Mechanisms of vertebrate segmentation

The paired homeobox gene Uncx4.1 specifies pedicles, transverse processes and proximal ribs of the vertebral column

The axial skeleton develops from the sclerotome, a mesenchymal cell mass derived from the ventral halves of the somites, segmentally repeated units located on either side of the neural tube. Cells from the medial part of the sclerotome form the axial perichondral tube, which gives rise to vertebral bodies and intervertebral discs; the lateral regions of the sclerotome will form the vertebral arches and ribs. Mesenchymal sclerotome cells condense and differentiate into chondrocytes to form a cartilaginous pre-skeleton that is later replaced by bone tissue. Uncx4.1 is a paired type homeodomain transcription factor expressed in a dynamic pattern in the somite and sclerotome. Here we show that mice homozygous for a targeted mutation of the Uncx4.1 gene die perinatally and exhibit severe malformations of the axial skeleton. Pedicles, transverse processes and proximal ribs, elements derived from the lateral sclerotome, are lacking along the entire length of the vertebral column. The mesenchymal anlagen for these elements are formed initially, but condensation and chondrogenesis do not occur. Hence, Uncx4.1 is required for the maintenance and differentiation of particular elements of the axial skeleton.

Chondroitin sulfates affect the formation of the segmental motor nerves in zebrafish embryos

Chondroitin sulfates have been implicated in the promotion and in the inhibition of axon growth. In the zebrafish embryo, chondroitin sulfates are present at the interface of the somites and the notochord where spinal motor axons extend ventrally to establish the midsegmental ventral motor nerves. Injection of chondroitinase ABC prior to motor axon outgrowth effectively removed all chondroitin sulfate immunoreactivity and induced abnormal axonal outgrowth in many (39%) of the ventral motor nerves. The most common abnormality was the formation of side branches, approximately half of which extended posteriorly, the others anteriorly. The effect was specific to the removal of chondroitin sulfates, since injections of vehicle solution or of heparinase III did not affect the ventral motor nerves. Electron microscopic examination demonstrated that the injections caused no damage to spinal cord, somite, and notochord. This suggests that chondroitin sulfates normally constrain the outgrowth of the ventral motor nerves. Consistent with this hypothesis, injections of soluble chondroitin sulfates, either as a mixture or individually, led to truncated or missing ventral motor nerves. Truncations were most frequent after injection of chondroitin sulfate-B (up to 23%) while chondroitin sulfate-A had a lesser, and chondroitin sulfate-C no apparent, effect.

Zebrafish Mesp family genes, mesp-a and mesp-b are segmentally expressed in the presomitic mesoderm, and Mesp-b confers the anterior identity to the developing somites

Segmentation of a vertebrate embryo begins with the subdivision of the paraxial mesoderm into somites through a not-well-understood process. Recent studies provided evidence that the Notch-Delta and the FGFR (fibroblast growth factor receptor) signalling pathways are required for segmentation. In addition, the Mesp family of bHLH transcription factors have been implicated in establishing a segmental prepattern in the presomitic mesoderm. In this study, we have characterized zebrafish mesp-a and mesp-b genes that are closely related to Mesp family genes in other vertebrates. During gastrulation, only mesp-a is expressed in the paraxial mesoderm at the blastoderm margin. During the segmentation period, both genes are segmentally expressed in one to three stripes in the anterior parts of somite primordia. In fused somites (fss) embryos, in which all early somite boundary formation is blocked, initial mesp-a expression at the gastrula stage remains intact, but the expression of mesp-a and mesp-b is not detected during the segmentation period. This suggests that these genes are downstream targets of fss at the segmentation stage. Comparison with her1 expression (Muller, M., von Weizsacker, E. and Campos-Ortega, J. A. (1996) Development 122, 2071–2078) suggests that, like her1, mesp genes are not expressed in primordia of the first several somites. Furthermore, we found that zebrafish her1 expression oscillates in the presomitic mesoderm. The her1 stripe, which first appears in the tailbud region, moves in a caudal to rostral direction, and it finally overlaps the most rostral mesp stripe. Thus, in the trunk region, both her1 and mesp transcripts are detected in every somite primordium posterior to the forming somites. Ectopic expression of Mesp-b in embryos causes a loss of the posterior identity within the somite primordium, leading to a segmentation defect. These embryos show a reduction in expression of the posterior genes, myoD and notch5, with uniform expression of the anterior genes, FGFR1, papc and notch6. These observations suggest that zebrafish mesp genes are involved in anteroposterior specification within the presumptive somites, by regulating the essential signalling pathways mediated by Notch-Delta and FGFR.

Notch signalling is required for cyclic expression of the hairy-like gene HES1 in the presomitic mesoderm

Somitic segmentation provides the framework on which the segmental pattern of the vertebrae, some muscles and the peripheral nervous system is established. Recent evidence indicates that a molecular oscillator, the ‘segmentation clock’, operates in the presomitic mesoderm (PSM) to direct periodic expression of c-hairy1 and lunatic fringe (l-fng). Here, we report the identification and characterisation of a second avian hairy-related gene, c-hairy2, which also cycles in the PSM and whose sequence is closely related to the mammalian HES1 gene, a downstream target of Notch signalling in vertebrates. We show that HES1 mRNA is also expressed in a cyclic fashion in the mouse PSM, similar to that observed for c-hairy1 and c-hairy2 in the chick. In HES1 mutant mouse embryos, the periodic expression of l-fng is maintained, suggesting that HES1 is not a critical component of the oscillator mechanism. In contrast, dynamic HES1 expression is lost in mice mutant for Delta1, which are defective for Notch signalling. These results suggest that Notch signalling is required for hairy-like genes cyclic expression in the PSM.

Isolation of a spontaneously fusing BC3H1 muscle cell line: fusion alters the response to serum stimulation

Differentiation of skeletal muscle cells involves two distinct events: exit from the cell cycle and expression of muscle-specific contractile genes and formation of multinucleated myocytes. Although many studies have shown that growth factors regulate the initial step of differentiation, little is known about regulation of fusion. BC3H1 cells are a skeletal muscle cell line characterized by a nonfusing phenotype and an ability to dedifferentiate. When subjected to serum or growth factors, differentiated BC3H1 cells lose muscle-specific gene expression and re-enter the cell cycle. In this study, we describe a spontaneously fusing clone of BC3H1 cells. We demonstrate that this fusion capability is not due to altered muscle regulatory factor or adhesion molecule expression. Furthermore, we show that fusion inhibits dedifferentiation. Multinucleated BC3H1 cells do not lose myosin expression, nor do they re-enter the cell cycle. Fused BC3HI cells react to serum stimulation with a hypertrophic response. Our results suggest that the state of differentiation, mono- or multi-nucleated, is essential to how myocytes react to growth stimulation and may provide a mechanism for how differentiation, fusion, and hypertrophy are regulated in vivo.

