Biology: Photoreceptors and the Vertebrate Eye

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I want to tell you a story about a friend of mine who is a physicist and really struggles. He likes biology and wants to learn biology, and he's struggling a little bit with the theory of evolution. He came to me one day and he said, "You know, George, just explain one thing to me." He said, "I can see the fossil evidence story and I can see that, but how do you explain something as incredible as the human eye?" I said, "You know, that's a good question."
But if you look over the course of evolution and you think about it, obviously the human eye couldn't have like, "Poof, it's there." There must be some kind of ancestral eyes that have developed. And let me refer you to an organism you probably saw back when you were in 9^th grade or taking biology. You probably saw a creature called "euglena." Now, a euglena is a simple, one-celled organism and it's flagellated, and it actually has something we call an "eye spot." Now, it's not an eye. But all an eye is literally photosensitive pigments imbedded in membranes. This is a one-celled organism that has a photosensitive pigment imbedded in its cell.
And now, if you take it to the next level, well then the eye must have developed. And so we look at something like this--and now we're going to stretch my artistic abilities to their limit. Have you ever seen one of these? This is a flatworm. And even they--now, the flatworm, in terms of animal evolution, is a fairly ancient organism, and even that has photosensitive eye spots. Certainly not the eye complexities that you and I have, but you see what's happening to the course of evolutionary history? There is this evolutionary adaptation that allows the capture of sensory input. There's sensory input captures of pressure, like our ears. There are sensory input captures of heat. In other words, any way you can change the environment, life is going to find a way to take advantage of that. Eyes are antennas for light. That is, in essence, what they are. They are antennas for light. How do they do that? With rhodopsin, the photosensitive pigment that's common to all creatures. All creatures have rhodopsin that pick up light.
With that being said, let's take a huge evolutionary jump to bugs--arthropods, crustaceans, flies. Let's take a look at what they have. Now what you'll notice here is that if you ever look at a bug's eye, like a fly, or a dragonfly, you'll see that they have what are called "compound eyes." This has gotten a lot of publicity and it's pretty well known that they have what are called "compound eyes." In essence, they have many eyes that make up the big eye. They may have as many as 10,000. A dragonfly has literally 10,000 of these mini eyes, which are called, by the way, ommatidia. That's a great word. That's the kind of word you can throw out at your friends and they'll think you're so smart. Ommatidium are the mini eyes, if you will, of an arthropods eye, and I want to talk to you a little bit about that before we get to our eye, because they kind of work the same way. Here's the thing--if you take a look at that. So an ommatidia is literally going to be this mini eye. Let's go to the cellular level of the ommatidia. Every one of these little mini eyes has literally seven cells called retinula cells. So you see, here we have them outlined in blue, and now we've broken inside of them and you see the small packets there. Those small packets are cells that are elongated and go all the way down through the eye. These are called retinula cells, and we're going to focus in on this one right here because we have that cut in longitudinal sections so we can see what toes on inside of there. So these are called retinula cells. Let's draw that right there.
Now, retinula cells, as I said, are elongated cells, and you'll notice they surround what looks here to be kind of a lens-like structure. It's called a crystalline cone. In essence, what seems to be happening here, If you imagine light coming into this ommatidia and it's hitting that cell, and it's being directed towards the crystalline cone, the crystalline cone looks like it's focusing, and it sure enough is, because the retinula cells, as they are elongated, they have inside of them, on a microscopic level, microvilli, all along in here. And what's going to happen in here is that the microvilli from this retinula is going to overlap with that, and all seven of them in that circle are going to form this central that looks like a rod down the middle. It's not. It's merely where the microvilli are overlapping, and those microvilli is where the rhodopsin is going to be. And so the light literally is channeled into this rod, which is called the rhabdom, and channeled down to the axons of nerves, where they are going to be conveyed to the insect's brain. In essence, a dragonfly sees 10,000 images at once. That is very cool. Maybe I'd like to be a dragonfly, I don't know.
But that's not what I want to talk to you about today. I want to talk to you about my eye and your eye, and actually I want to talk to you a little bit about another group, the mollusks, particularly the cephalopods like the squids, because they have an eye just like yours and mine. But we'll get to that. I want to talk to you about our eye. There it is. Like any other light-sensitive organ, it's going to have rhodopsin in it, and it's going to concentrate light onto that rhodopsin, and every structure in there is built for that one job, the concentration of light towards rhodopsin. Let's start out with the outside of the eye. Obviously, first of all, you guys know enough biology to think about this--do you want blood vessels on the outside of your eye? No. Do you want blood vessels on this part of your eye? No. Why? Any place light is going to be, you can't have blood vessels because then the light is going to be reflected and blocked by the blood vessels. So the outside portions of your eye, the light receivers, are clear. So on the outside of the eye is the cornea. You've all heard of the cornea. There's a hole in your eye called your pupil through which light passes, and it's good to have a pupil, because if you don't have a pupil light won't pass through. And you know, the pupil is surrounded by the iris of your eye. I don't think this is news to most of you, but it's a nice review. What I really want to talk about is this part right here for a second--the lens, because the lens is very cool. It's a clear structure, and it's a convex lens. There are some very interesting implications to the fact that that is convex.
