Role of noggin as an upstream signal in the lack of neuronal derivatives found in the avian caudal-most neural crest

Neural crest cells (NCCs) arising from trunk neural tube (NT) during primary and secondary neurulation give rise to melanocytes, glia and neurons, except for those in the caudal-most region during secondary neurulation (somites 47 to 53 in the chick embryo), from which no neurons are formed, either in vivo or in vitro. To elucidate this discrepancy, we have specifically analyzed caudal-most NCC ontogeny. In this region, NCCs emerge at E5/HH26, one day after full cavitation of the NT and differentiation of flanking somites. The absence of neurons does not seem to result from a defect in NCC specification as all the usual markers, with the exception of Msx1, are expressed in the dorsal caudal-most NT as early as E4/HH24. However, Bmp4-Wnt1 signaling, which triggers trunk NCC delamination, is impaired in this region due to persistence of noggin (Nog) expression. Concomitantly, a spectacular pattern of apoptosis occurs in the NT dorsal moiety. Rostral transplantation of either the caudal-most somites or caudal-most NT reveals that the observed features of caudal-most NCCs relate to properties intrinsic to these cells. Furthermore, by forced Nog expression in the trunk NT, we can reproduce most of these particular features. Conversely, increased Bmp4-Wnt1 signaling through Nog inhibition in the caudal-most NT at E4/HH24 induces proneurogenic markers in migratory NCCs, suggesting that noggin plays a role in the lack of neurogenic potential characterizing the caudal-most NCCs.CNRS, UPMC, FCT and AFM. L.O. is a recipient of a grant from FCT (SFRH/BD/11858/2003) and from AR

Neural crest ontogeny during secondary neurulation: a gene expression pattern study in the chick embryo

In the prospective lumbo-sacral region of the chick embryo, neurulation is achieved by cavitation of the medullary cord, a process called secondary neurulation. Neural crest cells (NCC) are generated in this region and they give rise to the same types of derivatives as in more rostral parts of the trunk where neurulation occurs by dorsal fusion of the neural plate borders (primary neurulation). However, no molecular data were available concerning the different steps of their ontogeny. We thus performed a detailed expression study of molecular players likely to participate in the generation of secondary NCC in chick embryos between Hamburger and Hamilton stages 18-20 (HH18-20) at the level of somites 30 to 43. We found that specification of secondary NCC involves, as in primary neurulation, the activity of several transcription factors such as Pax3, Pax7, Snail2, FoxD3 and Sox9, which are all expressed in the dorsal secondary neural tube as soon as full cavitation is achieved. Moreover, once specification has occurred, emigration of NCC from the dorsal neuroepithelium starts facing early dissociating somites and involves a series of changes in cell shape and adhesion, as well as interactions with the extracellular matrix. Furthermore, Bmp4 and Wnt1 expression precedes the detection of migratory secondary NCC and is coincident with maturation of adjacent somites. Altogether, this first study of molecular aspects of secondary NCC ontogeny has revealed that the mechanisms of neural crest generation occurring along the trunk region of the chick embryo are generally conserved and independent of the type of neurulation involved.We are grateful to our colleagues for helpful discussions. We thank Dr Jean-Loup Duband for fibronectin and NC1 antibodies and Dr James Briscoe for Sox9 plasmid. This work has been supported by Centre National de la Recherche Scientifique (CNRS), University Paris 6 (UPMC), Fundacao para Ciencia e Tecnologia (FCT), Association Francaise contre les Myopathies (AFM). LO is a recipient of a grant from FCT (SFRH/BD/1185812003)

Early- and late-migrating cranial neural crest cell populations have equivalent developmental potential in vivo

