Axial stem cells deriving both posterior neural and mesodermal tissues during gastrulation.

The posterior neural plate is primarily derived from the axial stem cells bipotential for neural and paraxial mesodermal development, which reside in the caudal lateral epiblast (CLE) of gastrulating amniote embryos. This process has been demonstrated only recently through cell lineage analyses and determination of Sox2 activation mechanisms. The alternative developmental pathways depend on the activation of either transcription factor genes Sox2 (neural) or Tbx6 (mesodermal); the latter occurs in association with cell ingression through the primitive streak. Tbx6 mutant embryos develop ectopic neural tubes at the expense of the paraxial mesoderm, as Sox2 is expressed even after cell ingression. While producing alternative somatic cell populations, the axial stem cells proliferatively maintain themselves through a process dependent on the Brachyury-Wnt3a coregulatory loop, and even contribute to a fraction of later stem cells of the tail bud in the chordoneural hinge (CNH). Experimental evidence for the above processes is discussed, and unsolved problems indicated.