Development of floor plate, neurons and axonal outgrowth pattern in the early spinal cord of the notochord-deficient chick embryo

Divergent axial morphogenesis and early shh expression in vertebrate prospective floor plate

Background

The notochord has organizer properties and is required for floor plate induction and dorsoventral patterning of the neural tube. This activity has been attributed to sonic hedgehog (shh) signaling, which originates in the notochord, forms a gradient, and autoinduces shh expression in the floor plate. However, reported data are inconsistent and the spatiotemporal development of the relevant shh expression domains has not been studied in detail. We therefore studied the expression dynamics of shh in rabbit, chicken and Xenopus laevis embryos (as well as indian hedgehog and desert hedgehog as possible alternative functional candidates in the chicken).

Results

Our analysis reveals a markedly divergent pattern within these vertebrates: whereas in the rabbit shh is first expressed in the notochord and its floor plate domain is then induced during subsequent somitogenesis stages, in the chick embryo shh is expressed in the prospective neuroectoderm prior to the notochord formation and, interestingly, prior to mesoderm immigration. Neither indian hedgehog nor desert hedgehog are expressed in these midline structures although mRNA of both genes was detected in other structures of the early chick embryo. In X. laevis, shh is expressed at the beginning of gastrulation in a distinct area dorsal to the dorsal blastopore lip and adjacent to the prospective neuroectoderm, whereas the floor plate expresses shh at the end of gastrulation.

Conclusions

While shh expression patterns in rabbit and X. laevis embryos are roughly compatible with the classical view of "ventral to dorsal induction" of the floor plate, the early shh expression in the chick floor plate challenges this model. Intriguingly, this alternative sequence of domain induction is related to the asymmetrical morphogenesis of the primitive node and other axial organs in the chick. Our results indicate that the floor plate in X. laevis and chick embryos may be initially induced by planar interaction within the ectoderm or epiblast. Furthermore, we propose that the mode of the floor plate induction adapts to the variant topography of interacting tissues during gastrulation and notochord formation and thereby reveals evolutionary plasticity of early embryonic induction.

Moving the Shh Source over Time: What Impact on Neural Cell Diversification in the Developing Spinal Cord?

A substantial amount of data has highlighted the crucial influence of Shh signalling on the generation of diverse classes of neurons and glial cells throughout the developing central nervous system. A critical step leading to this diversity is the establishment of distinct neural progenitor cell domains during the process of pattern formation. The forming spinal cord, in particular, has served as an excellent model to unravel how progenitor cells respond to Shh to produce the appropriate pattern. In recent years, considerable advances have been made in our understanding of important parameters that control the temporal and spatial interpretation of the morphogen signal at the level of Shh-receiving progenitor cells. Although less studied, the identity and position of Shh source cells also undergo significant changes over time, raising the question of how moving the Shh source contributes to cell diversification in response to the morphogen. Here, we focus on the dynamics of Shh-producing cells and discuss specific roles for these time-variant Shh sources with regard to the temporal events occurring in the receiving field.

Evaluation of the developmental toxicity of lead in the Danio rerio body

Lead has been utilized throughout history and is widely distributed and mobilized globally. Although lead in the environment has been somewhat mitigated, the nature of lead and its extensive uses in the past prohibit it from being completely absent from our environment and exposure to lead is still a public health concern. Most studies regarding lead toxicity have focused on the brain. However, little is found in the literature on the effects of lead in other tissues. Here, we utilize the zebrafish model system to investigate effects of lead exposure during early developmental time windows at 24, 48 and 72 h post fertilization in the body. We analyze whole body, notochord and somatic muscle changes, vascular changes of the body, as well as motor neuron alterations. We find lead exposure induces a curved body phenotype with concomitant changes in somite length, decreased notochord staining and abnormal muscle staining using live and in situ approaches. Furthermore, altered vasculature within the somatic regions, loss and/or alternations of motor neuron extension both dorsally and ventrally from the spinal cord, loss of Rohon-Beard sensory neurons, and increased areas of apoptosis were found. We conclude that lead is developmentally toxic to other areas of the developing embryo, not just the brain.

In vivo knockdown of Brachyury results in skeletal defects and urorectal malformations resembling caudal regression syndrome

The T-box transcription factor BRACHYURY (T) is a key regulator of mesoderm formation during early development. Complete loss of T has been shown to lead to embryonic lethality around E10.0. Here we characterize an inducible miRNA-based in vivo knockdown mouse model of T, termed KD3-T, which exhibits a hypomorphic phenotype. KD3-T embryos display axial skeletal defects caused by apoptosis of paraxial mesoderm, which is accompanied by urorectal malformations resembling the murine uro-recto-caudal syndrome and human caudal regression syndrome phenotypes. We show that there is a reduction of T in the notochord of KD3-T embryos which results in impaired notochord differentiation and its subsequent loss, whereas levels of T in the tailbud are sufficient for axis extension and patterning. Furthermore, the notochord in KD3-T embryos adopts a neural character and loses its ability to act as a signaling center. Since KD3-T animals survive until birth, they are useful for examining later roles for T in the development of urorectal tissues.

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

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

The Sonic hedgehog pathway independently controls the patterning, proliferation and survival of neuroepithelial cells by regulating Gli activity

During CNS development, the proliferation of progenitors must be coordinated with the pattern of neuronal subtype generation. In the ventral neural tube, Sonic hedgehog acts as a long range morphogen to organise the pattern of cell differentiation by controlling the activity of Gli transcription factors. Here, we provide evidence that the same pathway also acts directly at long range to promote the proliferation and survival of progenitor cells. Blockade of Shh signaling or inhibition of Gli activity results in cell autonomous decreases in progenitor proliferation and survival. Conversely, positive Gli activity promotes proliferation and rescues the effects of inhibiting Shh signaling. Analysis of neural cells indicates that Shh/Gli signaling regulates the G1 phase of cell cycle and the expression of the anti-apoptotic factor Bcl2. Furthermore, Shh signaling independently regulates patterning, proliferation and survival of neural cells, thus Shh/Gli activity couples these separate cellular responses of progenitors to coordinate neural development.

The intermediate type split cord malformation: hypothesis and case report

METHODS

A patient is described in which a complete osteofibrotic dorsally implanted septum was found in combination with a split cord malformation in a single dural tube. This case cannot be explained using the widely used theory as proposed by Pang et al. [Pang D, Dias MS, Ahab-Barmada M (1992) Split cord malformation, part I: A unified theory of embryogenesis for double spinal cord malformations. Neurosurgery 31:451-480] but must be regarded as a combination of type I and II split cord malformation.

