Development
The key events mediating rod versus S cone versus M cone differentiation are induced by several transcription factors, including RORbeta, OTX2, NRL, CRX, NR2E3 and TRbeta2. The S cone fate represents the default photoreceptor program, however differential transcriptional activity can bring about rod or M cone generation. L cones are present in primates, however there is not much known for their developmental program due to use of rodents in research. There are five steps to developing photoreceptors: proliferation of multi-potent retinal progenitor cells (RPCs); restriction of competence of RPCs; cell fate specification; photoreceptor gene expression; and lastly axonal growth, synapse formation and outer segment growth.
Early Notch signaling maintains progenitor cycling. Photoreceptor precursors come about through inhibition of Notch signaling and increased activity of various factors including achaete-scute homologue 1. OTX2 activity commits cells to the photoreceptor fate. CRX further defines the photoreceptor specific panel of genes being expressed. NRL expression leads to the rod fate. NR2E3 further restricts cells to the rod fate by repressing cone genes. RORbeta is needed for both rod and cone development. TRbeta2 mediates the M cone fate. If any of the previously mentioned factors' functions are ablated, the default photoreceptor is a S cone. These events take place at different time periods for different species and include a complex pattern of activities that bring about a spectrum of phenotypes. If these regulatory networks are disrupted, retinitis pigmentosa, macular degeneration or other visual deficits may result.
Read more about this topic: Photoreceptor Cell
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