Frog axis cheat sheet

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Frog axis cheat sheet – Zoo 470 2013 – Page 6 Step 9: The same signals lead to neural induction: In the ectoderm, BMPs and Wnt promote epidermis, but the organizer counteracts these signals in ectoderm near the Organizer, leading to neural induction Gilbert, 9e, Fig. 7.27 & 7.32 Step 9: Refinement of neural induction occurs: Early in gastrulation, the organizer appears to send signals to nearby ectoderm prior to involution of the dorsal mesoderm (“planar induction”). Later, after involution, regional induction results in anterior and posterior neural tissues (“vertical induction”).
Frog axis cheat sheet – Zoo 470 2013 – Page 7 Experiments indicate that both sorts of signals are important. Embryological experiments show that both of these kinds of signals probably act. Planar signals can be shown to occur via Keller explants, in which the ectoderm is adjacent to mesoderm., and cannot involute to lie underneath the ectoderm. Vertical signals can be shown to operate through “Einsteck” implants, in which anterior or posterior mesoderm is inserted into a host gastrula. The induced tissue is appropriate to the nature of the inserted mesoderm. This suggests that after involution, anteriorposterior refinement of neural pattern occurs via vertical inductive signaling. Late in gastrulation a posteriorizing signal, mediated by retinoic acid (RA), Wnts, and fibroblast growth factors (FGFs) reinforces posterior neural fates. Secreted Wnt antagonists such as Cerberus, produced by the anterior mesendoderm reinforce head-specific neural fates, as opposed to trunk spinal cord, where mainly BMPs are blocked. “Planar” induction: Keller explant (Wolpert 2e, Fig. 4.23) “Vertical” induction: Otto Mangold’s “Einsteck” experiment (Woilpert et al 2e, Fig. 4.20) Gilbert, 9e, Fig. 7.35
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