Genetic Mutations and Eye Color

Researchers at the University of Copenhagen have discovered a single genetic mutation that took place six to ten million years ago and is believed to be responsible for blue irises in human eyes. According to Professor Eiberg from the Department of Cellular and Molecular Medicine at the University of Copenhagen, “Originally, we all had brown eyes, but a genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a ‘switch’, which literally ‘turned off’ the ability to produce brown eyes”.

Of course, many people still have brown eyes. It is, in fact, the most common eye color. However, the OCA2 gene code, which is involved in the production of melanin that gives pigment to our eyes, “switched” millennia ago, diluting the color brown to blue in certain individuals.

A Common Ancestor

According to Professor Eiberg, “All blue-eyed individuals are linked to the same ancestor. They have all inherited the same switch at exactly the same spot in their DNA.”  By contrast, people with brown eyes have innumerable individual variations in the area of DNA which controls the production of melanin.

Beginning the study in 1996, Eiberg and a team of researchers examined mitochondrial DNA from individuals in countries including Denmark, Jordan, and Turkey. They have concluded that this switch has no profound positive or negative effects on vision, but does show “That nature is constantly shuffling the human genome, creating a genetic cocktail of human chromosomes and trying out different changes as it does so.”

No matter what color your eyes, frequent eye examinations are necessary to ensure optimal vision. Please use our Eye Doctor Directory to find an experienced ophthalmologist in your area.

Zebrafish Stem Cells and Retina Repair

Your retina is the inside back surface of your eye that contains the light sensitive cells responsible for converting light into neural information. The retina contains millions of cones that provide sharp vision and color perception in bright light along with millions more rods that allow for vision in low light situations.  Issues with the cones and rods in the retina can prevent information from reaching your optic nerve, resulting in a complete loss of vision.

It has long been believed that damaged photoreceptor cells in the retina could not be repaired, but researchers at the University of Alberta have recently discovered that zebrafish stem cells can selectively regenerate damaged rods and cones, and may be the key to returning vision to those with retinal disorders.

Continuing Research

Thus far, researchers have found that zebrafish stem cells can replace damaged cells in many components of human eye sight. The research to date has shown significant success in repairing damaged rods, but most of the tests have been done on nocturnal animals that have millions more rods than cones. It is still unclear as to whether or not these stem cells can be instructed to only replace damaged cones in the cone-dense human retina.

According to the researchers, eyes tend to regenerate the photoreceptor cells that are most prevalent. In humans, this would be cones. Animal studies would suggest that the tissue environment in the human eye would instruct stem cells how to react to cone damage, but the specific gene in zebrafish that activates cone repair has not yet been isolated.

As it stands, there is no cure for blindness, but there are steps that can be taken to slow damage being done by retinal problems. If you notice floaters or flashers, a primary indication of retinal problems, you should contact your ophthalmologist right away.

If you are experiencing any vision disruptions, please visit our eye doctor directory today to find an experienced ophthalmologist in your area.

Eye Color and Trustworthiness

When someone looks at your eyes, the first things they see are your iris and pupil, and what they think about the color of your iris may surprise you. In a study of 238 university students in Prague, participants were asked to view 40 pictures of men and 40 pictures of women to determine -from their photos alone- their level of trustworthiness. In the study, brown-eyed people were more often seen as trustworthy than those with blue or green eyes. However, when eye colors were switched on the same faces, the results changed for photos of males. This led the researchers to conclude that it was facial shape and characteristics, not eye color, that people viewed as more trustworthy.

Brown Eyes and Facial Characteristics

The researchers in this study found that men with brown eyes tended to have bigger noses and wider mouths. They also found brown eyed males had broader chins and more prominent eyebrows. Based on this, they concluded that it was these facial features that determined trustworthiness in males. However, as these same correlations could not be found in perceived trustworthiness of photographs of female faces, more research will need to be done for a final conclusion.

To learn more about eye anatomy, please contact an eye doctor in your area today.

Deep Sea Eyes Evolved to Protect from Predators

A recent study of the eyes of giant and colossal squid reveals they would likely have only given an advantage for detecting the presence of large predators, such as the sperm whale.

Giant squid have possibly the largest eyes of any creature that ever lived. At a diameter of 10.5 inches, with a pupil size of 3.5 inches and hard lenses as big as human eyes, these eyes are disproportionally sized compared to almost any other creature, which raises the question, “Why?” Any time a creature develops a highly unusual structure such as this type of eye, the typical assumption is that some selection pressure has driven its development. Now researchers believe they have found the answer to what selection pressure drove the development of squids’ huge eyes.

It has long been assumed that the huge eyes of giant squids were in some way an adaptation to living in very dark water. Larger eyes are able to take in more light and so presumably would allow the monstrous squid, which measure up to 49 feet (15 m) in length, to see better in dim light conditions. However, researchers found that for most situations, an eye size greater than 4 inches, the size of swordfish eyes, provided no benefit. See the chart below for a comparison of the relative size of creatures’ eyes.