Chemotactic migration of mesencephalic neural crest cells in the mouse

We examined the roles of fibroblast growth factor (FGF)-2 and FGF-8 in the migration of mesencephalic mouse neural crest cells. Our in vitro migration assay has shown that FGF-2 (basic FGF) and FGF-8 have chemotactic activity for these cells. Chemotaxis was inhibited by anti-FGF-2 and anti-FGF-8 neutralizing antibodies. In addition, anti-FGF-2 blocked neural crest cell migration in cranial organ cultures. This observation suggests that FGF-2 functions as a chemoattractant in migration of mesencephalic neural crest cells in vivo. In organ culture, the antagonist of FGF binding to a low-affinity fibroblast growth factor receptor (FGFR) heparan sulfate, inositolhexakisphosphate (InsP6), inhibited migration as well. Mesencephalic neural crest cells had high-affinity FGFRs, in particular FGFR-1 and FGFR-3. Thus, the chemotactic activities of FGF-2 can be mediated by the low-affinity FGFR alone or by a combination of low- and high-affinity FGFRs (FGFR-1, FGFR-3, or both). Moreover, differential localization of FGF-2 was found at the mesencephalic axial level of intact embryos during neural crest cell migration. FGF-2 protein expression was predominant in the target regions, in particular the mandibular mesenchyme, that are colonized by mesencephalic neural crest cells. This characteristic distribution supports the notion that FGF-2 acts as a chemoattractant in the mouse embryo that directs mesencephalic neural crest cell migration. Whereas FGF-8 showed chemotactic activity in vitro, neural crest cell dispersion was observed in explants that had been treated with anti-FGF-8 neutralizing antibodies. This result suggests that FGF-8 may not be a chemoattractant in vivo. However, the distribution of neural crest cells in explants treated with anti-FGF-8 differed from that in control explants or in intact embryos. Extreme FGF-2 distribution was observed in the mandibular arch and FGF-8 is expressed in the epithelium. FGF-8 may play a role in mesencephalic neural crest cell migration, and its role may be concerned with the differential localization of FGF-2. To establish this notion, we performed immunohistochemical examination of FGF-2 distribution in explants treated with FGF-8 and analysis of FGF-2 gene expression levels by reverse transcriptase-polymerase chain reaction by using RNA from explants. The data indicate that FGF-2 is distributed throughout the mesenchyme in FGF-8-treated explants and that expression of FGF-2 is promoted by FGF-8. Therefore, we conclude that the expression of FGF-8 in the mandibular arch epithelium is a prerequisite for the differential localization of FGF-2 and that the FGF-2 distribution pattern is essential for chemotaxis of mesencephalic neural crest cell migration.

SHH-N upregulates Sfrp2 to mediate its competitive interaction with WNT1 and WNT4 in the somitic mesoderm

Dorsoventral polarity of the somitic mesoderm is established by competitive signals originating from adjacent tissues. The ventrally located notochord provides the ventralizing signals to specify the sclerotome, while the dorsally located surface ectoderm and dorsal neural tube provide the dorsalizing signals to specify the dermomyotome. Noggin and SHH-N have been implicated as the ventralizing signals produced by the notochord. Members of the WNT family of proteins, on the other hand, have been implicated as the dorsalizing signals derived from the ectoderm and dorsal neural tube. When presomitic explants are confronted with cells secreting SHH-N and WNT1 simultaneously, competition to specify the sclerotome and dermomyotome domains within the naive mesoderm can be observed. Here, using these explant cultures, we provide evidence that SHH-N competes with WNT1, not only by upregulating its own receptor Ptc1, but also by upregulating Sfrp2 (Secreted frizzled-related protein 2), which encodes a potential WNT antagonist. Among the four known Sfrps, Sfrp2 is the only member expressed in the sclerotome and upregulated by SHH-N recombinant protein. We further show that SFRP2-expressing cells can reduce the dermomyotome-inducing activity of WNT1 and WNT4, but not that of WNT3a. Together, our results support the model that SHH-N at least in part employs SFRP2 to reduce WNT1/4 activity in the somitic mesoderm.

Segmentation: a view from the border

Segmentation, or metamerism, consists of the subdivision of the body into discrete units that subsequently acquire regional specializations. In vertebrates, the most obvious manifestation of this phenomenon is seen during the formation of the mesodermal somites and their derivatives. This review surveys three different models for how somites form, and how they relate to recent molecular data suggesting the involvement of transcription factors and cell surface molecules. A new model (the "Morse code" model) is proposed to convey positional information to somitogenic cells. Finally, the molecular events of boundary formation (during the initial epithelialization of somites) and boundary maintenance (between adjacent somite halves as well as in resegmentation) are discussed.

Segmentation of the paraxial mesoderm and vertebrate somitogenesis

Somites are the most obviously segmented features of the vertebrate embryo. Although the way segmentation is achieved in the fly is now well described, little was known about the molecular mechanisms underlying vertebrate somitogenesis. Through the recent identification of genes important for vertebrate somitogenesis and the analysis of their function, several theoretical models accounting for somitogenesis such as the clock and wavefront model, which have been proposed over the past 20 years, are now starting to receive experimental support. A molecular clock linked to somitogenesis has been identified which might act as a periodicity generator in the presomitic cells. This temporal periodicity is then translated into a tightly controlled spatial periodicity which is revealed by the expression of several genes. Analysis of mouse mutants in the Notch-Delta pathway suggest that this signaling mechanism might play an important role at this level. The final step of the cascade is to translate these genetically specified segments into morphological units: the somites. Importantly, these studies have helped in dissociating the segmentation and the somitogenesis processes in vertebrates. In addition, although segmentation was classically thought to have arisen independently in protostomes and deuterostomes, recent evidence suggests that part of the segmentation machinery might actually have been conserved. The conservation of segmentation mechanisms reported in the fly such as the pair-rule pattern, however, remain a subject of controversy.

The origin and morphogenesis of amphibian somites

The origin and development of the amphibian somitic mesoderm is summarized and reviewed with the goal of identifying issues most profitably pursued in these organisms. The location of the prospective somitic mesoderm as well as the cell movements bringing this tissue into its definitive position varies among amphibians. These variations have implications for the tissue interactions patterning the embryo, the design of the gastrulation movements, the role of the somitic mesoderm in early patterning and morphogenic processes, and the nature of the developmental pathway leading to somites. The presegmentation morphogenesis, the process of segmentation, and the subsequent, postsegmentation morphogenesis of the somitic mesoderm also varies considerably among amphibians. Although segmentation in amphibians shares what may be highly conserved and general patterning mechanisms with other vertebrates, the somitic developmental pathway as a whole is not conservative and has been capable of accommodating the use of a number of quite different morphogenic processes, all leading to very similar ends. The major challenges in studying amphibian somitogenesis are to develop molecular markers for major components of the somite, to determine the derivatives of the somite with better cell tracing experiments, and learning to work with the small dermatomal and sclerotomal cell populations found in most species. A potential advantage is that the diversity of somitogenesis among the amphibians makes this group ideal for studying the evolution of developmental processes. In addition, many amphibians allow direct observation of somitogenesis with great resolution and permit biomechanical analysis of tissues participating in morphogenesis, thus making it possible to analyze cellular mechanisms of morphogenesis in ways not possible in most other systems.

