Fig. 6. The role of Rho-family GTPases in mesoderm spreading. (A) A section through an embryo that lacks the function of Rac2. The mesoderm flattens down onto the ectoderm as in wild type, but does not adhere to the ectoderm properly. (B) The effect of reducing the maternal dose of RhoA. The interaction of the mesoderm with the ectoderm is not affected, but the shape of the cells in the mesoderm is abnormal. (C,D) Embryos derived from mothers with reduced dose of (C) Rac1 and Rac2, or (D) Rac1, Rac2 and Mtl. In these mutants, contact between the mesoderm and the ectoderm fails to be established properly. (E) Embryos that lack the materal and zygotic function of the Rac effector Pak show no effect upon the initation of mesodermal spreading. (F) A section of a homozygous pebble mutant embryo. Contact of the mesoderm with the ectoderm does not occur, and MAPK activation is not observed within the mesoderm at this stage of development. Embryos shown in A-E were stained with Twist and the embryo in Fwas stained with anti-dp-ERK.