Changes in dorsoventral but not rostrocaudal regionalization of the chick neural tube in the absence of cranial notochord, as revealed by expression of engrailed-2

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.

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.

Formation and patterning of the avian neuraxis: one dozen hypotheses

Formation of the neuraxis is dependent on cell-cell interactions and cell movements beginning during stages of gastrulation. Cell movements bring together new combinations of cells, allowing sequential inductive interactions to occur and leading to the specification of the neural plate and to its ultimate mediolateral (subsequently dorsoventral) and rostrocaudal patterning. Formation of the neural plate involves changes in the shape of its constituent cells and the first appearance of neural-specific cell markers. Shortly after the neural plate forms it undergoes 'shaping', in which the pseudostratified columnar epithelium constituting the neural plate thickens apicobasally, narrows transversely and extends longitudinally. Shaping is driven by three principal intrinsic types of cell behaviour: changes in cell shape, position and number. The next stage of neurulation begins while shaping is underway--bending of the neural plate. Bending involves two main processes, furrowing and folding. Furrowing of the neural plate is associated with the formation of the hinge points; these are localized, longitudinal areas where the neuroepithelium is attached to adjacent tissues and where wedging of neuroepithelial cells occurs. Cell wedging in the median hinge point occurs as a result of inductive interactions with the notochord; such wedging drives furrowing, thereby facilitating subsequent folding. Folding of the neural plate requires extrinsic forces generated largely by the surface ectoderm. Types of cell behaviour that could provide such forces include changes in cell shape, position and number. As a result of shaping and bending of the neural plate, the neural folds are brought into apposition in the dorsal midline. Final closure of the neural groove is mediated by cell surface glycoconjugates coating the apical surfaces of the neural folds. Patterning of the neuraxis begins during shaping of the neural plate and continues throughout stages of neurulation and into early postneurula stages. Patterning probably involves inductive interactions with adjacent tissues and the expression of putative positional identity genes such as homeobox-containing genes.

Positioning of the midbrain-hindbrain boundary organizer through global posteriorization of the neuroectoderm mediated by Wnt8 signaling

The organizing center located at the midbrain-hindbrain boundary (MHB)patterns the midbrain and hindbrain primordia of the neural plate. Studies in several vertebrates showed that the interface between cells expressing Otx and Gbx transcription factors marks the location in the neural plate where the organizer forms, but it is unclear how this location is set up. Using mutant analyses and shield ablation experiments in zebrafish, we find that axial mesendoderm, as a candidate tissue, has only a minor role in positioning the MHB. Instead, the blastoderm margin of the gastrula embryo acts as a source of signal(s) involved in this process. We demonstrate that positioning of the MHB organizer is tightly linked to overall neuroectodermal posteriorization, and specifically depends on Wnt8 signaling emanating from lateral mesendodermal precursors. Wnt8 is required for the initial subdivision of the neuroectoderm,including onset of posterior gbx1 expression and establishment of the posterior border of otx2 expression. Cell transplantation experiments further show that Wnt8 signaling acts directly and non-cell-autonomously. Consistent with these findings, a GFP-Wnt8 fusion protein travels from donor cells through early neural plate tissue. Our findings argue that graded Wnt8 activity mediates overall neuroectodermal posteriorization and thus determines the location of the MHB organizer.

Cell interactions underlying notochord induction and formation in the chick embryo

The development of the notochord in the chick is traditionally associated with Hensen's node (the avian equivalent of the organizer). However, recent evidence has shown that two areas outside the node (called the inducer and responder) are capable of interacting after ablation of Hensen's node to form a notochord. It was not clear from these studies what effect (if any) signals from these areas had on normal notochord formation. A third area, the postnodal region, may also contribute to notochord formation, although this has also been questioned. Using transection and grafting experiments, we have evaluated the timing and cellular interactions involved in notochord induction and formation in the chick embryo. Our results indicate that the rostral primitive streak, including the node, is not required for formation of the notochord in rostral blastoderm isolates transected at stages 3a/b. In addition, neither the postnodal region nor the inducer is required for the induction and formation of the most rostral notochordal cells. However, inclusion of the inducer results in considerable elongation of the notochord in this experimental paradigm. Our results also demonstrate that the responder per se is not required for notochord formation, provided that at least the inducer and postnodal region are present, although in the absence of the responder, formation of the notochord occurs far less frequently. We also show that the node is not specified to form notochord until stage 4 and concomitant with this, the inducer loses its ability to induce notochord from the responder. The coincident timing of these changes in the node and inducer suggests that notochord specification and the activity of the inducer are regulated through a negative feedback loop. We propose a model relating our results to the induction of head and trunk organizer activity in the node.

