How come the three layers of retina are so awkwardly arranged?

  • Thread starter garytse86
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How come the three layers of retina are so awkwardly arranged?
Light has to pass through the inner and intermediate layers before reaching the photoreceptor layer, was there any evolutionary advantage by having this arrangement?
 

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I'm not an expert, but I believe it is just an artifact of the evolution of the eye from simple photoreceptors, and not an evolutionary advantage per se. Sort of like the blind spot.
 
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The light sensitive cells (rods and cones) contain membranous discs that contain the light-sensitive photo-pigments and other chemicals that start the response to the incoming light. These discs have to be replaced after some time (in about ten days most of them have been replaced by new ones). The outer layer of the eye, called the pigment epithelium, plays a crucial role in removing those discs and recycling the parts thereof. It also has the chemical machineries that help to regenerate the photo-pigments after they have been exposed to light (the pigments must be transformed back to a shape in which they can respond to light again). Because of this, the light sensitive cells have to be close to this epithelium. However, the pigment epithelium is opaque and does not let light pass so it cannot be on the inside of the eye and therefore has to be on the outside (with the rods and cones as the first layer next to them).
 
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