Gene therapy of the human retina levels of light. What follows often is the death of cone cells, responsible for color vision and acuity, at light levels present during the day. Loss of cones leads to full blindness as early as five years old, but may not onset until many years later. There have been multiple hypotheses about how the lack of rod cells can lead to the death of cone cells. Pinpointing a mechanism for RP is difficult because there are more than 39 genetic loci and genes correlated with this disease. In an effort to find the cause of RP, there have been different
Retina horizontal cell proposed. Horizontal cells span across photoreceptors and summate inputs before synapsing onto photoreceptor cells. Horizontal cells may also synapse onto bipolar cells, but this remains uncertain. There is a greater density of horizontal cells towards the central region of the retina. In the cat, it is observed that A-type horizontal cells have a density of 225 cells/mm near the center of the retina and a density of 120 cells/mm in more peripheral retina. Horizontal cells and other retinal interneuron cells are less likely to be near neighbours of the same subtype than would occur by chance, resulting in ‘exclusion zones’