Genetic regulation of somite formation

Segmentation of the paraxial mesoderm into somites requires a strategy distinct from the division of a preexisting field of cells, as seen in the segmentation of the vertebrate hindbrain into rhombomeres and the formation of the body plan of invertebrates. Each new somite forms from the anterior end of the segmental plate; therefore, the conditions for establishing the anterior-posterior boundary must be re-created prior to the formation of the next somite. It has been established that regulation of this process is native to the anterior end of the segmental plate, however, the components of a genetic pathway are poorly understood. A growing library of candidate genes has been generated from hybridization screens and sequence homology searches, which include cell adhesion molecules, cell surface receptors, growth factors, and transcription factors. With the increasing accessibility of gene knockout technology, many of these genes have been tested for their role in regulating somitogenesis. In this chapter, we will review the significant advances in our understanding of segmentation based on these experiments.

Pax1 and Pax9 synergistically regulate vertebral column development

The paralogous genes Pax1 and Pax9 constitute one group within the vertebrate Pax gene family. They encode closely related transcription factors and are expressed in similar patterns during mouse embryogenesis, suggesting that Pax1 and Pax9 act in similar developmental pathways. We have recently shown that mice homozygous for a defined Pax1 null allele exhibit morphological abnormalities of the axial skeleton, which is not affected in homozygous Pax9 mutants. To investigate a potential interaction of the two genes, we analysed Pax1/Pax9 double mutant mice. These mutants completely lack the medial derivatives of the sclerotomes, the vertebral bodies, intervertebral discs and the proximal parts of the ribs. This phenotype is much more severe than that of Pax1 single homozygous mutants. In contrast, the neural arches, which are derived from the lateral regions of the sclerotomes, are formed. The analysis of Pax9 expression in compound mutants indicates that both spatial expansion and upregulation of Pax9 expression account for its compensatory function during sclerotome development in the absence of Pax1. In Pax1/Pax9 double homozygous mutants, formation and anteroposterior polarity of sclerotomes, as well as induction of a chondrocyte-specific cell lineage, appear normal. However, instead of a segmental arrangement of vertebrae and intervertebral disc anlagen, a loose mesenchyme surrounding the notochord is formed. The gradual loss of Sox9 and Collagen II expression in this mesenchyme indicates that the sclerotomes are prevented from undergoing chondrogenesis. The first detectable defect is a low rate of cell proliferation in the ventromedial regions of the sclerotomes after sclerotome formation but before mesenchymal condensation normally occurs. At later stages, an increased number of cells undergoing apoptosis further reduces the area normally forming vertebrae and intervertebral discs. Our results reveal functional redundancy between Pax1 and Pax9 during vertebral column development and identify an early role of Pax1 and Pax9 in the control of cell proliferation during early sclerotome development. In addition, our data indicate that the development of medial and lateral elements of vertebrae is regulated by distinct genetic pathways.

Notch around the clock

The establishment of a segmental pattern within the vertebrate body plan is achieved during embryogenesis by the somitogenesis process. Two molecular systems have been implicated in this phenomenon: a molecular clock linked to vertebrate segmentation and the Notch signalling pathway. Rhythmic expression of the Lunatic Fringe gene in the presomitic mesoderm has now provided a link between these two systems.

Zebrafish semaphorin Z1b inhibits growing motor axons in vivo

Zebrafish semaphorin 1b (sema Z1b) is a new member of the semaphorin family, related to mammalian sema D/III. It is expressed in rhombomeres three and five, and in the posterior half of newly formed somites which is avoided by ventrally extending motor axons. Embryos injected at the 1-2 cell stage with synthetic sema Z1b mRNA developed normally but many (63%) showed missing or severely stunted ventral motor nerves. Other axons, somites, and hindbrain rhombomeres were not affected. No abnormalities were seen in control embryos injected with lacZ mRNA. Sema Z1b might normally influence the midsegmental pathway choice of the ventrally extending motor axons by contributing to a repulsive domain in the posterior somite.

Fringe, Notch, and making developmental boundaries

Multiple mechanisms are involved in positioning and restricting specialized dorsal-ventral border cells in the Drosophila wing, including modulation of Notch signaling by Fringe, autonomous inhibition by Notch ligands, and inhibition of Notch target genes by Nubbin. Recent studies have revealed that Fringe also modulates a Notch-mediated signaling process between dorsal and ventral cells in the Drosophila eye, establishing an organizer of eye growth and patterning along the dorsal-ventral midline. Fringe-dependent modulation of Notch signaling also plays a key role in Drosophila leg segmentation and growth. Lunatic Fringe has been shown to be required for vertebrate somitogenesis, where it appears to act as a crucial link between a molecular clock and the regulation of Notch signaling.

The dysmorphic cervical spine in Klippel-Feil syndrome: interpretations from developmental biology

The authors conducted a study to identify radiological patterns of Klippel-Feil syndrome (KFS), and they present a new interpretation of the origin of these patterns based on recent advances in understanding of embryonic development of the spine and its molecular genetic control.

The authors studied radiographs and computerized tomography (CT) scans as well as magnetic resonance images or CT myelograms obtained in 30 patients with KFS who were referred for treatment between 1982 and 1996; the patients had complained of various neuroorthopedic complications. Homeotic transformation due to mutations or disturbed expression of Hox genes is a possible mechanism responsible for C-1 assimilation, which was found to have occurred in 19 cases (63%). Notochordal defects and/or signaling problems, which result in reduced or impaired Pax-1 gene expression, may underlie vertebral fusions. This, together with asymmetrical distribution of paraxial mesoderm cells and a possible lack of communication across the embryonic midline, could cause asymmetrical fusion patterns, which were present in 17 cases (57%). The wide and flattened shape of the fused vertebral bodies and their resemblance to the embryonic cartilaginous vertebrae as well as the process of progressive bone fusion with age suggest that the fusions occur before or, at the latest, during chondrification of vertebrae.

The authors suggest that the aforementioned mechanisms are likely to be, at least in part, responsible for the observed patterns in KFS that affect the craniovertebral junction and the cervical spine.

