great detail on a cellular and a molecular level. It begins already before gastrulation, when a separation of neural and epidermal cell fates occurs under the control of FGF and BMP/Wnt signalling, respectively. This early specification becomes further refined around the tip of the primitive streak, until finally the anterior-posterior level of the neuroectoderm becomes established through progressive caudalization. In this review we focus on processes in the chick embryo and put classical and more recent molecular data into a coherent scenario.

Sequential antagonism of early and late Wnt-signaling by zebrafish colgate promotes dorsal and anterior fates

The establishment of the vertebrate body plan involves patterning of the ectoderm, mesoderm, and endoderm along the dorsoventral and antero-posterior axes. Interactions among numerous signaling molecules from several multigene families, including Wnts, have been implicated in regulating these processes. Here we provide evidence that the zebrafish colgate(b382) (col) mutation results in increased Wnt signaling that leads to defects in dorsal and anterior development. col mutants display early defects in dorsoventral patterning manifested by a decrease in the expression of dorsal shield-specific markers and ectopic expression of ventrolaterally expressed genes during gastrulation. In addition to these early patterning defects, col mutants display a striking regional posteriorization within the neuroectoderm, resulting in a reduction in anterior fates and an expansion of posterior fates within the forebrain and midbrain-hindbrain regions. We are able to correlate these phenotypes to the overactivation of Wnt signaling in col mutants. The early dorsal and anterior patterning phenotypes of the col mutant embryos are selectively rescued by inactivation of Wnt8 function by morpholino translational interference. In contrast, the regionalized neuroectoderm posterioriorization phenotype is selectively rescued by morpholino-mediated inactivation of Wnt8b. These results suggest that col-mediated antagonism of early and late Wnt-signaling activity during gastrulation is normally required sequentially for both early dorsoventral patterning and the specification and patterning of regional fates within the anterior neuroectoderm.

Hensen's node gives rise to the ventral midline of the foregut: implications for organizing head and heart development

Patterning of the ventral head has been attributed to various cell populations, including endoderm, mesoderm, and neural crest. Here, we provide evidence that head and heart development may be influenced by a ventral midline endodermal cell population. We show that the ventral midline endoderm of the foregut is generated directly from the extreme rostral portion of Hensen's node, the avian equivalent of the Spemann organizer. The endodermal cells extend caudally in the ventral midline from the prechordal plate during development of the foregut pocket. Thus, the prechordal plate appears as a mesendodermal pivot between the notochord and the ventral foregut midline. The elongating ventral midline endoderm delimits the right and left sides of the ventral foregut endoderm. Cells derived from the midline endoderm are incorporated into the endocardium and myocardium during closure of the foregut pocket and fusion of the bilateral heart primordia. Bilateral ablation of the endoderm flanking the midline at the level of the anterior intestinal portal leads to randomization of heart looping, suggesting that this endoderm is partitioned into right and left domains by the midline endoderm, thus performing a function similar to that of the notochord in maintaining left-right asymmetry. Because of its derivation from the dorsal organizer, its extent from the forebrain through the midline of the developing face and pharynx, and its participation in formation of a single midline heart tube, we propose that the ventral midline endoderm is ideally situated to function as a ventral organizer of the head and heart.

Central nervous system embryogenesis and its failures

The well-orchestrated development of the central nervous system (CNS) requires highly integrated regulatory processes to ensure its precise spatial organization that provides the foundation for proper function. As emphasized in this review, the type, timing, and location of regulatory molecules influence the different stages of development from neuronal induction, regional specification, neuronal specification, and neuronal migration to axonal growth and guidance, neuronal survival, and synapse formation. The known molecular mechanisms are summarized from studies of invertebrates and lower vertebrates, in which we have learned more about the different ligands, receptors, transcription factors, and the intracellular signaling pathways that play specific roles in the different stages of development. Despite known molecular mechanisms of some disturbances, most of the clinical entities that arise from failures of CNS embryogenesis remain unexplained. As more novel genes and their functions are discovered, existing mechanisms will be refined and tenable explanations will be made. With these limitations, two specific clinical entities that have been relatively well studied, holoprosencephaly and neuronal migration defects, are discussed in more detail to illustrate the complexity of regulatory mechanisms that govern well-defined stages of CNS development.

Induction and initial patterning of the nervous system - the chick embryo enters the scene

Until recently, almost everything known about the molecular controls of early neural development came from studies in amphibians. It is now possible to misexpress factors in chick embryos at relatively late stages in development, allowing careful dissection of the timing of cell interactions. This is starting to contribute significantly to our understanding of neural induction and early patterning.

