Living optic fibres bypass the retina’s back-to-front structure

The human retina is back-to-front. Its silly structure means that light has to cross a tangle of nerves and blood vessels before it reaches the light sensors at the back. Now, scientists have found that the retina uses special cells called Muller cells that funnel light through the retina, in the style of living optic fibres.

If you were a designer tasked with creating a machine for collecting and processing light, the last thing you would come up with is the human eye. Darwin marvelled at the eye’s perfection, but in this, he was wrong. Aside from the many illusions that can fool it, our eyes have a major structural flaw.

The human retina is back-to-front and has the light sensing cells at the back.In humans and other back-boned animals, the light-sensing cells of the eye lie at the back of the retina (see image right; courtesy of University of Michigan).

In front of these sensors lie several layers of nerve cells that carry their signals, and blood vessels that supply them with nutrients. The nerves join to the main optic nerve which passes through a hole in the centre of the retina and connects to the brain.

It’s a stupid, back-to-front design. Light has to pass through several layers of nerves, not to mention blood vessels, before it hits the retina itself. It’s a bit like designing a camera, and sticking the wiring in front of the lens.

Octopuses and squid have a very similar eye to ours, but theirs’ are much more sensibly structured. Their nerves and blood vessels connect to the light sensors from behind so that light can hit the sensor cells without having to negotiate an obstacle course. And because their retina doesn’t need a hole to accommodate the optic nerve, they have no blind spot.

In our own retinas, nerves and vessels are random in their spacing and irregular in their shape. The light that shines past them is reflected, scattered and refracted.

It’s amazing that our eye can see at all. But even though there is clearly no designer, evolution does a pretty good job instead. It has a remarkable capacity for making the best of a bad job. In the case of our eye, some of the obscuring cells act as living optic fibres, to funnel light onto the sensors it covers.

Muller cells – living optic fibres

Muller cells behave like optic fibres.Kristian Franke and colleagues from the Paul Flechsig Institute for Brain Research first noticed these fibres by shining light onto the retinas of guinea pigs. They looked at a cross-section near where the light sensors lay and saw a very regular pattern of bright spots. Clearly, some parts of the retina were transmitting light far better than others.

As they looked at further cross-sections throughout the retina, they realised that the bright spots were the endpoints of long tubes that stretched throughout the retina. Near the top, the tubes widened into funnels.

Franze identified these tubes as Muller cells. The brain cells aren’t nerves themselves, but are part of their supporting cast. They are long cylinders arranged in columns across the entire retina, and provide a route for light to pass through the tangled morass of nerves and blood vessels.

How they work

The eye has evolved to produce images despite a silly back-to-front retina.The Muller cells gather light at the top of the retina and channel it to the light sensors as a tight beam. Along the way, the light is barely reflected or scattered and little is lost when it finally reaches the light sensors, just like modern optic fibres.

Light enters the Muller cells at a shallow angle and is slowed down considerably by the cells’ high refractive index. When it hits the cells’ boundaries, it is almost completely reflected back along the tube.

Their funnel shape allows the Muller cells to gather and transmit as much light as possible. But as they narrow in the middle, they take up a very small amount of space and leave plenty of room for the blood vessels and nerves that the retina needs.

On average, each Muller cell serves a single cone cell and several rod cells. This one-to-one system ensures that the images that eventually hit the light sensors keep strong contrast, and are not distorted.

Evolution has given the vertebrate eye a remarkably ingenious solution to its ludicrous inverted retina. The eye may not be the perfect organ that Darwin thought, but new insights into its’ evolution still provides us with awe-inspiring surprises.

 

Reference: Franze, Grosche, Skatchkov, Schinkinger, Foja, Schild, Uckermann, Travis, Reichenbach & Guck. 2007. Muller cells are living optical fibers in the vertebrate retina. PNAS 104: 8287-8292.

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11 Responses

  1. So if I understood this correctly, the Müller cells evolved only because of the inverted retina and have been found only in vertebrates, right? Fascinating!

    What surprises me is that Müller cells acting as fiber optics was discovered only now. Didn’t the guy named Müller notice their properties when he discovered them?🙂

  2. Yep, to my knowledge, Muller cells are a vertebrates-only invention.

    Maybe Muller was too busy creating a line of tasty yoghurts to properly characterise his cells🙂

  3. So how many millions of years did it take for this all knowing Evolution to give us the “solution” of Muller cells? What happened in the meantime? I suppose vertebrates just limped along barely able to see. Sounds like a strong indicator for survival to me???
    Or, just maybe there is more to the design that we have yet to understand.

