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.)
I guess you know the facts, fill us in.
Did your college class create an eye in a Petri dish? Maybe you are just making a few assumptions that they forgot to teach you.:-)< /sarcasm >
But seriously, as we discover how complex the inner workings of the cell are, we need to pause and determine if a 150-year-old process needs a serious checkup. Pulling teeth in the science community doesn't come easy.
When the nerve of the community finally dies, we might see more scientists choosing to get an evolutionary "root canal".
Surely you can glean many questions from my post. As a starter, why don't we have millions of chemicals in our bodies just waiting for evolution to use them?
He would need to gain some weight to get a horse down. ;-)
I've got horses in my backyard too.
Evolution states that there is variation and that some of the variation is adaptive and some is not, depending upon the demands of the environment at any given point in time. Adaptive variations tends to survive and non-adaptive do not. Those with adaptive variation tend to propagate such adaptations, because those who do not don't survive. The direction this all takes depends upon the selective pressures in the environment--not upon what we human beings think is a proper or reasonable direction. That the comlexity of it all overhwelms you is not evidence that there is an "intelligent" designer.
There would seem to be a niche for long-winded predators with legs as long as horses.
And this does not address my other problems mentioned above.
--Boris
Because in mother nature you do not get to define the destinations.
I think it's a pretty good question. On the planet, there is one animal who principally survives on the basis of intelligence. Scientists say whales are intelligent. But why is that? And what do they do with their intelligence? Chimps are smart (in a way). But where does it get them?
Intelligence is clearly better than fast legs or sharp claws. Yet, in billions of years, only one animal has evolved intelligence in a way that ensures replication of its genes.
There's something special about Man, I think.
We do.
Check out progesterone and its various--disparate uses--throughout the various species.
As mentioned above, eyes have evolved separately many times. It's good to see.
Echolocation evolved separately in the sea and in (e.g.) bats. It's good to hear.
Flight has evolved in mammals, birds, and fish. It's good to fly.
Color vision evolved in insects and fish. Plants evolved brilliant colors to advertise to insects. It is thought that very few mammals have full color vision: then why does Man prove to be the exception?
I agree.
But why wouldn't we go where the evidence leads us, rather than trying to perpetuate an untenable mess that continues to be more and more unlikely.
Small adaptations don’t necessarily lead to trans-species evolution. Common descent could easily be interpreted common designer. A few matching genetic errors could mean a short term exposure to the same mutating mechanisms...
I agree, and evolution does not detract from that.
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