RESULTS

The authors state that all types of split cord malformation can be reduced to a single derailment during development, with various degrees of severity.

CONCLUSIONS

The configuration of the malformation is determined by the way the median parts of the mesoderm come to development. Type I and II split cord malformation are not distinct entities.

Morphogens in motion: growth control of the neural tube

The entire vertebrate nervous system develops from a simple epithelial sheet, the neural plate which, along development, acquires the large number and wide variety of neuronal cell types required for the construction of a functional mature nervous system. These include processes of growth and pattern formation of the neural tube that are achieved through complicated and tightly regulated genetic interactions. Pattern formation, particularly in the vertebrate central nervous system, is one of the best examples of a morphogen-type of function. Cell cycle progression, however, is generally accepted to be dependent on cell-intrinsic factors. Recent studies have demonstrated that proliferation of neural precursors is also somehow controlled by secreted signaling molecules, well-known by their role as morphogens, such as fibroblast growth factor (FGF), vertebrate orthologs of the Drosophila wingless (Wnt), hedgehog (Hh), and transforming growth factor beta (TGF-beta) families, that in turn regulate the activity of factors controlling cell cycle progression. In this review we will summarize the experimental data that support the idea that classical morphogens can be reused to regulate proliferation of neural precursors.

An atypical case of segmental spinal dysgenesis

Spinal segmental dysgenesis is a complex closed dysraphism. The diagnostic criteria are: lumbar or thoracolumbar vertebral dysgenesis causing kyphosis, focal spinal cord narrowing without exiting roots, deformity of the lower limbs and paraplegia or paraparesis. We present a newborn who showed atypical features of bifocal spinal cord narrowing, without any vertebral abnormality at the proximal level. This seems to be a variant of this rare entity, whose early diagnosis is important, as surgical stabilisation of the spine is required.

The floor plate: multiple cells, multiple signals

One of the key organizers in the CNS is the floor plate - a group of cells that is responsible for instructing neural cells to acquire distinctive fates, and that has an important role in establishing the elaborate neuronal networks that underlie the function of the brain and spinal cord. In recent years, considerable controversy has arisen over the mechanism by which floor plate cells form. Here, we describe recent evidence that indicates that discrete populations of floor plate cells, with characteristic molecular properties, form in different regions of the neuraxis, and we discuss data that imply that the mode of floor plate induction varies along the anteroposterior axis.

Sonic hedgehog guides commissural axons along the longitudinal axis of the spinal cord

Dorsal commissural axons in the developing spinal cord cross the floor plate, then turn rostrally and grow along the longitudinal axis, close to the floor plate. We used a subtractive hybridization approach to identify guidance cues responsible for the rostral turn in chicken embryos. One of the candidates was the morphogen Sonic hedgehog (Shh). Silencing of the gene SHH (which encodes Shh) by in ovo RNAi during commissural axon navigation demonstrated a repulsive role in post-commissural axon guidance. This effect of Shh was not mediated by Patched (Ptc) and Smoothened (Smo), the receptors that mediate effects of Shh in morphogenesis and commissural axon growth toward the floor plate. Rather, functional in vivo studies showed that the repulsive effect of Shh on postcommissural axons was mediated by Hedgehog interacting protein (Hip).

Morphological Features of Tail Bud Development in Truncate Mouse Mutants

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

The mouse homeobox gene Not is required for caudal notochord development and affected by the truncate mutation

The floating head (flh) gene in zebrafish encodes a homeodomain protein, which is essential for notochord formation along the entire body axis. flh orthologs, termed Not genes, have been isolated from chick and Xenopus, but no mammalian ortholog has yet been identified. Truncate (tc) is an autosomal recessive mutation in mouse that specifically disrupts the development of the caudal notochord. Here, we demonstrate that truncate arose by a mutation in the mouse Not gene. The truncate allele (Nottc) contains a point mutation in the homeobox of Not that changes a conserved Phenylalanine residue in helix 1 to a Cysteine (F20C), and significantly destabilizes the homeodomain. Reversion of F20C in one allele of homozygous tc embryonic stem (ES) cells is sufficient to restore normal notochord formation in completely ES cell-derived embryos. We have generated a targeted mutation of Not by replacing most of the Not coding sequence, including the homeobox with the eGFP gene. The phenotype of NoteGFP/eGFP, NoteGFP/tc, and Nottc/tc embryos is very similar but slightly more severe in NoteGFP/eGFP than in Nottc/tc embryos. This confirms allelism of truncate and Not, and indicates that tc is not a complete null allele. Not expression is abolished in Foxa2 and T mutant embryos, suggesting that Not acts downstream of both genes during notochord development. This is in contrast to zebrafish embryos, in which flh interacts with ntl (zebrafish T) in a regulatory loop and is essential for development of the entire notochord, and suggests that different genetic control circuits act in different vertebrate species during notochord formation.

The role of retinoic acid in the morphogenesis of the neural tube

We have examined the role of the signalling molecule, retinoic acid, in the process of neurulation and the subsequent growth and differentiation of the central nervous system using quail embryos that have developed in the absence of retinoic acid. Such retinoic acid-free embryos undergo abnormal neural tube formation in terms of its shape and structure, but the embryos do not display spina bifida or exencephaly. The neural tubes have a wider floor plate, a thicker roof plate and a different dorsoventral shape. Phalloidin staining and electron microscopy revealed alterations in the actin filaments and the junctional complexes of the cell layer lining the lumen. Initially the neural tubes proliferated at the same rate as normal, but later the proliferation rate declined drastically and neuronal differentiation was highly deficient. There were very few motoneurons extending neurites into the periphery, and within the neural tube axon trajectories were chaotic. These results reveal several functions for retinoic acid in the morphogenesis and growth of the neural tube, many of which can be explained by defective notochord signalling, but they do not suggest that this molecule plays a role in neural tube closure.

Distinct modes of floor plate induction in the chick embryo

To begin to reconcile models of floor plate formation in the vertebrate neural tube, we have performed experiments aimed at understanding the development of the early floor plate in the chick embryo. Using real-time analyses of cell behaviour, we provide evidence that the principal contributor to the early neural midline, the future anterior floor plate, exists as a separate population of floor plate precursor cells in the epiblast of the gastrula stage embryo, and does not share a lineage with axial mesoderm. Analysis of the tissue interactions associated with differentiation of these cells to a floor plate fate reveals a role for the nascent prechordal mesoderm, indicating that more than one inductive event is associated with floor plate formation along the length of the neuraxis. We show that Nr1, a chick nodal homologue, is expressed in the nascent prechordal mesoderm and we provide evidence that Nodal signalling can cooperate with Shh to induce the epiblast precursors to a floor-plate fate. These results indicate that a shared lineage with axial mesoderm cells is not a pre-requisite for floor plate differentiation and suggest parallels between the development of the floor plate in amniote and anamniote embryos.