Evidence for collapsin-1 functioning in the control of neural crest migration in both trunk and hindbrain regions

Collapsin-1 belongs to the Semaphorin family of molecules, several members of which have been implicated in the co-ordination of axon growth and guidance. Collapsin-1 can function as a selective chemorepellent for sensory neurons, however, its early expression within the somites and the cranial neural tube (Shepherd, I., Luo, Y., Raper, J. A. and Chang, S. (1996) Dev. Biol. 173, 185–199) suggest that it might contribute to the control of additional developmental processes in the chick. We now report a detailed study on the expression of collapsin-1 as well as on the distribution of collapsin-1-binding sites in regions where neural crest cell migration occurs. collapsin-1 expression is detected in regions bordering neural crest migration pathways in both the trunk and hindbrain regions and a receptor for collapsin-1, neuropilin-1, is expressed by migrating crest cells derived from both regions. When added to crest cells in vitro, a collapsin-1-Fc chimeric protein induces morphological changes similar to those seen in neuronal growth cones. In order to test the function of collapsin-1 on the migration of neural crest cells, an in vitro assay was used in which collapsin-1-Fc was immobilised in alternating stripes consisting of collapsin-Fc/fibronectin versus fibronectin alone. Explanted neural crest cells derived from both trunk and hindbrain regions avoided the collapsin-Fc-containing substratum. These results suggest that collapsin-1 signalling can contribute to the patterning of neural crest cell migration in the developing chick.

Mouse semaphorin H inhibits neurite outgrowth from sensory neurons

Mouse semaphorin H (M-semaH) was structurally similar to semaphorin III/D, a mammalian homologue of collapsin 1 which was identified as a collapsing factor for sensory nerves. In this study we investigated the expression patterns of M-semaH mRNA and the protein binding sites in the trunk of mouse embryos. M-semaH mRNA was expressed in the mesenchymal tissues surrounding each dorsal root ganglia. These tissues include the caudal sclerotome and perinotochordal mesenchyme, which were thought to express factors repulsive to axons. M-semaH binding was detected on the spinal nerves. We further investigated, using in vitro co-culture assay, whether M-semaH acted as a chemorepulsive molecule on sensory axons. The results suggested that M-semaH was a candidate for a chemorepellent expressed in the mesenchyme surrounding the sensory ganglia, which is involved in the axonal guidance mechanism of sensory nerves in the trunk.

A Drosophila doublesex-related gene, terra, is involved in somitogenesis in vertebrates

The Drosophila doublesex (dsx) gene encodes a transcription factor that mediates sex determination. We describe the characterization of a novel zebrafish zinc-finger gene, terra, which contains a DNA binding domain similar to that of the Drosophila dsx gene. However, unlike dsx, terra is transiently expressed in the presomitic mesoderm and newly formed somites. Expression of terra in presomitic mesoderm is restricted to cells that lack expression of MyoD. In vivo, terra expression is reduced by hedgehog but enhanced by BMP signals. Overexpression of terra induces rapid apoptosis both in vitro and in vivo, suggesting that a tight regulation of terra expression is required during embryogenesis. Terra has both human and mouse homologs and is specifically expressed in mouse somites. Taken together, our findings suggest that terra is a highly conserved protein that plays specific roles in early somitogenesis of vertebrates.

Dynamic expression of lunatic fringe suggests a link between notch signaling and an autonomous cellular oscillator driving somite segmentation

The metameric organization of the vertebrate trunk is a characteristic feature of all members of this phylum. The origin of this metamerism can be traced to the division of paraxial mesoderm into individual units, termed somites, during embryonic development. Despite the identification of somites as the first overt sign of segmentation in vertebrates well over 100 years ago, the mechanism(s) underlying somite formation remain poorly understood. Recently, however, several genes have been identified which play prominent roles in orchestrating segmentation, including the novel secreted factor lunatic fringe. To gain further insight into the mechanism by which lunatic fringe controls somite development, we have conducted a thorough analysis of lunatic fringe expression in the unsegmented paraxial mesoderm of chick embryos. Here we report that lunatic fringe is expressed predominantly in somite -II, where somite I corresponds to the most recently formed somite and somite -I corresponds to the group of cells which will form the next somite. In addition, we show that lunatic fringe is expressed in a highly dynamic manner in the chick segmental plate prior to somite formation and that lunatic fringe expression cycles autonomously with a periodicity of somite formation. Moreover, the murine ortholog of lunatic fringe undergoes a similar cycling expression pattern in the presomitic mesoderm of somite stage mouse embryos. The demonstration of a dynamic periodic expression pattern suggests that lunatic fringe may function to integrate notch signaling to a cellular oscillator controlling somite segmentation.

Vertebrate segmentation: the clock is linked to Notch signalling

The periodic formation of somites during vertebrate segmentation has been suggested to involve a molecular 'segmentation clock'. Recent observations of cyclic Lunatic fringe expression in chick and mouse embryos link the segmentation clock to Delta-Notch signalling.

N-cadherin/catenin-mediated morphoregulation of somite formation

Somitogenesis during early stages in the chick and mouse embryo was examined in relation to N-cadherin-mediated adhesion. Previous studies indicated that N-cadherin localizes to the somite regions during their formation. Those observations were extended to include a spatiotemporal immunohistochemical analyses of beta-catenin and alpha-catenin, as well as a more detailed study of N-cadherin, during segmentation, compaction, and compartmentalization of the somite. N-cadherin and the catenins appear early within the segmental plate and are expressed as small patch-like foci throughout this tissue. The small foci of immunostaining coalesce into larger clusters of N-cadherin/catenin-expressing regions. The clusters subsequently coalesce into a region of centrally localized cells that express N-cadherin/catenins at their apical surfaces. The multiple clusters are spaced wide apart in the anterior segmental plates that form the first 6 somite pairs, as contrasted to segmental plates that form somites 7 and beyond. To examine the functional significance of N-cadherin, segmental plates were exposed to antibodies that perturb N-cadherin-mediated adhesion in the chick embryo. The multiple, anomalous somites that result in these experiments indicate that each N-cadherin/catenin-expressing cluster can give rise to a somitic structure. beta-Catenin involvement in somitogenesis suggests a role for Wnt-mediated signaling. Embryos treated with LiCl also show induction of similar anomalous somites indicating further the possibility that Wnt-mediated signaling may be involved in the clustering event. It is suggested that beta-catenin serves to initiate the adhesion process which is spread then by N-cadherin. Later during compartmentalization, N-cadherin/catenins remain expressed by the myotome compartment. Taken together, these results suggest that the Ca2+-dependent cell adhesion molecule N-cadherin and the intracellular catenins are important in segmentation and formation of the somite and myotome compartment. It is proposed that the N-cadherin-mediated adhesion process may serve as a common, evolutionarily conserved, link in the differentiation pathways of skeletal and cardiac muscle.

