Cellular contacts required for neural induction in Xenopus embryos: evidence for two signals

Development of the left-right axis in amphibians

The heart and viscera of vertebrates are formed from primordia that are apparently bilaterally symmetrical. This symmetry is broken during development, yielding organs that develop characteristic asymmetries along the left-right axis. Results from three lines of experimentation on embryos of the amphibian Xenopus laevis indicate that left-right asymmetries are established early in development and that cellular interactions transmit left-right information from one primordium to another. First, a cytoplasmic rearrangement that occurs during the first cell cycle after fertilization may establish left-right asymmetry in some regions of the embryo. Second, a variety of experimental results indicate that embryonic ectoderm or its basal extracellular matrix may transmit left-right axial information to cardiac mesoderm and visceral endoderm. Third, inhibition of proteoglycan synthesis during a narrow period of development, concurrent with the migration of the cardiac primordia to the ventral midline, prevents asymmetrical development of the heart.

Xenopus fibrillin is expressed in the organizer and is the earliest component of matrix at the developing notochord-somite boundary

We identify a Xenopus fibrillin homolog (XF), and show that its earliest developmental expression is in presumptive dorsal mesoderm at gastrulation, and that XF expression is regulated by mesoderm-inducing factors in animal cap assays. XF protein is also first detected in presumptive mesoderm, but is concentrated specifically into extracellular-matrix structures that begin to develop de novo by mid-gastrulation at both of the bilateral presumptive notochord-somite boundaries. Later in embryogenesis, XF protein is localized to the extracellular matrix at tissue boundaries, where it is found surrounding the notochord, the somites, and the neural tube, as well as under the epidermis. This pattern of protein deposition combines to give the appearance of an "embryonic skeleton," suggesting that one role for XF is to serve as a mechanical element in the embryo prior to bone deposition.

Regulatory principles of developmental signaling

Signaling between cells is a widely used mechanism by which cell fate and tissue patterning is determined in development. We review the mechanisms by which signaling between cells is regulated so that a cell receives the right amount of signal, at the right time, to achieve its intended developmental fate and position. In nearly all cases, we find that the supply of signal factor (ligand) is the limiting step in initiating a signaling process. Ligand supply is regulated by the transcription and localization of RNA, the spread of ligand from a source, and by inhibitors that operate at several different levels. We emphasize the different regulatory strategies that operate for threshold as opposed to concentration-dependent (morphogen) signaling. Threshold signaling is extensively regulated by feedback mechanisms. Morphogen signaling is regulated quantitatively by receptor loading and transduction flow.

Embryonic and postembryonic neurogenesis in the ventral nerve cord of the freshwater crayfish Cherax destructor

Previous studies of neurogenic activity in the thoracic neuromeres of indirect developing crustaceans indicated that the temporal patterns of neurogenesis can be correlated with the appearance of the thoracic appendages during larval and metamorphic development. To test further the idea that the temporal patterns of neurogenesis in crustaceans are related to their life histories, we examined neurogenesis in the ventral nerve cord of a direct developing crustacean, the freshwater crayfish Cherax destructor, whose life history contains neither larval stages nor metamorphoses. Neurogenesis was examined using the in vivo incorporation of bromodeoxyuridine into DNA. During late embryonic development the thoracic neuromeres of the crayfish contain arrays of mitotically active neuroblasts similar to those previously described in the spider crab and lobster. The arrays in the crayfish abdomen are, however, greatly reduced compared with those of the thorax. On hatching, both the thoracic and abdominal appendages of C. destructor are capable of movement. The pleopods, however, do not beat rhythmically until the second postembryonic stage whereas the pereiopods are not used in coordinated walking movements until the third stage. An examination of the time course of neurogenesis in the ventral nerve cord revealed that neurogenic activity in each neuromere ceases during or before the moult to the developmental stage in which its segmental appendage is first used in coordinated movements. These findings indicate that the patterns of neurogenesis in crustaceans are indeed related to the maturation of the segmental appendages and, in particular, to the maturation of motor behaviours.

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

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

Retinal lesions induce differential changes in the expression of flip and flop isoforms of the glutamate receptor subunit GluR1 in the chick optic tectum

A sensitive RNase protection assay was employed to determine the levels of mRNA encoding the GluR1 subunit flip and flop isoforms in the chick optic tectum and forebrain. We found that the flip GluR1 mRNA predominates in the forebrain, whereas the flop variant is more strongly expressed in the optic tectum. A temporal analysis of GluR1 variants in the embryonic and adult chick brain revealed that the flip isoform is more highly expressed at E12 than at P15-21, whereas mRNA levels of the flop isoform are higher at P15-21 than at E12. To study the effect of deafferentation on GluR1 expression, unilateral retinal lesions were performed. Two days later the mRNA levels of GluR1 flip and flop variants were decreased in the deafferented tectum, especially for the flop isoform. However, 7 days after the lesion, the mRNA levels of both GluR1 isoforms were increased, especially for the flip isoform. These results reveal an important control of the retinal input upon the expression of the different GluR1 isoforms. Furthermore, they indicate a differential spatial and temporal regulation of the flip and flop splice variants, suggesting the existence of a mechanism regulating differential splicing or possibly differential RNA stability.

