Fig. 4. Rescue of MAPK activation and spreading at early stages of mesoderm morphogenesis. Sections of mutants expressing transgenes in the mesoderm stained to reveal activated MAPK. (A) Expression of Htl in the mesoderm of an htl mutant embryo completely rescues mesoderm spreading. (B) An activated form of the EGF receptor expressed in an htl mutant fails to promote interaction of the mesoderm with the ectoderm and does not stimulate activation of MAPK within the mesoderm at this developmental stage. (C) An activated form of Raf does not induce mesoderm spreading or the activation of MAPK in a dof mutant embryo. Note that we chose a slightly older embryo, showing that even at a stage when the mesoderm collapses onto the ectoderm no MAPK activation is detectable. (D-I) Sections from a young embryo are shown on the left of each row, with older embryos towards the right. (D-F) Expression of an activated form of Htl rescues both early contact of the mesoderm with the ectoderm and the activation of MAPK, notably at early stages this is only evident in the mesodermal cells that make contact with the ectoderm. (G-I) Expression of an activated form of Btl also rescues mesoderm spreading, but the onset of spreading occurs slightly later than in wild-type embryos. Activated MAPK is only observed in mesodermal cells that are in contact with the ectoderm.