Expression of T Protein in the Primitive Streak Is Necessary and Sufficient for Posterior Mesoderm Movement and Somite Differentiation

A characteristic abnormality of chimeras composed of wildtype andT/T(Brachyury) mutant embryonic stem cells is the aggregation and accumulation of mutant cells in the primitive streak and its descendant, the tail bud (V. Wilson, L. Manson, W. C. Skarnes, and R. S. P. Beddington (1995).Development121, 877–886). To demonstrate that this aberrant behaviour of mutant cells in the streak is due only to the absence of wild-type T protein and to investigate dosage effects of T function on cell deployment during gastrulation, a vector expressingTunder the control of its own promoter (which results inTexpression in the primitive streak but not in the notochord) was introduced intoT/Tmutant ES cells carrying a ubiquitouslacZlineage marker. Four clones (TR clones) that express T appropriately in the streak and rescue abnormal chimeric morphology were recovered. In chimeras, these four clones fall into two distinct categories with respect to their ability to exit from the primitive streak and their subsequent tissue colonisation profile. TR1 and TR4 descendants no longer accumulated in the tail bud and gave rise to all types of mesoderm as well as colonising ventral neurectoderm. Interestingly, TR2 and TR5 cells (which express higher levels of T protein than TR1 and TR4in vitro) tended to exit the streak prematurely, showed a marked reduction in posterior mesoderm colonisation, and were virtually excluded from ventral neurectoderm. However, while descendants of all four TR clones can colonise dermomyotome at all axial levels, the parentT/Tmutant cells only contribute to this tissue rostral to the forelimb bud and are completely excluded from more caudal dermomyotome. These results show that the abnormal aggregation of mutant cells homozygous for theBrachyurydeletion (∼200 kb) can be ascribed solely to the lack of wild-type T protein, as can the failure ofT/Tcells to colonise caudal dermomyotome. They also suggest that patterns of cell recruitment from the streak can be influenced by the level of T expression.