Planar signalling is not sufficient to generate a specific anterior/posterior neural pattern in pseudoexogastrula explants from Xenopus and Triturus

Early observations on the morphology of total exogastrulae from urodeles (Axolotl) had provided evidence for essential vertical signalling mechanisms in the process of neural induction. Conversely, more recent studies with anurans (Xenopus laevis) making use of molecular markers for neural-specific gene expression appear to support the idea of planar signalling as providing sufficient information for neural differentiation along the anterior-posterior axis. In an attempt to resolve this apparent contradiction, we report on the comparative analysis of morphology and gene expression characteristics with explants prepared from both urodeles (Triturus alpestris) and anurans (Xenopus laevis). For this purpose, we have made use of a refined experimental protocol for the preparation of exogastrulae that is intended to combine the advantages of the Holtfreter type exogastrula and the Keller sandwich techniques, and which we refer to as pseudoexogastrula explants. Analysis of histology and expression of several neural and ectodermal marker genes in such explants suggests that neural differentiation is induced in both species, but only within the intermediate zone between ectoderm and endomesoderm. Therefore, experiments with Xenopus and Triturus explants described in this communication argue against planar signalling events as being sufficient to generate a specific anterior/posterior neural pattern.

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

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

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.

Role of activin and other peptide growth factors in body patterning in the early amphibian embryo

The amphibian body plan is established as the result of a series of inductive interactions. During early cleavage stages cells in the vegetal hemisphere induce overlying animal hemisphere cells to form mesoderm. The interaction represents the first major body-patterning event and is mediated by peptide growth factors. Various peptide growth factors have been implicated in mesoderm development, including most notably members of the transforming growth factor-beta superfamily. Identification of the so-called "natural" inducer from among the several candidate peptide growth factors is being achieved by employing several experimental strategies, including the use of a tissue explant assay for testing potential inducers, cloning of marker genes as indices of early induction events, and microinjection of altered peptide growth factor receptors to disrupt normal embryonic inductions. Activin emerges as the most likely choice for assignment of the role of endogenous mesoderm inducer, because it currently best fulfills the rigorous set of criteria expected of such an important embryonic signaling molecule. Activin, however, may not act alone in mesoderm induction. Other peptide growth factors such as fibroblast growth factor might be involved, especially in the regional patterning of the mesoderm. In addition, several genes (e.g., Wnt and noggin), which are expressed after the mesoderm is initially induced, probably assist in further definition of the mesoderm pattern. Following mesoderm induction, the primary embryonic organizer tissue (first described in 1924 by Spemann) develops and contributes further to body patterning by its action as a neural inducer. Peptide growth factors such as activin may also be involved in the inductive event, either directly (by facilitating gene expression) or indirectly (by serving to constrain pathways).

Regulation of ectodermal differentiation in Xenopus laevis animal caps treated with TPA and ammonium chloride

Animal caps isolated from Xenopus laevis embryos at the blastula stage were treated sequentially with NH4Cl, a known cement gland inducer, and with 12-O-tetradecanoyl phorbol-13-acetate (TPA), a known neural inducer. The two artificial inducers were also used in reverse order to see if they can mimic the natural inducers acting during the progressive determination of the ectodermal organ. Immunofluorescence and whole-mount in situ hybridization were used to study the expression of tubulin, taken to indicate an early step on the pathway of cell elongation, and neural cell adhesion molecule (N-CAM) taken to indicate an early step in the determination of the nervous system. The expression of XCG-1, a marker of early specification of the cement gland, was also studied. The results showed that the two artificial inducers can mimic the effects of the natural inducers in animal cap explants. The TPA behaves like a neural inducer, reducing the number and the extension of the cement gland when added to the medium in addition to NH4Cl, before or after NH4Cl treatment. In the process of cement gland/neural induction, it is possible to redirect the ectoderm already specified as cement gland to neural tissue, but it does not seem possible to respecify the neural tissue as cement gland. Moreover, the animal caps were also cut into dorsal and ventral parts and the two halves were treated separately. The results were similar to those obtained with treatment of the entire animal cap, suggesting that a dorsal-ventral pattern is not yet established before the gastrula stage, and that in normal embryos there are boundaries between the effects of different inducers.

Cellular mechanism underlying neural convergent extension in Xenopus laevis embryos

Convergent extension, the simultaneous narrowing and lengthening of a tissue, plays a major role in shaping and patterning the neural ectoderm in vertebrate embryos. In this paper, we characterize the cellular mechanism underlying convergent extension of the neural ectoderm in the Xenopus laevis late gastrula and neurula embryo. Neural ectoderm in X. laevis consists of two components, a superficial layer of epithelial cells overlying deep mesenchymal cells. To investigate the force contribution of the deep cells to convergent extension, we explanted single layers of neural deep cells from late gastrula stage embryos. These "neural deep cell explants" undergo active convergent extension autonomously, implying that these cells contribute force for neural convergent extension in vivo. Using time-lapse videorecording of these explants, we observed the neural deep cell behaviors (previously hidden behind an opaque epithelium) underlying convergent extension. We show that neural deep cells mediolaterally intercalate to form a longer, narrower tissue and that cell shape change and cell division contribute little to their convergent extension. Moreover, we characterize the neural deep cell motility driving mediolateral intercalation, also using time-lapse videorecordings. Analyses of these videos revealed that, on average, neural deep cells exhibit mediolaterally biased protrusive activity which is expressed in an episodic fashion. We propose that neural deep cells accomplish mediolateral intercalation by applying their protrusions upon one another, exerting traction, and pulling themselves between one another. This mechanism is similar to that previously described for convergent extension of the mesodermal cells. However, because the neural deep cells do not mediolaterally elongate during their convergent extension as the mesodermal cells do, we predict that a given intercalation will result in more extension for neural deep cells than for the mesodermal cells. Intercalation of neural cells also likely occurs in a more episodic manner than that of the mesodermal cells because the neural cells' mediolateral protrusive activity is episodic, whereas the protrusive activity of mesodermal cells is more continuous. These differences in protrusive activity and cell shape changes between the neural and mesodermal regions may reflect specializations of the same basic mechanism of mediolateral intercalation, tailored to accommodate other aspects of patterning and development of each tissue. These descriptions of the active cell motility underlying neural convergent extension in X. laevis are the first high-resolution video documentation of protrusive activity during neural convergent extension in any system. Our findings provide an important step in the investigation of neural convergent extension in X. laevis and further our understanding of convergent extension in general.