From cells to circuits: development of the zebrafish spinal cord

The ability of an animal to carry out its normal behavioral repertoire requires generation of an enormous diversity of neurons and glia. The relative simplicity of the spinal cord makes this an especially attractive part of the nervous system for addressing questions about the development of vertebrate neural specification and function. The last decade has witnessed an explosion in our understanding of spinal cord development and the functional interactions among spinal cord neurons and glia. Cellular, genetic, molecular, physiological and behavioral studies in zebrafish have all been important in providing insights into questions that remained unanswered by studies from other vertebrate model organisms. This is the case because many zebrafish spinal neurons can be individually identified and followed over time in living embryos and larvae. In this review, we discuss what is currently known about the cellular, genetic and molecular mechanisms involved in specifying distinct cell types in the zebrafish spinal cord and how these cells establish the functional circuitry that mediates particular behaviors. We start by describing the early signals and morphogenetic movements that form the nervous system, and in particular, the spinal cord. We then provide an overview of the cell types within the spinal cord and describe how they are specified and patterned. We begin ventrally with floor plate and proceed dorsally, through motoneurons and oligodendrocytes, interneurons, astrocytes and radial glia, spinal sensory neurons and neural crest. We next describe axon pathfinding of spinal neurons. Finally, we discuss the roles of particular spinal cord neurons in specific behaviors.

A sonic hedgehog-dependent signaling relay regulates growth of diencephalic and mesencephalic primordia in the early mouse embryo

Sonic hedgehog (Shh) is a key signal in the specification of ventral cell identities along the length of the developing vertebrate neural tube. In the presumptive hindbrain and spinal cord, dorsal development is largely Shh independent. By contrast, we show that Shh is required for cyclin D1 expression and the subsequent growth of both ventral and dorsal regions of the diencephalon and midbrain in early somite-stage mouse embryos. We propose that a Shh-dependent signaling relay regulates proliferation and survival of dorsal cell populations in the diencephalon and midbrain. We present evidence that Fgf15 shows Shh-dependent expression in the diencephalon and may participate in this interaction, at least in part, by regulating the ability of dorsal neural precursors to respond to dorsally secreted Wnt mitogens.

Magnetic resonance imaging of spinal dysraphism

Spinal cord development occurs through three consecutive periods. Gastrulation (weeks 2-3) is characterized by conversion of the embryonic disk from a bilaminar to a trilaminar arrangement and establishment of a notochord. Primary neurulation (weeks 3-4) produces the uppermost nine tenths of the spinal cord. Secondary neurulation and retrogressive differentiation (weeks 5-6) result in formation of the conus tip and filum terminale. Defects in these early embryonic stages produce spinal dysraphisms, which are characterized by anomalous differentiation and fusion of dorsal midline structures. Spinal dysraphisms may be categorized clinically into two subsets. In open spinal dysraphisms, the placode (non-neurulated neural tissue) is exposed to the environment. These disorders include myelomeningocele, myeloschisis, hemimyelomeningocele, and hemimyelocele, and are always associated with a Chiari II malformation. Closed spinal dysraphisms are covered by intact skin, although cutaneous stigmata usually indicate their presence. Two subsets may be identified based on whether a subcutaneous mass is present in the low back. Closed spinal dysraphisms with mass comprise lipomyeloschisis, lipomyelomeningocele, meningocele, and myelocystocele. Closed spinal dysraphisms without mass comprise complex dysraphic states (ranging from complete dorsal enteric fistula to neurenteric cysts, split cord malformations, dermal sinuses, caudal regression, and spinal segmental dysgenesis), bony spina bifida, tight filum terminale, filar and intradural lipomas, and persistent terminal ventricle. Magnetic resonance imaging is the imaging method of choice for investigation of this complex group of disorders.

Notochord Patterning of the Endoderm

Endodermally derived organs of the gastrointestinal and respiratory system form at distinct anterioposterior and dorsoventral locations along the vertebrate body axis. This stereotyped program of organ formation depends on the correct patterning of the endodermal epithelium so that organ differentiation and morphogenesis occur at appropriate positions along the gut tube. Whereas some initial patterning of the endoderm is known to occur early, during germ-layer formation and gastrulation, later signaling events, originating from a number of adjacent tissue layers, are essential for the development of endodermal organs. Previous studies have shown that signals arising from the notochord are important for patterning of the ectodermally derived floor plate of the neural tube and the mesodermally derived somites. This review will discuss recent evidence indicating that signals arising from the notochord also play a role in regulating endoderm development.

Dorsal spinal cord inhibits oligodendrocyte development

Oligodendrocytes are the myelinating cells of the mammalian central nervous system. In the mouse spinal cord, oligodendrocytes are generated from strictly restricted regions of the ventral ventricular zone. To investigate how they originate from these specific regions, we used an explant culture system of the E12 mouse cervical spinal cord and hindbrain. In this culture system O4(+) cells were first detected along the ventral midline of the explant and were subsequently expanded to the dorsal region similar to in vivo. When we cultured the ventral and dorsal spinal cords separately, a robust increase in the number of O4(+) cells was observed in the ventral fragment. The number of both progenitor cells and mature cells also increased in the ventral fragment. This phenomenon suggests the presence of inhibitory factor for oligodendrocyte development from dorsal spinal cord. BMP4, a strong candidate for this factor that is secreted from the dorsal spinal cord, did not affect oligodendrocyte development. Previous studies demonstrated that signals from the notochord and ventral spinal cord, such as sonic hedgehog and neuregulin, promote the ventral region-specific development of oligodendrocytes. Our present study demonstrates that the dorsal spinal cord negatively regulates oligodendrocyte development.

Somite formation and patterning

As a consequence of their segmented arrangement and the diversity of their tissue derivatives, somites are key elements in the establishment of the metameric body plan in vertebrates. This article aims to largely review what is known about somite development, from the initial stages of somite formation through the process of somite regionalization along the three major body axes. The role of both cell intrinsic mechanisms and environmental cues are evaluated. The periodic and bilaterally synchronous nature of somite formation is proposed to rely on the existence of a developmental clock. Molecular mechanisms underlying these events are reported. The importance of an antero-posterior somitic polarity with respect to somite formation on one hand and body segmentation on the other hand is discussed. Finally, the mechanisms leading to the regionalization of somites along the dorso-ventral and medio-lateral axes are reviewed. This somitic compartmentalization is believed to underlie the segregation of dermis, skeleton, and dorsal and appendicular musculature.

