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By Philip Filner, Ph. D.
Recently, MDF has received a number of inquiries from people who
listened to a brief broadcast news report about retinal transplants. Apparently , the newscast gave many people the impression that a new way to restore lost or impaired vision was
about to become available. The purpose of this report is to respond to those inquiries.
Some researchers do indeed think that it may one day be possible to restore some degree
of vision to people with damaged or malfunctioning retinas, by placing in their eyes either retinal transplants or retinal implants.
What is a retinal transplant?
A retinal transplant is a graft of
"good" retina tissue onto a non-working retina. The retinal transplant tissue might originate from another human, perhaps just deceased, or an aborted fetus. Less likely
origins of the tissue to be transplanted are human retina tissue which has proliferated in culture; or retina tissue from another animal species.
In order for a retinal
transplant to work, several technologies which do not yet exist have to be developed:
1.The cells in the transplant must stay alive for a long time, preferably for the life of
the recipient
2.Those cells must have, and maintain, the light-sensing activities of normal, healthy retina cells
3.Those cells must transmit electric or electrochemical
signals to the brain, which the brain can interpret as the experience of vision.
A handful of laboratories are currently trying to develop retinal transplant technology, or are
doing the research on which such technology might eventually be based (see links at the end of this page). Ophthalmic surgeons are still at the stage of testing techniques for placing
transplants in the retina. Methods for getting retinal transplants to stay healthy will most likely get worked out first in an animal species other than humans. Some neuroscientists
are trying to find conditions under which retinal tissue such as what might be used in a transplant, will produce an electrical signal. The hope of some researchers is that if the
transplant produces a 2-dimensional pattern of electrical signals in response to light, e.g. in the shape of an alphabetical character projected onto the transplant, the underlying
retina which is not light-responsive will still be able to pick up the signal pattern from the transplant and transmit it to the brain.
This research and development path is
"high risk", meaning that it is full of pitfalls, with a high probability that the goal may not be reached.
What is a retinal implant?
A retinal implant is a prosthetic retina, i.e.
a manmade device designed to approximately do the job of the retina.
The concept currently being explored, principally in an overly publicized joint project of MIT &
Harvard, but also in a project in Germany, is to develop a light-sensitive diode array which can be mounted on the retina. The person with the implant would wear on the head a
miniature electronic camera mounted in a unit resembling glasses. The image formed in the camera would be transferred to the diode array implanted on the retina. The diode array would
produce a two-dimensional pattern of electrical signals, which it is again hoped would be picked up by the underlying malfunctioning retina, and transmitted to the brain for
interpretation as vision.
This type of research is at an early stage: learning how to get electric signals from implants in an animal model, the eye of the rabbit.
There
is a related field called computer vision, which is concerned with developing electronic devices which approximate the light-responsive behavior of a retina. These silicon retinas can
be used to give robots the ability to see. It is likely that much of prosthetic retina technology will come from that field. However, the major uncertainty again is: how to transmit a
signal from the prosthetic retina to the brain which the brain can interpret as vision?
The path to a successful retinal implant device is high risk, like the path to a
successful retinal transplant technique. It will be many years before we know if either path has been traversed successfully to its goal. |
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