Epithelial cell wedging and neural trough formation are induced planarly in Xenopus, without persistent vertical interactions with mesoderm

In this study we investigate the induction of the cell behaviors underlying neurulation in the frog, Xenopus laevis. Although planar signals from the organizer can induce convergent extension movements of the posterior neural tissue in explants, the remaining morphogenic processes of neurulation do not appear to occur in absence of vertical interactions with the organizer (R. Keller et al. , 1992, Dev. Dyn. 193, 218-234). These processes include: (1) cell elongation perpendicular to the plane of the epithelium, forming the neural plate; (2) cell wedging, which rolls the neural plate into a trough; (3) intercalation of two layers of neural plate cells to form one layer; and (4) fusion of the neural folds. To allow planar signaling between all the inducing tissues of the involuting marginal zone and the responding prospective ectoderm, we have designed a "giant sandwich" explant. In these explants, cell elongation and wedging are induced in the superficial neural layer by planar signals without persistent vertical interactions with underlying, involuted mesoderm. A neural trough forms, and neural folds form and approach one another. However, the neural folds do not fuse with one another, and the deep cells of these explants do not undergo their normal behaviors of elongation, wedging, and intercalation between the superficial neural cells, even when planar signals are supplemented with vertical signaling until the late midgastrula (stage 11.5). Vertical interactions with mesoderm during and beyond the late gastrula stage were required for expression of these deep cell behaviors and for neural fold fusion. These explants offer a way to regulate deep and superficial cell behaviors and thus make possible the analysis of the relative roles of these behaviors in closing the neural tube.

The role of planar and early vertical signaling in patterning the expression of Hoxb-1 in Xenopus

In this paper we examine the contributions of planar and vertical signaling to the patterning of gene expression in neural development and we examine the routes of this neural induction. We have examined how the expression of Xenopus homeobox gene, Hoxb-1, is regulated by instruction from the mesoderm and/or endoderm and ask whether this instruction is by the vertical or planar routes. We investigated normal expression patterns of Hoxb-1 during early Xenopus development and Hoxb-1 expression in sandwich explants of the dorsal marginal zone, which putatively allow only planar signals to pass from the mesodermal and endodermal tissue (Spemann's organizer) to the prospective neural tissue. In the latter case we found significant variability of expression. Observations during dissections suggested that variable degrees of invasion of the mesodermal-endodermal tissue at the leading edge of the mesodermal mantle might be the cause of this variability. Alternatively, differing lengths of time that the prospective neural region spends in planar contact with tissues of the lateral or ventral regions of the embryo could also contribute to this variability. Analysis of staged Keller sandwich explants, "skewered" sandwiches, in which the degree of contact with underlying, involuted mesoderm-endodermal tissues was marked, and "over-the-pole" and "giant" sandwich explants, in which the degree of planar contact with lateral or ventral tissues was normalized, suggests that both planar and vertical signals are involved in induction and patterning of Hoxb-1 expression. The shift in Hoxb-1 expression from a broad, diffuse pattern to a local, focused pattern, characteristic of the ultimate expression pattern in vivo, does not reflect variable degrees of contact with ventral or lateral tissues, but rather reflects early vertical contact with underlying mesodermal-endodermal tissues. We observed such contact at early gastrula stages (stages 10 to 10+), stages commonly assumed not to have the potential for vertical signaling. As the bottle cells first begin to form, at stage 10-, a massive rotation of the lower involuting marginal zone occurs around an internal lip ("levre interne," Nieuwkoop and Florschutz, 1950). This rotation initiates the formation of the Cleft of Brachet from the floor of the blastocoele and brings the prospective mesoderm and endoderm at the leading edge of the marginal zone into vertical apposition with the prospective neural region quite early in gastrulation. The consequence and importance of recognizing these early internal rearrangements are that it pushes backward the time at which potential vertical inductive interactions between mesoderm and neurectoderm can occur. This means that a purely planar inductive situation can cease to exist as early as the inception of bottle cell formation and that neural patterning through vertical induction starts at the very beginning of gastrulation.

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.

