Expression of an engrailed-like gene during development of the early embryonic chick nervous system

Rostral optic tectum acquires caudal characteristics following ectopic engrailed expression

BACKGROUND

Expression of the homeobox-containing gene Engrailed (En) in an increasing rostral-to-caudal gradient in the dorsal mesencephalon is the earliest known marker for polarity of the chick optic tectum. In heterotopic transplantation experiments, En protein expression correlates well with the subsequent gradient of cytoarchitecture as well as the pattern of retinotectal projections. The En gradient also correlates with the expression of two putative retinal axon-guidance molecules, RAGS and ELF-1, which are Eph-like receptor tyrosine kinase ligands that may function in the establishment of retinotopic projections by excluding temporal axons from the caudal tectum.

RESULTS

To examine the function of En in determining tectal polarity, we used the replication-competent retroviral vector RCAS to misexpress mouse En-1 throughout the chick tectal primordium. Our results show that the rostral portion of the tectum adopts a caudal phenotype: the gradient of cytoarchitectonic differentiation is abolished, and the molecular markers RAGS and ELF-1 are strongly expressed rostrally. In addition, cell membranes from rostral tectum of RCAS En-1-infected embryos preferentially repel temporal axons in in vitro membrane stripe assays.

CONCLUSIONS

These results are consistent with a role for En in determining rostrocaudal polarity of the developing tectum. The demonstration that both RAGS and ELF-1 are upregulated following En misexpression provides a molecular basis for understanding the previous observation, also based on retrovirus-mediated En misexpression, that nasal axons form ectopic connections in rostral tectum, from which temporal axons are excluded.

A role for gradient en expression in positional specification on the optic tectum

The optic tectum, the primary visual center in non-mammalian vertebrates, receives retinal fibers in a topographically ordered manner. en (en-1 and en-2, homologs of the Drosophila segment polarity gene engrailed) is expressed in the tectal primordium in a rostrocaudal gradient, around the stage when the polarity of the retinotectal projection map is being determined. Here we report that scattered en expression, caused by retroviral gene transfer, perturbed the retinotopic order. Nasal retinal fibers, which normally recognize the caudal side of the tectum (strong en expression side) as a target, arborized at ectopic sites, as if they found their targets, or degenerated. Temporal retinal fibers, which normally recognize the rostral side of the tectum (weak en expression side) as a target, were also affected in some cases by degeneration or prevention of innervation in the tectum. These results suggest that gradient en expression defines the positional identity of the tectum along the rostrocaudal axis.

Involvement of Wnt-1 in the formation of the mes/metencephalic boundary

Wnt-1, a putative signaling molecule, is required before the 7 somite stage (E8.5) for the development of midbrain structures in the mouse. We show here that Wnt-1 is also needed for the formation of a boundary between the mesencephalic and metencephalic domains of the neural tube. In embryos homozygous for the Wnt-1sw allele, mesencephalic and metencephalic markers fail to segregate and the establishment of a straight limit of Otx-2 and Wnt-1 expression at the mid-hindbrain junction is impaired. In addition, as observed previously in heterotopic mes/metencephalic transplantation experiments in avian embryos, Wnt-1 expression is induced at the border of ectopic mes- and metencephalic islands observed in Wnt-1sw/sw mutants, suggesting that, in situ, interactions between mes- and metencephalic cells reinforce Wnt-1 expression at the boundary.

Identification of a neurogenic sublineage required for CNS segmentation in an Annelid

In embryos of leeches (phylum Annelida), metameric structures arise sequentially from a germinal plate comprising the descendants of five pairs of embryonic stem cells called teloblasts. It has been shown that transverse stripes of cells expressing ht-en (a homolog of engrailed, a Drosophila segment polarity gene), arise in the germinal plate prior to the appearance of segmental ganglia and that, in the main neurogenic lineage (derived from the N teloblasts), the stripe of cells expressing ht-en demarcates the boundary between prospective segmental ganglia. Previous lineage-tracing experiments had suggested that the clones of nf and ns primary blast cells in the N lineage are confined to within segmental borders. This conclusion was called into question by the observation that the cells expressing ht-en do not appear to be at the very posterior edge of the nf clone, from which they arise. To resolve this issue, we have injected individual primary blast cells with fluorescent lineage tracers; we find that cells in the nf clone actually straddle two adjacent ganglia. Moreover, using photoablation techniques, we find that the nf clone is required for proper morphogenesis of the segmentally iterated central nervous system (CNS).

Induction of ectopic engrailed expression and fate change in avian rhombomeres: intersegmental boundaries as barriers

We tested the possibility of inducing ectopic expression of the gene Engrailed-2 (En-2) in the rhombomeres of the 2-days-old chick embryo. The experimental procedure consisted of grafting En-2-expressing neuroepithelium (prospective isthmocerebellum) from a quail or a mouse embryo into different rostrocaudal levels in the hindbrain of a host chick embryo. The graft replaced a given excised host rhombomere, the rostral and caudal limits of which were either also excised or left intact in different experiments. Induction of En-2 occurred in the host hindbrain, but only when the graft did not contact with host interrhombomeric boundaries and only in the alar plate of the rhombomeres immediately contacting the graft. Long survival experimental embryos showed that induced rhombomeres give rise to a cerebellar phenotype in their transformed alar plates. We thus demonstrate here a pluripotential state of the early rhombomeres as well as a possible role of the interrhombomeric limits as barriers to morphogenetic influences.