Progressive induction of caudal neural character by graded Wnt signaling

Early in differentiation, all neural cells have a rostral character. Only later do posteriorly positioned neural cells acquire characteristics of caudal forebrain, midbrain and hindbrain cells. Caudalization of neural tissue in the chick embryo apparently involves the convergent actions of (i) fibroblast growth factor (FGF) signaling and (ii) signaling from the caudal paraxial mesoderm, or 'PMC activity', which has not yet been defined molecularly. Here we report evidence that Wnt signaling underlies PMC activity, and show that Wnt signals act directly and in a graded manner on anterior neural cells to induce their progressive differentiation into caudal forebrain, midbrain and hindbrain cells.

Analysis of spatial and temporal gene expression patterns in blastula and gastrula stage chick embryos

Studies on the genetic basis of rostral-caudal specification, neural induction, and head development require knowledge of the relevant gene expression patterns. Gaps in our understanding of gene expression have led us to examine the detailed spatiotemporal expression patterns of 19 genes implicated in early development, to learn more about their potential role in specifying and patterning early developmental processes leading to head formation. Here, we report the expression patterns of these markers in blastula- and gastrula-stage chick embryos, using whole-mount in situ hybridisation. Nodal, Fgf8, Bmp7, Chordin, Lim1, Hnf3beta, Otx2, Goosecoid, Cerberus, Hex, Dickkopf1, and Crescent are all already expressed by the time the egg is laid. When the primitive streak has reached its full length, a later group of genes, including Ganf, Six3, Bmp2, Bmp4, Noggin, Follistatin, and Qin (BF1), begins to be expressed. We reassess current models of early rostral patterning based on the analysis of these dynamic spatiotemporal expression patterns.

Dynamic domains of gene expression in the early avian forebrain

The expression domains of genes implicated in forebrain patterning often share borders at specific anteroposterior positions. This observation lies at the heart of the prosomeric model, which proposes that such shared borders coincide with proposed compartment boundaries and that specific combinations of genes expressed within each compartment are responsible for its patterning. Thus, genes such as Emx1, Emx2, Pax6, and qin (Bf1) are seen as being responsible for specifying different regions in the forebrain (diencephalon and telencephalon). However, the early expression of these genes, before the appearance of putative compartment boundaries, has not been characterized. In order to determine whether they have stable expression domains before this stage, we have compared mRNA expression of each of the above genes, relative both to one another and to morphological landmarks, in closely staged chick embryos. We find that, between HH stage 8 and HH stage 13, each of the genes has a dynamic spatial and temporal expression pattern. To test for autonomy of gene expression in the prosencephalon, we grafted tissue from this region to more caudal positions in the neural tube and analyzed for expression of Emx1, Emx2, qin, or Pax6. We find that gene expression is autonomous in prosencephalic tissue from as early as HH stage 8. In the case of Emx1, our data suggest that, from as early stage 8, presumptive telencephalic tissue also is committed to express this gene. We propose that early patterning along the anteroposterior axis of the presumptive telencephalon occurs across a field that is subdivided by different combinations of genes, with some overlapping areas, but without either sharp boundaries or stable interfaces between expression domains.

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.

Initial patterning of the central nervous system: how many organizers?

For three-quarters of a century, developmental biologists have been asking how the nervous system is specified as distinct from the rest of the ectoderm during early development, and how it becomes subdivided initially into distinct regions such as forebrain, midbrain, hindbrain and spinal cord. The two events of 'neural induction' and 'early neural patterning' seem to be intertwined, and many models have been put forward to explain how these processes work at a molecular level. Here I consider early neural patterning and discuss the evidence for and against the two most popular models proposed for its explanation: the idea that multiple signalling centres (organizers) are responsible for inducing different regions of the nervous system, and a model first articulated by Nieuwkoop that invokes two steps (activation/transformation) necessary for neural patterning. As recent evidence from several systems challenges both models, I propose a modification of Nieuwkoop's model that most easily accommodates both classical and more recent data, and end by outlining some possible directions for future research.