Positive and negative signals modulate formation of the Xenopus cement gland

The cement gland is a simple secretory organ that marks the anterior-most dorsal ectoderm in Xenopus embryos. In this study, we examine the timing of cement gland induction and the cell interactions that contribute to cement gland formation. Firstly, we show that the outer ectodermal layer, from which the cement gland arises, becomes specified as cement gland by mid-gastrula. Curiously, at early gastrula, the inner layer of the dorsal ectoderm, which does not contribute to the mature cement gland, is strongly and transiently specified as cement gland. Secondly, we show that the mid-gastrula dorsoanterior yolky endoderm, which comes to underlie the cement gland primordium, is a potent inducer of cement gland formation and patterning. The cement gland itself has an anteroposterior pattern, with the gene XA expressed only posteriorly. Dorsoanterior yolky endoderm greatly enhances formation of large, patterned cement glands in partially induced anterodorsal ectoderm, but is unable to induce cement gland in naive animal caps. Neural tissue is induced less frequently than cement gland by the dorsoanterior yolky endoderm, suggesting that the endoderm induces cement gland directly. Thirdly, we demonstrate that the ventral ectoderm adjacent to the cement gland attenuates cement gland differentiation late during gastrulation. The more distant ventral mesendoderm is also a potent inhibitor of cement gland formation. These are the first data showing that normal ventral tissues can inhibit cement gland differentiation and suggest that cement gland size and position may be partly regulated by negative signals. Previous work has shown that cement gland can be induced by neural plate and by dorsal mesoderm. Together, these data suggest that cement gland induction is a complex process regulated by multiple positive and negative cell interactions.

Differentiation processes in the amphibian brain with special emphasis on heterochronies

Amphibians and caecilians exhibit a great variety of adult morphologies, life histories, and developmental strategies (biphasic development, direct development, viviparity, and neoteny). While early brain development and the differentiation of neural tissues in the three amphibian orders follow a basic pattern, differences exist in the onset and offset as well as the rate of growth and differentiation processes. These differences are described within a phylogenetic framework, and special emphasis is laid on the relationship between altered ontogenies and phylogenetic diversity. We concentrate on ontogenetic differentiation processes in the motor, olfactory, and visual system. We discuss the morphological consequences of secondary simplification of the brain in the context of paedomorphosis, which has happened several times independently among amphibians and consists in the abbreviation or truncation of late developmental processes. We deal with the cellular and molecular basis of brain development and the consequences for the adult nervous system in representative species of the three amphibian orders. Our analysis reveals that differences in brain morphology are largely due to heterochrony (i.e., the desynchronization of ontogenetic processes), a phenomenon that in turn is related to changes in genome sizes and life histories.

The Xenopus homologue of hepatocyte growth factor-like protein is specifically expressed in the presumptive neural plate during gastrulation

Using a RT-PCR approach, we were able to isolate a cDNA encoding the Xenopus homologue of hepatocyte growth factor-like protein, which we have termed accordingly Xhl. The deduced Xhl protein consists of 717 amino acids, contains four putative kringle domains and a serine protease-like domain characteristic for mammalian HGF and HGF-like protein. The mRNA of Xhl is exclusively expressed in the midline of the prospective neural plate during the period of neural induction, only. Ectopic expression of Xhl causes a 'spina bifida'-like phenotype with enlargement of neural tissue. Activation of Xhl mRNA transcription can be induced by delayed reaggregation of animal caps and appears to require vertical rather than planar signals from the organizer. These data suggest that Xhl is involved in the formation of the embryonic nervous system of Xenopus.

Fibroblast growth factor is a direct neural inducer, which combined with noggin generates anterior-posterior neural pattern

Neural tissue in developing Xenopus embryos is induced by signals from the dorsal mesoderm. Induction of anterior neural tissue could be mediated by noggin, a secreted polypeptide found in dorsal mesoderm. We show that bFGF, a known mesoderm inducer of blastula staged ectoderm, induces neural tissue from gastrula stage ectoderm. The type of neural tissue induced by bFGF from stage 10.25 ectoderm is posterior, as marked by Hox B9 expression. When bFGF and noggin are combined on early gastrula stage ectoderm, a more complete neural pattern is generated and no mesodermal tissue is detected. Explants treated with noggin and bFGF elongate and display distinct anterior and posterior ends marked by otx2 and Hox B9 expression, respectively. Furthermore, treatment of early gastrula ectoderm with noggin and bFGF results in the induction of En-2, a marker of the midbrain-hindbrain junction and Krox 20, a marker of the third and fifth rhombomeres of the hindbrain. Neither of these genes is induced by noggin alone or bFGF alone at this stage, suggesting a synergy in anterior-posterior neural patterning. The response of later gastrula (stage 11–12) ectoderm to bFGF changes so that Krox 20 and En-2 are induced by bFGF alone, while induction of more posterior tissue marked by Hox B9 is eliminated. The dose of bFGF affects the amount of neural tissue induced, but has little effect on the anterior-posterior character, rather the age of the ectoderm treated is the determinant of the response. Thus, an FGF signal may account for posterior neural induction, and anterior-posterior neural patterning could be partly explained by the actions of noggin and FGF, together with the changing response of the ectoderm to these factors.

Conversion of Xenopus ectoderm into neurons by NeuroD, a basic helix-loop-helix protein

Basic helix-loop-helix (bHLH) proteins are instrumental in determining cell type during development. A bHLH protein, termed NeuroD, for neurogenic differentiation, has now been identified as a differentiation factor for neurogenesis because (i) it is expressed transiently in a subset of neurons in the central and peripheral nervous systems at the time of their terminal differentiation into mature neurons and (ii) ectopic expression of neuroD in Xenopus embryos causes premature differentiation of neuronal precursors. Furthermore, neuroD can convert presumptive epidermal cells into neurons and also act as a neuronal determination gene. However, unlike another previously identified proneural gene (XASH-3), neuroD seems competent to bypass the normal inhibitory influences that usually prevent neurogenesis in ventral and lateral ectoderm and is capable of converting most of the embryonic ectoderm into neurons. The data suggest that neuroD may participate in the terminal differentiation step during vertebrate neuronal development.

