title: Evidence for medial/lateral specification and positional information within the presomitic mesoderm.
abstract: In the vertebrate embryo, segmentation is built on repetitive structures, named somites, which are formed progressively from the most rostral part of presomitic mesoderm, every 90 minutes in the avian embryo. The discovery of the cyclic expression of several genes, occurring every 90 minutes in each presomitic cell, has shown that there is a molecular clock linked to somitogenesis. We demonstrate that a dynamic expression pattern of the cycling genes is already evident at the level of the prospective presomitic territory. The analysis of this expression pattern, correlated with a quail/chick fate-map, identifies a 'wave' of expression travelling along the future medial/lateral presomitic axis. Further analysis also reveals the existence of a medial/lateral asynchrony of expression at the level of presomitic mesoderm. This work suggests that the molecular clock is providing cellular positional information not only along the anterior/posterior but also along the medial/lateral presomitic axis. Finally, by using an in vitro culture system, we show that the information for morphological somite formation and molecular segmentation is segregated within the medial/lateral presomitic axis. Medial presomitic cells are able to form somites and express segmentation markers in the absence of lateral presomitic cells. By contrast, and surprisingly, lateral presomitic cells that are deprived of their medial counterparts are not able to organise themselves into somites and lose the expression of genes known to be important for vertebrate segmentation, such as Delta-1, Notch-1, paraxis, hairy1, hairy2 and lunatic fringe.
authors.affiliation: Instituto Gulbenkian de Ciência, Rua da Quinta Grande 6, 2780-156 Oeiras, Portugal.
authors.forename: C
authors.initials: C
authors.lastname: Freitas
authors.forename: S
authors.initials: S
authors.lastname: Rodrigues
authors.forename: J B
authors.initials: JB
authors.lastname: Charrier
authors.forename: M A
authors.initials: MA
authors.lastname: Teillet
authors.forename: I
authors.initials: I
authors.lastname: Palmeirim
mesh_terms: D000818:Animals; D030161:Avian Proteins; D051792:Basic Helix-Loop-Helix Proteins; D001683:Biological Clocks; D001717:Birds / Q000196:embryology*; D019521:Body Patterning; D019070:Cell Lineage; D002642:Chick Embryo; D003370:Coturnix; D016695:Glycosyltransferases; D018398:Homeodomain Proteins; D066298:In Vitro Techniques; D008648:Mesoderm; D010507:Periodicity; D011506:Proteins / Q000302:isolation & purification; D019170:Somites / Q000637:transplantation; D000072056:Transcription Factor HES-1; D014157:Transcription Factors / Q000302:isolation & purification
chemical_list: D030161:Avian Proteins; D051792:Basic Helix-Loop-Helix Proteins; D016695:Glycosyltransferases; D018398:Homeodomain Proteins; D011506:Proteins; D000072056:Transcription Factor HES-1; D014157:Transcription Factors; C489734:Hes1 protein, mouse; C109424:hairy1 protein, Gallus gallus; C498394:LFNG protein, Gallus gallus