Institut für Biologie

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Delimitation of the organizer from the posterior notochord : descriptive and functional studies in mouse and African clawed frog
(2009) Andre, Philipp; Blum, Martin
During vertebrate development, gastrulation is probably the most important phase, as the future body plan is established. Thereby the three body axes anterior-posterior, dorsal-ventral and left-right are determined as well. A central role thereby is taken by the Spemann organizer, as this part of the embryo governs the above mentioned processes. The left-right axis is specified by an extracellular leftward fluid-flow, which results in asymmetric gene expression of the TGFβ factor Nodal. In mice the ciliated epithelium responsible for the fluid-flow as well as the organizer are denominated as ?node?. In contrast to that two distinct entities are thought to be responsible for organizer function and fluid-flow in zebrafish, Xenopus and rabbit embryos. In the present study, it could be shown that this also applies for mouse embryos. In order to prevent further confusion the ciliated epithelium responsible for the fluid-flow was denominated as posterior notochord (PNC) as it is in continuity with the notochord but located anterior to the organizer (?node?). The latter is characterized by the expression of the homeobox gene Goosecoid (Gsc). Gsc possesses, like the tissue of the organizer, the potential to induce an almost complete axis and became therefore famous as ?the organizer gene?. However upon knockout of Gsc in the mouse, surprisingly no gastrulation defects could be detected. Therefore the function of Gsc during gastrulation was investigated using a gain-of-function approach. The analysis of this, in the present and previous studies, indicated that Gsc acts as a switch between two modes of cell movement. Accordingly, Gsc promotes active cell migration and inhibits convergent extension movements. Furthermore it was investigated whether the monoamines adrenaline and serotonin have an influence on the cilia and thus on the leftward fluid-flow, as it was reported in rat and Xenopus experiments. Thereby it could be detected that the addition of adrenaline led to a reduction of the ciliary beat frequency (CBF) and therefore the fluid-flow was attenuated. In contrast to that the addition of serotonin or its antagonists resulted only in minor changes of CBF and thus had no measurable effect on the fluid-flow. The consequences of a malformed PNC were analyzed using embryos mutant for Brachyury (T). Thereby, it was shown that embryos homozygous for this mutation did not develop a functional PNC and thus lacked the fluid-flow. Furthermore a possible cause for the absence of asymmetric Nodal in these embryos was brought into context of an attenuated expression of Fgf8. This indicated that T possesses two distinct roles in left-right development. On the one hand it is necessary for the correct formation of a PNC and on the other hand it is probably needed to maintain the expression of Fgf8, which is a prerequisite for the transcription of Nodal. Finally it was investigated whether these functions were conserved from the African clawed frog Xenopus. Thereby, it could be shown, that Xbra, the homologous gene of T in Xenopus, was also necessary for the formation of the gastrocoel roof plate, the homologous structure of the PNC. Additionally it was observed that the absence of Xbra led to an attenuation of Nodal expression in the midline of Xenopus embryos. This implied that not only the function of Brachyury, but also the process of laterality determination is highly conserved between mammals and amphibians.

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