Retinoic acid induces stage-specific antero-posterior transformation of rostral central nervous system

We report a time-course analysis of the effect of retinoic acid (RA) on the development of the mouse central nervous system (CNS) from the beginning of gastrulation throughout induction and patterning of the neural tube. RA administration induces three different, stage-specific alterations of brain development, indicating perturbation of different morphogenetic steps during the establishment of a neural pattern. In particular, treatment at mid-late streak stage (7.2-7.4 days post coitum (d.p.c.)) results in early repression of Otx2 expression in the posterior neuroectoderm of the head fold and in the ventral mid line, including the prechordal plate and the rostralmost endoderm, followed by loss of forebrain morphological and molecular identities, as revealed by analysis of the expression of regionally-restricted brain genes (Otx2, Otx1, Emx2, Emx1 and Dlx1). In these embryos, reduction of the Otx2 expression domain correlates with hindbrain expansion marked by rostral extension of the Hoxb-1 expression domain. Our analysis indicates that RA interferes with the correct definition of both planar and vertical morphogenetic signals at specific developmental stages by affecting gene expression in the regions which are likely either to produce or to respond to these signals. We suggest that retinoids may contribute to early definition of head from trunk structures by selecting different sets of regulatory genes.

XIPOU 2, a noggin-inducible gene, has direct neuralizing activity

XIPOU 2, a member of the class III POU domain family, is expressed initially in Spemann's organizer, and later, in discrete regions of the developing nervous system in Xenopus laevis. XIPOU 2 may act downstream from initial neural induction events, since it is activated by the neural inducer, noggin. To determine if XIPOU 2 participates in the early events of neurogenesis, synthetic mRNA was microinjected into specific blastomeres of the 32-cell stage embryo. Misexpression of XIPOU 2 in the epidermis causes a direct switch in cell fate from an epidermal to a neuronal phenotype. In the absence of mesoderm induction, XIPOU 2 has the ability to induce a neuronal phenotype in uncommitted ectoderm. These data demonstrate the potential of XIPOU 2 to act as a master regulator of neurogenesis.

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.

Widespread expression of the eve1 gene in zebrafish embryos affects the anterior-posterior axis pattern

The zygotic expression of the eve1 gene is restricted to the ventral and lateral cells of the marginal zone. At later stages, the mRNAs are localized in the most posterior part of the extending tail tip. An eve1 clone (pcZf14), containing a poly-A tail, has been isolated. In order to address eve1 gene function, pcZf14 transcript injections into zebrafish embryos have been performed. The injection into uncleaved eggs of a synthetic eve1 mRNA (12 pg), which encodes a protein of approximately 28 kd, produces embryos with anterior-posterior (A-P) axis defects and the formation of additional axial structures. The first category of 24 h phenotypes (87%) mainly displays a gradual decrease in anterior structures. This is comparable to previous phenotypes observed following Xhox3 messenger injection either in Xenopus or in zebrafish that have been classified according to the index of axis deficiency (zf-IAD). These phenotypes result in anomalies of the development of the neural keel, from microphthalmia to acephaly. The second category (13%) corresponds to the phenotypes described above together with truncal or caudal supernumerary structures. Additional truncal structures are the most prominent of these duplicated phenotypes, displaying a "zipper" shape of axial structures including neural keels and notochords. Caudal duplication presents no evident axis supernumerary structures. The observation of these phenotypes suggests an important role for the eve1 gene in mesodermal cell specification and in the development of the posterior region, and more particularly of the most posterior tail tip where endogenous eve1 messengers are found.

XASH genes promote neurogenesis in Xenopus embryos

Neural development in Drosophila is promoted by a family of basic helix-loop-helix (bHLH) transcription factors encoded within the Achaete Scute-Complex (AS-C). XASH-3, a Xenopus homolog of the Drosophila AS-C genes, is expressed during neural induction within a portion of the dorsal ectoderm that gives rise to the neural plate and tube. Here, we show that XASH-3, when expressed with the promiscuous binding partner XE12, specifically activates the expression of neural genes in naive ectoderm, suggesting that XASH-3 promotes neural development. Moreover, XASH-3/XE12 RNA injections into embryos lead to hypertrophy of the neural tube. Interestingly, XASH-3 misexpression does not lead to the formation of ectopic neural tissue in ventral regions, suggesting that the domain of XASH proneural function is restricted in the embryo. In contrast to the neural inducer noggin, which permanently activates the NCAM gene, the activation of neural genes by XASH-3/XE12 is not stable in naive ectoderm, yet XASH-3/XE12 powerfully and stably activates NCAM, Neurofilament and type III beta-tubulin gene expression in noggin-treated ectoderm. These results show that the XASH-3 promotes neural development, and suggest that its activity depends on additional factors which are induced in ectoderm by factors such as noggin.

Vertebrate embryonic induction: mesodermal and neural patterning

Within the fertilized egg lies the information necessary to generate a diversity of cell types in the precise pattern of tissues and organs that comprises the vertebrate body. Seminal embryological experiments established the importance of induction, or cell interactions, in the formation of embryonic tissues and provided a foundation for molecular studies. In recent years, secreted gene products capable of inducing or patterning embryonic tissues have been identified. Despite these advances, embryologists remain challenged by fundamental questions: What are the endogenous inducing molecules? How is the action of an inducer spatially and temporally restricted? How does a limited group of inducers give rise to diversity of tissues? In this review, the focus is on the induction and patterning of mesodermal and neural tissues in the frog Xenopus laevis, with an emphasis on families of secreted molecules that appear to underlie inductive events throughout vertebrate embryogenesis.

