The Eye

[mak10]

thanks for the info mokele... i'll look into this cephalopods thing when I've the time. but it would have been more appealing if i could find some real experiment being carried out on this... as to whther the non-inverted arrangement would work better in human eyes, but i understand that this might raise some ethical issues, so i'll stay quiet

 

but i will make some comments on your post,

 

Think of it as if the retinal epithelium in our eyes, photoreceptors and all, were taken out, flipped over, and placed back against the back wall of the eye (in terms of geometry, not that such an operation could work). They'd still be the same distance from the retinal epithelium, but facing the other way around.

 

but then, where would you place the choroidal capillaries, that suppy blood to the retinal epithelium?? also, i learned that the bipolar and ganglion cells present are actually very important in being where they are, i.e - in between the incoming light and the photoreceptors. I finally found a link to the American Scientist article I was talking about:

 

How the Retina Works

 

Its by Dr. Helga Kolb who is considered an authority in the human retina. The article is a bit long and heavily detailed, so i'll just quote a few passages from her paper to highlight my point:

 

First off, she starts off explaining the very thing we are discussing about (all emphasis in italics are mine):

 

Intuitively, one might expect that the surface of the retina (the layer exposed to the liquid in the eyeball's vitreous chamber) would contain the sensory cells, the photoreceptors, but actually these cells lie at the very back of the retina; light rays must pass through the entire retina before reaching pigment molecules to excite. This is because the pigment-bearing membranes of the photoreceptors have to be in contact with the eye's pigment epithelial layer, which provides a steady stream of the vital molecule, retinal or vitamin A. Retinal becomes fixed in the photoreceptors' opsin proteins, where this small molecule changes its conformation in response to photons, or packets of light.

 

Also unknown to me was the presence of horizontal cells, let alone their vital function. These are also present between the photoreceptors and the incoming light. About them she writes:

 

Horizontal cells respond to more than the photoreceptors that link to them. Feedback signals from the inner plexiform layer influence horizontal-cell activity as well. These feedback signals are transmitted via substances such as dopamine, nitric oxide and retinoic acid. The result is that horizontal cells modulate the photoreceptor signal under different lighting conditions—allowing signaling to become less sensitive in bright light and more sensitive in dim light—as well as shaping the receptive field of the bipolar cells, as we have seen.

 

She also gives us some good amount of very detailed information regarding the ganglion and amacrine cells that are also involved in the processing. I wouldn't wanna quote all that here since this can really lengthen the post and I don't like long posts myself ! Nonetheless, anyone interested can go ahead and read it up... its very high stuff but very interesting.

 

You also write:

 

so the ancestral invertebrates (from which the chordates had evolved) also had the inverted arrangement??

 

 

Yep. Photoreceptor cells are delicate, so exposing them to the external environment wouldn't be good. And, when you're small and pretty much transparent, inverting them solves the problem nicely.

 

but cephalopods are also invertebrates, arent they? i dont know if they're small or transparent, but they do appear to have the non-inverted arrangement, as you've said. that would mean that the ancestral invertebrates changed from the advantageous inverted retina to the not-so-advantageous non-inverted retina and it bloomed in cephalopods. is that evolutionarily possible?? i've got some more questions since this is all very interesting and stuff and i am no expert, (which is why i've to rely on authorities ). besides, i've to sleep... so later!

 

-mak10

[Mokele]

as to whther the non-inverted arrangement would work better in human eyes, but i understand that this might raise some ethical issues, so i'll stay quiet

 

Hence why I've always said there's a place in society for Mad Science

 

Nonetheless, anyone interested can go ahead and read it up... its very high stuff but very interesting.

 

Fascinating, and thanks for the link. However, I should point out that many of these adaptations in the human retina are likely specificly formed to deal with the inverted state and "make the most of the situation", so to speak. Had our retina been non-inverted, like the cephalopod, we'd've developed an entirely different, but similarly effective, set of adaptions.

 

but cephalopods are also invertebrates, arent they? i dont know if they're small or transparent, but they do appear to have the non-inverted arrangement, as you've said.

 

Most aren't, but the ancestors of vertebrates were transparent, near as we can tell, or at least translucent. Most things around that time we're exactly huge.

 

that would mean that the ancestral invertebrates changed from the advantageous inverted retina to the not-so-advantageous non-inverted retina and it bloomed in cephalopods. is that evolutionarily possible??

 

Evidently it is, since it happened. From what I've read, in vertebrates, the eyes develop as outgrowths from the brain, while in cephalopods, they develop as invaginations from the skin. This probably means that they have similar photoreceptor genes, but that the expression of those genes was altered so they were expressed in the skin rather than the embryonic brain tissue.

 

Mokele

[Mokele]

as to whther the non-inverted arrangement would work better in human eyes, but i understand that this might raise some ethical issues, so i'll stay quiet

 

Hence why I've always said there's a place in society for Mad Science

 

Nonetheless, anyone interested can go ahead and read it up... its very high stuff but very interesting.

 

Fascinating, and thanks for the link. However, I should point out that many of these adaptations in the human retina are likely specificly formed to deal with the inverted state and "make the most of the situation", so to speak. Had our retina been non-inverted, like the cephalopod, we'd've developed an entirely different, but similarly effective, set of adaptions.

 

but cephalopods are also invertebrates, arent they? i dont know if they're small or transparent, but they do appear to have the non-inverted arrangement, as you've said.

 

Most aren't, but the ancestors of vertebrates were transparent, near as we can tell, or at least translucent. Most things around that time we're exactly huge.