We present the first in vivo study of the long-term fate and potential of early-migrating and late-migrating mesencephalic neural crest cell populations, by performing isochronic and heterochronic quail-to-chick grafts. Both early- and late-migrating populations form melanocytes, neurons, glia, cartilage and bone in isochronic, isotopic chimeras, showing that neither population is lineage-restricted. The early-migrating population distributes both dorsally and ventrally during normal development, while the late-migrating population is confined dorsally and forms much less cartilage and bone. When the late-migrating population is substituted heterochronically for the early-migrating population, it contributes extensively to ventral derivatives such as jaw cartilage and bone. Conversely, when the early-migrating population is substituted heterochronically for the late-migrating population, it no longer contributes to the jaw skeleton and only forms dorsal derivatives. When the late-migrating population is grafted into a late-stage host whose neural crest had previously been ablated, it migrates ventrally into the jaws. Thus, the dorsal fate restriction of the late-migrating mesencephalic neural crest cell population in normal development is due to the presence of earlier-migrating neural crest cells, rather than to any change in the environment or to any intrinsic difference in migratory ability or potential between early- and late-migrating cell populations. These results highlight the plasticity of the neural crest and show that its fate is determined primarily by the environment

Endothelio-Mesenchymal Interaction Controls runx1 Expression and Modulates the notch Pathway to Initiate Aortic Hematopoiesis

SummaryHematopoietic stem cells (HSCs) are produced by a small cohort of hemogenic endothelial cells (ECs) during development through the formation of intra-aortic hematopoietic cell (HC) clusters. The Runx1 transcription factor plays a key role in the EC-to-HC and -HSC transition. We show that Runx1 expression in hemogenic ECs and the subsequent initiation of HC formation are tightly controlled by the subaortic mesenchyme, although the mesenchyme is not a source of HCs. Runx1 and Notch signaling are involved in this process, with Notch signaling decreasing with time in HCs. Inhibiting Notch signaling readily increases HC production in mouse and chicken embryos. In the mouse, however, this increase is transient. Collectively, we show complementary roles of hemogenic ECs and mesenchymal compartments in triggering aortic hematopoiesis. The subaortic mesenchyme induces Runx1 expression in hemogenic-primed ECs and collaborates with Notch dynamics to control aortic hematopoiesis

Recul du nœud de Hensen et croissance axiale de l’embryon

Le nœud de Hensen des Oiseaux est considéré comme « l’organisateur » c’est-à-dire le centre fonctionnel de la gastrulation au même titre que la lèvre dorsale du blastopore des Amphibiens, le nœud des Mammifères et le bouclier du Poisson zèbre. Il a été démontré récemment que cette structure contient tous les tissus progéniteurs de la ligne médiane de l’embryon (floorplate, notochorde et endoderme dorsal). Cependant, des expériences portant sur cette structure chez l’embryon de Poulet conduisent à penser que la fonction « organisatrice » doit être attribuée à une zone extrêmement limitée qui constitue la frontière entre les territoires présomptifs du tissu axial et du tissu paraxial. Cette région est essentielle pour le recul du nœud de Hensen et la mise en place progressive des structures axiales et paraxiales

De l'aorte primitive à l'aorte définitive : angioblastes et hémangioblastes au cours de l'hématopoïèse

