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Listen to part of a lecture in a biology class.

I think the lens of the eyes is just about the most fascinating part of the body. For one, it’s the only, I repeat, only transparent cellular tissue in the human body. In fact, truly transparent tissue is very uncommon in nature.

There’s some sort of clear creatures that live in oceans and rivers like the jellyfish. Their quasi-transparency allows them to blend in with the water to escape predators. I said quasi because jellyfish are translucent. They are not totally see-through like the lens is. So you have to wonder why the lens is. How is transparent tissue possible? Marlene?

There’s no direct blood supply to that part of the eye.

Good. Exactly. Marlene. The blood and blood vessels would color the lens, the lens would probably end up looking pinkish.

But that’s only part of the explanation. What else?

Well, there was something about crystals and fibers in the chapter we read. It said that the lens is sometimes called a biological crystal, not a true crystal, like a diamond, but it has a very regular arrangement of cells, the cells are exactly aligned and tightly compacted.

Right. It’s made up of layers of cells. But the cells are all of one type, a rigid, elongated cell that’s called a lens fiber. Because it’s all one kind of cell, and because they are aligned so regularly, you don’t have all the angles you get when different types of cells are fitted together, which refract light, bend it as it passes through, and that contributes to its transparency.But there’s something missing in a lens fiber. What doesn’t a lens fiber have?

Well, we said blood vessels, right?

The lens itself, right. Is that it? Anybody? No? This is very important, the lens fibers don’t have organelles. Organelles are parts that function like miniature organs, specialized parts of the cell, like the nucleus that stores the DNA, or mitochondria, which are the powerhouse of a cell.

Okay, now what’s interesting here is that lens fibers, as they’re first forming, they start out as cells with organelles, but as the eye develops, the cells lose all of them, including the nuclei.

And without organelles, the lens fiber is nice and clear and all those lens fibers packed together make a nifty, crystal-clear lens.

But that clarity comes at a cost. So, Marlene, what’s the downside of this?

Actually, well, if it doesn’t have organelles, I mean, DNA, or whatever, then what happens if the lens is damaged or something. And that’s the problem. The price of clear vision, of transparent lenses, is that the lens fibers can’t regenerate or repair themselves as other cells do, which leads me to this, as we get older, the lens can get cloudy or yellow, it can be cataracts or happen just from aging.

There are lots of possible causes of that. But when you get down to it, the fact remains that the lens fibers just can’t repair themselves, so when the lens gets discolored, what do you think happens to your sight?

Well, it would change your color perception, wouldn’t it?

Exactly. Discolored lenses absorb certain wavelengths of light, so some light rays won’t reach the retina, so some wavelengths aren’t processed, which will affect your color vision.

I just read an article about the French impressionist painter, Claude Monet. And it was really interesting because it showed examples of his painting and you could see from his later paintings what he was seeing, how his eyes, when his eyesight had changed. As he got older, certain colors began to dominate his work, yellows and reds and browns. He was mostly seeing those colors because his lenses had yellowed and weren’t transparent anymore.