Eph signaling is required for segmentation and differentiation of the somites

Somitogenesis involves the segmentation of the paraxial mesoderm into units along the anteroposterior axis. Here we show a role for Eph and ephrin signaling in the patterning of presomitic mesoderm and formation of the somites. Ephrin-A-L1 and ephrin-B2 are expressed in an iterative manner in the developing somites and presomitic mesoderm, as is the Eph receptor EphA4. We have examined the role of these proteins by injection of RNA, encoding dominant negative forms of Eph receptors and ephrins. Interruption of Eph signaling leads to abnormal somite boundary formation and reduced or disturbed myoD expression in the myotome. Disruption of Eph family signaling delays the normal down-regulation of her1 and Delta D expression in the anterior presomitic mesoderm and disrupts myogenic differentiation. We suggest that Eph signaling has a key role in the translation of the patterning of presomitic mesoderm into somites.

A distinct developmental programme for the cranial paraxial mesoderm in the chick embryo

Cells of the cranial paraxial mesoderm give rise to parts of the skull and muscles of the head. Some mesoderm cells migrate from locations close to the hindbrain into the branchial arches where they undergo muscle differentiation. We have characterised these migratory pathways in chick embryos either by DiI-labelling cells before migration or by grafting quail cranial paraxial mesoderm orthotopically. These experiments demonstrate that depending on their initial rostrocaudal position, cranial paraxial mesoderm cells migrate to fill the core of specific branchial arches. A survey of the expression of myogenic genes showed that the myogenic markers Myf5, MyoD and myogenin were expressed in branchial arch muscle, but at comparatively late stages compared with their expression in the somites. Pax3 was not expressed by myogenic cells that migrate into the branchial arches despite its expression in migrating precursors of limb muscles. In order to test whether segmental plate or somitic mesoderm has the ability to migrate in a cranial location, we grafted quail trunk mesoderm into the cranial paraxial mesoderm region. While segmental plate mesoderm cells did not migrate into the branchial arches, somitic cells were capable of migrating and were incorporated into the branchial arch muscle mass. Grafted somitic cells in the vicinity of the neural tube maintained expression of the somitic markers Pax3, MyoD and Pax1. By contrast, ectopic somitic cells located distal to the neural tube and in the branchial arches did not express Pax3. These data imply that signals in the vicinity of the hindbrain and branchial arches act on migrating myogenic cells to influence their gene expression and developmental pathways.

Somitogenesis: segmenting a vertebrate

The partitioning of the vertebrate body into a repetitive series of segments, or somites, requires the spatially and temporally co-ordinated behaviour of mesodermal cells. To date, it remains unknown how applicable our knowledge of the genetic mechanisms governing Drosophila segmentation will be to that of vertebrates, though recent results indicate some degree of conservation. Genetic studies in the mouse point to a major role for the Notch-Delta signalling pathway in somite formation. Furthermore, a molecular clock may be 'ticking' in the presomitic mesoderm.

Defects in somite formation in lunatic fringe-deficient mice

Segmentation in vertebrates first arises when the unsegmented paraxial mesoderm subdivides to form paired epithelial spheres called somites. The Notch signalling pathway is important in regulating the formation and anterior-posterior patterning of the vertebrate somite. One component of the Notch signalling pathway in Drosophila is the fringe gene, which encodes a secreted signalling molecule required for activation of Notch during specification of the wing margin. Here we show that mice homozygous for a targeted mutation of the lunatic fringe (Lfng) gene, one of the mouse homologues of fringe, have defects in somite formation and anterior-posterior patterning of the somites. Somites in the mutant embryos are irregular in size and shape, and their anterior-posterior patterning is disturbed. Marker analysis revealed that in the presomitic mesoderm of the mutant embryos, sharply demarcated domains of expression of several components of the Notch signalling pathway are replaced by even gradients of gene expression. These results indicate that Lfng encodes an essential component of the Notch signalling pathway during somitogenesis in mice.

Genetic rescue of segmentation defect in MesP2-deficient mice by MesP1 gene replacement

Gene knock-out and knock-in strategies are employed to investigate the function of MesP1. MesP1 belongs to the same family of bHLH transcription factors as MesP2. The early expression pattern observed in the early mesoderm at the onset of gastrulation is restricted to Mesp1, while the later expression pattern in the anterior presomitic mesoderm during somitogenesis is almost the same for Mesp1 as for Mesp2. Homozygous Mesp1 null mice exhibited growth retardation after 7.5 dpc and died before 10.5 dpc with many developmental defects. The function of MesP1 during somitogenesis was not clearly revealed because of their early death and the possible compensation by MesP2. In order to examine the functions of MesP1 during somitogenesis, we replaced the Mesp2 gene with Mesp1 cDNA, using a gene knock-in strategy. The introduced Mesp1 cDNA could rescue the defects caused by Mesp2 deficiency in a dosage-dependent manner. Mice which lacked Mesp2 expression but had four copies of the Mesp1 gene survived into the adulthood and were fertile. The skeletal defects and the reduction in expression of Notch1, Notch2 and FGFR-1 previously observed in Mesp2 null mice were almost completely rescued by the introduced MesP1. Thus, it is concluded that the functions of MesP1 during somitogenesis, like MesP2, are also mediated via notch-delta and FGF signaling systems.

The mouse pudgy mutation disrupts Delta homologue Dll3 and initiation of early somite boundaries

Pudgy (pu) homozygous mice exhibit clear patterning defects at the earliest stages of somitogenesis, resulting in adult mice with severe vertebral and rib deformities. By positional cloning and complementation, we have determined that the pu phenotype is caused by a mutation in the delta-like 3 gene (Dll3), which is homologous to the Notch-ligand Delta in Drosophila. Histological and molecular marker analyses show that the pu mutation disrupts the proper formation of morphological borders in early somite formation and of rostral-caudal compartment boundaries within somites. Viability analysis also indicates an important role in early development. The results point to a key role for a Notch-signalling pathway in the initiation of patterning of vertebrate paraxial mesoderm.

Clocked gene expression in somite formation

A recent paper describes a striking expression pattern during somite formation for a chick ortholog of the fly hairy gene. Before segmentation, c-hairy1 mRNA oscillates in the presomitic mesoderm such that three distinct spatial patterns are seen. The authors use a series of ingenious manipulations to show that these phases follow each other in time, adding up to a 90-minute periodicity in c-hairy1 expression. The discovery of this clock of gene expression emphasizes the importance of temporally regulated events in the establishment of spatial patterns.

Segmental distribution of the motoneurons innervating trunk muscles in the spinal cord of the cat and rat

The current progress in developmental biology suggests a genetically stable peripheral pathway formation. However, this may be incompatible with the variations or anomalies observed in the segmental origins of motor nerves in the mammals including the human. For the consideration of the causes raising this inconsistency, we examined the distribution of motoneurons for the serratus dorsalis cranialis muscle of the cat using a retrograde labeling method because this muscle consists of segmentally-arranged parts which receive segmental dual innervation. Consequently, the distribution of the labeled motoneurons for one part spread throughout the full extent of two spinal cord segments, while the distributions for the intercostal muscles in the cat and rat were segmental and in accordance with each spinal cord segment. This may indicate the more precise correspondence between the spinal nerve segments and the distribution of motoneurons projecting axons through them. We think, therefore, that segments of the spinal nerves supplying a given target exactly indicate the segmental levels of supplying motoneurons and suggest the segments of somites from which primordial cells of the target migrate.

