Retinal cell fate determination and bHLH factors

Retinal development is controlled antagonistically by multiple basic helix–loop–helix (bHLH) transcriptional activators and repressors. bHLH repressors suppress bHLH activators and promote maintenance of progenitors and generation of glial cells. In contrast, bHLH activators override activities of bHLH repressors and promote neuronal differentiation. However, bHLH activators alone are not sufficient but homeodomain factors are additionally required for neuronal subtype specification. It is likely that homeodomain factors regulate the layer specificity but not the neuronal fate while bHLH activators determine the neuronal fate within the homedomain factor-specified layers. Thus, combinations of proper bHLH and homeodomain factors are required for neuronal subtype specification.