Analysis of nuclear ribonucleoproteic structures during notochordal cell differentiation and maturation in chick embryos

The ultrastructure of notochordal cells and the quantitative changes of nuclear mRNA-containing particles were studied in several stages of the development of the chick embryo. The modifications in the frequency of perichromatin granules (PCG) were analyzed in embryos at 24 hr to 10 days of incubation (stages 6-36 of Hamburger and Hamilton). The ultrastructural and morphometric data show that notochordal cells undergo changes that can be systematized in four periods. Very early notochordal cells (stages 6-11), are characterized by the presence of large nucleoli and abundant PCG, traits probably related to the frequent mitotic division and the expression of inductive signals reported in numerous papers. During the second period (stages 16-21) the number of PCG and the size of the nucleolus decrease. These changes are coincident with the beginning of vacuolization. In the third period (stages 21-30), the notochordal cells undergo a second cytodifferentiation characterized by a large increase of cytoplasmic vacuolization and secretion of materials that thicken the perichordal sheath. During this period, the nucleolus becomes smaller and the number of PCG increases. Similar features were previously described during functional maturation of embryonic neurons and striated fibers at synaptogenesis, and epidermal cells. The fourth period, beginning at stage 30, is characterized by the decrease of the density of PCG and of the nucleolar volume and corresponds to cessation of mitosis and cell degeneration.

Sclerotome induction and differentiation

Inductive events in the development of the sclerotome and their possible underlying mechanisms were reviewed from the primary literature. A brief review of morphological and anatomical aspects of sclerotome development was given. The importance of the notochord and neural tube in sclerotome induction and somite chondrogenesis in vivo and in vitro was established. The functions and patterns of expression of different sclerotome markers were discussed. Shh and Noggin were discussed as two molecules produced by the neural tube and notochord that appear to maintain and initiate the sclerotome, respectively. While the abilities of the axial organs and Shh and Noggin to induce sclerotome marker expression in the somite was not disputed, the exact nature of these inductions was discussed with regard to possible effects on gene expression, effects on cell survival, and physical effects on the cells and it was argued that the fundamental nature of inductive events in the sclerotome is still unknown.

Dorsoventral differential distribution of collagen type XIV around the spinal cord is regulated by the ectoderm

Regional specification in the nervous system is a critical issue in nervous system morphogenesis. Along the dorsoventral axis of the spinal cord, ventral inductive signals of the notochord and floor plate, and dorsal ones of the epidermal ectoderm are essential. Collagen type XIV is uniquely distributed around the spinal cord with a gradient of dorsal high and ventral low at the early developmental stages of the chick embryo. In the present study it was found that collagen type XIV expression around the spinal cord was entirely regulated by the ectoderm and that even the ventralizing tissues, the notochord and floor plate, themselves could be influenced to express this molecule by the ectoderm.

Defining subregions of Hensen's node essential for caudalward movement, midline development and cell survival

Hensen's node, also called the chordoneural hinge in the tail bud, is a group of cells that constitutes the organizer of the avian embryo and that expresses the gene HNF-3(β). During gastrulation and neurulation, it undergoes a rostral-to-caudal movement as the embryo elongates. Labeling of Hensen's node by the quail-chick chimera system has shown that, while moving caudally, Hensen's node leaves in its wake not only the notochord but also the floor plate and a longitudinal strand of dorsal endodermal cells. In this work, we demonstrate that the node can be divided into functionally distinct subregions. Caudalward migration of the node depends on the presence of the most posterior region, which is closely apposed to the anterior portion of the primitive streak as defined by expression of the T-box gene Ch-Tbx6L. We call this region the axial-paraxial hinge because it corresponds to the junction of the presumptive midline axial structures (notochord and floor plate) and the paraxial mesoderm. We propose that the axial-paraxial hinge is the equivalent of the neuroenteric canal of other vertebrates such as Xenopus. Blocking the caudal movement of Hensen's node at the 5- to 6-somite stage by removing the axial-paraxial hinge deprives the embryo of midline structures caudal to the brachial level, but does not prevent formation of the neural tube and mesoderm located posteriorly. However, the whole embryonic region generated posterior to the level of Hensen's node arrest undergoes widespread apoptosis within the next 24 hours. Hensen's node-derived structures (notochord and floor plate) thus appear to produce maintenance factor(s) that ensures the survival and further development of adjacent tissues.

The specification of dorsal cell fates in the vertebrate central nervous system

The generation of distinct classes of neurons at defined positions within the developing vertebrate nervous system depends on inductive signals provided by local cell groups that act as organizing centers. Genetic and embryological studies have begun to elucidate the processes that control the pattern and identity of neuronal cell types. Here we discuss the cellular interactions and molecular mechanisms that direct neuronal cell fates in the dorsal half of the vertebrate central nervous system. The specification of dorsal neuronal cell fates appears to depend on a cascade of inductive signals initiated by cells of the epidermal ectoderm that flank the neural plate and propagated by roof plate cells within the neural tube. Members of the transforming growth factor-beta (TGF beta) family of secreted proteins have a prominent role in mediating these dorsalizing signals. Additional signals involving members of the Wnt and fibroblast growth factor (FGF) families may also contribute to the proliferation and differentiation of dorsal neuronal cell types.

Delta-mediated specification of midline cell fates in zebrafish embryos

BACKGROUND

Fate mapping studies have shown that progenitor cells of three vertebrate embryonic midline structures - the floorplate in the ventral neural tube, the notochord and the dorsal endoderm - occupy a common region prior to gastrulation. This common region of origin raises the possibility that interactions between midline progenitor cells are important for their specification prior to germ layer formation.

RESULTS

One of four known zebrafish homologues of the Drosophila melanogaster cell-cell signaling gene Delta, deltaA (dlA), is expressed in the developing midline, where progenitor cells of the ectodermal floorplate, mesodermal notochord and dorsal endoderm lie close together before they occupy different germ layers. We used a reverse genetic strategy to isolate a missense mutation of dlA, dlAdx2, which coordinately disrupts the development of floorplate, notochord and dorsal endoderm. The dlAdx2 mutant embryos had reduced numbers of floorplate and hypochord cells; these cells lie above and beneath the notochord, respectively. In addition, mutant embryos had excess notochord cells. Expression of a dominant-negative form of Delta protein driven by mRNA microinjection produced a similar effect. In contrast, overexpression of dlA had the opposite effect: fewer trunk notochord cells and excess floorplate and hypochord cells.

