GR-06 Early alignment of slow muscle cells prior to migration ensures the rapid establishment of a functional larval myotome

somitogenesis clock was evaluated by assessing the rate and number of somites formed and gene expression alterations upon notochord ablation. Chick embryo explants were cultured for different time periods with or without notochord. We found that notochord removal delays somite formation from the undetermined PSM and alters both clock and determination front gene expression. We show that Shh is the notochord-derived signal responsible for this effect since the notochord ablation phenotype is recapitulated by the use of Shh pathway inhibitors and rescued by the juxtaposition of Shh-producing cells. Moreover, we show that Shh is regulating the period of clock oscillations. Altogether, our data strongly suggest that embryo midlinederived Shh is overseeing somitogenesis clock periodicity and timely somite formation.