Nodal signaling patterns the organizer

Spemann's organizer plays an essential role in patterning the vertebrate em bryo. During gastrulation, organizer cells involute and form the prechordal plate anteriorly and the notochord more posteriorly, The fate mapping and gene expression analyses in zebrafish presented in this study reveal that t his anteroposterior polarity is already initiated in the organizer before g astrulation. Prechordal plate progenitors reside close to the blastoderm ma rgin and express the homeobox gene goosecoid, whereas notochord precursors are located further from the margin and express the homeobox gene floating head. The nodal-related genes cyclops and squint are expressed at the blast oderm margin and are required for prechordal plate and notochord formation. We show that differential activation of the Nodal signaling pathway is ess ential in establishing anteroposterior pattern in the organizer. First, ove rexpression of cyclops and squint at different doses leads to the induction of floating head at low doses and the induction of both goosecoid and floa ting head at higher doses. Second, decreasing Nodal signaling using differe nt concentrations of the antagonist Antivin inhibits goosecoid expression a t low doses and blocks expression of both goosecoid and floating head at hi gher doses. Third, attenuation of Nodal signaling in zygotic mutants for th e EGF-CFC gene one-eyed pinhead, an essential cofactor for Nodal signaling, leads to the loss of goosecoid expression and expansion of floating head e xpression in the organizer. Concomitantly, cells normally fated to become p rechordal plate are transformed into notochord progenitors. Finally, activa tion of Nodal signaling at different times suggests that prechordal plate s pecification requires sustained Nodal signaling, whereas transient signalin g is sufficient for notochord development. Together, these results indicate that differential Nodal signaling patterns the organizer before gastrulati on, with the highest level of activity required for anterior fates and lowe r activity essential for posterior fates.