Fig. 5. Alterations of ectoderm specification in embryos and chimeras in which vegetal signaling is blocked. (A) Co-injection of -cadherin and Antivin results in permanent blastulae with numerous unpatterned serotonergic neurons in an enlarged animal plate. Inset shows transmitted light image. (B,C) Embryos co-injected with -cadherin and BMP. (B) SpNk2.1 is expressed throughout. Inset shows transmitted light image. (C) Numerous serotonergic neurons are scattered in thickened epithelium. (D-F) Embryos co-injected with -cadherin and Nodal. SpNk2.1 (D) and SpHnf6 (E) expression is identical to -cadherin alone. However, fewer serotonergic neurons form (F). Inset in S shows transmitted light image. (G,H) Chimeras in which the entire embryo expresses -cadherin and half co-expresses Antivin (G) and myc as a lineage tracer or BMP (H) and myc. Serotonergic neurons form in similar numbers as embryos injected with -cadherin. (I) Chimeras expressing -cadherin throughout and Nodal in one half have fewer serotonergic neurons. The serotonergic neurons that form in the half expressing -cadherin alone are several cell diameters from the interface with Nodal-expressing cells (arrowhead). (J-L) SpGsc-injected embryos. (J) SpNk2.1 protein is in all cells of embryos expressing SpGsc. Inset shows transmitted light image. Neither SpHnf6 (K) nor serotonin (L) are detected. (M-O) Embryos co-injected with -cadherin and SpGsc. (M) SpNk2.1 is expressed uniformly throughout the embryo. Inset shows transmitted light image. Neither SpHnf6 (N) nor serotonergic neurons (O) are detected. (P) In chimeras expressing -cadherin throughout, one half co-expresses SpGsc (myc-positive). Serotonergic neurons differentiate in the half of the embryo expressing only -cadherin. The serotonergic neurons form at the interface with cells expressing SpGsc (arrowhead). (Q) SB431542-treated -cadherin-injected embryos have a similar number of unpatterned serotonergic neurons as untreated embryos expressing -cadherin. (R) Embryos co-expressing -cadherin and Nodal, then treated with SB431542 form serotonergic neurons. Blocking Nodal receptors rescues the formation of serotonergic neurons. Scale bar: 20 µm.