Distinct classes of neurons are generated from progenitor cells distributed
in characteristic dorsoventral patterns in the developing spinal neural tu
be. We define restricted neural progenitor populations by the discrete, non
overlapping expression of Ngn1, Math1, and Mash1. Crossinhibition between t
hese bHLH factors is demonstrated and provides a mechanism for the generati
on of discrete bHLH expression domains. This precise control of bHLH factor
expression is essential for proper neural development since as demonstrate
d in both loss- and gain-of-function experiments, expression of Math1 or Ng
n1 in dorsal progenitor cells determines whether LH2A/B- or dorsal Lim1/2-e
xpressing interneurons will develop. Together, the data suggest that althou
gh Math1 and Ngn1 appear to be redundant with respect to neurogenesis, they
have distinct functions in specifying neuronal subtype in the dorsal neura
l tube.