March 15, 2009;
Cell communication with the neural plate is required for induction of neural markers by BMP inhibition: evidence for homeogenetic induction and implications for Xenopus animal cap and chick explant assays.
In Xenopus, the animal cap
is very sensitive to BMP antagonists, which result in neuralization. In chick, however, only cells at the border of the neural plate
can be neuralized by BMP inhibition. Here we compare the two systems. BMP antagonists can induce neural plate border
markers in both ventral
and non-neural chick epiblast. However, BMP antagonism can only neuralize ectodermal cells when the BMP-inhibited cells form a continuous trail
connecting them to the neural plate
or its border, suggesting that homeogenetic neuralizing factors can only travel between BMP-inhibited cells. Xenopus animal cap
explants contain cells fated to contribute to the neural plate border
and even to the anterior neural plate
, explaining why they are so easily neuralized by BMP-inhibition. Furthermore, chick explants isolated from embryonic epiblast behave like Xenopus animal caps and express border markers. We propose that the animal cap
assay in Xenopus and explant assays in the chick are unsuitable for studying instructive signals in neural induction.
[+] show captions
Fig. 3. BMP inhibition together with eFGF activation induces neural marker expression indirectly in Xenopus. (A–L) Inhibition of BMP by injection of Smad6 into the A4 blastomere does not induce either Sox3 (A, B) or Sox2 (C, D) expression. eFGF together with BMP inhibition into the A4 blastomere induces Sox3 (E, F, N) and Sox2 (G, H, O). Neural induction by the former combination is inhibited when Nodal signaling is blocked: injection of Smad6 + eFGF together with CerS no longer induces Sox3 (I, J) or Sox2 (K, L). (M) Quantification of Sox3 and Sox2 expression in the different experiments described above. A, C, E, G, I and K dorsal views. B, D, F, H, J and L ventral views of the embryos to their left. N and O are enlargements of the areas enclosed by a square in F and H, respectively.
Fig. 4. BMP inhibition together with FGF8a does not induce neural marker expression in Xenopus. (A–B) Injection of FGF8a into one cell at the two-cell stage does not alter Brachyury expression at the gastrula stage (A) but does expand β-tubulin expression at the neurula stage (B); arrowheads indicate the injected side. (C–F) Injection of FGF8a into an A4 blastomere, alone (C, D) or in combination with the BMP inhibitor Smad6 (E, F) does not induce Sox3 expression in ventral epidermis. A: vegetal view; B, C and E are dorsal views; D and F are ventral views of the embryos to their left. Black squares show the area enlarged in the inset in panels D and F.
Fig. 5. Only the border of the neural plate is sensitive to BMP inhibition in Xenopus. (A–I) Smad6 (1 ng) injection into the A4 blastomere induces Pax3 (A–C), Slug (D–F) and Hairy2A (G–I). (A, D, and G: dorsal view; B, C, E, F, H and I: ventral view of the embryo to their left). (J–N) Injection into blastomere A2/3 expands Sox3 (J–L) and Slug (M, N). J, M: dorsal view; K, L and N are lateral views of the embryos to their left. White brackets in J show the extension of the neural plate in the injected and non-injected sides of the embryo. The black square in K indicates the area enlarged in L. Black arrows in L point to injected cells adjacent to the endogenous neural plate, expressing Sox3; the blue arrows point to injected cells distant from the endogenous neural plate, which do not express Sox3. Injected cells were recognized by FDX or LacZ (C, F, I, K, L and N, for embryos to their left). (O–P) Animal caps from Smad6-injected embryos at the 2-cell stage express Sox3 (O), which is not inhibited by CerS (P).