EFFECTS OF THE T-WIS MUTATION ON NOTOCHORD FORMATION AND MESODERMAL PATTERNING

The mouse T (Brachyury) gene is required for early mesodermal patterning. Mice homozygous for mutations in T die at midgestation and display defects in mesodermal tissues such as the notochord, the allantois and the semitic mesoderm. To examine the role of T in pattering of semitic and posterior mesoderm along the anterior-posterior axis, we have examined the expression of a panel of molecular markers normally localized to the sub-set of cell types affected in T-Wis mutant mice. Through the use of whole-mount antibody double labelling techniques, we have analysed the spatial relationships of distinct mesodermal populations relative to cells expressing the T protein. We have also examined the consequences of the T-Wis mutation on mesodermal populations recognised by these markers, We demonstrate that T-Wis homozygous mutants retain the ability to form notochordal precursor cells, as identified both by the T antibody and the expression of sonic hedgehog/vertebrate homolog of hedgehog 1 (Shh/vhh-1) and goosecoid, however, these cells fail to proliferate or differentiate. These early notochordal defects appear to result in aberrant semitic differentiation as revealed by the distribution of mox-l protein and twist RNA expression. Moreover, twist expression in paraxial mesoderm appears to be dependent on normal T activity, while Shh/vhh-1, goosecoid, mox-1 and cdx-4 are not T dependent. We propose that T is required for the maintenance of notochordal tissue and subsequent signals required for somite differentiation.