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The section of retina shows a layered arrangement of the various retinal cells. Starting from the inside (where the light strikes first) is the nerve fibre layer, ganglionic layer, amacrine cell layer, bipolar cell layer, horizontal cell layer and finally rods and cones (and that too in inverted position, i.e. the actual receptor portions are farther inwards).

Why is this stratification inverted? If the receptors would have been the innermost cells, they could have received the maximum quantity of unaltered light. Does this "inverted" arrangement, i.e. arrangement with the receptor cell behind a screen of several other layers not hinder the process of photoreception by blocking a lot of incident light and hence the formed image? What is, if any, advantage of this reverse arrangement?

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up vote 3 down vote accepted

I don't have a other explanation than pointing to evolution. It was obviously not a problem to evolve the way it did. One article I found (see "The Inverted Retina: Maladaptation or Pre-adaptation?") called it a sign of pre-adaptation, where forms exist before they get their final function. Another pretty comprehensive review can be found here: "Evolution of Phototransduction, Vertebrate Photoreceptors and Retina."

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Thank you. And great references, as usual. –  Satwik Pasani Dec 25 '13 at 10:04
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