Negative control of Xenopus GATA-2 by activin and noggin with eventual expression in precursors of the ventral blood islands

To increase our understanding of haematopoiesis during early vertebrate development, we have studied the expression pattern of the transcription factor GATA-2 in Xenopus embryos, and asked how this is regulated. We show that the blood island precursors of the ventral mesoderm express GATA-2 RNA at neural tube stages, some 5 hours before globin RNA is detected in their derivatives. Prior to this however, GATA-2 is expressed much more widely within the embryo. Maternal transcripts are uniformly distributed, and zygotic transcription is activated during gastrulation throughout ventral and lateral regions of the embryo, with expression highest in the sensorial ectoderm and only weak in the ventral mesoderm. The domain of GATA-2 expression in neurulae outlines the region of the neural plate and suggests a possible wider role in dorsoventral patterning. To identify the signals involved in regulating this pattern of expression, we performed experiments with embryo explants. GATA-2 is activated autonomously in isolated animal caps and this activation is suppressed by the mesoderm-inducing factor activin, but not by FGF. Thus, the down-regulation of GATA-2 observed in the region of the Spemann organiser may be a response to an activin-like signal emanating from the dorsal-vegetal region or Nieuwkoop centre. GATA-2 activation in animal caps and ventral marginal zones was suppressed by co-culturing with dorsal marginal zones, suggesting that a signal from the Spemann organiser is involved in suppression of GATA-2 in the dorsal region of the embryo. Expression of a candidate for this signal, noggin, had the same effect. Taken together, the observations presented here suggest that GATA-2 activation occurs by default in the absence of signals, that the restriction of its expression within the early embryo is controlled by negative signals emanating from the Nieuwkoop centre and the organiser, and that noggin and activin-like molecules play a role in these signalling pathways.

The winged-helix transcription factor HNF-3 beta is required for notochord development in the mouse embryo

HNF-3 beta, a transcription factor of the winged-helix family, is expressed in embryonic and adult endoderm and also in midline cells of the node, notochord, and floor plate in mouse embryos. To define the function of HNF-3 beta, a targeted mutation in the HNF-3 beta locus was generated by homologous recombination in embryonic stem cells. Mice lacking HNF-3 beta die by embryonic day (E) 10-11. Mutant embryos examined from E6.5 to E9.5 do not form a distinct node and lack a notochord. In addition, mutant embryos show marked defects in the organization of somites and neural tube that may result from the absence of the notochord. The neural tube of mutant embryos exhibits overt anteroposterior polarity but lacks a floor plate and motor neurons. Endodermal cells are present but fail to form a gut tube in mutant embryos. These studies indicate that HNF-3 beta has an essential role in the development of axial mesoderm in mouse embryos.

Goosecoid expression in neurectoderm and mesendoderm is disrupted in zebrafish cyclops gastrulas

RNA from goosecoid, a homeobox-containing gene expressed during gastrulation in the anterior mesoderm of vertebrate embryos, can generate organizer activity when injected into ventral mesoderm, resulting in a secondary body axis; it is not yet understood, however, how goosecoid performs its organizer function. We report here that in the zebrafish gastrula, a domain of goosecoid expression arises in presumptive anterior neurectoderm which lies directly above goosecoid-expressing mesendodermal cells. From this position, goosecoid expression then spreads gradually across the ectodermal layer. In cyclops mutant embryos, which lack a ventral anterior brain, expression of goosecoid is abnormal in the mesendoderm and completely absent in the overlying neurectoderm. These results indicate that cyclops is required for correct specification of the mesendoderm and suggest that goosecoid expression in the ectoderm may result from vertical induction from the mesoderm. We propose that in the gastrula head, goosecoid may be important in organizing the ventral neurectoderm.

Expression of achaete-scute homolog 3 in Xenopus embryos converts ectodermal cells to a neural fate

In Drosophila, the proneural genes of the achaete-scute complex encode transcriptional activators that can commit cells to a neural fate. We have isolated cDNAs for two Xenopus achaete-scute homologs, ASH3a and ASH3b, which are expressed in a subset of central nervous system (CNS) neuroblasts during early neurogenesis. After expressing either ASH3 protein in developing Xenopus embryos, we find enlargement of the CNS at the expense of adjacent non-neural ectoderm. Analysis of molecular markers for neural, epidermal, and neural crest cells indicates that CNS expansion occurs as early as neural plate formation. ASH3-dependent CNS enlargement appears to require neural induction, as it does not occur in animal cap explants. Inhibition of DNA synthesis shows that additional CNS tissue does not depend on cell division--rather it reflects conversion of prospective neural crest and epidermal cells to a neural fate. The differentiation of the early forming primary neurons also seems to be prevented by ASH3 expression. This may be secondary to the observed activation of Xotch transcription by ASH3.

