Defects of the body plan of mutant embryos lacking Lim1, Otx2 or Hnf3 betaactivity

The orientation of the anterior-posterior (A-P) axis was examined in gastru la-stage Hnf3 beta, Otx2 and Lim1 null mutant embryos that display defectiv e axis development. In situ hybridization analysis of the expression patter n of genes associated with the posterior germ layer tissues and the primiti ve streak (T, Wnt3 and Fgf8) and anterior endoderm (Cer1 and Sox17) reveale d that the A-P axis of mutant embryos remains aligned with the proximo-dist al plane of the gastrula. Further analysis revealed that cells which expres s Chrd activity are either absent in Hnf3 beta mutant embryos or localised in heterotopic sites in Lim1 and Ofx2 null mutants. Lim1-expressing cells a re present in the Hnf3 beta mutant embryo albeit in heterotopic sites. In a ll three mutants, Gsc-expressing cells are missing from the anterior mesend oderm. These findings suggest that although some cells with organizer activ ity may be present in the mutant embryo, they are not properly localised an d fail to contribute to the axial mesoderm of the head. By contrast, in T/T mutant embryos that display normal head fold development, the expression d omains of organizer, primitive streak and anterior endoderm genes are regio nalised correctly in the gastrula.