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.

Plasticity of axial identity among somites: cranial somites can generate vertebrae without expressing Hox genes appropriate to the trunk

Classic studies have shown that the presomitic mesoderm is already committed to a specific morphological fate, for example, the ability to generate a rib. Hox gene expression in the paraxial mesoderm has also been shown to be fixed early and not susceptible to modulation by an ectopic environment. This is in contrast to the plasticity of Hox expression in neuroectodermal derivatives. We reexamine here the potential of somites for morphological plasticity by transplanting the cranial (occipital) somites 1-4, that normally produce small contributions to the skull, to the trunk of avian embryos. Surprisingly, the transposed cranial somites are able to form reasonably normal vertebral anlage. In addition, the cranial somitic mesoderm produces intervertebral disks, structures not normally found in the skull. These somites are however unable to generate some elements of the vertebrae, such as the costal process. In contrast to the morphogenetic plasticity of the occipital somites, their characteristic inability to support survival of dorsal root ganglia was not significantly modified by posterior transplantation. Dorsal root ganglia initially developed and then degenerated with the same morphological stages as normally observed. In striking contrast to the plasticity of morphology, we found that all four members of the of the fourth paralogous group of Hox genes that are expressed endogenously at the level of the graft are not upregulated in the caudad-transposed cranial mesoderm. It therefore appears that genes other than those of the Hox family normally expressed at this axial level control the position-specific morphogenesis of ectopic vertebrae formed from cranial somites. In evolutionary terms, the present results imply that occipital somites that were incorporated into the "New Head" retain the ability to develop according to their original morphogenetic fate, into vertebrae.

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.

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.

Reconstitution of the organizer is both sufficient and required to re-establish a fully patterned body plan in avian embryos

Lateral blastoderm isolates (LBIs) at the late gastrula/early neurula stage (i.e., stage 3d/4) that lack Hensen's node (organizer) and primitive streak can reconstitute a functional organizer and primitive streak within 10–12 hours in culture. We used LBIs to study the initiation and regionalization of the body plan. A complete body plan forms in each LBI by 36 hours in culture, and normal craniocaudal, dorsoventral, and mediolateral axes are re-established. Thus, reconstitution of the organizer is sufficient to re-establish a fully patterned body plan. LBIs can be modified so that reconstitution of the organizer does not occur. In such modified LBIs, tissue-type specific differentiation (with the exception of heart differentiation) and reconstitution of the body plan fail to occur. Thus, the reconstitution of the organizer is not only sufficient to re-establish a fully patterned body plan, it is also required. Finally, our results show that formation and patterning of the heart is under the control of the organizer, and that such control is exerted during the early to mid-gastrula stages (i.e., stages 2–3a), prior to formation of the fully elongated primitive streak.

Granule cell raphes and parasagittal domains of Purkinje cells: complementary patterns in the developing chick cerebellum

The extensive migration of granule cells and the parasagittal organization of Purkinje cells are two prominent features of cerebellar development. Using granule cell markers, we observed that the inward migration of a subset of granule cells occurs in streams that appear to be restricted to specific areas in the developing chick cerebellum. These streams are organized into a stereotypical series of parasagittal linear arrays, similar to the "granule cell raphes" described previously by . Similar raphes were found in the developing cerebellum of other avian species but not in the mouse cerebellum. During the period when granule cell raphes are apparent, Purkinje cells appear to be segregated into discrete parasagittal domains, interrupted by Purkinje cell-poor areas that correspond to the granule cell raphes. Purkinje cells in each domain exhibit a domain-specific expression profile of genes, including Bmp-7, EphA5/Cek-7, EphA4/Cek-8, and several chick homologs of Drosophila segmentation genes. From embryonic day 12 (E12) to E15, most of these genes gradually cease to be expressed differentially in parasagittal stripes, concurrent with the disappearance of the granule cell raphes by E15-E16. The spatial and temporal correlations of granule cell raphes and Purkinje cell parasagittal domains suggest a novel interaction between these two cell types and a potentially critical period of parasagittal patterning of the chick cerebellum.

