Question

B. (3 points) Explain how the organizer is specified according to the figure in Part A above and the figure below. Your answer should include the role of B-catenin and the paracrine (Xnr) signal from the Nieuwkoop center. man TGF-family Owner Chuda, Nagin DEVELOPMENTAL BIOLOGY

explain how the organizer is specificed according to figure A above

Answer 1

The dorsal region of blastula embryo plays an important role in patterning the axial mesoderm, endoderm and neural tissues in amphibians, zebra fish, chick and mouse. In amphibian, the formation of the Spemann organizer involves 2 sequential steps. The first is a process caleed cortical rotation, which takes place after fertilization and triggers the relocalization of dorsal determinants toward the future dorsal side of the embryo, thereby creating a first organizing centre, The Nieuwkoop centre in the dorsal vegetal cells. The Nieuwkoop centre then emits signals that induce the Spemann organizer in the overlying equatorial region of the embryo known as the dorsal blastopore lip. Transplantation of the organizer to the ventral side induces a secondary axis through the secretion of the BMP antagonists, Noggin, Chordin and Follistatin. These proteins bind to and inhibit bone morphogenetic protein (BMP) extracellularly in the ectoderm overlying the organizer thereby inducing the formation of neural tissue. When expression of the secreted organizer molecules is blocked and BMP signaling is maintained on the dorsal side of the embryo, the neural plate and CNS do not form. β-catenin is present in the nuclei on the dorsal side but not on the ventral side. β-catenin levels are regulated by GSK-3. When active, GSK-3 phosphorylates free β-catenin, which is then targeted for degradation. There are two possible molecules that might regulate GSK-3: GBP (GSK-3 Binding Protein) and Dishevelled. The current model is that these act together to inhibit GSK-3 activity. Dishevelled is able to induce a secondary axis when overexpressed and is present at higher levels on the dorsal side after cortical rotation. Depletion of Dishevelled, however, has no effect whereas GBP has an effect both when depleted and overexpressed.