Expression of the homeobox-containing gene En-2 during the development of the chick central nervous system

The expression of the homeobox-containing gene En-2 was analysed with the monoclonal antibody 4D9 in the chick central nervous system throughout embryogenesis. Confirming previous studies, early expression of the En-2 protein [beginning at stage 9 of Hamburger and Hamilton (HH9)] is restricted to a portion of the neural tube containing the primordia of the cerebellum, the isthmic region and the mesencephalic grisea, and forms a double gradient decreasing both caudally and rostrally from a high point located around the midbrain-hindbrain constriction. This mes-isthmo-cerebellar region contains all the En-2-positive germinative cells and the great majority of the En-2-positive postmitotic neurons throughout embryogenesis. Nevertheless, as the postmitotic neurons appear, En-2 expression also occurs outside this region: in two columns of non-motoneuron cells in rhombomeres two to four (between HH20 and HH30) and, from HH24 onwards, throughout the grey matter of the lumbar and thoracic spinal cord, with the exception of the ventral motoneuron columns. Here, a detailed description of En-2 expression is provided for the mes-isthmo-cerebellar region at stages HH30-32 [embryonic day (E) 7], HH37 (E11) and HH46 (E21, hatching). This allows the visualization of cellular groups with heterogeneous patterns of En-2 expression, which are specific for each group in the intensity of En-2 expression, the distribution of the labelled cells and the temporal regulation of the gene. The use of tyrosine hydroxylase antiserum shows coexpression of the tyrosine hydroxylase enzyme and En-2 protein in the caudal part of the nuclei tegmenti pedunculo-pontinus, the area ventralis of Tsai and the substantia grisea centralis, but not in the locus coeruleus. In the cerebellum, the first expression, which is located in the deep nuclei and parasagittal bands of Purkinje cells, is down-regulated when the molecular layer interneurons and the granular cells begin to express the gene, at the end of embryogenesis. Finally, at hatching, En-2 expression permits the visualization in the cerebellum of a population of small En-2-negative cells located around the Purkinje cells that may correspond to those described in chick/quail chimaeras as having an origin different from that of the bulk of granular neurons.

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.

The status of the neural segment

A crucial phase of development in the vertebrate rhombencephalon involves transient organization into segments. Recent studies on the diencephalon and telencephalon pose the question of whether segmentation might also play a role in the development of more rostral brain regions. Criteria for segmentation formulated for the hindbrain might be met by the diencephalon, although there is disagreement as to the number and arrangement of segmental units. In contrast to the hindbrain, these segments appear when neurogenesis has begun, and might represent definitive functional units. Regarding the telencephalon, it is at present unclear whether domains of gene expression are associated with other features that are characteristic of segmental development, or whether other mechanisms control specification of this region.

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

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

Ectopic induction and reorganization of Wnt-1 expression in quail/chick chimeras

When grafted ectopically into the diencephalon of a chick host embryo, a portion of met-mesencephalon straddling the met-mesencephalic constriction has the capacity to induce En-2 expression in the surrounding host tissue. Subsequently, tectal and cerebellar structures, composed of both host and grafted cells, are reconstructed in this ectopic location at the expense of the host diencephalon. Previous experiments indicated that the induction of En-2 was correlated with Wnt-1 expression within the graft. The aim of the present study was: (i) to determine whether Wnt-1 expression was spatially regulated within the graft, (ii) to investigate whether host Wnt-1-expressing cells were also involved in the ectopic met-mesencephalic development and, if so, (iii) to localize these Wnt-1-positive domains in relation to the patterning of the ectopically developing met-mesencephalic territory. We studied the expression profile of Wnt-1, in relation with that of other positional markers, in quail/chick chimeras where various portions of met-mesencephalon had been grafted into the diencephalon. We found that Wnt-1 expression was reorganized within the graft, and that it was also induced in the host in contact with the graft. Moreover, these ectopic expressions of Wnt-1, in both the grafted and the surrounding host tissues, were organized in concert to form a continuous positive line at the host/graft junction, the location of which depended on the precise origin of the graft. Finally, we found that this line was frequently located at the limit between territories expressing different positional markers. We propose that Wnt-1 expression is turned on at the junction between domains of different phenotypes, and may be used as a border to stabilize these adjacent differently committed territories.

Plasticity in mesencephalic and retinal polarity formation in avian embryos

Polarity and its plasticity in the avian mesencephalon and the retina are reviewed. Rostrocaudal polarity of the tectum (dorsal mesencephalon) is revealed by the gradient of engrailed gene expression, by its cytoarchitecture, and by positional specification of tectal cells which retinal fibers read to find their targets. In the ventral mesencephalon, the polarity is revealed by directional growth of tectofugal axons. To examine the plasticity in rostrocaudal polarity, rostrocaudal orientation of the mesencephalon was surgically reversed on E2. Rostrocaudal polarity of the reversed mesencephalon was all adjusted to the host pattern. This suggests that the mesencephalic polarity is not fixed on E2 and is established under environmental influences. Retinal polarity is revealed by the distribution of certain molecules, by retinal structures, and by the retinotectal projection pattern. Retinal structures and retinotectal projection pattern were kept as in the original if retinal primordium was rotated or partially excised after stages 12-14 of Hamburger and Hamilton. The location of an optic fissure and the retinal fiber pattern were adjusted to those of normal eyes when the rotation was performed before stage 12, therefore the retinal polarity was supposed to be plastic before stage 12 and to be fixed around the stages 12-14 under environmental influences.

The molecular basis of retinotectal topography

Over 50 years have passed since Roger Sperry formulated a simple model of how visual space, as seen by the retina, can be projected onto the brain in a two-dimensional, topographic map during development. Sperry posited a set of two orthogonal gradients in the retina that gives each cell a positional identity. He further suggested that these molecules could be used to match up with complementary gradients in the target field of the retinal projection, the tectum. While some investigators hold that the existence of such molecules may not be necessary to establish retinotectal maps, recent work has identified several cell surface proteins whose distributions are of the type predicted by Sperry. An unexpected twist comes from culture assays demonstrating that inhibitory activities on tectal membranes can guide the growth of processes from retinal neurons. Moreover, the expression patterns of several enzymes and three transcription factors suggest that these proteins are candidates for regulatory agents in the determination of cell position in the retina. In addition, results from perturbation experiments support the candidacy of two of the enzymes, and a new mutant screen has uncovered several as yet unidentified genes that are required for establishment of the proper retinotectal map. A number of these results were presented at a recent meeting on neurospecificity held in Cargese, Corsica and sponsored by NATO and NSF.

