Table 1: A summary of cell replacement therapies currently in development for retinal repair, including the main challenges and clinical trial information (adapted from http://clinicaltrials.gov)

Cell Therapy

Main Challenges

Clinical Trial Examples

Embryonic stem cells (ESCs)

>  Obtaining safe, stable and effective cell    sources

>  Finding a realistic up-scaling strategy

>  Chance of immunorejection

 >  Advanced Dry Macular Degeneration treatment (California, US) [52]

 >  Age related Macular Degeneration treatment (Israel) [53]

 >  Stargardt Macular Dystrophy treatment (US) [54]

 >  Advanced Dry Age related Macular Degeneration treatment (US)  [55] + long term follow up study [56]

 >  Advanced Dry Age related Macular Degeneration treatment (Korea) [57]

Induced pluripotent stem cells (iPSCs)

>  Potential time + cost of autologous cell therapy

>  May require gene correction before autologous transplantation

 >  Investigation of genetic retinal diseases (Minnesota, US) [58]

 >  Investigation of retinal degenerative diseases (Maryland, US) [59]

 >  iPSCs derived from retinoblastoma patients (Tennessee, US) [60]

 >  Age related Macular Degeneration treatment (UK) [61]

Mesenchymal stem cells (MSCs)

>  Large scale differentiation of MSCs into functional cells makes cell replacement therapy problematic

 >  Stem cell ophthalmology treatment study (Florida, US) [62]

Adipose stem cells (ASCs)

>  Risk of transplanted cells migrating towards non-targeted regions

>  Potential costs of autologous cell therapy

 >  Glaucomatous neurodegeneration treatment (Russia) [63]

Retinal progenitor cells (RPCs)

> Ability to integrate into retinal circuitry

> Chance of immunorejection

 >  Retinitis pigmentosa treatment (UK + US) [46,47]

 >  Retinitis pigmentosa treatment (California, US) [64]

Retinal pigmented epithelium (RPE) transplantation

>  High chance of immunorejection

>  Survival + function of transplanted RPE difficult on aged and deteriorated Bruch's membrane

 >  Acute Wet Macular Degeneration treatment (London, UK) [65]

 >  Age related Macular Degeneration treatment (Austria) [66]