| Summary: | This thesis is concerned with further analysis of the classical mouse mutant <I>Brachyury (T)</I>. Developmental genetics has shown that this gene plays a key role in mesoderm formation during murine gastrulation. Although the <I>Brachyury</I> gene is only expressed in the primitive streak, nascent mesoderm and notochord of the gastrulating mouse embryo, homozygous null <I>T/T</I> mutants have a defective allantois, kinked neural tube and disorganized somites in addition to a bulky primitive streak and disrupted notochord. To study the cell autonomy of the <I>T</I> mutation, we have isolated and genetically characterised embryonic stem (ES) cell lines derived from heterozygous <I>T/+</I> matings and studied their behaviour in chimaeras. T/+ ↔ +/+ form normal chimaeras whereas T/T ↔ +/+ chimaeras mimic the T/T mutant phenotype. In order to assess how <I>Brachyury</I> may interact with other genes which may also be required for mesoderm formation and both rostrocaudal and dorsoventral patterning, I decided to compare the expression patterns of several genes in homozygous and wildtype embryos using a non-radioactive wholemount <I>in situ</I> hybridisation technique. The expression pattern of three <I>Pax</I> genes (<I>Pax-1, Pax-3</I> and <I>Pax-6</I>) which exhibit dorsoventral patterning in the neural tube and/or somites are altered in <I>T/T</I> embryos, whilst the pattern of <I>Hox-7</I> which is expressed in the dorsal part of the neural tube remains unaltered. The <I>T</I> gene is also necessary to maintain the normal expression level of <I>Wnt-5a</I> and <I>Evx-1</I> in 8.5 and 9.5 day embryos respectively. In contrast <I>Wnt-3a</I> and <I>BMP-4</I> expression appear to be unaffected by the absence of the <I>T</I> gene product.
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