Somite number and vertebrate evolution

Variation in segment number is an important but neglected feature of vertebrate evolution. Some vertebrates have as few as six trunk vertebrae, while others have hundreds. We examine this phenomenon in relation to recent models of evolution and development. Surprisingly, differences in vertebral number are foreshadowed by different somite counts at the tailbud stage, thought to be a highly conserved (phylotypic) stage. Somite number therefore violates the ‘developmental hourglass’ model. We argue that this is because somitogenesis shows uncoupling or dissociation from the conserved positional field encoded by genes of the zootype. Several other systems show this kind of dissociation, including limbs and feathers. Bmp-7 expression patterns demonstrate dissociation in the chick pharyngeal arches. This makes it difficult to recognise a common stage of pharyngeal development or ‘pharyngula’ in all species. Rhombomere number is more stable during evolution than somite number, possibly because segmentation and positional specification in the hindbrain are relatively interdependent. Although developmental mechanisms are strongly conserved, dissociation allows at least some major evolutionary changes to be generated in phylotypic stages.

Mediolateral patterning of somites: multiple axial signals, including Sonic hedgehog, regulate Nkx-3.1 expression

The axial structures, the notochord and the neural tube, play an essential role in the dorsoventral patterning of somites and in the differentiation of their many cell lineages. Here, we investigated the role of the axial structures in the mediolateral patterning of the somite by using a newly identified murine homeobox gene, Nkx-3.1, as a medial somitic marker in explant in vitro assays. Nkx-3.1 is dynamically expressed during somitogenesis only in the youngest, most newly-formed somites at the caudal end of the embryo. We found that the expression of Nkx-3.1 in pre-somitic tissue explants is induced by the notochord and maintained in newly-differentiated somites by the notochord and both ventral and dorsal parts of the neural tube. We showed that Sonic hedgehog (Shh) is one of the signaling molecules that can reproduce the effect of the axial structures by exposing explants to either COS cells transfected with a Shh expression construct or to recombinant SHH. Shh could induce and maintain Nkx-3.1 expression in pre-somitic mesoderm and young somites but not in more mature, differentiated ones. The effects of Shh on Nkr-3.1 expression were antagonized by a forskolin-induced increase in the activity of cyclic AMP-dependent protein kinase A. Additionally, we confirmed that the expression of the earliest expressed murine myogenic marker, myf 5, is also regulated by the axial structures but that Shh by itself is not capable of inducing or maintaining it. We suggest that the establishment of somitic medial and lateral compartments and the early events in myogenesis are governed by a combination of positive and inhibitory signals derived from the neighboring structures, as has previously been proposed for the dorsoventral patterning of somites.

Cell death in the avian sclerotome

In this study the occurrence of apoptotic cells in chick embryo trunk somites, between 2.5 and 4 days of development, has been examined using an in situ nick-end-labeling method (TUNEL) to identify nuclei in which DNA is undergoing fragmentation. At 2.5 days of development, apoptotic cells were found in the sclerotome with a distribution that depended on the rostrocaudal level in the trunk. At the most rostral levels (somites 1-18), dying cells were present primarily in the rostral half of the ventral sclerotome; at midlevels (somites 19-26), they were present throughout the ventral sclerotome; and at caudal levels (somites 27-32), no dying cells were present. By 4 days of development, the number of dying cells in the sclerotome was sharply reduced, and those present were primarily distributed to the caudal side of the intrasclerotomal fissure. Double labeling of cells for both TUNEL and the HNK-1 epitope, at 2.5 days, indicated that the majority of the dying cells were not neural crest cells. Further, dying cells in the rostral somite half were present largely in regions of the sclerotome that labeled poorly for HNK-1. It was confirmed that apoptotic neural crest cells retain the HNK-1 epitope and therefore would have been observed if present. Neural crest cells only appeared to be apoptotic in relatively small numbers and only at the ventral border of the sclerotome. Examination of DiI-labeled neural crest cells confirmed that the dying cells in the body of the somite were not primarily neural crest cells. Two hypotheses regarding the TUNEL-positive cells in the sclerotome were experimentally tested. First, that they originate from the somitocoel compartment of the somite, because their distribution patterns at 4 days were similar to those of somitocoel cells. To test this, somitocoel cells were labeled with carboxyfluorescein and grafted into host embryos in ovo. Results showed that these cells did not become apoptotic and that the dying cells were therefore not derived from the somitocoel. Second, the hypothesis was tested that the distribution patterns of the dying cells in the sclerotome are determined by factors outside the somite itself. Somites and segmental plates were transplanted into hosts in ovo with reversed orientation, after which the patterns of dying cells were examined using nile blue sulfate staining. The results indicated that the patterns were unchanged after a further 2 days incubation, suggesting that the patterns of cell death in the sclerotome are not determined solely from within the somite. The distribution of the cell death-associated gene products, bcl-2, bax, and interleukin-1 beta converting enzyme, indicates that although these proteins are segmentally distributed in the dermomyotome and in the rostrodorsal quadrant of the sclerotome, their patterns are not directly correlated with the distribution of dying cells.

Avian hairy gene expression identifies a molecular clock linked to vertebrate segmentation and somitogenesis

We have identified and characterized c-hairy1, an avian homolog of the Drosophila segmentation gene, hairy. c-hairy1 is strongly expressed in the presomitic mesoderm, where its mRNA exhibits cyclic waves of expression whose temporal periodicity corresponds to the formation time of one somite (90 min). The apparent movement of these waves is due to coordinated pulses of c-hairy1 expression, not to cell displacement along the anteroposterior axis, nor to propagation of an activating signal. Rather, the rhythmic c-hairy mRNA expression is an autonomous property of the paraxial mesoderm. These results provide molecular evidence for a developmental clock linked to segmentation and somitogenesis of the paraxial mesoderm, and support the possibility that segmentation mechanisms used by invertebrates and vertebrates have been conserved.

