Fig. 2. Expression of Spry2-IRES-alkaline phosphatase in the mouse second heart field using Mesp1Cre. (A-D) Alkaline phosphatase activity is absent from control sprouty 2 gain-of-function (Spry2-GOF) (A,C), but present in the second heart field (SHF, white bracket) and heart tube in Spry2-GOF;Mesp1Cre (B,D) embryos at E9.5. Pharyngeal arches are numbered. (E-H) In situ hybridization for Spry2 mRNA at E8.5 in control (E,G) and Spry2-GOF;Mesp1Cre (F,H) embryos, on whole-mounts (E,F) and transverse sections (G,H). White brackets indicate SHF; arrow in E points to the OFT myocardium; arrows and arrowheads in G,H indicate the OFT and unlabeled endothelial cells, respectively. (I-J') Immunochemistry on E8.5 whole-mount (I,J) and sectioned (I',J') control (I,I') and Spry2-GOF;Mesp1Cre (J,J') embryos shows decreased phosphoERK1/2 (pERK) staining in the SHF of mutants (white brackets in whole-mounts, red arrowheads in sections). Pharyngeal arch 1 is numbered. (K,L) In situ hybridization shows expression of Erm in right-sided whole-mount views of control (K) and Spry2-GOF;Mesp1Cre (L) embryos at E9.5. White brackets indicate SHF; arrows in L point to epithelial expression domains that are not affected (as expected); arrowhead in L indicates decreased somite expression.