Formation and function of Spemann's organizer

The organizer is formed in an equatorial sector of the blastula stage amphibian embryo by cells that have responded to two maternal agents: a general mesoendoderm inducer (involving the TFG-beta signaling pathway) and a dorsal modifier (probably involving the Wnt signaling pathway). The meso-endoderm inducer is secreted by most vegetal cells, those containing maternal materials that had been localized in the vegetal hemisphere of the oocyte during oogenesis. As a consequence of the inducer's distribution and action, the competence domains of prospective ectoderm, mesoderm, and endoderm are established in an animal-to-vegetal order in the blastula. The dorsal modifier signal is secreted by a sector of cells of the animal and vegetal hemispheres on one side of the blastula. These cells contain maternal materials transported there in the first cell cycle from the vegetal pole of the egg along microtubules aligned by cortical rotation. The Nieuwkoop center is the region of blastula cells secreting both maternal signals, and hence specifying the organizer in an equatorial sector. Final steps of organizer formation at the late blastula or early gastrula stage may involve locally secreted zygotic signals as well. At the gastrula stage, the organizer secretes a variety of zygotic proteins that act as antagonists to various members of the BMP and Wnt families of ligands, which are secreted by cells of the competence domains surrounding the organizer. BMPs and Wnts favor ventral development, and cells near the organizer are protected from these agents by the organizer's inducers. The nearby cells are derepressed in their inherent capacity for dorsal development, which is apparent in the neural induction of the ectoderm, dorsalization of the mesoderm, and anteriorization of the endoderm. The organizer also engages in extensive specialized morphogenesis, which brings it within range of responsive cell groups. It also self-differentiates to a variety of axial tissues of the body.

De novo induction of the organizer and formation of the primitive streak in an experimental model of notochord reconstitution in avian embryos

We have developed a model system for analyzing reconstitution of the notochord using cultured blastoderm isolates lacking Hensen's node and the primitive streak. Despite lacking normal notochordal precursor cells, the notochord still forms in these isolates during the 36 hours in culture. Reconstitution of the notochord involves an inducer, which acts upon a responder, thereby inducing a reconstituted notochord. To better understand the mechanism of notochord reconstitution, we asked whether formation of the notochord in the model system was preceded by reconstitution of Hensen's node, the organizer of the avian neuraxis. Our results show not only that a functional organizer is reconstituted, but that this organizer is induced from the responder. First, fate mapping reveals that the responder forms a density, morphologically similar to Hensen's node, during the first 10–12 hours in culture, and that this density expresses typical markers of Hensen's node. Second, the density, when fate mapped or when labeled and transplanted in place of Hensen's node, forms typical derivatives of Hensen's node such as endoderm, notochord and the floor plate of the neural tube. Third, the density, when transplanted to an ectopic site, induces a secondary neuraxis, identical to that induced by Hensen's node. And fourth, the density acts as a suppressor of notochord reconstitution, as does Hensen's node, when transplanted to other blastoderm isolates. Our results also reveal that the medial edge of the isolate forms a reconstituted primitive streak, which gives rise to the normal derivatives of the definitive primitive streak along its rostrocaudal extent and which expresses typical streak markers. Finally, our results demonstrate that the notochordal inducer also induces the reconstituted Hensen's node and, therefore, acts like a Nieuwkoop Center. These findings increase our understanding of the mechanism of notochord reconstitution, provide new information and a novel model system for studying the induction of the organizer and reveal the potential of the epiblast to regulate its cell fate and patterns of gene expression during late gastrula/early neurula stage in higher vertebrates.

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

The avian equivalent of Spemann's organizer, Hensen's node, begins to lose its ability to induce a nervous system from area opaca epiblast cells at stage 4+, immediately after the full primitive streak stage. From this stage, the node is no longer able to induce regions of the nervous system anterior to the hindbrain. Stage 4+ is marked by the emergence from the node of a group of cells, the prechordal mesendoderm. Here we have investigated whether the prechordal region possesses the lost functions of the organizer, using quail-chick chimaeras to distinguish graft- and host-derived cells, together with several region-specific molecular markers. We find that the prechordal region does not have neural inducing ability, as it is unable to divert extraembryonic epiblast cells to a neural fate. However, it can confer more anterior character to prospective hindbrain cells of the host, making them acquire expression of the forebrain markers tailless and Otx-2. It can also rescue the expression of Krox-20 and Otx-2 from nervous system induced by an older (stage 5) node in extraembryonic epiblast. We show that these properties reflect a true change of fate of cells rather than recruitment from other regions. The competence of neuroectoderm to respond to anteriorizing signals declines by stages 7–9, but both posteriorizing signals and the ability of neuroectoderm to respond to them persist after this stage.

