Snail1b modulates extension movements during gastrulation. (A,F,K) Knockdown of snail1b caused the posterior expansion of the spt expression domain, whereas overexpression of snail1b mRNA produced anterior compression. Similar effects were observed on the expression of hgg1 (B,G,L). (D,I,N) The expression of cdh1 follows that of hgg1 in the prechordal plate of control, as well as in snail1b morphants or overexpressing embryos. (C,H,M) Parasagittal sections obtained from hgg1-hybridised embryos reveal the position of the presumptive prechordal plate cells. Sections obtained from cdh1-hybridised embryos (E,J,O) confirm the similarity between hgg1 and cdh1 expression. (P-S) Increased cellular adhesion seems to account for the defects in extension movements in snail1b morphants, since the phenotype can be ameliorated by impairing cadherin activity with the injection of cdh1 MOs. The morphology of the polster is altered in the snail1b morphants (Q), although this effect can be reverted by the injection of cdh1 MOs (S). Asterisks indicate the defective posterior region where cells detach from the embryo both in the cdh1 (R) and in the double morphants (S). (T-W) Higher magnification of the images shown in P-S at the level of the prechordal plate to allow a better visualisation of the polster. The arrows indicate the territory it occupies along the anteroposterior axis. All embryos in this figure are at tail bud stages.