Expression of the LIM class homeobox gene Xlim-1 in pronephros and CNS cell lineages of Xenopus embryos is affected by retinoic acid and exogastrulation

The LIM class homeobox gene Xlim-1 is expressed in Xenopus embryos in the lineages leading to (i) the notochord, (ii) the pronephros, and (iii) certain cells of the central nervous system (CNS). In its first expression phase, Xlim-1 mRNA arises in the Spemann organizer region, accumulates in prechordal mesoderm and notochord during gastrulation, and decays in these tissues during neurula stages except that it persists in the posterior tip of the notochord. In the second phase, expression in lateral mesoderm begins at late gastrula, and converges to the pronephros at tailbud stages. Expression in a central location of the neural plate also initiates at late gastrula, expands anteriorly and posteriorly, and becomes established in the lateral regions of the spinal cord and hindbrain at tailbud stages. Thus Xlim-1 expression precedes morphogenesis, suggesting that it may be involved in cell specification in these lineages. Enhancement of Xlim-1 expression by retinoic acid (RA) was first detectable in the dorsal mesoderm at initial gastrula. During gastrulation and early neurulation, RA strongly enhanced Xlim-1 expression in all three lineages and also expanded its expressing domains; this overexpression correlated well with RA phenotypes such as enlarged pronephros and hindbrain-like structure. Exogastrulation reduced Xlim-1 expression in the lateral mesoderm and ectoderm but not in the notochord, suggesting that the second phase of Xlim-1 expression requires mesoderm/ectoderm interactions. RA treatment of exogastrulae did not revert this reduction.

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.

Inhibition of activin receptor signaling promotes neuralization in Xenopus

Expression of a truncated activin type II receptor, which blocks signaling by activin, neuralizes explants of embryonic cells that would otherwise become epidermal cells. This neuralization is direct and does not require the presence of mesoderm. The induced neural tissue expresses general molecular markers of the central nervous system as well as an array of neural markers along the anteroposterior axis. In the context of the whole embryo, expression of this truncated activin receptor diverts prospective ectoderm and endoderm to a neural fate. We propose that inhibition of the activin type II receptor signaling causes the cells of Xenopus embryos to adopt a neural fate. These results, along with previous experiments performed in Drosophila, suggest that the formation of the nervous system in vertebrates and invertebrates occurs by a common strategy.

An increase in intracellular pH during neural induction in Xenopus

In this paper, we show that an intracellular alkalinization of the dorsal ectoderm cells is among the earliest responses to neural induction in Xenopus. Planar explants of the dorsal marginal zone were prepared from embryos that had been microinjected during cleavage stages with the fluorescent pH indicator bis-carboxyethyl-carboxyfluorescein-dextran (BCECF-dextran), and intracellular pH (pHi) was monitored continuously by emission ratio microfluorimetry. During stage 10.5, the dorsal ectoderm cells undergo a sustained intracellular alkalinization of approximately 0.1 pH units in response to neural induction; in the absence of the inductive signal, the pH of the dorsal ectoderm cells decreases slightly. Ectoderm cells within planar explants of the ventral marginal zone show little change in pH during a similar period. This increase in intracellular pH is inhibited by 4, 4′-dihydrodiisothiocyanatostilbene-2, 2′-disulfonate (H2DIDS) or a low Na+/high Cl- medium, treatments that presumably affect anion transport. Under these conditions, expression of the anterior neural-specific homeobox gene engrailed is not detected, while the notochord-specific epitope recognized by the Tor-70 antibody is expressed in the presence of H2DIDS. This characteristic alkalinization is not evoked by pharmacological agents that reportedly alter ectodermal developmental pathways in Xenopus embryos, such as NH4Cl, phorbol esters, or cAMP-dependent protein kinase agonists. Our results suggest that an ionic regulatory event may participate in the regulation of gene expression in response to neural induction.

Inducing factors in Xenopus early embryos

Recent results make it possible to postulate credible candidates for each of the known inducing signals that act to determine cell fate during Xenopus early development. Experiments on biological activity, expression patterns and inhibition of function suggest that Vg-1 and Wnt-11 may act as the primary mesoderm-inducing signals, FGF and activin may serve to relay their effects, and noggin may be a major component of the dorsalizing and neural-inducing signals from the organizer.

Genetic control of early neuronal development in vertebrates

The specification of neuronal fate starts with cell commitment and determination. These early events are accompanied by rearrangement and reshaping of presumptive neural cells. Later, the neural differentiation begins, and its course can be followed using specific molecular markers. Such events take place long before the cells acquire a typical neuronal phenotype. Primary neurons of lower vertebrates differ from secondary neurons by their size, position, timing of differentiation and length of axon. Primary neurons start to express early markers of neural differentiation at the end of gastrulation. Recent data indicate that in lower vertebrates the neural induction of primary neurons differs from the induction of secondary neurons; however, neural induction in higher vertebrates appears to be similar to the induction of secondary neurons in lower vertebrates.

Tail formation as a continuation of gastrulation: the multiple cell populations of the Xenopus tailbud derive from the late blastopore lip

Three lines of evidence suggest that tail formation in Xenopus is a direct continuation of events initiated during gastrulation. First, the expression of two gene markers, Xbra and Xnot2, can be followed from the blastopore lip into distinct cell populations of the developing tailbud. Second, the tip of the tail retains Spemann's tail organizer activity until late stages of development. Third, lineage studies with the tracer DiI indicate that the cells of the late blastopore are fated to form specific tissues of the tailbud, and that intercalation of dorsal cells continues during tail elongation. In particular, the fate map shows that the tip of the tail is a direct descendant of the late dorsal blastopore lip. Thus, the tailbud is not an undifferentiated blastema as previously thought, but rather consists of distinct cell populations which arise during gastrulation.

