The role of nodal signaling in patterning the zebrafish embryo
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Gastrulation is a critical event that leads the formation of the three embryonic germ layersectoderm, mesoderm and endoderm. During this time, an impressive series of events needs to becoordinated, leading to the spatial positioning of the three germ layers along the body axis andtheir patterning. nodal-related genes comprise a highly conserved subclass of the transforminggrowth factor-beta (TGF-²) superfamily of secreted proteins that regulate key events invertebrate embryogenesis. Members of this gene family are essential to instruct cells to adoptmesodermal, endodermal and ventral neural-ectodermal cell fates in all vertebrates. In addition,they are required to pattern all three germ layers and to establish the left-right body axis. There are three known nodal-related genes in zebrafish, squint (sqt), cyclops (cyc) andsouthpaw (spaw). The two nodal-related genes, squint and cyclops are necessary and sufficientto instruct cells to adopt mesodermal and endodermal cell types. The third nodal-related genes,spaw is expressed late in development and is involved in establishing the left-right asymmetry.In zebrafish signals from the extraembryonic yolk syncytial layer (YSL) are required to patternthe embryo. Both sqt and cyc are expressed in the extraembryonic YSL and in future mesodermand endoderm (mesendoderm). The present has three aims: (I) (Nodal-related signals are required to specify different celltypes at different times) (II) (squint and cyclops from the YSL are required to pattern thezebrafish embryo) and (III) (the role of maternal TGF-² signals including squint, Activin andVg1 in zebrafish development).The results presented here establish how Nodal-related proteins act during normaldevelopment to instruct cell fates. My data demonstrates that cells respond to Nodal-relatedproteins by a novel time-dependent mechanism that is not yet fully characterized. I also showedthat Nodal signals from the extraembryonic tissues are required to pattern embryonic tissues.Lastly, maternal TGF-² signals are required after the mid-blastula transition for mesoderm andendoderm formation and patterning.