  4. Firstly, evolution has no plan and thus isn’t ‘all-knowing’ – that would be the other guy.

    And you’re supposing that the vertebrate eye evolved first and the Muller cells later. Which is not what the paper nor my article imply.

  5. You are saying that cephalopods like the octopus have a sensible eye design. And that is why we say “he’s got a sharp vision like a… octopus!” Because eagles have their retina in wrong. Hmmmm… An eagle can see a fish under the surface a mile away, and all that with an upside down “poorly designed” retina. I have not heard of octopi doing similar feats.
    Besides, the human eye can see a single photon once adjusted to darkness. How can you improve that? The resolution is just limited by the optics (lens etc) not by the retina. Poorly designed??
    There is a logical flaw in your argument. You take the end of your proof and place it at the start as a supposition. You say that evolution rescued a bad design by adding Muller cells etc. So you are supposing that they were not there in the first instance. This is not a question. That is what you are doing.
    As the vertebrate eye is a pretty high-powered and accurate instrument with high needs, there are a few simple reasons that the photo receptor cells are in upside down. The pigment layer or pigment epithelium under the photoreceptor cells is full of blood vessels to supply the receptor cells. This is vital as the photoreceptor cells are the most metabolically active cells in the human body and have to be replaced every 7 days as they “burn out”. If the receptor cells were “the right side up” then the blood-filled pigment epithelium would have to be above them, and all we would see is red. Besides, as the cells are replaced the old cells are shed. Where would the trash go if they were the “right” side up? Float into the vitreous humour?
    Many other features also, even through to Muller cells, show off the amazing design of the eye. For any one of these features to evolve on their own would give no evolutionary advantage. In fact, for all of them to emerge but one would give no advantage.
    I once thought like you, but if we really dig deeper instead of laughing off the idea of a designer at the surface, we just might discover that there is something there. Perhaps that’s what people are scared of. It’s easy to quote Dr. So-and-so and feel like we don’t have to dig for ourselves. I don’t mean to appear arrogant and I respect your beliefs, but it just seems so logical to me that I can’t help myself. Thanks.

  6. And like many pro-ID arguments, this one seems to be based on crazy logical leaps and faulty science.

    Some points in retort:

    1) Photoreceptors aren’t replaced once lost. If they were, I’m sure the many people suffering from macular degeneration would be a lot happier and companies working on photoreceptor transplantation would be less interesting…

    2) The idea that eye is so complex that it only works if all the elements turn up at once has been refuted in several other sources. I recommend the Counter-Creationist’s Handbook, or even Wikipedia, for further reading.

    3) Saying that the cephalopod design is more sensible in no way equates to saying that they have the best vision. Nor are comparisons with eagles relevant. As stated in the article, the vertebrate eye has evolved solutions to cope with an otherwise limiting structure.

    Okay enough now.

  7. Are there animal eyes which have back-to-front retina design as in vertebrate eyes, but do not have Muller cells?
    And, do you think the apparently unreasonal back-to-front design of vertebrate eyes’ retina is a valid argument against the theory of ID?

  8. The first question is a good one – I don’t know the answer, but if anyone else does, please chip in.

    As to the second question, the monumental amount of evidence in favour of evolution is ‘valid argument’ enough against “the theory of ID”.

    And let’s just make this clear here. This blog will take it as granted that evolution is real and that ID is daft. If this doesn’t chime with your own views, this isn’t the blog for you and I will not miss your readership. If you want to contest this point, please go bother PZ at Pharyngula or the folks at the Panda’s Thumb. They will happily rant at you about how wrong you are – I have neither the time nor inclination.

  9. Mr. Fong, why do you assume that evolution must necessarily rule out the possibility of intelligent design? Evolution is a beautiful and elegant solution; I’m sure if I were the designer, I would be very proud of having created it.

  10. […] 1) Living optic fibres bypass the retina’s back-to-front structure […]

  11. Mr Thotpoizn, the proposition that evolution or natural selection is a mechanism used by a designer (aka God) to bring about the diversity of life on Earth, is simply a belief or faith, not science.

    From the fossil records and our study of organisms on Earth, we can see that evolution has no purpose other than replication, and no foresight other than adaptation to the environment. Evolution is thus mutually exclusive with intelligent design, which has purpose and foresight.

    Moreover, the argument that “any complex structures that are so impossibly to have occured by chance or evolved from unconscious, non-purposeful forces, that they must have a designer” will inevitably fall into a circular argument: there must a “super designer” to create the designer, and a “super super designer” to design the “super designer” and so on. Would you believe that?

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