Fig. 11. Transcription factors for retinal cell fate specification. (A) Model for fate switches. The time course of ganglion, amacrine, bipolar and Müller glial cell genesis (Young, 1985) and the bHLH genes that regulate generation of these cells are shown. Because ganglion cell genesis is overlapped with amacrine cell genesis, the cells that are blocked from differentiation into ganglion cells may adopt the amacrine cell fate. By contrast, as amacrine cell genesis is overlapped with ganglion and Müller glial cell genesis, the cells that are blocked from differentiation into amacrine cells may adopt the ganglion and Müller glial cell fates. Because bipolar and Müller glial cells are the last cell types to be generated, the cells that are blocked from differentiation into bipolar cells may adopt Müller glial cell fate. (B) The transcription factor code for retinal cell fate specification. Proper combinations of bHLH and homeobox genes are important for retinal cell fate specification.