[Photoreceptor Transplantation into the Degenerative Retina]

Vision impairment and blindness due to photoreceptor loss represents one of the major causes for disability in industrialized societies. Whereas rod photoreceptors allow vision under dim light conditions, cone photoreceptors provide high-acuity vision in daylight conditions and color detection. Several therapeutic strategies are currently developed to repair vision loss, including cell-based interventions. Within the last decade, major progress regarding the replacement of photoreceptors by transplantation has been made in pre-clinical animal models. This includes defining the necessary conditions, like the optimal ontogenetic stage of transplantable donor photoreceptors, cell-specific enrichment procedures and robust transplantation technologies. Moreover, first studies provided evidence for functional improvements by photoreceptor transplantation in mouse models of retinal dysfunction. Furthermore, advances in cell culture technology were made by introducing methods to generate photoreceptor-containing retinal organoids, derived from pluripotent stem cells, that provide theoretically unlimited sources for the production of photoreceptor transplants. Interestingly, the recently identified transfer of cytoplasmic material between donor and host photoreceptors might represent an additional treatment option for cell transplantation approaches. Within this review, we focus on the main developments within the photoreceptor transplantation field and discuss important achievements, challenges and hurdles to develop photoreceptor transplantation towards clinical applications.