Posted on 09/08/2003 4:58:18 PM PDT by bondserv
How the Eye Lens Stays Clear 08/28/2003
To act as a true lens that can focus light, the lens of the eye must remain transparent for a lifetime. Yet the eye lens is not a piece of glass, but a growing, living tissue made up of cells. How can such a tissue stay clear, when the cells must be nourished, and when they contain organelles and chromosomes that would tend to obscure light?
Actually, that is exactly the problem with cataracts, one of the leading causes of blindness, in which the lens becomes clouded. Scientists at Bassnet Labs at Washington University (St. Louis, Missouri) have been studying how the eye maintains transparency, and found an enzyme that, when it fails, leads to cataracts in mice. The job of this enzyme is to chop up and dispose of DNA in lens cells. In a normal eye, “Light can pass through the lens because the cells break down their internal structures during development,” reports Science Now. Nagata et al. at the lab found large amounts of an enzyme named DLAD in mouse lens cells that chops up DNA for disposal. Mice lacking this enzyme developed cataracts. Failures in this enzyme, or the gene that codes for it, are also probably implicated in cataract development in humans.
Their work, published in Nature Aug. 28, explains how lens cells develop: “The eye lens is composed of fibre cells, which develop from the epithelial cells on the anterior surface of the lens. Differentiation into a lens fibre cell is accompanied by changes in cell shape, the expression of crystallins and the degradation of cellular organelles.” Until now it was not known how the cell dismantled its organelles and DNA. The fibre cells have their nuclei removed during maturation, but the DNA remains. It is the job of DLAD to act like a chipper and degrade the long DNA molecules into fragments that can be expelled. Even if the other aspects of fibre-cell cleanup succeed, this study shows that DNA stragglers are enough to cause cataracts.
So normal eye operation depends on the successful cleanup and removal of construction equipment and blueprints: organelles and DNA. Science Now tells a little more about these remarkable lens cells: “Even so, these cells aren’t simply empty; they house a highly organized network of proteins called crystallins* that transmit and focus the light passing through. Any disruption in this sophisticated scaffolding can cloud the lens, causing cataracts.” (Emphasis added.)
Here is an electron micrograph from Birkbeck College, UK showing how the fibre cells in the lens are stacked in neat rows like lumber with hexagonal edges for close packing.
What an amazing thing a living, transparent lens is. Did you ever think about this process, that a sophisticated molecular machine had to be produced from the DNA library that could chop up DNA into fragments, so that they could be removed and not obstruct the light path? Undoubtedly this is not the only enzyme involved in the cleanup job. Each fibre cell needs organelles and DNA during development, but they must be cleared away at the right time, and in the right order before the lens is deployed into operation, or else the user is denied the wonder of sight. This is just one tiny aspect of dozens of complex systems that all must work for vision to work.
Think of an eagle, detecting from high in the air a fish below the water, and using its visual sensors to accurately gauge its approach velocity, pitch, yaw and roll in order for it to capture food for the young in the nest, whose eyes are just opening to the world. Muscles, nerves, specialized tissues, detectors, software, image processing, cleanup, maintenance, lubrication and systems integration are just a few subsystems that must be accurately designed and coordinated in this, just one of many such complex sensory organs in the body.
Evolution is a fake fur that gives warm fuzzies to people who think in glittering generalities. Those who put on lab coats and examine the details and try to fit them into an evolutionary history get cold shudders.*A National Library of Medicine paper describes one of these crystallin proteins: “alpha-Crystallin is a major lens protein, comprising up to 40% of total lens proteins, where its structural function is to assist in maintaining the proper refractive index in the lens. In addition to its structural role, it has been shown to function in a chaperone-like manner. The chaperone-like function of alpha-crystallin will help prevent the formation of large light-scattering aggregates and possibly cataract. ... Reconstructed images of alpha B-crystallin obtained with cryo-electron microscopy support the concept that alpha B-crystallin is an extremely dynamic molecule and demonstrated that it has a hollow interior. Interestingly, we present evidence that native alpha-crystallin is significantly more thermally stable than either alpha A- or alpha B-crystallin alone. In fact, our experiments suggest that a 3:1 ratio of alpha A to alpha B subunit composition in an alpha-crystallin molecule is optimal in terms of thermal stability. This fascinating result explains the stoichiometric ratios of alpha A- and alpha B-crystallin subunits in the mammalian lens.” (Emphasis added.)