Foregut endoderm is required at head process stages for anteriormost neural patterning in chick

Anterior definitive endoderm, the future pharynx and foregut lining, emerges from the anterior primitive streak and Hensen's node as a cell monolayer that replaces hypoblast during chick gastrulation. At early head process stages (4+ to 6; Hamburger and Hamilton) it lies beneath, lateral to and ahead of the ingressed axial mesoderm. Removal of the monolayer beneath and ahead of the node at stage 4 is followed by normal development, the removed cells being replaced by further ingressing cells from the node. However, similar removal during stages 4+ and 5 results in a permanent window denuded of definitive endoderm, beneath prechordal mesoderm and a variable sector of anterior notochord. The foregut tunnel then fails to form, heart development is confined to separated lateral regions, and the neural tube undergoes no ventral flexures at the normal positions in brain structure. Reduction in forebrain pattern is evident by the 12-somite stage, with most neuraxes lacking telencephalon and eyes, while forebrain expressions of the transcription factor genes GANF and BF1, and of FGF8, are absent or severely reduced. When the foregut endoderm removal is delayed until stage 6, later forebrain pattern appears once again complete, despite lack of foregut formation, of ventral flexure and of heart migration. Important gene expressions within axial mesoderm (chordin, Shh and BMP7) appear unaffected in all embryos, including those due to be pattern-deleted, during the hours following the operation when anterior brain pattern is believed to be determined. A specific system of neural anterior patterning signals, rather than an anterior sector of the initially neurally induced area, is lost following operation. Heterotopic lower layer replacement operations strongly suggest that these patterning signals are positionally specific to anteriormost presumptive foregut. The homeobox gene Hex and the chick Frizbee homologue Crescent are both expressed prominently within anterior definitive endoderm at the time when removal of this tissue results in forebrain defects, and the possible implications of this are discussed. The experiments also demonstrate how stomodeal ectoderm, the tissue that will, much later, form Rathke's pouch and the anterior pituitary, is independently specified by anteriormost lower layer signals at an early stage.

The role of prechordal mesendoderm in neural patterning

Prechordal mesendoderm is formed in response to Nodal and maternal beta-Catenin signaling and is regulated by signals from anterior endoderm and chordamesoderm. Prechordal mesendodermal cells are involved in neural induction and in anteroposterior and dorsoventral neural patterning. Inhibitors of Wnt and BMP growth factors secreted by prechordal mesendoderm mediate neural induction and anteroposterior and dorsoventral patterning, whereas SHH and TGF betas mediate dorsoventral patterning.

The role of Xenopus dickkopf1 in prechordal plate specification and neural patterning

Dickkopf1 (dkk1) encodes a secreted WNT inhibitor expressed in Spemann's organizer, which has been implicated in head induction in Xenopus. Here we have analyzed the role of dkk1 in endomesoderm specification and neural patterning by gain- and loss-of-function approaches. We find that dkk1, unlike other WNT inhibitors, is able to induce functional prechordal plate, which explains its ability to induce secondary heads with bilateral eyes. This may be due to differential WNT inhibition since dkk1, unlike frzb, inhibits Wnt3a signalling. Injection of inhibitory antiDkk1 antibodies reveals that dkk1 is not only sufficient but also required for prechordal plate formation but not for notochord formation. In the neural plate dkk1 is required for anteroposterior and dorsoventral patterning between mes- and telencephalon, where dkk1 promotes anterior and ventral fates. Both the requirement of anterior explants for dkk1 function and their ability to respond to dkk1 terminate at late gastrula stage. Xenopus embryos posteriorized with bFGF, BMP4 and Smads are rescued by dkk1. dkk1 does not interfere with the ability of bFGF to induce its immediate early target gene Xbra, indicating that its effect is indirect. In contrast, there is cross-talk between BMP and WNT signalling, since induction of BMP target genes is sensitive to WNT inhibitors until the early gastrula stage. Embryos treated with retinoic acid (RA) are not rescued by dkk1 and RA affects the central nervous system (CNS) more posterior than dkk1, suggesting that WNTs and retinoids may act to pattern anterior and posterior CNS, respectively, during gastrulation.

Vertebrate anteroposterior patterning: the Xenopus neurectoderm as a paradigm

This review discusses formation of the vertebrate anteroposterior (AP) axis, focusing on the dorsal ectoderm, which gives rise to the nervous system, using the frog Xenopus as a model. After summarizing classical models of AP neural patterning, we describe recent molecular studies that are encouraging re-examination of these models. Such studies have shown that AP ectodermal patterning occurs by the onset of gastrulation, much earlier than previously thought. The identity of tissues that determine AP pattern is discussed, and the definition of the Organizer is reconsidered. The activity of factors secreted by inducing tissues in early patterning decisions is assessed and formulated into a revised model for Xenopus AP neural patterning. Finally, AP ectodermal patterning in Xenopus dorsal ectoderm is compared to that of other germ layers, and to other vertebrates.