Vertical induction of engrailed-2 and other region-specific markers in the early chick embryo

We investigated the role of vertical signals in the regulation of Engrailed-2, a regionally restricted (mesencephalon/metencephalon) neuroectodermal marker, using epiblast grafted from prospective neuroectoderm or prospective trunk mesoderm at mid-stage 3 in the gastrulating chick embryo. Grafts that were isolated from the rostral (prospective neuroectodermal) epiblast and placed rostral to or at the future mesencephalon/metencephalon level, between the endoderm and epiblast of stage 3d to stage 8 host embryos, expressed Engrailed-2 after 24 hr in culture, whereas these same grafts failed to express this marker when placed at a more caudal level. Grafts from caudal = (prospective trunk mesodermal) epiblast, which would ordinarily not express Engrailed-2, also expressed this marker when placed at the mesencephalon/metencephalon level, and failed to express it when grafted more caudally. The expression of four other markers, L5, Fgf8, Wnt-1, and paraxis, were also evaluated. Collectively, our results show that regionally restricted vertical signals are capable of inducing neuroectoderm from naive tissue, and of patterning epiblast to express some but not all mesencephalon/metencephalon isthmus markers. Experiments using grafts taken from older embryos indicated that the competence of prospective neuroectoderm to become regionally patterned by vertical signals is gradually lost between stage 3c and stage 7. Similarly, prospective mesoderm from the caudal epiblast becomes unable to respond to vertical, neural-inductive signals at these stages. These observations support a role for vertical signals in the induction and patterning of the neuroectoderm at gastrula and early neurula stages.

Transcription factors and head formation in vertebrates

Evidence from Drosophila and also vertebrates predicts that two different sets of instructions may determine the development of the rostral and caudal parts of the body. This implies different cellular and inductive processes during gastrulation, whose genetic requirements remain to be understood. To date, four genes encoding transcription factors expressed in the presumptive vertebrate head during gastrulation have been studied at the functional level: Lim-1, Otx-2, HNF-3 beta and goosecoid. We discuss here the potential functions of these genes in the formation of rostral head as compared to posterior head and trunk, and in the light of recent fate map and expression analyses in mouse, chick, Xenopus and zebrafish. These data indicate that Lim-1, Otx-2 and HNF-3 beta may be involved in the same genetic pathway controlling the formation of the prechordal mesendoderm, which is subsequently required for rostral head development. goosecoid may act in a parallel pathway, possibly in conjunction with other, yet unidentified, factors.

BMP-7 influences pattern and growth of the developing hindbrain of mouse embryos

The expression pattern of bone morphogenetic protein-7 (BMP-7) in the hindbrain region of the headfold and early somite stage developing mouse embryo suggests a role for BMP-7 in the patterning of this part of the cranial CNS. In chick embryos it is thought that BMP-7 is one of the secreted molecules which mediates the dorsalizing influence of surface ectoderm on the neural tube, and mouse surface ectoderm has been shown to have a similar dorsalizing effect. While we confirm that BMP-7 is expressed in the surface ectoderm of mouse embryos at the appropriate time to dorsalize the neural tube, we also show that at early stages of hindbrain development BMP-7 transcripts are present in paraxial and ventral tissues, within and surrounding the hindbrain neurectoderm, and only later does expression become restricted to a dorsal domain. To determine more directly the effect that BMP-7 may have on the developing hindbrain we have grafted COS cells expressing BMP-7 into the ventrolateral mesoderm abutting the neurectoderm in order to prolong BMP-7 expression in the vicinity of ventral hindbrain. Three distinct actvities of BMP-7 are apparent. Firstly, as expected from previous work in chick, BMP-7 can promote dorsal characteristics in the neural tube. Secondly, we show that it can also attenuate the expression of sonic hedgehog (Shh) in the floorplate without affecting Shh expression in the notochord. Finally, we find that ectopic BMP-7 appears to promote growth of the neurectoderm. These findings are discussed with respect to possible timing mechanisms necessary for the coordination of hindbrain dorsoventral patterning.