Plasticity of avian mesencephalic polarity revealed by trajectories of tectofugal axons

In normal E6 (sixth day of incubation) quail mesencephalons, circumferential axons originate in the tectum (the dorsal part of a mesencephalon), course in a ventral direction and split into three trajectories in the ventral region of the mesencephalon; two of them turned on the ipsilateral side of the mesencephalon, one rostrally and the other caudally, while the third crossed the ventral midline and turned caudally on the contralateral side. In this study, we examined whether the ventral part of the mesencephalon has plasticity in its rostrocaudal polarity formation. We transplanted quail mesencephalons at the 9-10 somite stage (E2) with reversed rostrocaudal orientation prior to axon outgrowth. In reversely transplanted mesencephalons, circumferential axons at E6 took the same trajectory pattern as that of normal embryos; one turned rostrally and the second caudally on the ipsilateral side, and the third one turned caudally on the contralateral side. These results indicate that the rostrocaudal polarity of the mesencephalon is not fixed at E2, and that it may develop under the influence of tissues surrounding the mesencephalon.

Tracing neuroepithelial cells of the mesencephalic and metencephalic alar plates during cerebellar ontogeny in quail-chick chimaeras

The quail-chick chimaera system was used to investigate the origin of the various neuronal cell types of the cerebellum from the mesencephalic and metencephalic brain vesicles at the 11- to 14-somite stage in the avian embryo. We have already demonstrated that the cerebellum is derived from both the mesencephalic and metencephalic brain vesicles. The mesencephalic contribution to the cerebellum is restricted to a mediodorsal territory inserted as a V-shaped area into the primitive metencephalic vesicle through complex morphogenetic movements taking place from day 2 to day 4 of embryonic development. Here we report that the cerebellar presumptive territory extends along the anteroposterior axis, over the caudal half of the mesencephalon and the rostral half of the metencephalon. Along the dorsoventral axis, the cerebellar anlage is located in the alar plates at the exclusion of the roof and basal plates, i.e. lies in the lateral walls of the neuroepithelium in the area included between approximately 25 and 120 degrees with respect to the sagittal plane. We also report that the neuroepithelium corresponding to the cerebellar presumptive territory also yields other brain structures (e.g. part of the optic tectum in the mesencephalon). The external granular layer (EGL) arises only from the rostral metencephalon, undergoing extensive tangential movements which we have analysed in detail: the more ventral the position of cells in the metencephalic alar plates the more rostral and lateral is their position in the EGL. Finally, we discuss the fact that the cerebellar cortex, an integrative structure of the brain, arises from the alar plates of the neural tube. This is consistent with the general spatial organization of the neural anlage of the vertebrate embryo, in which this part of the neuroepithelium is devoted to the production of interneurons, whereas the basal plate and the neural folds yield motor structures and primary sensory neurons respectively.

Segmental migration of the hindbrain neural crest does not arise from its segmental generation

The proposed pathways of chick cranial neural crest migration and their relationship to the rhombomeres of the hindbrain have been somewhat controversial, with differing results emerging from grafting and DiI-labelling analyses. To resolve this discrepancy, we have examined cranial neural crest migratory pathways using the combination of neurofilament immunocytochemistry, which recognizes early hindbrain neural crest cells, and labelling with the vital dye, DiI. Neurofilament-positive cells with the appearance of premigratory and early-migrating neural crest cells were noted at all axial levels of the hindbrain. At slightly later stages, neural crest cell migration in this region appeared segmented, with no neural crest cells obvious in the mesenchyme lateral to rhombomere 3 (r3) and between the neural tube and the otic vesicle lateral to r5. Focal injections of DiI at the levels of r3 and r5 demonstrated that both of these rhombomeres generated neural crest cells. The segmental distribution of neural crest cells resulted from the DiI-labelled cells that originated in r3 and r5 deviating rostrally or caudally and failing to enter the adjacent preotic mesoderm or otic vesicle region. The observation that neural crest cells originating from r3 and r5 avoided specific neighboring domains raises the intriguing possibility that, as in the trunk, extrinsic factors play a major role in the axial patterning of the cranial neural crest and the neural crest-derived peripheral nervous system.

Cloning and sequence comparison of the mouse, human, and chicken engrailed genes reveal potential functional domains and regulatory regions

We have isolated and characterized genomic DNA clones for the human and chicken homologues of the mouse En-1 and En-2 genes and determined the genomic structure and predicted protein sequences of both En genes in all three species. Comparison of these vertebrate En sequences with the Xenopus En-2 [Hemmati-Brivanlou et al., 1991) and invertebrate engrailed-like genes showed that the two previously identified highly conserved regions within the En protein ]reviewed in Joyner and Hanks, 1991] can be divided into five distinct subregions, designated EH1 to EH5. Sequences 5' and 3' to the predicted coding regions of the vertebrate En genes were also analyzed in an attempt to identify cis-acting DNA sequences important for the regulation of En gene expression. Considerable sequence similarity was found between the mouse and human homologues both within the putative 5' and 3' untranslated as well as 5' flanking regions. Between the mouse and Xenopus En-2 genes, shorter stretches of sequence similarity were found within the 3' untranslated region. The 5' untranslated regions of the mouse, chicken and Xenopus En-2 genes, however, showed no similarly conserved stretches. In a preliminary analysis of the expression pattern of the human En genes, En-2 protein and RNA were detected in the embryonic and adult cerebellum respectively and not in other tissues tested. These patterns are analogous to those seen in other vertebrates. Taken together these results further strengthen the suggestion that En gene function and regulation has been conserved throughout vertebrate evolution and, along with the five highly conserved regions within the En protein, raise an interesting question about the presence of conserved genetic pathways.