However, there was one situation in which the very large eyes of the squid would be very helpful: the detection of approaching sperm whales. As sperm whales cut through the deep water, they disturb numerous smaller organisms, which give off bioluminescence. The massive eyes of the giant squid species would allow it to detect and interpret the pattern of disturbed organisms to see sperm whales approaching from a distance of nearly 400 feet, which would allow the squid to potentially avoid approaching whales.

Looking at the chart above, we notice that another creature had eyes about the same size as the giant squid, the Temnodontosaurus platydon. Based on the models provided by researchers, it seems likely that the function of these eyes was the same: to detect the approach of a very large predator.

But what ate a temnodontosaurus? Shown above in comparison with a human, temnodontosaurus was one of the largest known creatures in Jurassic seas.

One possible speculation is the presence of a very large pliosaur. Some estimate that pliosaurs, similar to liopleurodon (image below), may have been up to 50 feet long, about half again the length of temnodontosaurus, which may very well have made it a viable predator. Another possibility, though, is the existence of a kraken, a gigantic cephalopod predator, which would not be well preserved in the fossil record.

What is remarkable about these eyes is their extraordinary ability to function under demanding conditions. If your eyes are not properly functioning, please contact a local ophthalmologist to talk about options for improving your vision.

Study Focuses on the Development of Nearsightedness in Children

Myopia—or nearsightedness—is thought to affect more than 30 percent of American children, and it can occur when the length of the eyeball from front to back extends too far for the cornea and lens to focus properly.

What was not known was how myopia developed in children who previously had normal vision. But a recent study indicates that nearsightedness in children may occur when the lens no longer counterbalances for the continued growth of the eye.

To determine why children with normal vision could seemingly suddenly develop myopia, researchers led by Dr. Donald O. Mutti of The Ohio State University College of Ophthalmology examined changes in eye growth for children between 6 and 14 who developed nearsightedness at varying ages against children of the same ages whose vision remained normal.

In children whose vision did not change, researchers found that the eye lenses grew progressively thinner and flatter as the eyeball grew, maintaining normal vision through the growth period. In children who developed nearsightedness, the eye lenses stopped changing optical power as the length of the eyeball increased.

While it remains unclear why the lens stops adapting to the growth of the eye in some children, the study indicated that the change happens rapidly. Research showed the imbalance occurred approximately one year before the onset of myopia.

Findings of the study were published in the March issue of Optometry and Vision Science, which is the journal of the American Academy of Optometry.

If you believe you or your child may have myopia, please use the doctor locator to find an experienced ophthalmologist in your area.

Giant Cambrian Predator Had Excellent Vision

Recent analysis of fossils of an ancient predator reveal that the creature had excellent vision. The predator, Anomalocaris, had a worldwide distribution and is though to have been the top predator in the Cambrian seas. The existence of the predator was postulated before the creature was known. In 1979, a paleontologist studying trilobites explained that wounds on the bodies of certain trilobites might be caused by a large predator. At the same time, another paleontologist realized that what had previously been identified as a shrimp body was actually an appendage on a larger creature, which might be responsible for trilobite injuries. When the full body of the creature was described in 1985, it was characterized as “a formidable predator.”

Now we know that vision was an essential tool for this predator. 515 million year old fossils attributed to the creature show extremely complicated compound eyes. The eyes are speculated to have had as many as 16,0000 individual lenses, which is comparable to modern dragonflies, which may have as many as 30,000 lenses per eye. By contrast, triDragonflies use their excellent vision for hunting prey in a three-dimensional space, much like the Anomalocaris, which, with its newly-discovered vision could locate prey more effectively than competitors. In response, it is likely that prey developed anti-vision defenses, such as camouflage.

But what did Anomalocaris eat? Some researcher claim its teeth were too soft to eat trilobites, but fossil feces too large to be from any other known animal contain trilobite pieces. So, until another candidate is discovered, Anomalocaris is the prime culprit.

Because the fossils in question are not directly attached to Anomalocaris, some dispute that the eyes actually belonged to the predator. However, it is unknown what other creature they might belong to. Again, Anomalocaris is the only creature large enough to have owned them. However, because preservation of lenses is rare in fossils of this age, there may be many creatures with similarly sharp eyesight, which had been unknown before this discovery. It also likely means that arthropods likely developed eyes before they developed their characteristic jointed skeletons. For more information on this predator and its vision, see the Discover blog Not Exactly Rocket Science.

This type of eye is very different from human eyes. Instead of having a pair of lenses (the cornea and the crystalline lens) that focuses light onto a bank of nerve endings (the retina), compound eyes have many separate lenses that each focus light onto a separate photoreceptor. Thus, each separate lens represents a pixel in the creature’s vision, creating an arrangement that is ideally suited for capturing movement, but less well suited to capturing detail. An evolutionary arms race between camouflage and vision would drive increasing numbers of pixels that could resolve detail to allow predators to spot hiding prey, even when static.