L'hématopoïèse aortique est un mécanisme transitoire, extrêmement conservé dans toutes les classes de vertébrés. Elle est caractérisée par la production de véritables Cellules Souches Hématopoïétiques (CSH) qui naissent à partir de l'endothélium ventral du vaisseau par modification phénotypique des Cellules Endothéliales (CE). Ces CSH colonisent alors les organes hématopoïétiques définitifs. Nous nous sommes interrogés sur les mécanismes mis en place pour maintenir l'intégrité vasculaire pendant cette étape de production hématopoïétique. Il a été précédemment démontré que la vascularisation embryonnaire est assurée par deux contingents distincts de CE. L'un issu de la splanchnopleure, donnant des CE et des cellules hématopoïétiques (CH), l'autre provenant du somite et ne fournissant que des CE. Nous avons, à l'aide du modèle de greffe interspécifique caille/poulet, étudié la formation de l'aorte avant, pendant et après l'hématopoïèse. Nous avons pu montrer que 1) avant l'hématopoïèse le toit de l'aorte, initialement d'origine splanchnopleurale, est entièrement colonisé par des CE provenant des somites. Ce vaisseau subit donc un premier remodelage qui aboutit à la formation d'un nouveau toit et de nouveaux côtés constitués de CE d'origine somatopleurale alors que le cadran ventral reste formé par des hémangioblastes issus de la splanchnopleure ; 2) pendant l'hématopoïèse, les CE somitiques commencent à coloniser la partie ventrale du vaisseau. Cette colonisation s'effectue par intercalation des CE sous les bourgeonnements de CSH ; 3) après l'hématopoïèse, les hémangioblastes aortiques ont disparu du plancher de l'aorte et sont remplacés par les CE somitiques. L'aorte subit donc un deuxième remodelage qui conduit à rendre la totalité des CE du vaisseau d'origine somitique ; 4) enfin, nous avons identifié une nouvelle population cellulaire issue du somite qui contribue à l'élaboration de la tunique musculaire lisse. Cette population colonise l'aorte de manière distincte des CE. Cette dichotomie a été confirmée par des expériences de traçage cellulaire, à l'aide de vecteurs rétroviraux nonréplicatifs, qui démontrent que les CE aortiques ne produisent pas de cellules musculaires lisses. Ces résultats apportent un éclairage nouveau sur la production hématopoïétique de l'aorte et le devenir de l'endothélium hémogénique. Ils fournissent aussi des explications sur la nature transitoire de l'hématopoïèse aortique, observée dans toutes les classes de vertébrés

Horloge moléculaire et segmentation des vertébrés : qui fait quoi?

Chez les vertébrés, les somites sont les structures responsables de la mise en place du squelette axial segmenté et de la segmentation du système nerveux périphérique. Les mécanismes moléculaires qui contrôlent la formation progressive des somites le long de l’axe rostro-caudal de l’embryon n’ont pas encore été complètement élucidés. L’analyse des profils d’expression de quelques gènes à expression cyclique dans le mésoderme présomitique et dans son territoire présomptif au niveau de la ligne primitive révèle que les cellules précurseurs des cellules somitiques possèdent dès leur origine une information positionnelle, non seulement selon l’axe rostrocaudal, mais aussi selon l’axe médio-latéral. De plus, ces cellules semblent présenter des propriétés différentes en ce qui concerne le processus de segmentation.Fundação para a Ciência e a Tecnologia (FCT) – Praxis XXI/BD/21583/99 , projet nº 34599.Fundação Calouste Gulbenkian (FCG). Instituto Gulbenkian de Ciência (IGC).Centre National de la Recherche Scientifique (CNRC).Association Française pour la Recherche contre le Cancer (ARC) - contrat nº 5578

Relationship between neural crest cells and cranial mesoderm during head muscle development

Background: In vertebrates, the skeletal elements of the jaw, together with the connective tissues and tendons, originate from neural crest cells, while the associated muscles derive mainly from cranial mesoderm. Previous studies have shown that neural crest cells migrate in close association with cranial mesoderm and then circumscribe but do not penetrate the core of muscle precursor cells of the branchial arches at early stages of development, thus defining a sharp boundary between neural crest cells and mesodermal muscle progenitor cells. Tendons constitute one of the neural crest derivatives likely to interact with muscle formation. However, head tendon formation has not been studied, nor have tendon and muscle interactions in the head. Methodology/Principal Findings: Reinvestigation of the relationship between cranial neural crest cells and muscle precursor cells during development of the first branchial arch, using quail/chick chimeras and molecular markers revealed several novel features concerning the interface between neural crest cells and mesoderm. We observed that neural crest cells migrate into the cephalic mesoderm containing myogenic precursor cells, leading to the presence of neural crest cells inside the mesodermal core of the first branchial arch. We have also established that all the forming tendons associated with branchiomeric and eye muscles are of neural crest origin and express the Scleraxis marker in chick and mouse embryos. Moreover, analysis of Scleraxis expression in the absence of branchiomeric muscles in Tbx1 2/2 mutant mice, showed tha