THE CHICKEN CDXA HOMEOBOX GENE AND AXIAL POSITIONING DURING GASTRULATION

The chicken homeobox containing gene, CdxA (formerly CHox-cad), was pr eviously shown to be expressed during gastrulation. Localization of Cd xA transcripts by in situ hybridization to tissue sections revealed th at, during gastrulation, expression of this gene exhibits a posterior localization along the primitive streak. The transcripts are localized to epiblast cells in the vicinity of the primitive streak, to cells o f the primitive streak itself and in the definitive endoderm as it rep laces the hypoblast. In order to study in greater detail the pattern o f expression of the CdxA gene during gastrulation, we expressed the fu ll-length CdxA protein as a fusion protein in E. coli and generated mo noclonal antibodies against it. Chicken embryos at different stages of gastrulation were processed for whole-mount immunohistochemical local ization of the protein using anti-CdxA antibodies. Once the pattern of expression in the whole embryo was determined, the same embryos were sectioned to determine the identity of the cells expressing the CdxA p rotein. Detailed analysis of the CdxA protein in embryos, from the ons et of primitive streak formation to the beginning of the tail bud stag e (stages 2 to 10), has shown different patterns of expression during primitive streak elongation and regression. The CdxA protein is initia lly detected at the posterior marginal zone and the expression moves r ostrally into the primitive streak during mid-streak stages. As the pr imitive streak elongates, the CdxA stripe of expression moves anterior ly. By definitive streak stages, the CdxA stripe of expression delinea tes a position along the anterior-posterior axis in the primitive stre ak. CdxA, like its Drosophila homologue cad, is expressed during gastr ulation in a stripe localized to the posterior region of the embryo. T hese observations suggest that CdxA as a homeobox gene may be part of a regulatory network coupled to axial determination during gastrulatio n in the early chick embryo.