The counter-intuitive discovery that the default option of the ectoderm (animal-hemisphere-derived cells) is neural tissue was a surprise. Where does BMP4 come from? Where is its gradient situated in the embryo? BMP4 comes from vegetal pole cells? Gradient is highest ventrally and lowest dorsally. Why did it not have a neural plate (or a dorso-ventral axis, for that matter?) UV prevented Organizer from forming, so the inhibitors couldn't be secreted, so BMP4 and Xwnt8 were active throughout the embryo, so there was no axis specification. As a secreted protein, BMP4 binds to the ventral ectoderm that will become the skin of the tadpole - it can diffuse to the dorsal side but is bound by inhibitor proteins produced by the Spemann Organizer -> therefore the dorsal ectoderm cells cannot become skin (so defaults to becoming the CNS) - This is all happening in the animal hemisphere b/c the vegetal hemisphere is engulfed into the archenteron Without the Spemann organizer to make antagonists or inhibitors to BMP4, no neural plate (*central nervous system) would form. The DLB contains BMP4 inhibitors (such as noggin, chordin and follistatin) that protect the future neural plate from being induced to form epidermis by BMP4. Organizer is a busy place b/c many proteins are expressed in the nuclei of the cells that comprise the dorsal lip - the cells have many TFs in their nuclei, but they also secrete many proteins -> the secreted proteins are spread out as the involuting cells move and acting on those moving cells, creating regional specificity Dorsal mesoderm defines the anteroposterior axis of the embryo Along with the earlier established expression domains of VegT, Vg1, Xnr, BMP4 and Xwnt8, DLB formation sets in motion a precise series of cell-signaling events that collectively result in the regional specificity of embryonic induction. Because the DLB is a moving entity consisting of a small group of IMZ cells involuting against the overlying animal hemisphere, the primary induction process that the DLB affects (makes possible) has important consequences for axis specification. In addition to goosecoid protein, which regulates cell migration, the region called the dorsal lip of the blastopore (DLB) also uniquely expresses various transcription factors (recall siamois, for example) and juxtacrine factors, secreted proteins that act as ligands and affect adjacent or nearby cells.
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