Regional specification of motoneurons along the anterior-posterior axis is independent of the notochord

Cek8 and low affinity NGF receptor (LNGFR) are expressed at high levels on the chick spinal motoneurons of the brachial and lumbar segments from embryonic day (E) 5 to E7, but weakly on the motoneurons of the non-limb-innervating segments. We determined by means of heterotopic neural tube transplantation, that the expression of these molecules was already intrinsically determined at E2. We used these spatiotemporal specific molecules as markers of motoneuron subpopulations. To analyze how motoneurons acquire regional specification along the anterior-posterior (A-P) axis and in the transverse plane, we observed the expression of these molecules on ectopic motoneurons induced by implanting a supernumerary notochord or floor plate at E2. The ectopic motoneurons induced by the graft obtained from either the thoracic or lumbar segments had the same expression profile as the normal motoneurons at each A-P level. These findings suggest that regional specification of motoneurons, at least of Cek8 and LNGFR expression, is independent of the notochord and the floor plate and that the whole neural tube appears to be committed to differentiate into the motoneuron subtypes along the A-P axis at the operative stages.

Expression of osteopontin in the head process late in gastrulation in the rat

Osteopontin (OPN) is a secreted, glycosylated phosphoprotein. Previous studies have implicated OPN in calcification/decalcification and inflammation. We became interested in developmental expression of OPN because of our previous findings that suggest OPN may play a role in formation of new vascular layers after injury, and because of our observation of developmental regulation in vascular smooth muscle cells. While OPN was expressed at low levels in vessels, only late in development, examination of very early expression patterns revealed a surprising finding. OPN was expressed exclusively in the head process, on day 9.0 late in gastrulation in the rat. In vitro studies of vascular smooth muscle cells have previously demonstrated that OPN promotes cell adhesion and migration. It is suggested that OPN may promote migration of cells, during gastrulation, via integrins which may be present in the head process or overlying ectoderm.

Experimental analyses of the rearrangement of ectodermal cells during gastrulation and neurulation in avian embryos

The rearrangement of ectodermal cells was studied in chimeras in which grafts were transplanted during late gastrula and early neurula stages to heterotopic locations in avian embryos. Three types of experiments were done. In all experiments, Hensen's node was extirpated completely and replaced with an epithelial plug derived from 1 of 3 regions of the prospective ectoderm. In type-1 experiments, Hensen's node was replaced with a plug consisting of precursor cells of the floor plate of the neural tube. In type-2 experiments, Hensen's node was replaced with a plug consisting of precursor cells of the lateral wall of the neural tube. In type-3 experiments, Hensen's node was replaced with a plug consisting of precursor cells of the epidermal ectoderm. In all experiments, the amount and direction of cell rearrangement that occurred in the transplanted ectodermal plug was essentially typical for prospective ectodermal cells normally residing within Hensen's node. That is, transplanted ectodermal cells underwent lateral-to-medial cell-cell intercalation and contributed to the ventral midline of the neural tube along its entire rostrocaudal extent. In most embryos, a notochord was reconstituted from host cells, despite the fact that Hensen's node--the prime source of prospective notochordal cells in intact embryos--was extirpated completely; however, a few embryos had long notochordal gaps. In such essentially notochordless embryos, the ventral midline of the neural tube still derived from grafted cells, but it failed to form a floor plate, providing further confirmation of the results of several previous studies that the notochord is required to induce the floor plate.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Mesodermal patterning during avian gastrulation and neurulation: experimental induction of notochord from non-notochordal precursor cells

The cells that are normally fated to form notochord occupy a region at the rostral tip of the primitive streak at late gastrula/early neurula stages of avian and mammalian development. If these cells are surgically removed from avian embryos in culture, a notochord will nonetheless form in the majority of cases. The origin of this reconstituted notochord previously had not been investigated and was the objective of this study. Chick embryos at late gastrulal early neurula stages were cultured, and the rostral tip of the primitive streak including Hensen's node was removed and replaced with non-node cells from quail epiblast to ensure that the cells normally fated to be notochord would be absent and that healing of the blastoderm would occur. Embryos were allowed to develop for 24 hr, and the presence and origin (host or graft) of the notochord were assessed using antibodies against notochord or quail cells. Two notochords typically developed; both were almost exclusively of host origin. The primitive streak, and in some cases adjacent tissues, was removed from another group of embryos in an attempt to estimate the mediolateral position and extent of the cells required to form reconstituted notochord. Additional experimental embryos with and without grafts were transected at various rostrocaudal levels in an attempt to estimate the rostrocaudal extent of the cells required to form reconstituted notochord. Finally, various levels of the primitive streak either were placed in a neutral environment (the germ cell crescent) or were grafted in place of the node. Collective results from all experiments indicate that the areas lateral to the rostral portion of the primitive streak, estimated to have a rostrocaudal span of less than 500 microns and a mediolateral extent of less than 250 microns, are critical for formation of the reconstituted notochord. Fate mapping and histological examination of this region identify 4 possible precursor cell populations. Further studies are underway to determine which of the 4 possible precursor cell types forms or induces the reconstituted notochord, and which tissue interactions underlie this change in cell fate.