Structure and early embryonic expression of the zebrafish engrailed-2 gene

The Drosophila homeobox gene engrailed (en) is needed for correct embryonic development, and related sequences are active during vertebrate embryogenesis. Here we report the protein coding sequence and embryonic expression pattern of the zebrafish engrailed-2 gene (eng-2) which is directly homologous to En-2 in mice and Xenopus. The predicted zebrafish Eng-2 protein shares 65% overall identity to its Xenopus counterpart. In addition to the highly conserved homeodomain region, sequence conservation is present within three short stretches in the N-terminal region. The embryonic expression of the eng-2 gene was analysed by in situ hybridization to whole-mount embryos and tissue sections. Transcripts are first detected in two lateral bands at the 10-h stage, when epiboly is completed. Within the next 2 h of development, these two bands migrate and fuse at the midline. By the time the neural keel becomes visible (11-12 h), a transverse stripe of eng-2 expressing cells is seen at the presumptive midbrain-hindbrain boundary. Later this stripe becomes significantly compressed along the AP axis, and in 24-h embryos eng-2 transcripts are detected mainly in the posterior midbrain. In the hindbrain, eng-2 expression seems restricted to the primordium of the cerebellum. A second site of activity was observed in each somite where specific myotomal cells, the muscle pioneers, express eng-2. Our observations are discussed in relation to early regionalization of the central nervous system (CNS) and the generation of morphological borders.

Reversal of axonal pathways from rhombomere 3 correlates with extra Hox expression domains

Exposure of midgastrulation mouse embryos to retinoic acid induced anteriorized expression of the Hoxa-1 (Hox-1.6) and Hoxb-1 (Hox-2.9) genes. Separate, extra domains of Hoxb-1 expression were detected as stripes and patches up to the midbrain boundary within rhombomeres r3, r2, and r1. Morphological alterations were studied in embryos of the transgenic line L17, which allowed staining of cranial ganglia, motor neurons, and axons by means of the beta-galactosidase reaction. Axons of motor neurons in r3 normally project laterally, before they turn sharply rostrally to exit with the trigeminal nerve from r2. Altered projection patterns were observed for single neurons, groups of neurons, or the complete set of r3 motor neurons in different embryos exposed to retinoic acid. Here r3 axons turned in the opposite direction and exited as facial nerves from r4. These changes of neuroectodermal fates indicate a linkage between axonal pathfinding and intrinsic neuronal specification by Hox codes.

Two enhancer regions in the mouse En-2 locus direct expression to the mid/hindbrain region and mandibular myoblasts

An En-2/lacZ gene fusion containing 9.5 kb of En-2 genomic DNA was capable of directing lacZ expression in an En-2-specific manner both temporally and spatially during embryogenesis and in the adult. lacZ expression was confined in the embryo to cells within the mid/hindbrain and mandibular arch regions and in the adult to cells of the molecular and granular layers of the cerebellum, and within the pons and colliculi regions. Interestingly, in the adult, transgene expression patterns within the cerebellum in two lines appeared to mark distinct anterior-posterior compartments. Analysis of the expression pattern of this transgene, in fetal and adult mice lacking a functional En-2 protein, provided evidence that the En-2 gene in mouse is not autoregulated. Deletion analysis of the En-2 genomic region and the use of a heterologous promoter identified two enhancer-containing regions of 1.5 and 1.0 kb in length, 5′ of the transcribed sequences, which independently directed expression in the embryo to either the mid/hindbrain region or mandibular myoblasts, respectively. The 1.5 kb fragment contains the most anterior neural enhancer and the 1.0 kb fragment, the earliest myogenic enhancer thus far characterized. These En-2-specific regulatory regions can now be used in a biochemical analysis to identify proteins important in anterior-posterior patterning of the vertebrate CNS and in the specification of muscle identity as well as in a mutational analysis to direct expression of other developmentally important genes to these regions.

Cell lineage analysis of the expression of an engrailed homolog in leech embryos

ht-en is an engrailed-class gene that is expressed during early development and neurogenesis in embryos of the leech Helobdella triserialis. During the early development of this annelid (stages 7–9), ht-en is expressed in each of the ectodermal and mesodermal teloblast lineages that contributes progeny to the definitive segments. ht-en is expressed transiently by individually identified cells within the segmentally iterated primary blast cell clones. Its expression is correlated with the age of the primary blast cell clone. After consegmental primary blast cell clones from the different teloblast lineages have come into segmental register, cells that express ht-en during stages 7–9 are clearly confined to a transverse region corresponding to the posterior portion of the segmental anlage, but not all cells within this region express ht-en. Only a minority of the identified cells that express ht-en during terminal differentiation of the segmental ganglia and body wall (stages 10–11) are descendants of cells that express ht-en in early development (stages 7–9).

Topographic maps and molecular gradients

Topographically organized patterns of connectivity occur throughout the central and peripheral nervous systems. It is commonly supposed that gradients of recognition molecules underlie this form of synaptic specificity. Recent studies have led to new ideas about how such gradients might arise in the retinotectal system, and initiated molecular analyses of position-dependent gene expression in the peripheral motor system.

Embryonic neural chimaeras in the study of brain development

The quail-chick cell-marking technique consists of constructing chimaeras in ovo by grafting quail embryonic rudiments into chick embryos or vice versa. Quail and chick cells can be recognized at any time in the chimeras owing to the structures of their interphase nuclei that can be visualized after cytological staining of the DNA. Recently, antibodies that recognize species-specific determinants carried by either neuronal cell bodies or neurites of one or other of the species have greatly enhanced the analytic capacities of this technique, particularly for studying brain development. This article describes the application of the quail-chick chimaera technique to the study of the development of the cerebellum and the optic tectum in the avian embryo. The use of the interspecific chimaeras for behavioural studies is also illustrated by experiments in which certain genetic characteristics of the quail song pattern have been transferred to the chick by neural transplants.