Reconciling different models of forebrain induction and patterning: a dual role for the hypoblast

Several models have been proposed for the generation of the rostral nervous system. Among them, Nieuwkoop's activation/transformation hypothesis and Spemann's idea of separate head and trunk/tail organizers have been particularly favoured recently. In the mouse, the finding that the visceral endoderm (VE) is required for forebrain development has been interpreted as support for the latter model. Here we argue that the chick hypoblast is equivalent to the mouse VE, based on fate, expression of molecular markers and characteristic anterior movements around the time of gastrulation. We show that the hypoblast does not fit the criteria for a head organizer because it does not induce neural tissue from naive epiblast, nor can it change the regional identity of neural tissue. However, the hypoblast does induce transient expression of the early markers Sox3 and Otx2. The spreading of the hypoblast also directs cell movements in the adjacent epiblast, such that the prospective forebrain is kept at a distance from the organizer at the tip of the primitive streak. We propose that this movement is important to protect the forebrain from the caudalizing influence of the organizer. This dual role of the hypoblast is more consistent with the Nieuwkoop model than with the notion of separate organizers, and accommodates the available data from mouse and other vertebrates.

Formation of the definitive endoderm in mouse is a Smad2-dependent process

TGFbeta growth factors specify cell fate and establish the body plan during early vertebrate development. Diverse cellular responses are elicited via interactions with specific cell surface receptor kinases that in turn activate Smad effector proteins. Smad2-dependent signals arising in the extraembryonic tissues of early mouse embryos serve to restrict the site of primitive streak formation and establish anteroposterior identity in the epiblast. Here we have generated chimeric embryos using lacZ-marked Smad2-deficient ES cells. Smad2 mutant cells extensively colonize ectodermal and mesodermal populations without disturbing normal development, but are not recruited into the definitive endoderm lineage during gastrulation. These experiments provide the first evidence that TGFbeta signaling pathways are required for specification of the definitive endoderm lineage in mammals and identify Smad2 as a key mediator that directs epiblast derivatives towards an endodermal as opposed to a mesodermal fate. In largely Smad2-deficient chimeras, asymmetric nodal gene expression is maintained and expression of pitx2, a nodal target, is also unaffected. These results strongly suggest that other Smad(s) act downstream of Nodal signals in mesodermal populations. We found Smad2 and Smad3 transcripts both broadly expressed in derivatives of the epiblast. However, Smad2 and not Smad3 mRNA is expressed in the visceral endoderm, potentially explaining why the primary defect in Smad2 mutant embryos originates in this cell population.

Development of chick axial mesoderm: specification of prechordal mesoderm by anterior endoderm-derived TGFbeta family signalling

Two populations of axial mesoderm cells can be recognised in the chick embryo, posterior notochord and anterior prechordal mesoderm. We have examined the cellular and molecular events that govern the specification of prechordal mesoderm. We report that notochord and prechordal mesoderm cells are intermingled and share expression of many markers as they initially extend out of Hensen's node. In vitro culture studies, together with in vivo grafting experiments, reveal that early extending axial mesoderm cells are labile and that their character may be defined subsequently through signals that derive from anterior endodermal tissues. Anterior endoderm elicits aspects of prechordal mesoderm identity in extending axial mesoderm by repressing notochord characteristics, briefly maintaining gsc expression and inducing BMP7 expression. Together these experiments suggest that, in vivo, signalling by anterior endoderm may determine the extent of prechordal mesoderm. The transforming growth factor (beta) (TGFbeta) superfamily members BMP2, BMP4, BMP7 and activin, all of which are transiently expressed in anterior endoderm mimic distinct aspects of its patterning actions. Together our results suggest that anterior endoderm-derived TGFbetas may specify prechordal mesoderm character in chick axial mesoderm.