Dorsoventral patterning of the avian mesencephalon/metencephalon: role of the notochord and floor plate in suppressing Engrailed-2

Transcription factors that are spatially and temporally restricted within the embryo may be used for dorsoventral and rostrocaudal positional information during development. The Engrailed-2 (En-2) gene is expressed across the mesencephalon/metencephalon (mes/met) boundary in the cerebellar primordium with strong dorsolateral expression and limited expression in the floor plate. In a previous experiment we demonstrated that, after removal of Hensen's node, embryos lacked a notochord in the head and the pattern of En-2 expression was normal rostrocaudally, but it was expanded into the ventral midline of the neural tube. This suggested that the notochord suppresses En-2 in the ventral neural tube during normal development. To test further the ability of the notochord (and floor plate) to suppress En-2, we transplanted ventral midline tissues from HH 5-9 quail embryos beneath the rostral neural plate of HH 4-6 chick embryos. After 24 hours in culture, 90% of the embryos with quail notochord or floor plate near the mes/met of the host lacked En-2 expression adjacent to the graft, and suppression was distance dependent. Enzymatically isolated notochords also suppressed En-2 (71%), but the results from isolated floor plates were inconclusive. Other grafts served as controls and included tissues from the trunk ventral midline, mes/met level dorsolateral neural plate, and trunk dorsolateral neural plate/somite. Collectively, the results suggest that during normal development the notochord and possibly the floor plate are important regulators of normal En-2 expression.

Patterns of cell behaviour underlying somitogenesis and notochord formation in intact vertebrate embryos

We have made a detailed analysis of cell behaviour using high resolution time-lapse microscopy of the earliest cellular interactions taking place during morphogenesis of the notochord and somites in intact teleost embryos. Notochord formation is typified by active intercalation of paraxial mesenchyme cells into the lateral surfaces of the primordium. Following this recruitment phase, complete immiscibility develops between cells of the notochord and the presomitic mesenchyme. Dorso-ventral and rostro-caudal expansion of the notochord is characterised by translocation of cells within dorso-ventral planes of section and is supported by elongation of the remaining cells and reduction in width across its latero-medial axis. A lateral palisading of paraxial mesenchyme against the lateral aspects of the notochord precedes overt segmentation. Intersomitic furrows form by localised de-adhesion at small foci at the nascent intersomitic planes, which are consolidated by coalescence of such areas by de-adhesion to produce the interface. It is not possible to predict precisely where cells would initiate de-adhesion since there is a stochastic element to the phenomenon. Once formed, boundaries between somites are stable and provide no opportunity for mixing, except across the first formed furrow, which disintegrates at the 4-6 somite stage. The first ten somites form at a constant rate of 2.3 somites/hr, during which time we recorded constant relative displacement of the segmental plate against the rostro-caudally elongating notochord. Unlike teleost epiboly and gastrulation, no large-scale movements of individual cells can be detected during elaboration of the embryonic axis.

Pattern formation in the vertebrate neural plate

Recent advances have been made in the understanding of the cellular and molecular mechanisms involved in the formation and patterning of the neural plate of vertebrate embryos. Both planar and vertical signaling pathways appear to be involved in the neural induction and axial patterning of the neural plate. The neural plate, behaving as a developmental field, might be patterned by signals emanating from boundary regions: the organizer region and the midline and edges of the neural plate. Here, A. Ruiz i Altaba describes a possible model for anteroposterior patterning involving ;lanar signals for amphibian, avian and mammalian embryos, compares the axial patterning of the neural plate with the patterning of insect epithelia, and discussed possible roles of noggin, follistatin and hedgehog-related genes in neural induction and patterning.

Delayed formation of the floor plate after ablation of the avian notochord

We have examined the long-term effects of notochord ablation at chick stages 9-10 on formation of the floor plate and motor neurons. Although missing or reduced 2 days postablation, the floor plate and motor neurons were morphologically normal by 4 postoperative days. When isolated whole or ventral, but not lateral, neural plate fragments from stage 9 embryos were cultured for 4 days in collagen gels, floor plate and neural markers were observed. Our results suggest that floor plate and motor neurons can form in a delayed fashion in vivo after notochord ablation and in vitro from isolated neural plates. This suggests that either there is an early induction of floor plate by the chordamesoderm of Hensen's node, or only limited interactions between the neural plate and notochord immediately after neurulation are required for floor plate determination.

