POURQUIE, Olivier; Stowers Institute for Medical Research: Segmental Patterning of the Vertebrate Axis

The vertebrate body can be subdivided along the antero-posterior (AP) axis into repeated structures called segments. This periodic pattern is established during embryogenesis by the somitogenesis process. Somites are generated in a rhythmic fashion from the paraxial mesoderm and subsequently differentiate to give rise to the vertebrae and sekeletal muscles of the body. Somite formation involves an oscillator, the segmentation clock whose periodic signal is converted into the periodic array of somite boundaries. This clock drives the dynamic expression of cyclic genes in the presomitic mesoderm and requires Notch and Wnt signaling. Whereas the segmentation clock is thought to set the pace of vertebrate segmentation, the translation of this pulsation into the reiterated arrangement of segment boundaries along the AP axis involves FGF signaling. The FGF pathway controls the positioning of the wavefront, which corresponds to the level of the presomitic mesoderm where cells respond to the clock. fgf8 mRNA is only transcribed in tail bud precursors and it progressively decays in newly formed paraxial mesoderm cells, thus forming a dynamic mRNA gradient. This mRNA gradient is then translated into a graded FGF signaling response used to position the wavefront. This mechanism provides an efficient means to couple the spatio-temporal activation of segmentation to the posterior elongation of the embryo. Recent research focussed on the molecular characterization of the segmentation clock and on how the clock and the wavefront interact to specify segment boundaries in the presomitic mesoderm will be discussed.