title: Anteroposterior patterning is required within segments for somite boundary formation in developing zebrafish.
abstract: Somite formation involves the establishment of a segmental prepattern in the presomitic mesoderm, anteroposterior patterning of each segmental primordium and formation of boundaries between adjacent segments. How these events are co-ordinated remains uncertain. In this study, analysis of expression of zebrafish mesp-a reveals that each segment acquires anteroposterior regionalisation when located in the anterior presomitic mesoderm. Thus anteroposterior patterning is occurring after the establishment of a segmental prepattern in the paraxial mesoderm and prior to somite boundary formation. Zebrafish fss(-), bea(-), des(-) and aei(-) embryos all fail to form somites, yet we demonstrate that a segmental prepattern is established in the presomitic mesoderm of all these mutants and hox gene expression shows that overall anteroposterior patterning of the mesoderm is also normal. However, analysis of various molecular markers reveals that anteroposterior regionalisation within each segment is disturbed in the mutants. In fss(-), there is a loss of anterior segment markers, such that all segments appear posteriorized, whereas in bea(-), des(-) and aei(-), anterior and posterior markers are expressed throughout each segment. Since somite formation is disrupted in these mutants, correct anteroposterior patterning within segments may be a prerequisite for somite boundary formation. In support of this hypothesis, we show that it is possible to rescue boundary formation in fss(-) through the ectopic expression of EphA4, an anterior segment marker, in the paraxial mesoderm. These observations indicate that a key consequence of the anteroposterior regionalisation of segments may be the induction of Eph and ephrin expression at segment interfaces and that Eph/ephrin signalling subsequently contributes to the formation of somite boundaries.
authors.affiliation: Department of Anatomy and Developmental Biology, University College London, Gower Street, London, WC1E 6BT, UK. l.durbin@ucl.ac.uk
authors.forename: L
authors.initials: L
authors.lastname: Durbin
authors.forename: P
authors.initials: P
authors.lastname: Sordino
authors.forename: A
authors.initials: A
authors.lastname: Barrios
authors.forename: M
authors.initials: M
authors.lastname: Gering
authors.forename: C
authors.initials: C
authors.lastname: Thisse
authors.forename: B
authors.initials: B
authors.lastname: Thisse
authors.forename: C
authors.initials: C
authors.lastname: Brennan
authors.forename: A
authors.initials: A
authors.lastname: Green
authors.forename: S
authors.initials: S
authors.lastname: Wilson
authors.forename: N
authors.initials: N
authors.lastname: Holder
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001368:Axis, Cervical Vertebra; D051792:Basic Helix-Loop-Helix Proteins; D019521:Body Patterning / Q000502:physiology*; D002970:Cleavage Stage, Ovum / Q000502:physiology; D003001:Cloning, Molecular; D005326:Fetal Proteins / Q000235:genetics / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D018257:Helix-Loop-Helix Motifs; D008969:Molecular Sequence Data; D020794:Receptor Protein-Tyrosine Kinases / Q000235:genetics / Q000378:metabolism; D036122:Receptor, EphA4; D019170:Somites; D014157:Transcription Factors / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D015027:Zebrafish / Q000196:embryology* / Q000235:genetics; D029961:Zebrafish Proteins
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D005326:Fetal Proteins; D020794:Receptor Protein-Tyrosine Kinases; D036122:Receptor, EphA4; D014157:Transcription Factors; D029961:Zebrafish Proteins; C410084:mespaa protein, zebrafish