Induction and axial patterning of the neural plate: planar and vertical signals

In this review I summarize recent findings on the contributions of different cell groups to the formation of the basic plan of the nervous system of vertebrate embryos. Midline cells of the mesoderm--the organizer, notochord, and prechordal plate--and midline cells of the neural ectoderm--the notoplate and floor plate--appear to have a fundamental role in the induction and patterning of the neural plate. Vertical signals acting across tissue layers and planar signals acting through the neural epithelium have distinct roles and cooperate in induction and pattern formation. Whereas the prechordal plate and notochord have distinct vertical signaling properties, the initial anteroposterior (A-P) pattern of the neural plate may be induced by planar signals originating from the organizer region. Planar signals from the notoplate may also contribute to the mediolateral (M-L) patterning of the neural plate. These and other findings suggest a general view of neural induction and axial patterning.

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

In amphibians and other vertebrates, neural development is induced in the ectoderm by signals coming from the dorsal mesoderm during gastrulation. Classical embryological results indicated that these signals follow a "vertical" path, from the involuted dorsal mesoderm to the overlying ectoderm. Recent work with the frog Xenopus laevis, however, has revealed the existence of "planar" neural-inducing signals, which pass within the continuous sheet or plane of tissue formed by the dorsal mesoderm and presumptive neurectoderm. Much of this work has made use of Keller explants, in which dorsal mesoderm and ectoderm are cultured in a planar configuration with contact along only a single edge, and vertical contact is prevented. Planar signals can induce the full anteroposterior (A-P) extent of neural pattern, as evidenced in Keller explants by the expression of genes that mark specific positions along the A-P axis. In this review, classical and modern molecular work on vertical and planar induction will be discussed. This will be followed by a discussion of various models for vertical induction and planar induction. It has been proposed that the A-P pattern in the nervous system is derived from a parallel pattern of inducers in the dorsal mesoderm which is "imprinted" vertically onto the overlying ectoderm. Since it is now known that planar signals can also induce A-P neural pattern, this kind of model must be reassessed. The study of planar induction of A-P pattern in Xenopus embryos provides a simple, manipulable, two-dimensional system in which to investigate pattern formation.

Cell behaviors underlying notochord formation and extension in avian embryos: quantitative and immunocytochemical studies

Formation and extension of the notochord is one of the earliest and most obvious events of axis development in vertebrate embryos. In birds, prospective notochord cells arise from Hensen's node and come to lie beneath the midline of the neural plate, where they assist in the process of neurulation and initiate the dorsoventral patterning of the neural tube through sequential inductive interactions. In the present study, we examined notochord development in avian embryos with quantitative and immunological procedures. Extension of the notochord occurs principally through accretion, that is, the addition of cells to its caudal end, a process that involves considerable cell rearrangement at the notochord-Hensen's node interface. In addition, cell division and cell rearrangement within the notochord proper contribute to notochord extension. Thus, extension of the notochord occurs in a manner that is significantly different from that of the adjacent, overlying, midline region of the neural plate (i.e., the median hinge-point region or future floor plate of the neural tube), which as shown in one of the previous studies from our laboratory (Schoenwolf and Alvarez: Development 106:427-439, 1989), extends caudally as its cells undergo two rounds of mediolateral cell-cell intercalation and two-three rounds of cell division.

Midline cells and the organization of the vertebrate neuraxis

Vertebrate embryos exhibit a striking midline axis of symmetry that can be recognized in the overall body plan, the framework of skeletal structures and the organization of the nervous system. Cells located at the midline of the embryo during gastrulation have a crucial influence on the establishment of cell identity and pattern within the nervous system. The identification of transcription factors and secreted proteins that are expressed by these midline cell groups has begun to provide a molecular characterization of the organizing centers that establish early neural identity and pattern.

Segregation of cell lineage in the neural crest

Following neurulation, neural crest cells emerge from the neural tube and undergo extensive migrations. At the onset of migration, multipotent stem cells exist within the neural crest population. Eventually, these assume one of a number of possible fates, ranging from neurons and glia of the peripheral nervous system to pigment cells and cells of the facial skeleton. Neural crest cells follow migratory pathways and differentiate into derivatives that often are characteristic of their axial level of origin. Based on their stereotyped patterns of migration, limited intermixing and distinct homeobox-gene codes, some populations of neural crest cells may have a rostrocaudal regional identity imprinted prior to their emigration.

