NEUROGENIN1 and NEUROGENIN2 control two distinct waves of neurogenesis in developing dorsal root ganglia

Different classes of sensory neurons in dorsal root ganglia (DRG) are gener ated in two waves: large-diameter trkC(+) and trkB(+) neurons are born firs t, followed by small-diameter trkA(+) neurons. All such neurons require eit her neurogenin (ngn) 1 or 2, two neuronal determination genes encoding basi c helix-loop-helix (bHLH) transcription factors, ngn2 is required primarily if not exclusively for the generation of trkC+ and trkB+ neurons, whereas the generation of most or all trkA(+) neurons requires ngn1. Comparison wit h previous lineage tracing data in the chick suggests that this dichotomy r eflects a requirement for the two ngns in distinct sensory precursor popula tions. The neurogenesis defect in ngn2(-/-) embryos is transient and later compensated by ngn1-dependent precursors, suggesting that feedback or compe titive interactions between these precursors may control the proportion of different neuronal subtypes they normally produce. These data reveal remark able parallels in the roles of bHLH factors during neurogenesis in the DRG, and myogenesis in the neighboring myotome.