As with these early predators, your vision is a great tool, and visual acuity can make the difference in facing your competition. To make sure your vision is at its peak, please contact a local ophthalmologist today.

Dinovision: How We Know What We Know about Dinosaur Vision

Studying the vision of current and prehistoric animals can help us understand the origins of our vision and track down causes for visual defects or shortcomings. Studying the vision of current animals is relatively easy. Researchers have devised a number of vision tests that allow us to test the functional vision of animals. We can also look at the structure of their eyes to determine their vision on the basis of the optical structures they possess.

It is much harder to understand the visual systems of extinct animals, since the eyes, as soft tissue, are rarely preserved, and then only in hard-bodied animals whose eyes were part of their exoskeleton, such as insects or arthropods.

In the case of something like dinosaurs, the problem is much more complicated. We can get a lot of answers from the morphology of the skull. The skull tells us a lot about the structure of the eyes, including whether they had a scleral ring to support their large eyes. A scleral ring is a bone that sits inside the sclera of the eye to give it support and tells us the size of the iris of an animal and therefore how much light it could admit

The shape of the skull also tells us the size and shape of the animal’s brain. We know that the brain performs numerous tricks in helping us to see, including selecting between different images, which is helpful in the case of people receiving multifocal intraocular lenses after cataract surgery.

Another important tool for determining the way extinct animals see is looking at their evolutionary relatives. We know, for example, that dinosaurs are closely related to crocodiles and birds. By looking at the vision of an animal’s living relatives, we can extrapolate how they may have been able to see.

Based on all this evidence, we have actually learned a lot about what and how well dinosaurs see, something we will explore periodically.

To learn more about your own vision, please contact a local eye doctor for a consultation.

What Is the Cornea?

The cornea is the clear outer window at the front of the eye. You can see the cornea when looking at the eye from the side, it bulges out visibly from the eye’s spherical shape.

Directly behind the cornea is the eye’s anterior chamber, filled with aqueous humor, one of the eye’s intraocular fluids. This fluid is essential for nourishing the cornea, which does not have blood vessels.

In terrestrial animals, the cornea is the first of the eye’s two refractive lenses. The second is the crystalline lens, which sits behind the iris. Although the crystalline lens can change focus, the cornea’s focal power is essentially fixed for life. In aquatic animals, such as fish and whales, the cornea is actually not a focal lens because its density is too close to that of water to perform any significant refraction of light. In whales, it is unknown how they focus, because the lens does not change shape.

The cornea has five layers:

  • Epithelium
  • Bowman’s layer
  • Stroma–about 90% of the cornea’s thickness
  • Descemet’s membrane
  • Endothelium

As the eye’s primary focal lens, problems with the cornea have the potential to impair or completely disable a person’s vision. The most common defect with the cornea is simple refractive errors, whether myopia, hyperopia, or astigmatism. The most common correction methods for treating this type of problem with the cornea are glasses, contacts, and LASIK or other laser vision correction procedures.

Keratoconus and Fuchs’ dystrophy are two other types of corneal conditions that may threaten your vision. Sometimes keratoconus can be treated with a special contact lens. Other times, it requires a cornea transplant. Fuchs’ dystrophy always requires a cornea transplant.

Unlike the eye’s crystalline lens, there is no artificial substitute for the cornea. Also, corneas from other animals cannot be used for cornea transplants. The only source of cornea transplants is corneas donated by people.

Remember to donate your corneas if you can. Even if you have refractive errors, you can still be a good cornea donor.

To learn more about your cornea or other parts of your eye anatomy, please contact a local eye doctor for a consultation.

LASIK and Corneal Scars

A corneal scar is an injury to your eye’s cornea. Having a corneal scar doesn’t necessarily disqualify you from LASIK eye surgery, but certain types of scars can have a negative effect on the results of surgery. You ophthalmologist should carefully examine your corneal scar to consider the following factors:

  • The depth of the scar – If your scar is superficial, it is possible to remove it during LASIK surgery. A deeper scar can be reduced and improved with a different laser treatment (such as PRK), but it still might not be completely eliminated.
  • The location of the scar – Unless your scar is close to the center of your eye, you are probably still a candidate for LASIK. However, if it is too close to your visual axis and has an effect on your vision, your ophthalmologist may recommend an alternative laser treatment.
  • The cause of the scar – If the scar was the result of an infection, the exact type of infection may affect your LASIK candidacy. For example, if viral keratitis caused your scar, there is a slight risk that LASIK surgery would re-activate the virus.

LASIK surgery should only be performed on patients who are ideal candidates. In some situations, corneal scars can be reduced or eliminated by Laser Vision Correction. But if your ophthalmologist believes your corneal scar would pose an undue risk, he or she can work with you to find an alternative solution.

If you would like more information about LASIK and scarred eyes, contact an experienced LASIK eye surgeon in your area today.