Serotonin and morphogenesis. Transient expression of serotonin uptake and binding protein during craniofacial morphogenesis in the mouse

This study describes the timecourse of expression of low-affinity serotonin uptake sites in the developing craniofacial region of the mouse embryo. Whole mouse embryos were incubated in the presence of various serotonergic compounds followed by immunocytochemical localization of serotonin (5-HT) and its binding protein. In the gestational day 9 embryo (3-5 somites), 5-HT uptake was observed in the myocardium of the heart, the visceral yolk sac and foregut. A specific and transient pattern of 5-HT uptake was observed in the hindbrain neuroepithelium from day 9.5-11, where it was localized in rhombomeres 2-5 in the day 9.5 embryo. By day 10, when rhombomeres were no longer evident, uptake was present in the dorso-lateral neuroepithelium surrounding the fourth ventricle (rhombic lip; cerebellar anlage). Uptake of 5-HT was initially observed in the surface epithelium of the craniofacial region at day 10 (20-25 somites) and was greatly increased at day 11. The invaginating lens, nasal placode epithelium and otocyst also took up 5-HT at day 11. During these stages a 45 kD serotonin-binding protein (SBP) was expressed in craniofacial mesenchyme, and became progressively restricted to regions subjacent to epithelial uptake sites. These staining patterns were shown to be specific for 5-HT and SBP by their absence in embryos stained using preabsorbed antisera. The timecourse of these patterns are correlated with critical events in craniofacial morphogenesis including (1) onset of inductive epithelial-mesenchymal interactions, (2) invagination and fusion of placodal structures, (3) presence of rhombomeres, and (4) regions of low proliferative activity.

Relationship between Wnt-1 and En-2 expression domains during early development of normal and ectopic met-mesencephalon

Grafting a met-mesencephalic portion of neural tube from a 9.5-day mouse embryo into the prosencephalon of a 2-day chick embryo results in the induction of chick En-2 (ChickEn) expression in cells in contact with the graft (Martinez et al., 1991). In this paper we investigate the possibility of Wnt-1 being one of the factors involved in En-2 induction. Since Wnt-1 and En-2 expression patterns have been described as diverging during development of the met-mesencephalic region, we first compared Wnt-1 and En-2 expression in this domain by in situ hybridization in mouse embryos after embryonic day 8.5. A ring of Wnt-1-expressing cells is detected encircling the neural tube in the met-mesencephalic region at least until day 12.5. This ring consistently overlapped with the En-2 expression domain, and corresponds to the position of this latter gene's maximal expression. We subsequently studied ChickEn ectopic induction in chick embryos grafted with various portions of met-mesencephalon. When the graft originated from the level of the Wnt-1-positive ring, ChickEn induction was observed in 71% of embryos, and in these cases correlated with Wnt-1 expression in the grafted tissue. In contrast, this percentage dropped significantly when the graft was taken from more rostral or caudal parts of the mesencephalic vesicle. Taken together, these results are compatible with a prolonged role of Wnt-1 in the specification and/or development of the met-mesencephalic region, and show that Wnt-1 could be directly or indirectly involved in the regulation of En-2 expression around the Wnt-1-positive ring during this time. We also provide data on the position of the Wnt-1-positive ring relative to anatomical boundaries in the neural tube, which suggest a more general role for the Wnt-1 protein as a positional signal involved in organizing the met-mesencephalic domain.

Edgeworth's legacy of cranial muscle development with an analysis of muscles in the ventral gill arch region of batoid fishes (Chondrichthyes: Batoidea)

A series of studies by Edgeworth demonstrated that cranial muscles of gnathostome fishes are embryologically of somitic origin, originating from the mandibular, hyoid, branchial, epibranchial, and hypobranchial muscle plates. Recent experimental studies using quail-chick chimeras support Edgeworth's view on the developmental origin of cranial muscles. One of his findings, the existence of the premyogenic condensation constrictor dorsalis in teleost fishes, has also been confirmed by molecular developmental studies. Therefore, developmental mechanisms for patterning of cranial muscles, as described and implicated by Edgeworth, may serve as structural entities or regulatory phenomena responsible for developmental and evolutionary changes. With Edgeworth's and other studies as background, muscles in the ventral gill arch region of batoid fishes are analyzed and compared with those of other gnathostome fishes. The spiracularis is regarded as homologous at least within batoid fishes, but its status within elasmobranchs remains unclear; developmental modifications of the spiracularis proper are evident in some batoid fishes and in several shark groups. The peculiar ventral extension of the spiracularis in electric rays and some stingrays may represent convergence, probably facilitating ventilation and/or feeding in both groups. The evolutionary origin of the "internus" and "externus" remains uncertain, despite the fact that a variety of forms of the constrictor superficiales ventrales in batoid fishes indicates an actual medio-ventral extension of the "externus." The intermandibularis is probably present only in electric rays. The "X" muscle occurs only in electric rays and is considered to be Edgeworth's intermandibularis profundus. Its association with the adductor mandibular complex in narkinidid and narcinidid electric rays may relate to its functional role in lower jaw movement. Contrary to common belief, in most batoid fishes as well as some sharks, muscles that originate from the branchial muscle plate and extend medially in the ventral gill arches do exist: the medial extension of the interbranchiales in most batoid fishes and some sharks and the "Y" muscle in the pelagic stingrays Myliobatos and Rhinoptera. The latter is another example of the medial extension of the "internus." Whether the interbranchiales and "Y" muscle are homologous within elasmobranchs and whether homologous with the obliques ventrales and/or transversi ventrales of osteichthyan fishes await further research. Four hypobranchial muscles are recognized in batoid fishes: the coracomandibularis, coracohyoideus, coracoarcualis, and coracohyomandibularis. The coracohyoideus is discrete from the coracoarcualis; its complete structural separation from the latter occurs in several groups of batoid fishes.(ABSTRACT TRUNCATED AT 400 WORDS)