You guys remember physics? Here's your worst nightmare--a physics lesson in the middle of biology. Do you remember any diagrams like this where you had like an arrow and they had the light rays going through there and they would have it come down like this and then they'd have a ray come down like this. What happened is you always got an image converted that was upside down because of the bending of the light. Well, guess what? That's exactly what happens here. When light passes through the lens and through this gooey stuff called the vitreous humor. We like to call it "eye juice" in my classroom. Vitreous humor sounds gross enough. It's going to be projected upside down on a structure that's down here, which is called your retina, and that's where all your photosensitivity is going to happen, right down there at the retina. So everything in here is built to transmit light right down there so that eventually the message can go from the retina out into this thing called the optic nerve, which, of course, is the connection to the brain.
Let's talk about the retina. How does the retina work? Does it work like those ommatidia? No. First of all, the retina has a series of cells in it. Here's a typical retinal cell. There's two different kinds of these cells. What I want you to see is that they are kind of modified neurons. You've got dendrites up here. Remember, neurons are for conveying impulses. You have a cell nucleus, so they're real cells. You have mitochondria. They need energy. You have a Golgi apparatus and an ER so they need proteins, and down at the bottom of the cell is where your rhodopsin is going to be. So keep this in mind as we go through this, because I want to tell you about the two types of eye cells.
First of all, there's the rods--sound familiar?--and the cones. That's the great thing about biology at this level. We can take the stuff you learned back in junior high and make it make sense. Now, at the phovia of the retina, which is kind of the focal point where everything goes, there's about 160,000 of these sensors of your eye, as compared to something with a little bit sharper vision, a hawk, which has about a million of these sensors. In fact, a hawk has two phovias. So let's talk about the vision of a hawk. No, we don't have time.
Anyway, rods--there are about three million cones, and most are in the center. These cones, which are tapered at the end, that's one of the things that makes them different from the rods, are there for picking up color and sharpness. Rods, on the other hand, they have no ability to pick up color. They are very light-sensitive, and they're to the outside. Here's what's interesting about this. Did you ever notice when you're trying to look at something... These are light-sensitive, but they're to the outside. And sometimes like if you're trying to put a key in a lock and it's dark and you're looking and you can't see it, and if you look--try this--if it's in the dark just try looking to the side a little bit and you'll be able to see it better because you can direct the light off of your focal point where the cones are and direct it to the rods, and they're better at picking up the light.
So these are much more light-sensitive, these are much more for color sharpness. They do color vision. In fact, for those of you who are color-blind, and that's not so usual to be color-blind, this is usually a defect in either cones or some of the chemicals in the cones. So, for example, there's three different kinds of cones that pick up three different combinations of colors. If one of your cones is defective, you might have what is called red/green color-blindness.
I've got one more story to tell you. I want to tell you how the rods and cones and everything work in a nutshell. Light comes in, and here's what's so bizarre. You have one, two, three, four, five layers of cells in your eye, and one, two, three, the first four layers are completely transparent. Why? Because your receptors are way down at the bottom, and this is where your rhodopsin is. Here are your rods and cones. So what has to happen is this. You have to get through five layers of cells. In a nutshell, here's what happens. Light comes through, it passes through what is called the "ganglia" or the "ganglion layer," past what is called the amacrine cells, past what is called the bipolar cells, past what is called the horizontal cells, and eventually to the rods and cones where there's a message that is sent to the brain that says, "Hey, I've been hit with a photon." But how does it get to the brain? That's the best part of all. After these get stimulated, these remedies and neurons, there's going to be an impulse and it's going to be sent back up through here, back up through here, back up through here, back up through here, sideways, through the horizontal cells and the amacrine cells, back up through the ganglia cells, and then and only then to the optic nerve. Unbelievable.
And one last unbelievable story. Convergent evolution. You know that mollusks and humans branched off hundreds of millions of years ago, and yet a squid has an eye very much like yours and mine. Through convergent evolution they have solved the problem--through natural selection we and the squids have solved that very same problem of how to pick up an image, how to convert that image from upside down to right side up, very much like a camera does. My friend was right. Vision is an incredible thing. It's even more incredible when you really understand it.
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