A role for WNT proteins in induction of dermomyotome

Dorsoventral patterning of somites into sclerotome and dermomyotome involves antagonistic actions of ventralizing and dorsalizing signals originating from tissues surrounding the somites. The notochord and the floor plate of the neural tube provide a ventralizing signal(s) directing sclerotome development, whereas the surface ectoderm and dorsal neural tube provide a dorsalizing signal(s) directing dermomyotome development. Evidence has been provided that Sonic Hedgehog mediates the ventralizing effects of notochord and floor plate, but the dorsalizing signal(s) that patterns the dermomyotome has not been identified. The documented expression of Wnt1 and Wnt3a in the dorsal neural tube and of Wnt4 and Wnt6 in the surface ectoderm at the time of dermomyotome specification prompted us to investigate the involvement of WNT proteins in patterning the dermomyotome. Here we show that tissue culture cells expressing these WNT family members can maintain and induce dermomyotome marker expression in presomitic mesoderm explants, supporting the hypothesis that WNT proteins mediate the dorsalizing effects of the surface ectoderm and dorsal neural tube on somites.

Cerberus-like is a secreted factor with neutralizing activity expressed in the anterior primitive endoderm of the mouse gastrula

We report the isolation of mouse cerberus-like (cer-l), a gene encoding a novel secreted protein that is specifically expressed in the anterior visceral endoderm during early gastrulation. Expression in the primitive endoderm starts before the appearance of the primitive streak and lasts until the head-fold stage. In later stages, a second region of expression is found in newly formed somites. Mouse cer-l shares some sequence similarity with Xenopus cerberus (Xcer). In Xenopus assays cer-l, like Xcer, mRNA acts as a potent neuralizing factor that induces forebrain markers and endoderm, but is unable to induce ectopic head-like structures as Xcer does. In addition to cer-l, anterior visceral endoderm was found to express the transcription factors Lim1, goosecoid and HNF-3beta that are also present in trunk organizer cells. A model of how head and trunk development might be regulated is discussed. Given its neuralizing activity, the secreted protein Cer-l is a candidate for mediating inductive activities of anterior visceral endoderm.

Acetylcholine receptor formation in mouse-chick chimera

This study investigated possible interactions between motoneurons and somitic-derived muscle cells in the formation of neuromuscular synapses in the myotome. The peculiarities of the neuromuscular synaptic pattern in chick and mouse embryos provided a model for studying the achievement of synaptogenesis between chick motoneurons and mouse muscle cells. In chick embryo, initial AChR clustering occurs well before innervation of the myotome, whereas in mouse embryo nerve axons invade the myotome extensively before the appearance of AChR clusters. Our approach was to replace somites from a chick host embryo with those derived from mouse donor embryos. We show that muscle cells from mouse myotome can differentiate in the chick embryo environment and form neuromuscular contacts with chick motor axons. Host axons invaded in ovo differentiating mouse myotome at a time when they had not yet reached the host myotome. This particular ingrowth of motor nerves was attributable to the mouse transplant since use of a quail somite did not produce the same effect as the mouse somite, which suggests that developing mouse muscles specifically modify the time course of chick axogenesis. The synaptic areas formed between chick motor axons and mouse myotubes developed according to the mouse pattern. Both the timing of their appearance and their morphology correlated perfectly with events in mouse synaptogenesis. These results indicate the important role played by postsynaptic membrane in controlling the first steps of AChR formation.

Cloning and characterization of chicken Paraxis: a regulator of paraxial mesoderm development and somite formation

To investigate the molecular regulation of embryonic somite formation and development, we have cloned the full-length cDNA and characterized the embryonic expression profile of chicken Paraxis, a member of a novel family of basic helix-loop-helix (bHLH) proteins, which has been suggested to play a role in paraxial mesoderm development. Chicken Paraxis encodes a 1.35-kb mRNA and contains a 53-amino-acid residue bHLH domain, identical in sequence to that found in the mammalian Paraxis genes of mouse, hamster, and human. Northern analysis revealed significant Paraxis expression in the early embryo up to the 30- to 35-somite stage, declining from Incubation Day 4 on and becoming undetectable by Day 5. By whole-mount in situ hybridization, Paraxis expression is first seen distinctly in the emerging paraxial mesoderm of the primitive streak stage chick embryo. During gastrulation, Paraxis expression in the mesoderm defines bilaterally symmetric crescents located immediately rostral to Hensen's node and appears to pre-configure the emerging somitic mesoderm. During somite development, Paraxis expression is evident in the rostral segmental plate and the newly formed somites, although the level of expression clearly decreases in the more mature somites. By the 10-12th pair of somites, counting from the caudal end, Paraxis expression appears to be preferentially localized to the medial aspect of individual somites. Histological analysis showed that Paraxis expression is evenly distributed in the newly formed caudal epithelial somites, then localized to the medial portion of maturing somites, and preferentially localized in the dermomyotome of more rostral somites before diminishing to undetectable levels in the most cranial somites. The functional involvement of Paraxis in somite development was assessed by perturbing its expression in somitic stage chick embryos using a Paraxis-specific antisense oligonucleotide. Disruption of somite formation from the paraxial mesoderm was observed in 67% of the surviving topically treated embryos, whereas control embryos treated with sense or random sequence oligonucleotides did not show similar effects. In addition, direct injection of Paraxis-specific antisense oligonucleotide into the paraxial mesoderm produced discrete segmentation anomalies which correlated spatially with the site of injection. Whole-mount in situ hybridization revealed that the regions defective in somite formation displayed perturbed Paraxis expression and a reduction of Pax-1 expression, a marker for epithelial somites and sclerotome. Histological analysis indicated poor condensation and/or epithelization of the somitic mesoderm. Finally, embryos treated with valproic acid, a known teratogen which affects somite segmentation, showed perturbed Paraxis expression, suggesting that the mechanism of action of this teratogen involves a pathway(s) requiring Paraxis activity. These data provide evidence that Paraxis acts as an important regulator of paraxial mesoderm and somite development and functions in axial patterning of the chick embryo.

Mouse Dll3: a novel divergent Delta gene which may complement the function of other Delta homologues during early pattern formation in the mouse embryo

Mouse delta-like 3 (Dll3), a novel vertebrate homologue of the Drosophila gene Delta was isolated by a subtracted library screen. In Drosphila, the Delta/Notch signalling pathway functions in many situations in both embryonic and adult life where cell fate specification occurs. In addition, a patterning role has been described in the establishment of the dorsoventral compartment boundary in the wing imaginal disc. Dll3 is the most divergent Delta homologue identified to date. We confirm that Dll3 can inhibit primary neurogenesis when ectopically expressed in Xenopus, suggesting that it can activate the Notch receptor and therefore is a functional Delta homologue. An extensive expression study during gastrulation and early organogenesis in the mouse reveals a diverse and dynamic pattern of expression. The three major sites of expression implicate Dll3 in somitogenesis and neurogenesis and in the production of tissue from the primitive streak and tailbud. A careful comparison of Dll3 and Dll1 expression by double RNA in situ hybridisation demonstrates that these genes have distinct patterns of expression, but implies that together they operate in many of the same processes. We postulate that during somitogenesis Dll3 and Dll1 coordinate in establishing the intersomitic boundaries. We confirm that, during neurogenesis in the spinal cord, Dll1 and Dll3 are expressed by postmitotic cells and suggest that expression is sequential such that cells express Dll1 first followed by Dll3. We hypothesise that Dll1 is involved in the release of cells from the precursor population and that Dll3 is required later to divert neurons along a specific differentiation pathway.