CONCLUSION

Our results indicate that Delta signaling is important for the specification of midline cells. The results are most consistent with the hypothesis that developmentally equivalent midline progenitor cells require Delta-mediated signaling prior to germ layer formation in order to be specified as floorplate, notochord or hypochord.

Tenascin-C expression in the trunk of wild-type, cyclops and floating head zebrafish embryos

The function and the regulation of the expression of the extracellular matrix molecule tenascin-C during embryonic development are still unclear. In the present study, the expression of tenascin-C was analyzed in the trunk of zebrafish at the end of the first embryonic day. An antiserum raised against a zebrafish tenascin-C (TN-C) fusion protein reacted with 220 (doublet), 200, and 160 KD peptides. In situ hybridization showed that in the zebrafish wild-type embryo, tn-c mRNA was expressed by somites, neural crest cells, roof plate, notochord, hypochord, and tail fin bud. Thus, the expression of tn-c mRNA is an excellent marker for the differentiation of most zebrafish trunk structures. Immunolabelling with the anti-TN-C antibody was detected in the migratory pathway of neural crest cells and in the intersomitic furrows. In situ hybridization analysis of the zebrafish cyclops mutants, lacking the midline floor plate cells, showed normal expression of tn-c mRNA in all trunk structures. Analysis of the floating-head mutant, lacking the notochord, showed that tn-c mRNA expression in neural crest cells, roof plate, and tail fin bud is normal, but it is defective in the somites. By showing that the notochord, but not the floor plate, cells are required for normal tn-c expression in the trunk, this work provides new information on the role played by the embryonic axial structures in the regulation of the expression of tn-c during the development of zebrafish and allows new conclusions about somite patterning in the cyclops and floating-head zebrafish mutants.

Apoptosis of epaxial myotome in Danforth's short-tail (Sd) mice in somites that form following notochord degeneration

Danforth's short-tail (Sd) mouse is a semidominant mutation that prevents completion of notochord development. In homozygous mutant mice, the notochord completely degenerates at embryonic day 9.5 (E9. 5), whereas the neural tube and somites continue to form, permitting analysis of somite development in the absence of inductive signals from the notochord and floor plate. In the somites formed after notochord degeneration, Myf5 expression initiates in a normal temporal sequence, but instead of the normal restriction to the dorsomedial somite, its expression extends into the ventral somite. MyoD, myogenin, and myosin heavy chain are normally expressed in the ventral myotome and there is normal development of hypaxial muscles. In contrast, subsequent to initial Myf5 expression, muscle gene expression was not detected in the dorsal myotome and a high level of apoptosis was observed with significantly decreased formation of epaxial muscles. The apoptosis of epaxial muscle in somites that formed after notochord degeneration is consistent with a role for the notochord in the survival and differentiation of the dorsal myotome.

The relationships between notochord and floor plate in vertebrate development revisited

By using the quail-chicken chimera system, we have previously shown that during development of the spinal cord, floor plate cells are inserted between neural progenitors giving rise to the alar plates. These cells are derived from the regressing Hensen's node or cordoneural hinge (HN-CNH). This common population of HN-CNH cells gives rise to three types of midline descendants: notochord, floor plate, and dorsal endoderm. Here we find that HNF3beta, an important gene in the development of the midline structures, is continuously expressed in the HN-CNH cells and their derivatives, floor plate, notochord, and dorsal endoderm. Experiments in which the notochord was removed in the posterior region of either normal chicken or of quail-chicken chimeras in which a quail HN had been grafted showed that the floor plate develops in a cell-autonomous manner in the absence of notochord. Absence of floor plate observed at the posterior level of the excision results from removal of HN-CNH material, including the future floor plate, and not from the lack of an inductive signal of notochord origin.

Voltage- and ligand-gated ion channels in floor plate neuroepithelia of the rat

Whole-cell patch-clamp recordings were used to characterize the membrane properties and ion channel complement of floor plate neuroepithelia in embryonic and neonatal rats. The average resting potential was close to -60 mV, the capacitance was approximately 7 pS and the membrane time constant averaged 31 ms, in both neonates and embryos. Two types of K+ current were identified (i) a slowly activating, slowly inactivating current that was present in all cells, and (ii) a rapidly inactivating current that was present in 39% of cells from neonates and 64% of cells from embryos. K+ currents were significantly larger in neonates than embryos. Na+ currents were absent from all neuroepithelial cells examined. In contrast, the majority of floor plate cells exhibited a significant Ca2+ current. Biophysically this current activated at potentials positive to 60 mV and exhibited fast, voltage-dependent, inactivation. The Ca2+ current was equipermeant to Ca2+ and Ba2+, sensitive to 40-120 microM Ni2+ and only slightly inhibited by 100 microM Cd2+. These and other observations indicated this current is mediated by low-voltage-activated (i.e. T-type) Ca2+ channels. The majority of floor plate cells tested also exhibited responses to the neurotransmitter GABA which produced robust inward currents at negative membrane potentials, in chloride-loaded cells. Both the pharmacology and voltage-dependence of the GABA-activated currents indicated they arose from activation of GABA(A) receptors.

Noggin-mediated antagonism of BMP signaling is required for growth and patterning of the neural tube and somite

Embryonic patterning in vertebrates is dependent upon the balance of inductive signals and their specific antagonists. We show that Noggin, which encodes a bone morphogenetic protein (BMP) antagonist expressed in the node, notochord, and dorsal somite, is required for normal mouse development. Although Noggin has been implicated in neural induction, examination of null mutants in the mouse indicates that Noggin is not essential for this process. However, Noggin is required for subsequent growth and patterning of the neural tube. Early BMP-dependent dorsal cell fates, the roof plate and neural crest, form in the absence of Noggin. However, there is a progressive loss of early, Sonic hedgehog (Shh)-dependent ventral cell fates despite the normal expression of Shh in the notochord. Further, somite differentiation is deficient in both muscle and sclerotomal precursors. Addition of BMP2 or BMP4 to paraxial mesoderm explants blocks Shh-mediated induction of Pax-1, a sclerotomal marker, whereas addition of Noggin is sufficient to induce Pax-1. Noggin and Shh induce Pax-1 synergistically. Use of protein kinase A stimulators blocks Shh-mediated induction of Pax-1, but not induction by Noggin, suggesting that induction is mediated by different pathways. Together these data demonstrate that inhibition of BMP signaling by axially secreted Noggin is an important requirement for normal patterning of the vertebrate neural tube and somite.

