The organizer\u27s role in germ layer specification in Xenopus

Genetics of the Spemann-Mangold organizer. Maternally derived proteins set up an Animal/Vegetal axis in the unfertilized egg, including VegT and Vg1 in the Vegetal pole inner cortex (orange). After fertilization, cortical rotation brings the Vegetal pole outer cortex protein Dishevelled (Dsh, green) into contact with the Animal inner cortex, driving the accumulation of b-catenin in dorsal cells. Vegetal pole proteins activate the expression of TGFb (BMP) and Nodal. Nodal converts the adjacent Nodal- cells into mesoderm (red), resulting in a stripe of mesoderm just above the vegetal pole. Meanwhile, b-catenin enhances Nodal expression in the dorsal-most part of the embryo - resulting in high Nodal expression in the dorsal vegetal inner cortex. This, plus b-catenin, drives the expression of organizer genes like Chordin and Noggin, while repressing ventral mesoderm fates like muscle. Chordin and Noggin specify the notochord, which will elongate and induce the dorsal ectoderm (all ectoderm is blue) that lies over it to become neurectoderm. BMP represses organizer activity on the ventral side of the embryo, in particular ventral ectoderm will form skin, not neurectoderm. The notochord itself has a leading (anterior) and lagging (posterior) edge which express different genes and will induce different parts of the central nervous system.https://scholarlycommons.pacific.edu/open-images/1026/thumbnail.jp