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

Notochord has been implicated in previous studies in both the dorsoventral and rostrocaudal patterning of the developing neural tube. This possibility has been further explored by analyzing the expression of Engrailed-2 in chick embryos developing with cranial notochord defects. Control embryos containing intact notochords expressed Engrailed-2 protein within the neural tube and in a subset of the neural crest and overlying surface ectoderm at the future mesencephalon and cranial metencephalon levels. Within the neural tube, expression was confined to cell nuclei in the roof plate and lateral walls; floor plate nuclei directly overlying the notochord typically failed to show expression. After surgical removal of Hensen's node, the source of notochord precursor cells, embryos were cultured through neurulation and assayed for expression of Engrailed-2 protein. All embryos that partially or completely lacked cranial notochord expressed Engrailed-2 in a pattern similar to that of control embryos containing intact notochords, except that when notochord and floor plate were absent, Engrailed-2 was also expressed in the most ventral part of the neural tube. These results indicate that 1) Engrailed-2 expression is suppressed in the most ventral neural tube owing to induction of the floor plate by the notochord, and 2) that the presence of an underlying notochord is not required for correct rostrocaudal expression, suggesting that multiple pathways act in the patterning of the rudiment of the central nervous system.

Rostrocaudal polarity of the tectum in birds: Correlation of en gradient and topographic order in retinotectal projection

Rostrocaudal polarity of the tectum is first detectable at embryonic day 2 (E2) in the chick by the rostrocaudal gradient of en expression. When ectopic tectum is produced by heterotopic transplantation at E2 in the diencephalon, the gradient of en expression is inverted from the host polarity by environmental influences. Here we report that the retinal fibers project to the ectopic tectum in a topographic order in accord with the inverted gradient of en expression. Moreover, if ectopic tectum was produced at E3, when the en pattern was preserved, the retinotectal projection pattern was also in accord with the en pattern, suggesting that rostrocaudal polarity of retinotectal order follows en expression patterns.

Molecular characterization and silk gland expression of Bombyx engrailed and invected genes

Genetic analysis in Drosophila has shown that engrailed (en) plays an important role in segmentation and neurogenesis. A closely related gene, invected (in), is coexpressed with en in the posterior developmental compartments where en is known to specify cell state. We report here the isolation of two en-like cDNAs from the middle silk glands of Bombyx mori larvae. Sequence analysis revealed that they are the counterparts of Drosophila en and in. Four highly conserved domains, including the homeodomain, were identified in these En and In proteins from Bombyx and Drosophila. In addition, two en-specific and one in-specific domains could also be found. These structurally homologous genes might share a similar role in Bombyx development. They were found to be coexpressed in the middle silk gland but not in the posterior silk gland during the fourth molt/fifth intermolt period. We speculate that these Bombyx en-like genes might be involved in the compartmentalization of the silk gland.

The formation of axonal pathways in developing cranial nerves

The roles of a variety of molecules including cell adhesion molecules and growth factors in the development of cranial nerves have begun to be understood in detail. In the course of embryonic development, cranial nerves are differentiated in concordance with the development of the metameric facial structure called 'ectomeres'. Each ectomere parallels the segmentation of the hindbrain called the 'rhombomere', in which pairs of metameric units cooperate to generate the repeating sequence of cranial branchiomotor nerves. A number of genes, including homeobox genes, are expressed in a rhombomere-specific pattern. For the formation of the olfactory nerve, it is suggested that several carbohydrate residues play important roles in receptor-target specificity. In the optic nerve, a combination of multiple cell adhesion molecules contributes to neurite growth in a developmental stage-specific manner. The development of the trigeminal nerve is under the control of both cell adhesion molecules and several growth factors. There is evidence that some of the adhesion molecules are expressed in a modality-specific way. There are also several molecules, such as 11p15 or TAG1/SNAP which are expressed only in selected cranial nerves. The growth rate of neurites also varies according to the individual nerves. Thus each cranial nerve has its own intrinsic properties and their outgrowth is the outcome of these properties and their interactions with surrounding non-neuronal tissues.

Expression of the homeobox engrailed gene during the embryonic development of the nervous system of the trout (Salmo fario L.)

The expression of the engrailed homeobox gene during trout embryogenesis has been examined using immunohistochemistry and the monoclonal antibody 'Mab 4D9'. Engrailed has been suggested to play an important role during development by controlling position-specific characteristics in the CNS of the early embryo. In the present study we have analyzed the expression of engrailed at 5 stages of embryonic development of the trout (Salmo fario L.). The earliest stage analyzed was when the optic vesicles appear. Engrailed was then expressed in the posterior mesencephalon and anterior metencephalon, in a caudorostrally decreasing gradient. As the embryo develops, the pattern of the engrailed expression increases in spatial complexity. Thus, in the later stages of development, just before hatching, engrailed was found in hypothalamic areas, the germinative matrix layer of the cerebellum, the mesencephalic tegmentum, the caudal optic tectum and in the area of the trigeminal motor nucleus. This is similar to the distribution of engrailed in embryos of amphibians, birds and mammals.

Transfer of genetic epilepsy by embryonic brain grafts in the chicken

In the Fayoumi chicken, a spontaneous recessive autosomal mutation (F.Epi) is responsible for high susceptibility to seizures that are especially inducible by intermittent light stimulation. Substitution of defined areas of the encephalic neuroepithelium in normal chicken embryos at 2 days of incubation by their counterparts from homozygous F.Epi embryos generates the epileptic phenotype in the chimeras. It was found that grafting primordia of both prosencephalon and mesencephalon of homozygous F.Epi birds is necessary and sufficient for transfer of the full disease. When grafted alone, the homozygous F.Epi prosencephalon, although showing the typical epileptic interictal electroencephalogram, does not allow the complete epileptic seizures to occur in the hosts. Grafts of mesencephalon and/or rhombencephalon modify neither the behavior nor the electroencephalographic pattern of the recipient chickens. Cooperation of forebrain and midbrain activities is therefore required to yield epileptic seizures in this model.