Gli1 is a target of Sonic hedgehog that induces ventral neural tube development

The vertebrate zinc finger genes of the Gli family are homologs of the Drosophila gene cubitus interruptus. In frog embryos, Gli1 is expressed transiently in the prospective floor plate during gastrulation and in cells lateral to the midline during late gastrula and neurula stages. In contrast, Gli2 and Gli3 are absent from the neural plate midline with Gli2 expressed widely and Gli3 in a graded fashion with highest levels in lateral regions. In mouse embryos, the three Gli genes show a similar pattern of expression in the neural tube but are coexpressed throughout the early neural plate. Because Gli1 is the only Gli gene expressed in prospective floor plate cells of frog embryos, we have investigated a possible involvement of this gene in ventral neural tube development. Here we show that Shh signaling activates Gli1 transcription and that widespread expression of endogenous frog or human glioma Gli1, but not Gli3, in developing frog embryos results in the ectopic differentiation of floor plate cells and ventral neurons within the neural tube. Floor-plate-inducing ability is retained when cytoplasmic Gli1 proteins are forced into the nucleus or are fused to the VP16 transactivating domain. Thus, our results identify Gli1 as a midline target of Shh and suggest that it mediates the induction of floor plate cells and ventral neurons by Shh acting as a transcriptional regulator.

Mesodermal subdivision along the mediolateral axis in chicken controlled by different concentrations of BMP-4

Molecular mechanisms by which the mesoderm is subdivided along the mediolateral axis in early chicken embryos have been studied. When the presomitic mesoderm (medial mesoderm) was transplanted into the lateral plate, the graft was transformed into lateral plate tissue, indicating that the primitive somite was not fully committed and that the lateral plate has a cue for mesodermal lateralization. Since the lateral plate expresses a high level of BMP-4 mRNA, a member of the TGF-beta family, we hypothesized that it is the molecule responsible for the lateralization of the somite. To test this, we transplanted COS cells producing BMP-4 into the presomitic region. Those cells locally prevented the presomitic cells from differentiating into somites, converting them instead into lateral plate mesoderm, which was revealed by expression of cytokeratin mRNA, a marker for the lateral plate. The effect was dependent on the level of effective BMP-4: with a high level of BMP-4, the somite was transformed completely to lateral plate; with a low level, the somite formed but was occupied by the lateral somitic component expressing cSim 1, a marker for the lateral somite. These results suggest that different thresholds of effective BMP-4 determine distinct subtypes of the mesoderm as a lateralizer during early development.

Coordination of early neural tube development by BDNF/trkB

Neurotrophins signal through members of the trk family of tyrosine kinase receptors and are known to regulate several neuronal properties. Although initially characterized by their ability to prevent naturally occurring cell death of subsets of neurons during development, neurotrophins can also regulate the proliferation and differentiation of precursor cells. Here we report a novel involvement of neurotrophins in early development of the neural tube. We demonstrate that a functional trkB receptor is expressed by motor neuron progenitors in the ventral neural tube and that treatment of ventral neural tube explants with the trkB ligand Brain-Derived Neurotrophic Factor (BDNF) leads to a significant increase in the number of motor neurons. The only BDNF expression detectable at this stage is by a subset of ventrally projecting interneurons in the dorsal neural tube; ablating this region in vivo leads to a reduction of motor neuron numbers. This loss can be prevented by simultaneous treatment with BDNF. We propose that BDNF produced by dorsal interneurons stimulates proliferation and/or differentiation of motor neuron progenitors after anterograde axonal transport and release in proximity to the trkB-expressing motor neuron precursors, thereby coordinating development between dorsal and ventral regions of the neural tube.

The one-eyed pinhead gene functions in mesoderm and endoderm formation in zebrafish and interacts with no tail

The zebrafish locus one-eyed pinhead (oep) is essential for the formation of anterior axial mesoderm, endoderm and ventral neuroectoderm. At the beginning of gastrulation anterior axial mesoderm cells form the prechordal plate and express goosecoid (gsc) in wild-type embryos. In oep mutants the prechordal plate does not form and gsc expression is not maintained. Exposure to lithium, a dorsalizing agent, leads to the ectopic induction and maintenance of gsc expression in wild-type embryos. Lithium treatment of oep mutants still leads to ectopic gsc induction but not maintenance, suggesting that oep acts downstream of inducers of dorsal mesoderm. In genetic mosaics, wild-type cells are capable of forming anterior axial mesoderm in oep embryos, suggesting that oep is required in prospective anterior axial mesoderm cells before gastrulation. The oep gene is also essential for endoderm formation and the early development of ventral neuroectoderm, including the floor plate. The loss of endoderm is already manifest during gastrulation by the absence of axial-expressing cells in the hypoblast of oep mutants. These findings suggest that oep is also required in lateral and ventral regions of the gastrula margin. The sonic hedgehog (shh).gene is expressed in the notochord of oep animals. Therefore, the impaired floor plate development in oep mutants is not caused by the absence of the floor plate inducer shh. This suggests that oep is required downstream or in parallel to shh signaling. The ventral region of the forebrain is also absent in oep mutants, leading to severe cyclopia. In contrast, anterior-posterior brain patterning appears largely unaffected, suggesting that underlying prechordal plate is not required for anterior-posterior pattern formation but might be involved in dorsoventral brain patterning. To test if oep has a wider, partially redundant role, we constructed double mutants with two other zebrafish loci essential for patterning during gastrulation. Double mutants with floating head, the zebrafish Xnot homologue, display enhanced floor plate and adaxial muscle phenotypes. Double mutants with no tail (ntl), the zebrafish homologue of the mouse Brachyury locus, display severe defects in midline and mesoderm formation including absence of most of the somitic mesoderm. These results reveal a redundant function of oep and ntl in mesoderm formation. Our data suggest that both oep and ntl act in the blastoderm margin to specify mesendodermal cell fates.