The morphogenetic role of midline mesendoderm and ectoderm in the development of the forebrain and the midbrain of the mouse embryo

The anterior midline tissue (AML) of the late gastrula mouse embryo comprises the axial mesendoderm and the ventral neuroectoderm of the prospective forebrain, midbrain and rostral hindbrain. In this study, we have investigated the morphogenetic role of defined segments of the AML by testing their inductive and patterning activity and by assessing the impact of their ablation on the patterning of the neural tube at the early-somite-stage. Both rostral and caudal segments of the AML were found to induce neural gene activity in the host tissue; however, the de novo gene activity did not show any regional characteristic that might be correlated with the segmental origin of the AML. Removal of the rostral AML that contains the prechordal plate resulted in a truncation of the head accompanied by the loss of several forebrain markers. However, the remaining tissues reconstituted Gsc and Shh activity and expressed the ventral forebrain marker Nkx2.1. Furthermore, analysis of Gsc-deficient embryos reveals that the morphogenetic function of the rostral AML requires Gsc activity. Removal of the caudal AML led to a complete loss of midline molecular markers anterior to the 4th somite. In addition, Nkx2.1 expression was not detected in the ventral neural tube. The maintenance and function of the rostral AML therefore require inductive signals emanating from the caudal AML. Our results point to a role for AML in the refinement of the anteroposterior patterning and morphogenesis of the brain.

SPC4/PACE4 regulates a TGFβ signaling network during axis formation

In vertebrates, specification of anteroposterior (A/P) and left–right (L/R) axes depends on TGFβ-related signals, including Nodal, Lefty, and BMPs. Endoproteolytic maturation of these proteins is probably mediated by the proprotein convertase SPC1/Furin. In addition, precursor processing may be regulated by related activities such as SPC4 (also known as PACE4). Here, we show that a proportion of embryos lacking SPC4 develop situs ambiguus combined with left pulmonary isomerism or complex craniofacial malformations including cyclopia, or both. Gene expression analysis during early somite stages indicates that spc4 is genetically upstream of nodal, pitx2, lefty1, and lefty2 and perhaps maintains the balance between Nodal and BMP signaling in the lateral plate that is critical for L/R axis formation. Furthermore, genetic interactions betweennodal and spc4, together with our analysis of chimeric embryos, strongly suggest that during A/P axis formation, SPC4 acts primarily in the foregut. These findings establish an important role for SPC4 in patterning the early mouse embryo.

Positional regulation of Krox-20 and mafB/kr expression in the developing hindbrain: potentialities of prospective rhombomeres

Krox-20 and mafB/kr encode transcription factors involved in the control of hindbrain development and are expressed in rhombomeres (r) 3 and 5 and 5 and 6, respectively. To analyse the regulation of the expression of these genes by positional cues, focusing on the stages just preceding the formation of rhombomeres, we have performed ectopic grafts involving single prospective rhombomeres (pr) or couples of pr on 4-6 somite avian embryos. Transplantation of pr6 in the pr5 position leads to Krox-20 activation and grafting of pr7 in the pr5 position results in mafB/kr activation. Furthermore, pr6 grafted in the pr5 position develops an r5-like cytoarchitecture. These data establish that rostral transplantation can lead to anteriorization within the hindbrain. However, additional experiments indicate that the competence of the transplanted tissue for such anteriorization appears limited and that transformations corresponding to shifts of a single rhombomere are favoured. We also show that caudal transplantation of pr5 into the pr6 position can lead to a down-regulation of Krox-20 expression consistent with posteriorization, suggesting that caudalizing influences are present within the nonsomitic hindbrain after the 4- to 6-somite stage. Finally, combinations of extirpation and grafting experiments suggest that the regulation of mafB/kr expression in the r6-r7 region may involve anteriorizing influences in addition to previously identified posteriorizing signals from the somitic region.

Head in the WNT: the molecular nature of Spemann's head organizer

Formation of the head during vertebrate embryogenesis has been one of the most-studied topics in development, probably because we are such cephalized beings ourselves. Early experimenters found that the head is induced during gastrulation by Spemann's organizer. In 1999 we celebrate the 75th anniversary of the discovery of the organizer by Spemann and Mangold, a group of cells in amphibia that secretes powerful signalling molecules. Recently, advances have been made in identifying candidate head inducers. Not surprisingly, these inducers act in familiar molecular pathways, namely transforming growth factor beta (TGF-beta) and WNT signalling.

Getting a-head of the organizer: anterior-posterior patterning of the forebrain

The molecular mechanisms that drive the development of embryonic tissues are being uncovered rapidly. One such fascinating example is the development of the forebrain, the most anterior part of the nervous system. In this review, we will discuss the mechanisms that induce the formation of the forebrain in multiple vertebrate systems, placing emphasis on a recent article published by Grinblat et al. ((1)) Using zebrafish as a model system, these authors combine elegant embryological manipulations with the use of early markers of the presumptive forebrain, to show that initial induction and patterning of this tissue occurs near the onset of gastrulation. In addition, their results confirm observations made in other systems that planar signals, those traveling in the plane of the ectoderm, are involved in forebrain induction and patterning.