Hugs
The proper question is; How did an undirected natural force know how to clear an eye lens up, before knowing clarity is an issue? Did it call a temp enzyme service to check which did the trick?
If you, a mere human being, cannot fathom how this could be done then you must assume creationism to be true.
Or you could go to school and learn something about physiology and anatomy.
Nothing "knew" to clear lenses up, and no critter had to think, "Gee, you know, if I only had clearer lenses everything would be better." Those critters with clearer lenses (or, more sharply focussed images, or more light-sensitive retinas) could find food or evade predators better than those with cloudier (or blurrier, or dimmer) images, and they are the ones whose genes propagated to the next generation with greater probability. Lather, rinse, repeat ... for millions if not billions of years.
Flowing water doesn't "know" to make pebbles round, but a sharp corner sticking out is more likely to be rubbed off than a smooth surface. Repeat a million times, and you get wonderful, smooth spheres even without any design or designer. (This is admittedly a poor analogy to natural selection: no reproduction is present.)
Side note: the eye has evolved several times--indpendently, as in cephalopods.
Why do horses have long legs? To run from predators, says the evolution 'just-so' story. Why then do predators not develop long legs?...
What's more, a 'genetically superior' horse is not gonna pop out with half-a-meter of extra leg. He's likely to emerge with a centimeter or less extra leg. The probability is that a tree will fall on him, he'll contract an infection, or a predator will get him anyway. In other words it is difficult to see how a small delta in leg length can lead to sufficient survival advantage for the individual animal.
Other traits, such as different eye coloration in humans, are hard to "justify" as promoting survival. Who is fittest--the blue-eyes or the brown?
Finally, if intelligence is such a wonderful evolutionary destination, why isn't everything intelligent?...
--Boris
Because in mother nature you do not get to define the destinations.
Like a fully functioning eye! Throw reasonable probabilities to the wind.
So how did clearer eye tissues happen to evolve at the same place as the some-several dozen proteins necessary for sight, any one of which if not present would prevent sight completely? And why not an eye in the back of my head? Wouldn't that be handy?I'm not a disbeliever in evolution because it doesn't affect my religious beliefs one way or the other, and I'm absolutely convinced that evolution happens at certain scales. I have no quarrel with the concept -- even as a Christian -- that humans evolved from apes. However, as one who has degrees in chemistry and chemical engineering (and a whole lot of math since school), the odds for all these amazing things happening in the same place -- like the eyeball -- seem pretty slim, no matter how much time you give it.
Darwinism seems nice and neat and tidy from a macro standpoint, like when looking at the similarities between a shark and a tuna, but when you get down to the level of chemical compounds, evolution all of a sudden becomes immensely more complicated than was dreamed of even a few years ago. To come up with something as complicated as the eye, our bodies ought to be full of millions of different chemicals just hanging around for evolution to perhaps find them useful. However, our bodies just aren't that way and we seem to have exactly the chemicals we need -- no more, no less -- in exactly the right places. The old-school Darwinists need to come up with some better answers.
It seems you are awestruck by your own ignorance of science and evolution. And, since you are so overwhelmed you fail to understand.
"If ignorance isn't bliss then I don't know what is."
I like that -- it's exactly what all the traditional Darwinist answers are!The scary fact that never gets mentioned is that there are significant numbers of honest-to-goodness scientists who aren't religious fanatics who have big doubts about evolution as currently taught. I find that chemists are among the biggest doubters, but perhaps that's because I was trained as one and still run into quite a few.
"And that, boys and girls, is how the tiger got his stripes."
What's the question?
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