Spatial and temporal effects of axial structures on myogenesis of developing somites

To elucidate the precise roles of axial structures in the myogenic differentiation of the somite, we have examined the effects of the axial organs' precise spatial position during migration and differentiation of somitic cells by using in vivo transplantation of the neural tube and of the notochord directly into the paraxial mesoderm. Differentiation of myotomal cells was identified through the use of Quox 1 antibody which recognizes specifically a quail homeoprotein Quox 1. We have demonstrated that both ectopic neural tube and notochord are able to influence the myogenesis in somites, but that the spatial position of axial organs and the degree of somite maturation at grafting time are decisive. At the level of the somites which were already formed and developmentally advanced (somites III-VI), both neural tube and notochord promote myogenesis, and the promoting effect of notochord is more efficient than that of the neural tube. In the newly formed somites (I-II) and/or the segmental plate mesoderm, the notochord inhibits the myogenesis of somites, whereas the neural tube plays an evident myogenic promoting role. But the myogenic effect of the neural tube depends not only upon the stage of developing somites and presomitic mesoderm, but also on the developmental maturation of the neural tube. We have demonstrated that the myogenic effect of the rostral part of neural tube is stronger than that of its caudal part. This observation suggests that there is a gradient of myogenic effect along the rostrocaudal axis of the neural tube, which depends on the developmental maturation of neural tube, and that the generation of skeletal muscle during somitogenesis may be in relation with the rostrocaudal gradient of the capacity of the neural tube to stimulate myogenesis since somites are also distributed along an anteroposterior axis.

A spinal cord fate map in the avian embryo: while regressing, Hensen's node lays down the notochord and floor plate thus joining the spinal cord lateral walls

The spinal cord of thoracic, lumbar and caudal levels is derived from a region designated as the sinus rhomboidalis in the 6-somite-stage embryo. Using quail/chick grafts performed in ovo, we show the following. (1) The floor plate and notochord derive from a common population of cells, located in Hensen's node, which is equivalent to the chordoneural hinge (CNH) as it was defined at the tail bud stage. (2) The lateral walls and the roof of the neural tube originate caudally and laterally to Hensen's node, during the regression of which the basal plate anlage is bisected by floor plate tissue. (3) Primary and secondary neurulations involve similar morphogenetic movements but, in contrast to primary neurulation, extensive bilateral cell mixing is observed on the dorsal side of the region of secondary neurulation. (4) The posterior midline of the sinus rhomboidalis gives rise to somitic mesoderm and not to spinal cord. Moreover, mesodermal progenitors are spatially arranged along the rest of the primitive streak, more caudal cells giving rise to more lateral embryonic structures. Together with the results reported in our study of tail bud development (Catala, M., Teillet, M.-A. and Le Douarin, N.M. (1995). Mech. Dev. 51, 51–65), these results show that the mechanisms that preside at axial elongation from the 6-somite stage onwards are fundamentally similar during the complete process of neurulation.

Inhibition of floor plate differentiation by Pax3: evidence from ectopic expression in transgenic mice

The Pax genes containing a complete paired-type homeobox are expressed in restricted dorsoventral domains of the undifferentiated neuroepithelium. Their expression respond to signals that pattern the neural tube and which emanate from the notochord, floor plate and overlying ectoderm. In order to determine whether the dorsally restricted Pax3 gene can influence the fate of cells within the neural tube, we produced transgenic mice overexpressing Pax3 in the entire neural tube under the Hoxb-4 promoter/region A enhancer. In two distinct transgenic mouse lines, we observed embryos with abnormal limb, eye, brain and neural tube development. Expression of Pax3 was not sufficient to dorsalize cells from the ventral intermediate zone, which still expressed ventral markers. However, in these embryos, expression of Pax3 at the ventral midline was associated with the absence of floor plate differentiation. Under these conditions, motor neurons still differentiated in the ventral spinal cord, although in fewer numbers.

Notochord signals control the transcriptional cascade of myogenic bHLH genes in somites of quail embryos

Microsurgical, tissue grafting and in situ hybridization techniques have been used to investigate the role of the neural tube and notochord in the control of the myogenic bHLH genes, QmyoD, Qmyf5, Qmyogenin and the cardiac alpha-actin gene, during somite formation in stage 12 quail embryos. Our results reveal that signals from the axial neural tube/notochord complex control both the activation and the maintenance of expression of QmyoD and Qmyf5 in myotomal progenitor cells during the period immediately following somite formation and prior to myotome differentiation. QmyoD and Qmyf5 expression becomes independent of axial signals during myotome differentiation when somites activate expression of Qmyogenin and alpha-actin. Ablation studies reveal that the notochord controls QmyoD activation and the initiation of the transcriptional cascade of myogenic bHLH genes as epithelial somites condense from segmental plate mesoderm. The dorsal medial neural tube then contributes to the maintenance of myogenic bHLH gene expression in newly formed somites. Notochord grafts can activate ectopic QmyoD expression during somite formation, establishing that the notochord is a necessary and sufficient source of diffusible signals to initiate QmyoD expression. Myogenic bHLH gene expression is localized to dorsal medial cells of the somite by inhibitory signals produced by the lateral plate and ventral neural tube. Signaling models for the activation and maintenance of myogenic gene expression and the determination of myotomal muscle in somites are discussed.

The murine homeobox gene Msx-3 shows highly restricted expression in the developing neural tube

The mouse homeobox-genes Msx-1 and Msx-2 are expressed in several areas of the developing embryo, including the neural tube, neural crest, facial processes and limb buds. Here we report the characterisation of a third mouse Msx gene, which we designate Msx-3. The embryonic expression of Msx-3 was found to differ from that of Msx-1 and -2 in that it was confined to the dorsal neural tube. In embryos with 5-8 somites a segmental pattern of expression was observed in the hindbrain, with rhombomeres 3 and 5 lacking Msx-3 while other rhombomeres expressed Msx-3. This pattern was transient, however, such that in embryos with 18 or more somites expression was continuous throughout the dorsal hindbrain and anterior dorsal spinal cord. Differentiation of dorsal cell types in the neural tube can be induced by addition of members of the Tgf-beta family. Additionally, Msx-1 and -2 have been shown to be activated by addition of the Tgf-beta family member Bmp-4. To determine if Bmp-4 could activate Msx-3, we incubated embryonic hindbrain explants with exogenous Bmp-4. The dorsal expression of Msx-3 was seen to expand into more ventral regions of the neurectoderm in Bmp-4-treated cultures, implying that Bmp-4 may be able to mimic an in vivo signal that induces Msx-3.

Dependence of cranial motor neuron formation on ventromedial brain stem

The formation of motor neurons in the spinal cord is dependent on inductive signals from the floor plate and notochord. Motor neurons in the brain stem, on the other hand, develop in the absence of both structures. This suggests that either the germinal epithelium is specified intrinsically to form specific cranial motor nuclei or that the inductive signals for the formation of cranial motor neurons arise from some other structure. These possibilities were investigated experimentally by using the formation of trochlear motor neurons in the midbrain of duck embryos as a model system. The trochlear motor neurons, which form the nucleus of the fourth cranial nerve, developed normally after early damage to the prospective germinal epithelium, suggesting that it is unlikely to be specified intrinsically to form these cranial motor neurons. Instead, their development was found to be dependent on the cells within, or associated with, the ventromedial region of the brain stem, as the extirpation of this region results in the absence of motor neuron formation. These results show that structures other than the floor plate and notochord provide inductive signals for the cellular differentiation and patterning of the developing central nervous system. The raise the possibility that the inductive signals for motor neuron differentiation in the spinal cord and the brain stem may not be necessarily identical.