 

that would mean that the ancestral invertebrates changed from the advantageous inverted retina to the not-so-advantageous non-inverted retina and it bloomed in cephalopods. is that evolutionarily possible??

 

Evidently it is, since it happened. From what I've read, in vertebrates, the eyes develop as outgrowths from the brain, while in cephalopods, they develop as invaginations from the skin. This probably means that they have similar photoreceptor genes, but that the expression of those genes was altered so they were expressed in the skin rather than the embryonic brain tissue.

 

Mokele

[mak10]

Had our retina been non-inverted, like the cephalopod, we'd've developed an entirely different, but similarly effective, set of adaptions.

 

maybe... possible. but to say that the non-inverted retina would work much better in humans than the present inverted retina that we have.... is not exactly very agreeable, taking in account all the functional features of the present arrangement.

 

Evidently it is, since it happened. From what I've read, in vertebrates, the eyes develop as outgrowths from the brain, while in cephalopods, they develop as invaginations from the skin. This probably means that they have similar photoreceptor genes, but that the expression of those genes was altered so they were expressed in the skin rather than the embryonic brain tissue.

 

from what i understand, natural selection, even though it isn't perfect, couldn't be going though all those developmental constraints, that you mentioned, to change the inverted invertebrate eye to the non-inverted arrangement that we see in cephalopods, had that not been an advantage. on the other hand, the inverted eye remained unchanged throughout the millenia in chordates.. and that wouldn't have been possible, if it weren't advantageous for us. since, as we have seen, it could have been wholly possible to change the arrangment as it did in cephalopods.

 

i don't know what to attribute this inconsistency to. i guess we still have a lot to learn but at the present state, i can think of two possible reasons: 1) either natural selection is conscious and dont like chordates, so stuck them with a supposedly disadvatageous eye arrangement or 2) the inverted eye is actually advantageous for chordates, in the context of their habitual environment and the non-inverted system is advantageous for the cephalopods, in context of theirs. take yr pick!

 

-mak10

[mak10]

Had our retina been non-inverted, like the cephalopod, we'd've developed an entirely different, but similarly effective, set of adaptions.

 

maybe... possible. but to say that the non-inverted retina would work much better in humans than the present inverted retina that we have.... is not exactly very agreeable, taking in account all the functional features of the present arrangement.

 

Evidently it is, since it happened. From what I've read, in vertebrates, the eyes develop as outgrowths from the brain, while in cephalopods, they develop as invaginations from the skin. This probably means that they have similar photoreceptor genes, but that the expression of those genes was altered so they were expressed in the skin rather than the embryonic brain tissue.

 

from what i understand, natural selection, even though it isn't perfect, couldn't be going though all those developmental constraints, that you mentioned, to change the inverted invertebrate eye to the non-inverted arrangement that we see in cephalopods, had that not been an advantage. on the other hand, the inverted eye remained unchanged throughout the millenia in chordates.. and that wouldn't have been possible, if it weren't advantageous for us. since, as we have seen, it could have been wholly possible to change the arrangment as it did in cephalopods.

 

i don't know what to attribute this inconsistency to. i guess we still have a lot to learn but at the present state, i can think of two possible reasons: 1) either natural selection is conscious and dont like chordates, so stuck them with a supposedly disadvatageous eye arrangement or 2) the inverted eye is actually advantageous for chordates, in the context of their habitual environment and the non-inverted system is advantageous for the cephalopods, in context of theirs. take yr pick!

 

-mak10

[premjan]

wouldn't a non-inverted retina raise the problem of not having a uniformly curved (spherical ideally) retina? it would be like projecting onto a wavy screen unless the retinal cells (long structures as I gather they are) were substantially rigid.

[premjan]

wouldn't a non-inverted retina raise the problem of not having a uniformly curved (spherical ideally) retina? it would be like projecting onto a wavy screen unless the retinal cells (long structures as I gather they are) were substantially rigid.

[Skye]

but cephalopods are also invertebrates, arent they? i dont know if they're small or transparent, but they do appear to have the non-inverted arrangement, as you've said. that would mean that the ancestral invertebrates changed from the advantageous inverted retina to the not-so-advantageous non-inverted retina and it bloomed in cephalopods. is that evolutionarily possible?? i've got some more questions since this is all very interesting and stuff and i am no expert, (which is why i've to rely on authorities ). besides, i've to sleep... so later!

No, cephalopod eyes appear to have evolved completely seperately from the eyes we have. Remember that vertebrates are just one tiny part of the animal kingdom, the vast bulk is invertebrates. So they have evolved eyes seperately numerous times, like in insects, the worms I posted previously, some jellyfish, the cephalopods, and others I forget.

[Skye]

but cephalopods are also invertebrates, arent they? i dont know if they're small or transparent, but they do appear to have the non-inverted arrangement, as you've said. that would mean that the ancestral invertebrates changed from the advantageous inverted retina to the not-so-advantageous non-inverted retina and it bloomed in cephalopods. is that evolutionarily possible?? i've got some more questions since this is all very interesting and stuff and i am no expert, (which is why i've to rely on authorities ). besides, i've to sleep... so later!

No, cephalopod eyes appear to have evolved completely seperately from the eyes we have. Remember that vertebrates are just one tiny part of the animal kingdom, the vast bulk is invertebrates. So they have evolved eyes seperately numerous times, like in insects, the worms I posted previously, some jellyfish, the cephalopods, and others I forget.