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First published online November 11, 2004
doi: 10.1242/10.1242/dev.01519

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Coupling segmentation to axis formation

Julien Dubrulle^* and Olivier Pourquié

Stowers Institute for Medical Research, 1000E 50th street, Kansas City, MO 64110, USA

View larger version (31K): [in a new window]   Fig. 1. The successive steps in somitogenesis. (A) The caudal part of a 2-day-old chicken embryo, representing the events taking place in the presomitic mesoderm (PSM) until somite formation. Somite nomenclature, as defined by Pourquie and Tam (Pourquie and Tam, 2001) is indicated on the left. Paraxial mesoderm production is indicated by gray arrows. (B) A parasagittal section through the PSM illustrating the cellular organization of cells along the AP axis. Caudal PSM cells are of a loose mesenchymal character and epithelialization begins anterior to the determination front level.

View larger version (32K): [in a new window]   Fig. 2. Categories of gene expression patterns that are associated with paraxial mesoderm segmentation and maturation in the chick embryo. (A-E) In situ hybridization gene expression patterns in the presomitic mesoderm (PSM). (A) Genes that regulate the wavefront progression: the caudal-to-rostral Fgf8 (fibroblast growth factor 8) gradient (left) and the rostral-to-caudal Raldh2 (retinaldehyde dehydrogenase 2) gradient (right). (B) Expression of a cyclic gene, chick Hairy1, in three different phases that identify the segmentation clock. (C) Striped expression of genes involved in segment specification and rostrocaudal compartmentalization, such as these belonging to the Mesp/Meso2/Thylacine1 bHLH transcription factor family. (D) Expression pattern of paraxis (Tcf15), which is involved in the epithelialization process during somite formation, in the anterior PSM and somites. (E) Genes highlighting the rostrocaudal compartmentalization of the formed somites, such as Uncx4.1, a paired homeobox transcription factor that has restricted expression in the caudal part of the somites.

View larger version (34K): [in a new window]   Fig. 3. A model for somitogenesis. (A) Double in situ hybridization of a 2-day-old chicken embryo with Raldh2 (retinaldehyde dehydrogenase 2) and Fgf8 (fibroblast growth factor 8) probes. Anterior is towards the top. These genes participate in the establishment of mutually inhibitory, antagonistic gradients of retinoic acid (RA) and fibroblast growth factor (FGF) signaling. (B) Molecular mechanisms leading to a segmental pattern. Segment patterning genes are periodically activated by the segmentation clock, whose main regulators are the Notch and Wnt signaling pathways. The spatial activation of the segment patterning genes is defined by the RA and FGF antagonistic gradients: RA positively regulates their transcription, whereas FGF signaling represses RA activity and inhibits presomitic mesoderm maturation.