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.

The dorsal neural tube organizes the dermamyotome and induces axial myocytes in the avian embryo

Somites, like all axial structures, display dorsoventral polarity. The dorsal portion of the somite forms the dermamyotome, which gives rise to the dermis and axial musculature, whereas the ventromedial somite disperses to generate the sclerotome, which later comprises the vertebrae and intervertebral discs. Although the neural tube and notochord are known to regulate some aspects of this dorsoventral pattern, the precise tissues that initially specify the dermamyotome, and later the myotome from it, have been controversial. Indeed, dorsal and ventral neural tube, notochord, ectoderm and neural crest cells have all been proposed to influence dermamyotome formation or to regulate myocyte differentiation. In this report we describe a series of experimental manipulations in the chick embryo to show that dermamyotome formation is regulated by interactions with the dorsal neural tube. First, we demonstrate that when a neural tube is rotated 180 degrees around its dorsoventral axis, a secondary dermamyotome is induced from what would normally have developed as sclerotome. Second, if we ablate the dorsal neural tube, dermamyotomes are absent in the majority of embryos. Third, if we graft pieces of dorsal neural tube into a ventral position between the notochord and ventral somite, a dermamyotome develops from the sclerotome that is proximate to the graft, and myocytes differentiate. In addition, we also show that myogenesis can be regulated by the dorsal neural tube because when pieces of dorsal neural tube and unsegmented paraxial mesoderm are combined in tissue culture, myocytes differentiate, whereas mesoderm cultures alone do not produce myocytes autonomously. In all of the experimental perturbations in vivo, the dorsal neural tube induced dorsal structures from the mesoderm in the presence of notochord and floorplate, which have been reported previously to induce sclerotome. Thus, we have demonstrated that in the context of the embryonic environment, a dorsalizing signal from the dorsal neural tube can compete with the diffusible ventralizing signal from the notochord. In contrast to dorsal neural tube, pieces of ventral neural tube, dorsal ectoderm or neural crest cells, all of which have been postulated to control dermamyotome formation or to induce myogenesis, either fail to do so or provoke only minimal inductive responses in any of our assays. However, complicating the issue, we find consistent with previous studies that following ablation of the entire neural tube, dermamyotome formation still proceeds adjacent to the dorsal ectoderm. Together these results suggest that, although dorsal ectoderm may be less potent than the dorsal neural tube in inducing dermamyotome, it does nonetheless possess some dermamyotomal-inducing activity. Based on our data and that of others, we propose a model for somite dorsoventral patterning in which competing diffusible signals from the dorsal neural tube and from the notochord/floorplate specify dermamyotome and sclerotome, respectively. In our model, the positioning of the dermamyotome dorsally is due to the absence or reduced levels of the notochord-derived ventralizing signals, as well as to the presence of dominant dorsalizing signals. These dorsal signals are possibly localized and amplified by binding to the basal lamina of the ectoderm, where they can signal the underlying somite, and may also be produced by the ectoderm as well.

Formation of the cranial motor neurons in the absence of the floor plate

The inductive signals for the differentiation of motor neurons in the spinal cord have been experimentally shown to arise from cells in the midventral region of the neural tube, often referred to as the floor plate, and from the notochord. Although the prevailing view is that a similar mechanism accounts for the differentiation of motor neurons in the brain stem, supporting experimental evidence is lacking. Here, using the formation of the trochlear nucleus in the midbrain of duck embryos as a model system, we report that the floor plate and the notochord are not necessary for the development of these motor neurons in the brain stem. Early damage to the floor plate or extirpation of the floor plate and notochord does not prevent the development of these cranial motor neurons. Thus, either the inductive signals for the formation of these cranial motor neurons arise from some other structure or the germinal epithelium of the cranial neural tube is intrinsically programmed to form specific cranial motor nuclei.

Myogenesis in paraxial mesoderm: preferential induction by dorsal neural tube and by cells expressing Wnt-1

Previous studies have demonstrated that the neural tube/notochord complex is required for skeletal muscle development within somites. In order to explore the localization of myogenic inducing signals within the neural tube, dorsal or ventral neural tube halves were cultured in contact with single somites or pieces of segmental plate mesoderm. Somites and segmental plates cultured with the dorsal half of the neural tube exhibited 70% and 85% myogenic response rates, as determined by immunostaining for myosin heavy chain. This response was slightly lower than the 100% response to whole neural tube/notochord, but was much greater than the 30% and 10% myogenic response to ventral neural tube with and without notochord. These results demonstrate that the dorsal neural tube emits a potent myogenic inducing signal which accounts for most of the inductive activity of whole neural tube/notochord. However, a role for ventral neural tube/notochord in somite myogenic induction was clearly evident from the larger number of myogenic cells induced when both dorsal neural tube and ventral neural tube/notochord were present. To address the role of a specific dorsal neural tube factor in somite myogenic induction, we tested the ability of Wnt-1-expressing fibroblasts to promote paraxial mesoderm myogenesis in vitro. We found that cells expressing Wnt-1 induced a small number of somite and segmental plate cells to undergo myogenesis. This finding is consistent with the localized dorsal neural tube inductive activity described above, but since the ventral neural tube/notochord also possesses myogenic inductive capacity yet does not express Wnt-1, additional inductive factors are likely involved.

Dorsal differentiation of neural plate cells induced by BMP-mediated signals from epidermal ectoderm

The cellular interactions that control the differentiation of dorsal cell types from neural progenitors have been examined in neural plate explants. Certain genes that are expressed in the dorsal neural tube are initially expressed uniformly within the neural plate and appear to achieve their dorsal restriction through a Sonic hedgehog (SHH)-mediated repressive signal from the notochord. The acquisition of definitive dorsal cell fates, however, requires a contact-dependent signal from the epidermal ectoderm. BMP4 and BMP7 are expressed in the epidermal ectoderm, and both proteins mimic its inductive activity. BMP4 and a related gene, DSL1, are subsequently expressed by cells in the dorsal neural tube. The differentiation of dorsal cell types, therefore, appears to be initiated at the neural plate stage and to involve the opponent activities of a BMP-mediated dorsalizing signal from the epidermal ectoderm and a SHH-mediated ventralizing signal from the notochord.