Induction of a mesencephalic phenotype in the 2-day-old chick prosencephalon is preceded by the early expression of the homeobox gene en

The homeobox gene en, homologous to the gene en-grailed of Drosophila, is expressed in the metencephalic-mesencephalic segment of the vertebrate neural tube. Using quail-chick chimeras, an antibody against en proteins, and cytoarchitectonic techniques, we demonstrate that metencephalon transplanted to prosencephalon, at E2, maintains a high level of en proteins and its presumptive cerebellar fate. The ectopic metencephalon induces in the contiguous host prosencephalon the expression of en and, subsequently, a mesencephalic phenotype. These related genetic and phenotypic expressions indicate that the transcriptional regulatory en gene is involved in cerebellar and mesencephalic cyto-differentiation. The expression of en can also be induced in chick prosencephalon by a mammalian metencephalic graft, indicating that the factors regulating the transcription of en are phylogenetically well conserved.

Subtle cerebellar phenotype in mice homozygous for a targeted deletion of the En-2 homeobox

The two mouse genes, En-1 and En-2, that are homologs of the Drosophila segmentation gene engrailed, show overlapping spatially restricted patterns of expression in the neural tube during embryogenesis, suggestive of a role in regional specification. Mice homozygous for a targeted mutation that deletes the homeobox were viable and showed no obvious defects in embryonic development. This may be due to functional redundancy of En-2 and the related En-1 gene product during embryogenesis. Consistent with this hypothesis, the mutant mice showed abnormal foliation in the adult cerebellum, where En-2, and not En-1, is normally expressed.

Regulation of the rostrocaudal axis of the optic tectum: histological study after rostrocaudal rotation in quail-chick chimeras

Transplantation of the quail tectum primordia after rotation of its rostrocaudal axis through 180 degrees into a chick embryo was carried out at around the 10-somite stage. Our previous study showed that the rostrocaudal specificity was not determined at the stage when the transplantation was carried out in view of the retinotectal map formation. Here, we report the cytoarchitectonic development of the rotated tectum by comparing it with the quail tectum development. In the quail and chick embryos, the rostral part of the tectum had a thicker wall and was in more advanced cytoarchitectonic differentiation than the caudal part. Such cytoarchitectonic gradient along the rostrocaudal axis of the tectum became discernible on day 5.5 of incubation (E5.5) in quail embryos. In the rotated tectum, the rostral part, though it was originally caudal, had a thicker wall and more advanced cytoarchitecture than the caudal part at E5.5 and E6.5. Our results suggest that the rostrocaudal gradient of the cytoarchitecture is regulated to that of the host by E5.5.

Specification of jaw muscle identity in zebrafish: correlation with engrailed-homeoprotein expression

Molecules that determine the specific features of individual muscles in vertebrates are unknown. Antibody labeling studies described here revealed a molecular difference among muscles in the zebrafish head, in that two functionally related jaw muscles (the levator arcus palatini and the dilator operculi), and not other head muscles, expressed engrailed-homeodomain proteins. Expression began in mesoderm-derived muscle-precursor cells in the paraxial mesenchyme and continued during muscle morphogenesis and differentiation. Growth cones of trigeminal motoneurons that innervate these muscles associated with the precursors within a few hours of the time they began to express engrailed. It is proposed that the engrailed proteins in these cells establish muscle identity and neuromuscular target recognition.

Expression of the homeobox Chick-en gene in chick/quail chimeras with inverted mes-metencephalic grafts

The homeobox gene Chick-en, sharing homologies to the engrailed gene of Drosophila, is expressed, during early steps of development, in a restricted area of the chick embryo including mes-metencephalic neuroepithelia. The expression of the Chick-en gene has been analyzed in chick/quail chimeric embryos in which a portion of the 2-day-old mes-metencephalic neuroepithelium has been transplanted in an inverted position. By means of a monoclonal antibody, "Mab 4D9," recognizing engrailed proteins, it is shown that the expression of the Chick-en gene is regulated in the inverted neuroepithelium according to its new position in the host neural tube. The regulation takes place within 20 hr after transplantation. These results, together with previous data demonstrating that the phenotypic expression of the inverted neuroepithelium depends, also, on its new position in the host neural tube, strongly suggest that the engrailed protein could play an important role in the positional specification of the mes-metencephalic neuroepithelium.

Pluripotentiality of the 2-day-old avian germinative neuroepithelium

In a previous study using chick/quail chimeric embryos with homotopic transplants (Martinez & Alvarado-Mallart, 1989b), we have delimited in the 2-day-old avian embryo the areas of the neural tube giving rise to optic tectum and mesencephalic grissea as well as to isthmic grissea and cerebellum: respectively, "mesencephalic" and "metencephalic" alar plates. To investigate the determination or the competence of these areas, portions of these germinative neuroepithelia from a quail embryo were transplanted in substitution for other areas of the chick neural tube. The analysis of the chimeric brains was done by comparing alternating transverse sections stained for cytoarchitecture and with two different techniques to recognize transplanted versus host cells: either the Feulgen and Rossenbeck DNA histochemical reaction and/or immunohistochemical methods with a monoclonal antibody recognizing quail but not chick cells. The eventual visual innervation of the quail graft was analyzed in many cases by injecting anterograde axonal tracers in the eye contralateral to the graft. The results are as follows: (1) caudal metencephalon transferred to mesencephalon maintained in all cases its presumptive cerebellar phenotype, whereas (2) rostral metencephalon transferred to mesencephalon changed its fate to a tectal phenotype but maintained its cerebellar fate when transferred to diencephalon; (3) caudal mesencephalon maintained its tectal fate in 65% of the cases when transferred to diencephalon, whereas (4) rostral mesencephalon transferred to a cerebellar domain changed its fate and became influenced by the surrounding structures in all cases, but only in 85% of the cases when it was transplanted to diencephalon; (5) the in situ host diencephalon, isolated from its normal environment by a mesencephalic graft, is competent to change its fate and express a mesencephalic phenotype. These results demonstrate that at least some regions of the germinative neuroepithelium from either metencephalon, mesencephalon, and diencephalon are still pluripotent in the 2-day-old avian embryo and that their fate seems to be under the influence of the surrounding structures. Rostral mesencephalon and rostral metencephalon have been more easily influenced by environmental factors than their caudal counterparts, suggesting that regions providing instructive positional factors exist within the 2-day-old germinative neuroepithelium. These regions might play an important role in the determination of the various segments of the neural tube.

