EPH SIGNALING IS REQUIRED FOR SEGMENTATION AND DIFFERENTIATION OF THESOMITES

Somitogenesis involves the segmentation of the paraxial mesoderm into units along the anteroposterior axis. Here we show a role for Eph and ephrin signaling in the patterning of presomitic mesoderm and formatio n of the somites. Ephrin-A-L1 and ephrin-B2 are expressed in an iterat ive manner in the developing somites and presomitic mesoderm, as is th e Eph receptor EphA4. We have examined the role of these proteins by i njection of RNA, encoding dominant negative forms of Eph receptors and ephrins. Interruption of Eph signaling leads to abnormal somite bound ary formation and reduced or disturbed myoD expression in the myotome. Disruption of Eph family signaling delays the normal down-regulation of her1 and Delta D expression in the anterior presomitic mesoderm and disrupts myogenic differentiation. We suggest that Eph signaling has a key role in the translation of the patterning of presomitic mesoderm into somites.