Bill Holton
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If someone asked you, “What is the connection between your eye and your teeth?” you would likely chew things over for a second or two, then reply, “Well, I do have two eyeteeth.” This may be true, albeit no one knows for sure how those teeth, also called canines, received their ocular moniker. Some say it’s because these teeth are positioned directly beneath your eyes. Others insist the reason is that these teeth have extremely long roots, extending nearly all the way to your orbs.
These aren’t the only interesting connections between teeth and eyes. For example, five years ago researchers at the Pittsburgh University School of Medicine demonstrated that it’s possible to turn stem cells obtained from the dental pulp of extracted teeth into the specialized cells that keep corneas healthy and free of blinding scars caused by illness or injury. These preliminary studies anticipate a day when corneal damage can be repaired using the patient’s own body tissue—especially helpful when a transplanted cornea is rejected by the recipient or in places where there is a shortage of donated corneas.
Here are two more connections between teeth and eyes for you to chew on.
A First Bite
Age-related macular degeneration (AMD) is the leading cause of blindness in people age 65 and older. Currently, an estimated13 million people in the US suffer from AMD, and the problem is expected to grow as the population ages. One large study found that people in middle age have about a 2 percent risk of developing AMD but by age 75 this risk increases to nearly 30 percent.
The most common type of macular degeneration is the early or “dry” form of the disease. It occurs when tiny, yellow cholesterol-rich deposits called soft drusen build up between the retinal pigment epithelium (RPE) cells and the underlying layer of blood vessels. Drusen block the retina’s ability to rid itself of waste products and to receive essential nutrients from the underlying blood supply. “In wet AMD there is an over proliferation of blood vessels,” says Graeme Wistow, chief of the National Eye Institute’s Section on Molecular Structure and Functional Genomics. “In dry AMD it seems to be the opposite. There is evidence that blood vessels under the retinal epithelium seem to be withering and shriveling in the same areas where people develop dry AMD.”
Without adequate nourishment from a healthy blood supply over time the rods and cones that enable us to see begin to weaken and die. This is a particular problem in the central part of the retina, the cone-rich macula. As cones die, straight-ahead vision begins to dim and fail. Eventually the disease reaches a tipping point and further vision loss can be both sudden and dramatic. It’s not uncommon for a patient to go from 20-20 to legally blind in less than six months. In 20 percent of cases the disease will progress to the even more damaging “wet” form, which can lead to near total blindness. Recently, researchers discovered a calcium-containing mineral compound called hydroxyapatite (HAP) associated with cholesterol in soft drusen in dry AMD. They theorized that these spherules are produced as a reaction to stress in the RPE cells. To test this theory Wistow and fellow researchers grew human RPE cells in a cell culture. They then stressed the cells by withholding nourishment. After nine days the stressed RPE cells responded by depositing HAP, and expressing a protein called amelotin, which spurred mineralization of the HAP’s calcium. When expression of amelotin was blocked, so was the accumulation of HAP. When they looked at samples from human donor eyes, they found amelotin in drusen from patients with dry AMD, but not in normal eyes or those with wet AMD.
The expression of amelotin in dry AMD was a complete surprise. ”Most of the existing research on amelotin is on teeth,” says Wistow. “It turns out amelotin is responsible for creating the hard, shiny, outermost layer of tooth enamel.”
The gene that prompts the production of amelotin is found in every cell of the body, but it usually only expresses itself in teeth. Which left Wistow and other researchers wondering, “What if we could block amelotin in retinal cells?”
Wistow and others now plan to experiment with compounds that can block production or function of amelotin, either throughout the body or within the eye only. They have created a line of mice that express amelotin in RPE to aid in their research.
“We have a lot of questions,” says Wistow. “At this point we can’t even say for sure if the amelotin, HAP, and cholesterol actually cause dry AMD, or they are merely a symptom. For all we know, it may turn out they are the body’s last ditch defense to battle some other, more elusive mechanism that’s actually causing the disease. Even if that turns out to be the case, at the very least we will be lead to new and exciting, and potentially more productive future lines of research.”
An Eyetooth for an Eye?
Modified Osteo-odonto-keratoprosthesis MOOKP for short, is a rare procedure used to treat the most profound cases of corneal burns, scarring, and other damage. The procedure was pioneered by Italian ophthalmic surgeon Professor Benedetto Strampelli in the early 1960s, before the current corneal transplant surgery was widely available. It's still used in rare cases today, however, primarily when standard transplants don’t work.
There are two chief reasons why a donor cornea transplant might fail. The first is because the eye is so damaged it no longer produces the tears necessary to keep the new tissue adequately lubricated. Tissue rejection is also possible, at which time it may be worth making a last ditch effort to restore sight.
The MOOKP surgery is a two-step procedure that takes place over a period of several months. First, a surgeon extracts one of the patient’s teeth. An eyetooth is usually chosen, because it has the longest roots. Root tissue is required for the procedure, along with the periosteum, which is the fibrous layer of collagen and nerve fibers that covers bone tissue. Meanwhile, during the same operation, a second surgery scrapes away the scar tissue from one of the patient’s corneas and often covers the cleaned surface with tissue taken from the inside of the cheek.
Next, the tooth is carved into a smaller, thinner shape and a hole is drilled into the center. The hole is filled with a plastic cylinder, which is destined to be the patient’s new lens. The prosthetic, often called a bio-integrated prosthetic unit, is sewn into the patient’s inner cheek, where it is left for up to four months. During this time the prosthetic becomes surrounded by new blood vessels, providing the circulation it will need to survive the final step.
Finally, the prosthetic is removed from the cheek, a hole is made in the opaque, rebuilt cornea, and the new lens, surrounded by compatible bone tissue, is sewn in place. When the surgery works, partial and even complete vision is restored almost immediately. Long-term success rates approach 70 percent.
The surgery is more commonly performed in Europe than here in the US. Indeed, the only successful case I could find mentioned online was the very first such operation that took place in the US back in 2009. Then 60 year-old Mississippi grandmother Sharon Thornton had been blind for nearly a decade due to Stevens-Johnson syndrome, a rare disease that destroys skin and corneal cells. Her cornea was too scarred for a transplant and stem cell therapy failed. However after receiving MOOKP her vision improved to 20/70.
Meanwhile, also in 2009, British builder Martin Jones regained full 20/20 vision after 12 years of blindness caused by a tub of white hot scrapyard aluminum exploding in his face. "The doctors took the bandages off and it was like looking through water and then I saw this figure and it was her,” he explained to the Telegraph about seeing his wife, whom he had met after he was blinded. “She's wonderful and lovely. It was unbelievable to see her for the first time.”
This article is made possible in part by generous funding from the James H. and Alice Teubert Charitable Trust, Huntington, West Virginia.
Related articles:
- Vision Tech: Corneal Clarity by Bill Holton
- Vision Tech: Several Gene Therapies for Blindness Reach Clinical Trials
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