Mapping of the two mouse engrailed-like genes: close linkage of En-1 to dominant hemimelia (Dh) on chromosome 1 and of En-2 to hemimelic extra-toes (Hx) on chromosome 5

The mouse genome contains two genes, En-1 and En-2, with sequence similarity to the engrailed gene of Drosophila. Using conventional linkage crosses, we have shown that En-1 maps approximately 0.28 cM distal to the dominant hemimelia (Dh) gene on chromosome 1 and that En-2 maps approximately 1.1 cM proximal to the hemimelic extra-toes (Hx) gene on chromosome 5. We have also shown by Northern blot analysis that En-1 transcripts in Dh homozygotes and En-2 transcripts in Hx homozygotes are of normal size and abundance. These data, in conjunction with previously published studies of the patterns of En-1 and En-2 expression in developing mouse embryos, suggest that Dh and Hx are very unlikely to be mutant alleles of En-1 or En-2, respectively. Instead, we suggest that En-1-Dh and En-2-Hx represent paralogous linkage groups that evolved following duplication of a common ancestral chromosome segment.

Expression of engrailed proteins in arthropods, annelids, and chordates

engrailed is a homeobox gene that has an important role in Drosophila segmentation. Genes homologous to engrailed have been identified in several other organisms. Here we describe a monoclonal antibody that recognizes a conserved epitope in the homeodomain of engrailed proteins of a number of different arthropods, annelids, and chordates; we use this antibody to isolate the grasshopper engrailed gene. In Drosophila embryos, the antibody reveals engrailed protein in the posterior portion of each segment during segmentation, and in a segmentally reiterated subset of neuronal cells during neurogenesis. Other arthropods, including grasshopper and two crustaceans, have similar patterns of engrailed expression. However, these patterns of expression are not shared by the annelids or chordates we examined. Our results provide the most comprehensive view that has been obtained of how expression patterns of a regulatory gene vary during evolution. On the basis of these patterns, we suggest that engrailed is a gene whose ancestral function was in neurogenesis and whose function was co-opted during the evolution of segmentation in the arthropods, but not in the annelids and chordates.

Genetics and early development of zebrafish

Zebrafish genes and development are being studied in a growing number of laboratories. Given that many other organisms are already being exploited by large numbers of investigators, and that our general knowledge about the zebrafish embryo and genome is at present rather sketchy, why should we now concern ourselves with how this tropical fish develops? Whereas the zebrafish embryo is similar in important ways to other vertebrate embryos, it is relatively simple and unusually accessible for both cellular and genetic analyses.

Rostral Cerebellum Originates from the Caudal Portion of the So-Called 'Mesencephalic' Vesicle: A Study Using Chick/Quail Chimeras

Homotopic and isochronic transplantations of the right dorsal half of the mesencephalic vesicle have been performed between chick and quail embryos at the stage of 10 - 14 somites. Analysis of the extension of the graft, by means of the quail nucleolar marker, combined with cytoarchitectonic analysis has disclosed that the transplanted neuroepithelium gives rise to isthmic nuclei and to a portion of rostral cerebellum, in addition to the optic tectum and mesencephalic dorsal grisea. These results show that the rostral portion of the cerebellar primordium is located in the so-called 'mesencephalic' alar plate, thus considerably more rostrally than previously supposed. This has been confirmed by two other types of chimeric embryos resulting from homotopic transplantation of either: (i) the quail right alar plate of the first rhombencephalic vesicle, which gives rise to caudal but not rostral cerebellum in the operated side, or (ii) the right alar portion of a segment of the quail neural tube including both the caudal third of the mesencephalic vesicle and the rostral half of the first rhombencephalic vesicle, which gives rise to the whole hemicerebellum in the operated side. Moreover, in chimeric embryos with transplants restricted to the mesencephalic alar plate, the grafted portion of the cerebellar primordium gives rise both to deep cerebellar neurons and to all types of cortical neurons. Among the quail cortical neurons, the Purkinje cells, although intermingled with host Purkinje cells, are organized, at E18, in a characteristic longitudinal band which is strongly reminiscent of the longitudinal functional and morphological organization of the cerebellum. Other types of quail cortical neurons, that is, Golgi cells, granule cells, and molecular layer interneurons, are also observed within this sagittal band. In addition, quail granule cells and molecular layer interneurons as well as quail glial cells, extend over a larger territory on both sides of the longitudinal band containing quail Purkinje cells and even cross the midline and invade the contralateral hemicerebellum. In all types of chimeric embryos, the proliferation, migration, and differentiation of quail transplanted neurons, both in the isthmic region and in the cerebellum, evolve asynchronously from the host homologous ones, following a more precocious and faster developmental schedule. This asynchrony in the development of grafted and host isthmic and cerebellar homologous areas confirms and extends previous findings concerning the proliferation and migration of quail tectal cells in chick quail chimeric embryos (Senut and Alvarado-Mallart, 1987).