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.

Spatially distinct head and heart inducers within the Xenopus organizer region

BACKGROUND

The mouse anterior visceral endoderm, an extraembryonic tissue, expresses several genes essential for normal development of structures rostral to the anterior limit of the notochord and has been termed the head organizer. This tissue also has heart-inducing activity and expresses mCer1 which, like its Xenopus homolog cerberus, can induce markers of cardiac specification and anterior neural tissue when ectopically expressed. We investigated the relationship between head and heart induction in Xenopus embryos, which lack extraembryonic tissues.

RESULTS

We found three regions of gene expression in the Xenopus organizer: deep endoderm, which expressed cerberus; prechordal mesoderm, which showed overlapping but non-identical expression of genes characteristic of the murine head organizer, such as XHex and XANF-1; and leading-edge dorsoanterior endoderm, which expressed both cerberus and a subset of the genes expressed by the prechordal mesoderm. Microsurgical ablation of the cerberus-expressing endoderm decreased the incidence of heart, but not head, formation. Removal of prechordal mesoderm, in contrast, caused deficits of anterior head structures. Finally, although misexpression of cerberus induced ectopic heads, it was unable to induce genes thought to participate in head induction.

CONCLUSIONS

In Xenopus, the cerberus-expressing endoderm is required for heart, but not head, inducing activity. Therefore, this tissue is not the topological equivalent of the murine anterior visceral endoderm. We propose that, in Xenopus, cerberus is redundant to other bone morphogenetic protein (BMP) and Wnt antagonists located in prechordal mesoderm for head induction, but may be necessary for heart induction.

Axial mesendoderm refines rostrocaudal pattern in the chick nervous system

There has long been controversy concerning the role of the axial mesoderm in the induction and rostrocaudal patterning of the vertebrate nervous system. Here we investigate the neural inducing and regionalising properties of defined rostrocaudal regions of head process/prospective notochord in the chick embryo by juxtaposing these tissues with extraembryonic epiblast or neural plate explants. We localise neural inducing signals to the emerging head process and using a large panel of region-specific neural markers, show that different rostrocaudal levels of the head process derived from headfold stage embryos can induce discrete regions of the central nervous system. However, we also find that rostral and caudal head process do not induce expression of any of these molecular markers in explants of the neural plate. During normal development the head process emerges beneath previously induced neural plate, which we show has already acquired some rostrocaudal character. Our findings therefore indicate that discrete regions of axial mesendoderm at headfold stages are not normally responsible for the establishment of rostrocaudal pattern in the neural plate. Strikingly however, we do find that caudal head process inhibits expression of rostral genes in neural plate explants. These findings indicate that despite the ability to induce specific rostrocaudal regions of the CNS de novo, signals provided by the discrete regions of axial mesendoderm do not appear to establish regional differences, but rather refine the rostrocaudal character of overlying neuroepithelium.

Head induction in the chick by primitive endoderm of mammalian, but not avian origin

Different types of endoderm, including primitive, definitive and mesendoderm, play a role in the induction and patterning of the vertebrate head. We have studied the formation of the anterior neural plate in chick embryos using the homeobox gene GANF as a marker. GANF is first expressed after mesendoderm ingression from Hensen's node. We found that, after transplantation, neither the avian hypoblast nor the anterior definitive endoderm is capable of GANF induction, whereas the mesendoderm (young head process, prechordal plate) exhibits a strong inductive potential. GANF induction cannot be separated from the formation of a proper neural plate, which requires an intact lower layer and the presence of the prechordal mesendoderm. It is inhibited by BMP4 and promoted by the presence of the BMP antagonist Noggin. In order to investigate the inductive potential of the mammalian visceral endoderm, we used rabbit embryos which, in contrast to mouse embryos, allow the morphological recognition of the prospective anterior pole in the living, pre-primitive-streak embryo. The anterior visceral endoderm from such rabbit embryos induced neuralization and independent, ectopic GANF expression domains in the area pellucida or the area opaca of chick hosts. Thus, the signals for head induction reside in the anterior visceral endoderm of mammals whereas, in birds and amphibia, they reside in the prechordal mesendoderm, indicating a heterochronic shift of the head inductive capacity during the evolution of mammalia.