Tag: Ophthalmology

The evolution of complex and physiologically remarkable structures such as the vertebrate eye has long been a focus of intrigue and theorizing by biologists. In work reported this week in Current Biology, the evolutionary history of a critical eye protein has revealed a previously unrecognized relationship between certain components of vertebrate eyes and those of the more primitive light-sensing systems of invertebrates. The findings help clarify our conceptual framework for understanding how the vertebrate eye, as we know it, has emerged over evolutionary time. The work is reported by Sebastian Shimeld at the University of Oxford and colleagues at the University of London and Radboud University in The Netherlands.

Our sight relies on the ability of our eye to form a clear, focused image on the retina. The critical component in focusing is the eye lens, and the physical properties that underlie the transparency of the lens, as well as its ability to precisely refract light, arise from the high concentrations of special proteins called crystallins found in lens cells. Fish, frogs, birds and mammals all experience image-forming vision, thanks to the fact that their eyes all express crystallins and form a lens; however, the vertebrates’ nearest invertebrate relatives, such as sea squirts, have only simple eyes that detect light but are incapable of forming an image. This has lead to the view that the lens evolved within the vertebrates early in vertebrate evolution, and it raises a long-standing question in evolutionary biology: How could a complex organ with such special physical properties have evolved?

In their new work, Shimeld and colleagues approached this question by examining the evolutionary origin of one crystallin protein family, known as the betagamma-crystallins. Focusing on sea squirts, invertebrate cousins of the vertebrate lineage, the researchers found that these creatures possess a single crystallin gene, which is expressed in its primitive light-sensing system. The identification of the sea squirt’s crystallin strongly suggests that it is the single gene from which the vertebrate betagamma-crystallins evolved.

The researchers also found that, remarkably, expression of the sea squirt crystallin gene is controlled by genetic elements that also respond to the factors that control lens development in vertebrates: The researchers showed that when regulatory regions of the sea squirt gene are transferred to frog embryos, these regulatory elements drive gene expression in the tadpoles’ own visual system, including the lens. This strongly suggests that prior to the evolution of the lens, there was a regulatory link between two tiers of genes: those that would later become responsible for controlling lens development, and those that would help give the lens its special physical properties. This combination of genes appears to have then been co-opted in an early vertebrate during the evolution of its visual system, giving rise to the lens.

Source: Cell Press

Bio.com
October 11, 2005

Original web page at Bio.com

A bionic eye that allows blind people to see has now got a protective coat of diamond that should significantly improve its performance. The silicon chip retinal implant is being developed by Second Sight, a company based in Sylmar, California, along with a consortium of university researchers. The device needs a hermetic case to prevent it from reacting with fluids in the eye. “It’s as if you’re throwing a television into the ocean and expecting it to work,” says the company’s president, Robert Greenberg. “The approach until now has been to lock it in a big waterproof can, but it’s very big and bulky,” he explains.

So researchers have developed an ultrananocrystalline diamond (UNCD) film that is guaranteed to be safe, long-lasting, electrically insulating and extremely tough. The coating can also be applied at low temperatures that do not melt the chip’s microscopic circuits. The UNCD film is the first coating to meet all the necessary criteria for the implant, says Xingcheng Xiao, a materials scientist at Argonne National Laboratory, Illinois, who developed the film. The tiny diamond grains that make up the film are about 5 millionths of a millimetre across. They grow from a mixture of methane, argon and hydrogen passing over the surface of the five-millimetre-square chip at about 400 °C. Xiao and his colleagues have already tested the implants in rabbits’ eyes, and saw no adverse reaction after six months. He will present the results on 1 April at the Materials Research Society meeting in San Francisco, California.

A healthy retina contains rod and cone cells that convert light into electrical impulses, which fly to the brain through the optic nerve. But for millions of people with diseases such as retinitis pigmentosa or macular degeneration, these cells do not work properly. The retinal implant bypasses these diseased cells by electrically stimulating healthy cells that sit beneath them at the back of the eyeball. The patient ‘sees’ using a pair of glasses carrying a tiny video camera that sends digitized images to the implant using radio waves.

The first human trial of Second Sight’s artificial retina has been running since 2002, and it has enabled a formerly blind patient to distinguish between objects such as cups and plates, and even to make out large letters. But with only 16 electrodes, the device does not allow the patient to see a clear picture. For that, thousands of electrodes are needed on the same size of chip, making it even more delicate.

In the first trial, the electronics package was separated from the electrodes and implanted behind the patient’s ear because its casing was so bulky, explains Greenberg. The team’s goal is to integrate everything into a single unit that fits neatly into the eye. Xiao adds that a UNCD coating could be equally useful for other implantable devices, such as biosensors to monitor a patient’s health.

“They’ve managed to create a diamond film that is of considerably higher quality than other methods have managed,” says Doug Shire, a materials engineer at Cornell University in Ithaca, New York, who is part of the Boston Retinal Implant Project. “But the ultimate potential of these devices will only be told in long-term trials,” he adds. Second Sight is planning clinical trials of a 60-electrode device this year, but it may be several more years before the diamond-coated chip is used in humans, says Greenberg.

Nature
April 12, 2005

Original web page at Nature