Temperature and neural development of the Atlantic herring (Clupea harengus L.)

Embryos of Atlantic herring (Clupea harengus L.) from the Buchan (Northern North Sea) stock were incubated from fertilisation until hatching at temperatures of 5, 8, 12, and 15 degrees C. The relative timing of development of the Kolmer-Agduhr (KA) neurons, the posterior lateral line nerve, the motor neurons, and myotubes were determined with respect to somite stage of the embryo. Development of the KA neurons, the lateral line nerve, and the myotubes was similar at all temperatures. In contrast, timing of outgrowth of the motor neuron axons with respect to somite stage was earlier at higher (> or = 12 degrees C) than at lower temperatures (< or = 8 degrees C) although it reached a similar point at all temperatures by the 58-somite stage. Our hypothesis to explain these observations is that delayed motor axon outgrowth in the lower temperature groups is probably due to a delay in a signalling interaction between motor neurons and the somite.

New insights into segmentation and patterning during vertebrate somitogenesis

Understanding how muscles and the skeleton develop is essential to understanding how distinct form arises in different vertebrate organisms. The meristic somites give rise to the serially-repeated precursors of the axial skeleton and musculature. Recent studies of somitogenesis have shed light upon the mechanisms of segmentation in vertebrates and the relationship between segmentation and patterning. Significant advances have also been made in connecting signalling molecules that possess somite patterning activity with downstream effector genes.

Mesp2: a novel mouse gene expressed in the presegmented mesoderm and essential for segmentation initiation

We isolated a novel bHLH protein gene Mesp2 (for mesoderm posterior 2) that cross-hybridizes with Mesp1 expressed in the early mouse mesoderm. Mesp2 is expressed in the rostral presomitic mesoderm, but down-regulated immediately after the formation of the segmented somites. To determine the function of MesP2 protein (MesP2) in somitogenesis, we generated Mesp2-deficient mice by gene targeting. The homozygous Mesp2 (-/-) mice died shortly after birth and had fused vertebral columns and dorsal root ganglia, with impaired sclerotomal polarity. The earliest defect in the homozygous embryos was a lack of segmented somites. Their disruption of the metameric features, altered expression of Mox-1, Pax-1, and Dll1, and lack of expression of Notch1, Notch2, and FGFR1 suggested that MesP2 controls sclerotomal polarity by regulating the signaling systems mediated by notch-delta and FGF, which are essential for segmentation.

A family of mammalian Fringe genes implicated in boundary determination and the Notch pathway

The formation of boundaries between groups of cells is a universal feature of metazoan development. Drosophila fringe modulates the activation of the Notch signal transduction pathway at the dorsal-ventral boundary of the wing imaginal disc. Three mammalian fringe-related family members have been cloned and characterized: Manic, Radical and Lunatic Fringe. Expression studies in mouse embryos support a conserved role for mammalian Fringe family members in participation in the Notch signaling pathway leading to boundary determination during segmentation. In mammalian cells, Drosophila fringe and the mouse Fringe proteins are subject to posttranslational regulation at the levels of differential secretion and proteolytic processing. When misexpressed in the developing Drosophila wing imaginal disc the mouse Fringe genes exhibit conserved and differential effects on boundary determination.

Regulation of paraxis expression and somite formation by ectoderm- and neural tube-derived signals

During vertebrate embryogenesis, the paraxial mesoderm becomes segmented into somites, which form as paired epithelial spheres with a periodicity that reflects the segmental organization of the embryo. As a somite matures, the ventral region gives rise to a mesenchymal cell population, the sclerotome, that forms the axial skeleton. The dorsal region of the somite remains epithelial and is called dermomyotome. The dermomyotome gives rise to the trunk and limb muscle and to the dermis of the back. Epaxial and hypaxial muscle precursors can be attributed to distinct somitic compartments which are laid down prior to overt somite differentiation. Inductive signals from the neural tube, notochord, and overlying ectoderm have been shown to be required for patterning of the somites into these different compartments. Paraxis is a basic helix-loop-helix transcription factor expressed in the unsegmented paraxial mesoderm and throughout epithelial somites before becoming restricted to epithelial cells of the dermomyotome. To determine whether paraxis might be a target for inductive signals that influence somite patterning, we examined the influence of axial structures and surface ectoderm on paraxis expression by performing microsurgical operations on chick embryos. These studies revealed two distinct phases of paraxis expression, an early phase in the paraxial mesoderm that is dependent on signals from the ectoderm and independent of the neural tube, and a later phase that is supported by redundant signals from the ectoderm and neural tube. Under experimental conditions in which paraxis failed to be expressed, cells from the paraxial mesoderm failed to epithelialize and somites were not formed. We also performed an RT-PCR analysis of combined tissue explants in vitro and confirmed that surface ectoderm is sufficient to induce paraxis expression in segmental plate mesoderm. These results demonstrate that somite formation requires signals from adjacent cell types and that the paraxis gene is a target for the signal transduction pathways that regulate somitogenesis.

Sequence and embryonic expression of the amphioxus engrailed gene (AmphiEn): the metameric pattern of transcription resembles that of its segment-polarity homolog in Drosophila

Vertebrate segmentation has been proposed as an evolutionary inheritance either from some metameric protostome or from a more closely related deuterostome. To address this question, we studied the developmental expression of AmphiEn, the engrailed gene of amphioxus, the closest living invertebrate relative of the vertebrates. In neurula embryos of amphioxus, AmphiEn is expressed along the anteroposterior axis as metameric stripes, each located in the posterior part of a nascent or newly formed segment. This pattern resembles the expression stripes of the segment-polarity gene engrailed, which has a key role in establishing and maintaining the metameres in embryos of Drosophila and other metameric protostomes. Later, amphioxus embryos express AmphiEn in non-metameric patterns - transiently in the embryonic ectoderm and dorsal nerve cord. Nerve cord expression occurs in a few cells approximately midway along the rostrocaudal axis and also in a conspicuous group of anterior cells in the cerebral vesicle at a level previously identified as corresponding to the vertebrate diencephalon. Compared to vertebrate engrailed expression at the midbrain/hindbrain boundary, AmphiEn expression in the cerebral vesicle is relatively late. Thus, it is uncertain whether the cerebral vesicle expression marks the rostral end of the amphioxus hindbrain; if it does, then amphioxus may have little or no homolog of the vertebrate midbrain. The segmental expression of AmphiEn in forming somites suggests that the functions of engrailed homologs in establishing and maintaining a metameric body plan may have arisen only once during animal evolution. If so, the protostomes and deuterostomes probably shared a common segmented ancestor.