Figure 5.

Changes in progenitor domain identity and neuronal fate in the spinal cord of Nkx6.1 mutant embryos. (A) In wild-type mouse embryos, cells in the Nkx6.1 progenitor domain give rise to three classes of ventral neurons: V2, motor (MN), and V3 neurons. V3 neurons derive from cells in the ventral most region of Nkx6.1 expression that also express Nkx2.2 and Nkx2.9. V1 neurons derive from progenitor cells that express Dbx2 but not Nkx6.1. (B). In Nkx6.1 mutant embryos the domain of Dbx2 expression by progenitor cells expands ventrally and by embryonic day 12 (E12) occupies the entire dorsoventral extent of the ventral neural tube, excluding the floor plate. Checked blue indicates the gradual onset of ventral Dbx2 expression. This ventral shift in Dbx2 expression is associated with a marked decrease in the generation of V2 and MN neurons and a ventral expansion in the domain of generation of V1 neurons. A virtually complete loss of MN and V2 neurons is observed at cervical levels of the spinal cord. The generation of V3 neurons (and cranial visceral motor neurons at hindbrain levels) is unaffected by the loss of Nkx6.1 or by the ectopic expression of Dbx2.