09 mai 19331933/05/09 (A13,N4496)-1933/05/09

Christoph Viebahn (4812567)

Kristine Henningfeld (4812564)

Nikoloz Tsikolia (4812561)

Stanislav Kremnyov (3510680)

EvoDevo

Abstract Background The notochord has organizer properties and is required for floor plate induction and dorsoventral patterning of the neural tube. This activity has been attributed to sonic hedgehog (shh) signaling, which originates in the notochord, forms a gradient, and autoinduces shh expression in the floor plate. However, reported data are inconsistent and the spatiotemporal development of the relevant shh expression domains has not been studied in detail. We therefore studied the expression dynamics of shh in rabbit, chicken and Xenopus laevis embryos (as well as indian hedgehog and desert hedgehog as possible alternative functional candidates in the chicken). Results Our analysis reveals a markedly divergent pattern within these vertebrates: whereas in the rabbit shh is first expressed in the notochord and its floor plate domain is then induced during subsequent somitogenesis stages, in the chick embryo shh is expressed in the prospective neuroectoderm prior to the notochord formation and, interestingly, prior to mesoderm immigration. Neither indian hedgehog nor desert hedgehog are expressed in these midline structures although mRNA of both genes was detected in other structures of the early chick embryo. In X. laevis, shh is expressed at the beginning of gastrulation in a distinct area dorsal to the dorsal blastopore lip and adjacent to the prospective neuroectoderm, whereas the floor plate expresses shh at the end of gastrulation. Conclusions While shh expression patterns in rabbit and X. laevis embryos are roughly compatible with the classical view of “ventral to dorsal induction” of the floor plate, the early shh expression in the chick floor plate challenges this model. Intriguingly, this alternative sequence of domain induction is related to the asymmetrical morphogenesis of the primitive node and other axial organs in the chick. Our results indicate that the floor plate in X. laevis and chick embryos may be initially induced by planar interaction within the ectoderm or epiblast. Furthermore, we propose that the mode of the floor plate induction adapts to the variant topography of interacting tissues during gastrulation and notochord formation and thereby reveals evolutionary plasticity of early embryonic induction

Stanislav Kremnyov

Kristine Henningfeld

Christoph Viebahn

Nikoloz Tsikolia

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

Confédération générale du travail (France). Auteur du texte

Gallica, Bibliotheque Numerique

Le Peuple : organe quotidien du syndicalisme

Additional file 3: Fig. 3. Schematic transversal view of a chicken embryo at the level of the posterior notochord. A: ventral to dorsal floor plate induction in the classical view—shh is first expressed in the notochord and induces shh in floor plate. Shh protein from both sources forms a gradient. B: modified model demonstrating early shh expression and shh gradient formation in the floor plate. Labeling: pink—notochord, blue—floor plate, green—hypoblast/endoderm, brown—paraxial mesoderm, arrows—gradient formation and induction

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Le Peuple : organe quotidien du syndicalisme

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