Control of cell pattern in the neural tube: regulation of cell differentiation by dorsalin-1, a novel TGF beta family member

Distinct cell types differentiate along the dorsoventral axis of the neural tube. We have cloned and characterized a novel member of the TGF beta gene family, dorsalin-1 (dsl-1), that appears to regulate cell differentiation within the neural tube. dsl-1 is expressed selectively in the dorsal neural tube, and its pattern of expression appears to be restricted by early signals from the notochord. Exposure of neural plate cells to dsl-1 promotes the differentiation of cells with neural crest-like properties and inhibits the induction of motor neurons by signals from the notochord and floor plate. These findings suggest that dsl-1 regulates the differentiation of cell types along the dorsoventral axis of the neural tube, acting in conjunction with distinct ventralizing signals from the notochord and floor plate.

Anterior mesendoderm induces mouse Engrailed genes in explant cultures

We have developed germ layer explant culture assays to study the role of mesoderm in anterior-posterior (A-P) patterning of the mouse neural plate. Using isolated explants of ectodermal tissue alone, we have demonstrated that the expression of Engrailed-1 (En-1) and En-2 genes in ectoderm is independent of mesoderm by the mid- to late streak stage, at least 12 hours before their onset of expression in the neural tube in vivo at the early somite stage. In recombination explants, anterior mesendoderm from headfold stage embryos induces the expression of En-1 and En-2 in pre- to early streak ectoderm and in posterior ectoderm from headfold stage embryos. In contrast, posterior mesendoderm from embryos of the same stage does not induce En genes in pre- to early streak ectoderm but is able to induce expression of a general neural marker, neurofilament 160 × 10(3) M(r). These results provide the first direct evidence for a role of mesendoderm in induction and regionalization of neural tissue in mouse.

Regulative ability of the prospective cardiogenic and vasculogenic areas of the primitive streak during avian gastrulation

Four types of microsurgical experiments were conducted to analyze heart and blood vessel development during gastrula stages of avian embryos, stages during which prospective cardiogenic and vasculogenic cells reside within the primitive streak. Experiments addressed whether cells not normally destined to form heart could form heart when given the opportunity to do so and vice versa; cells destined to form rostral levels of the heart (or, alternatively, head blood vessels) could form caudal levels of the heart (or, alternatively, trunk blood vessels) and vice versa; the early endoderm imparts rostrocaudal organization to the heart and associated blood vessels; and ingression of cells from the primitive streak and their subsequent migration into the mesodermal mantle is a cell-autonomous event for migrating cells. Our results demonstrate the lability of prospective cardiogenic and vasculogenic cells of the primitive streak, both in terms of the type of mesodermal structures they are capable of forming (or of being formed from) and in terms of the rostrocaudal patterning of the cardiovascular system. In addition, our results show that cell ingression and migration is directed by environmental cues and is not a cell-autonomous process for migrating cells. Finally, our results suggest that patterning of the cardiovascular system occurs after cells enter the mesodermal mantle, presumably through cell-cell inductive interactions. However, the early endoderm is not the primary source of the patterning influence. What is the source remains to be established.

Monoclonal antibodies identifying subsets of ectodermal, mesodermal, and endodermal cells in gastrulating and neurulating avian embryos

The goal of our laboratory research is to elucidate the mechanisms underlying gastrulation and neurulation, using the avian embryo as a model system. In previous studies, we used two approaches to map the morphogenetic movements involved in these processes: (1) we constructed quail/chick transplantation chimeras in which grafted quail cells could be identified within chick host embryos by the presence of nucleolar-associated heterochromatin, and (2) we microinjected exogenous cell markers. However, it would be advantageous to be able to detect endogenous markers to demarcate various subsets of cells within the unmanipulated embryo. To elucidate such a series of natural markers, we have used monoclonal antibodies to identify epitopes found on subsets of ectodermal, mesodermal, and endodermal cells. Antibodies were made by immunizing mice against either homogenized ectoderm (i.e., prospective neural plate and surface ectoderm) or primitive streak, which had been microdissected from stage 3 chick embryos. Additionally, we screened a panel of antibodies made against soluble protein obtained from isolates of cell nuclei from late embryonic chick brain. Here, we describe the labeling patterns of three monoclonal antibodies, called MAb-GL1, GL2, and GL3 (GL, germ layer), during avian gastrulation and neurulation. Our results show that labeling early avian embryos with monoclonal antibodies can reveal previously undetected distributions of cells bearing shared epitopes, providing new labels for subsets of cells in each of the three primary germ layers.