Visual information processing by intracerebral retinal transplants in rats

Ectopic eyes outside the head in Xenopus tadpoles provide sensory data for light-mediated learning

A major roadblock in the biomedical treatment of human sensory disorders, including blindness, has been an incomplete understanding of the nervous system and its ability to adapt to changes in sensory modality. Likewise, fundamental insight into the evolvability of complex functional anatomies requires understanding brain plasticity and the interaction between the nervous system and body architecture. While advances have been made in the generation of artificial and biological replacement components, the brain's ability to interpret sensory information arising from ectopic locations is not well understood. We report the use of eye primordia grafts to create ectopic eyes along the body axis of Xenopus tadpoles. These eyes are morphologically identical to native eyes and can be induced at caudal locations. Cell labeling studies reveal that eyes created in the tail send projections to the stomach and trunk. To assess function we performed light-mediated learning assays using an automated machine vision and environmental control system. The results demonstrate that ectopic eyes in the tail of Xenopus tadpoles could confer vision to the host. Thus ectopic visual organs were functional even when present at posterior locations. These data and protocols demonstrate the ability of vertebrate brains to interpret sensory input from ectopic structures and incorporate them into adaptive behavioral programs. This tractable new model for understanding the robust plasticity of the central nervous system has significant implications for regenerative medicine and sensory augmentation technology.

Retinal transplantation: progress and problems in clinical application

There is currently no real treatment for blinding disorders that stem from the degeneration of cells in the retina and affect at least 50 million individuals worldwide. The excitement that accompanied the first studies showing the potential of retinal cell transplantation to alleviate the progress of blindness in such diseases as retinitis pigmentosa and age-related macular degeneration has lost some of its momentum, as attempts to apply research to the clinic have failed so far to provide effective treatments. What these studies have shown, however, is not that the approach is flawed but rather that the steps that need to be taken to achieve a viable, clinical treatment are many. This review summarizes the course of retinal transplant studies and points to obstacles that still need to be overcome to improve graft survival and efficacy and to develop a protocol that is effective in a clinical setting. Emphasis is given particularly to the consequences of introducing transplants to sites that have been considered immunologically privileged and to the role of the major histocompatibility complex classes I and II molecules in graft survival and rejection.

Long-term survival of retinal cell cultures on retinal implant materials

The aim of the present study was to evaluate cell adhesion and cell survival of mammalian retinal neurons on different materials used for the production of multi-photodiode arrays (MPDAs) intended for implantation in the subretinal space of patients suffering from progressive photoreceptor cell loss. The survival rates of different types of retinal neurons and glia cells were monitored by conventional histochemical techniques and immunocytochemistry up to 4 weeks. Whereas most of the materials tested showed good biocompatibility, cell survival of retinal glia and neurons was markedly reduced on titanium nitride (TiN), especially for culturing periods longer than 2 weeks. The effect was not mediated by diffusible factors released from TiN material. In conclusion, most of the materials tested in this study are suitable for the production of functional MPDAs and no complications are to be expected from long-term implantations of them in the subretinal space.

Development of a silicon retinal implant: cortical evoked potentials following focal stimulation of the rabbit retina with light and electricity

There are currently many experimental efforts to develop a treatment for retinitis pigmentosa (RP) and age-related macular degeneration (AMD). The efforts reported here are part of the Retinal Implant Project, a collaborative effort of the Massachusetts Eye and Ear Infirmary and the Massachusetts Institute of Technology, to develop an independent functioning epiretinal prosthesis to restore vision in patients with disease of the outer retina such as RP and AMD. The epiretinal prosthesis will electrically stimulate the inner retina, bypass the damaged photoreceptors, and hopefully result in meaningful vision. One way to monitor the epiretinal implant is to record focal (stimulation of a small area of retina) evoked cortical potentials (EEP) upon electrical current stimulation. EEP recordings have 3 uses: (1) verify that focal retinal electrical stimulation sends a signal to the cortex that is similar to that elicited by light; (2) develop an animal model to chronically monitor the epiretinal implant; and (3) investigate optimal stimulus parameters. Five dutch pelted rabbits were placed under anesthesia and a 250 microm concentric bipolar stimulating electrode was introduced on the vitreal surface of the retina under visual guidance to record the EEP. Stimuli were charge-balanced pulses and recording electrodes were extradural. Focal VEPs were also recorded and compared to the focal EEP to ascertain cortical origin of the EEP, determine similarities of the EEP to the VEP and determine focality of electrical stimulus. EEP recordings were elicited in 3 out of the 5 rabbits. Current amplitudes which produced detectable responses ranged from 50-1000 microA. In our best series, amplitudes increased by 47-206 microV in increasing current from 50-600 microA. In comparing latencies of the focal EEP and focal VEP, the EEP latencies were 60-70 ms faster, favoring a cortical signal origin. In comparing amplitudes, VEP and EEP responses behaved similarly with changes in stimulus strength and stimulating frequency. In conclusion, EEPs were obtained with parameters of electrode size and current threshold level that would be used by the epiretinal implant, enabling use of the EEP response to monitor the epiretinal implant.