The vertebrate tail bud is thought to contain multipotent progenitor cell populations that generate the embryo's axial structures (the neural tube,notochord and paraxial mesoderm). Now, by grafting tissue from a new transgenic chick line in which all embryonic cells express GFP into unlabelled embryos, McGrew and colleagues identify three progenitor cell populations in the avian tail bud (see ). Cells from the embryonic chordoneural hinge, they report,contribute descendants to all of these axial structures, whereas cells from the dorsoposterior tail bud yield mesodermal tissue only. Both these populations are likely to be `long-term axial progenitors' because they are retained in the tail bud after serial transplantation. By contrast, a ventral tail bud cell population, which also generates paraxial mesoderm, is not retained after serial transplantation. Finally, by showing that transplantation of tail bud progenitor cells into earlier embryos resets their Hox expression (which determines the anteroposterior identity of axial cells),the researchers challenge the idea that Hox identity is fixed during gastrulation.