Sequential expression of HNF-3 beta and HNF-3 alpha by embryonic organizing centers: the dorsal lip/node, notochord and floor plate

Axial patterning in the nervous system of vertebrate embryos depends on inductive signals that derive from the organizer region (the dorsal lip in amphibians and the node in birds and mammals) and leter from the notochord and floor plate. Previous studies have shown that Pintallavis, a member of the HNF-3/fork head transcription factor family, is expressed selectively by these cell groups in frog embryos and may be involved in regulating neural development. We report here that in early rat and mouse embryos, the embryonic endoderm, the node, the notochord and the floor plate express two related transcription factors, HNF-3 alpha and HNF-3 beta, which also function in the control of liver cell differentiation. Early embryonic tissues express variant forms of HNF-3 beta which derive from the use of 5' alternative exons. Within the organizer region and notochord, HNF-3 beta and HNF-3 alpha have distinct temporal patterns of expression and appear in partially overlapping domains. The early expression pattern of mammalian HNF-3 beta in the node, notochord and midline neural plate cells is similar to that of Pintallavis in frog embryos. There does not appear to be a Pintallavis homologue in mice. This prompted us to isolate and analyze the expression of the frog HNF-3 beta gene. In frog embryos, HNF-3 beta is expressed in the dorsal lip, pharyngeal endoderm and floor plate. In contrast to mammalian HNF-3 beta, the onset of frog HNF-3 beta expression in neural tissue occurs after neural tube closure. Thus, the combined expression patterns of Pintallavis and HNF-3 beta in frogs is equivalent to that of HNF-3 beta in rats and mice. Within neural tissue, the onset of expression of these regulatory genes define successive stages in the differentiation of floor plate cells. The results reported here show that closely related members of the HNF-3/fork head gene family are expressed by axial midline cell groups involved in neural induction and patterning and suggest the involvement of these genes in the development of the vertebrate neuraxis.

Neural induction by the secreted polypeptide noggin

The Spermann organizer induces neural tissue from dorsal ectoderm and dorsalizes lateral and ventral mesoderm in Xenopus. The secreted factor noggin, which is expressed in the organizer, can mimic the dorsalizing signal of the organizer. Data are presented showing that noggin directly induces neural tissue, that it induces neural tissue in the absence of dorsal mesoderm, and that it acts at the appropriate stage to be an endogenous neural inducing signal. Noggin induces cement glands and anterior brain markers, but not hindbrain or spinal cord markers. Thus, noggin has the expression pattern and activity expected of an endogenous neural inducer.

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.

Heterotopic transplantation of presumptive placodal ectoderm changes the fate of sensory neuron precursors

The placode-derived cranial sensory neurons of the vestibular and nodose ganglia in avian embryos exhibit differences in neurite growth rate and the duration of neurotrophin-independent survival in vitro that arise prior to gangliogenesis and target contact (Davies, A. M. (1989) Nature 337, 553–555; Vogel, K. S. and Davies, A. M. (1991) Neuron 7, 819–830). To ascertain the state of commitment of presumptive placodal ectoderm to differentiate into neurons of the vestibular or nodose type, we performed heterotopic transplantation of labelled presumptive placodal ectoderm at E1.5 in the chicken embryo. We then assayed transplant-derived neurons for hindbrain innervation patterns, neurite growth and survival at E3.5. We show that presumptive placodal ectoderm is not determined to give rise to neurons of the vestibular or nodose phenotype at E1.5. Explantation of presumptive placodal ectoderm at E1.5 showed that this ectoderm is also not specified to differentiate into neurons at this stage. In addition, we demonstrate that non-neurogenic ectoderm from the trunk can give rise to nodose-type neurons when transplanted heterotopically to the nodose region.

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.

Induction of neuronal differentiation by planar signals in Xenopus embryos

The induction of the central nervous system in amphibian embryos is mediated both by early planar signals produced by mesoderm at the dorsal lip and later vertical signals emanating from the dorsal mesoderm after involution. We have examined the role and spatial extent of planar signals in the induction of neuronal differentiation. Planar explants that included only the deep layer of the dorsal marginal zone, comprising both the dorsal mesoderm and the contiguous dorsal ectoderm, were isolated at the beginning of gastrulation. After removal of the epithelial layer, explants were maintained in modified Danilchik's medium until mid-neurula stages, when they were transferred to modified Danilchik's medium + 0.1% bovine serum albumin and cultured on laminin. Neurite outgrowth occurred in 90% of these planar explants. In contrast, little or no neuronal differentiation occurred in either ventral planar explants or explants of ectoderm alone. Video analysis of cell movements shows that large-scale cell mixing does not occur between mesoderm cells and ectoderm cells in planar explants. Retrograde labelling of neuronal cell bodies indicates that cells throughout the ectoderm undergo neuronal differentiation; neurons also differentiate in cultures of distal ectoderm isolated at early neurula stages from planar explants prepared at the beginning of gastrulation. These observations indicate that planar signals act over an extended range to induce neuronal differentiation. The inductive capacity of vertical signals was examined by recombining animal caps from ultra-violet (UV) irradiated embryos with involuted mesoderm from normal midgastrula embryos. Differentiation of either neurons or anterior neural structures occurred in 73% of vertical recombinates. Our results demonstrate that planar signals from the dorsal lip of the blastopore are capable of inducing neuronal differentiation over a considerable distance in the absence of epithelial confinement, convergence and extension, and mixing between the mesoderm and ectoderm.