Functional 3-Dimensional Retinal Organoids: Technological Progress and Existing Challenges

Abstract

Stem cell scientists have developed methods for the self-formation of artificial organs, often referred to as organoids. Organoids can be used as model systems for research in multiple biological disciplines. Yoshiki Sasai’s innovation for deriving mammalian retinal tissue from in vitro stem cells has had a large impact on the study of the biology of vision. New developments in retinal organoid technology provide avenues for in vitro models of human retinal diseases, studies of pathological mechanisms, and development of therapies for retinal degeneration, including electronic retinal implants and gene therapy. Moreover, these innovations have played key roles in establishing models for large-scale drug screening, studying the stages of retinal development, and providing a human model for personalized therapeutic approaches, like cell transplants to replace degenerated retinal cells. Here, we first discuss the importance of human retinal organoids to the biomedical sciences. Then, we review various functional features of retinal organoids that have been developed. Finally, we highlight the current limitations of retinal organoid technologies.

Retina organoids: Window into the biophysics of neuronal systems

“…In particular, recent work has shown functional networks in cerebral organoids via a combination of recording technologies. 21,96 In ROs, however, the existence of major neuronal circuits has not been reported so far 97 and is currently being tackled via different approaches. While traditional patch clamp experiments have offered unprecedented timing information on individual RO neurons, 39 it prevents the readout of a large set of neurons organized in a network.…”

The Role of Mitochondria in Optic Atrophy With Autosomal Inheritance

“…Mitochondrial function can also be studied with relative ease in retinal organoids through the use of transmission electron microscopy ( Duong et al, 2021 ) and the use of bioassays measuring viability ( Das et al, 2020 ), ATP production ( Duong et al, 2021 ) and oxygen consumption rate ( Roy-Choudhury and Daadi, 2019 ). These organoids are capable of forming functional synapses and are responsive to light stimulation ( Mellough et al, 2015 ), however, they still lack many features of the native retina – such as vasculature ( Laschke and Menger, 2012 ) and the presence of microglial cells ( Fathi et al, 2021 ) – which can somewhat limit their use in the context of modeling optic neuropathies. Retinal organoids are becoming an increasingly attractive model for the study of inherited ocular disorders as they provide researchers with the unique opportunity to model patient-specific mutations through the use of iPSCs ( Bell et al, 2020 ).…”

“…However, there are still many issues that need to be addressed in order for retinal organoids to reach their full potential. Despite being able to produce all of the major retinal cell types ( Chichagova et al, 2019 ), as time in culture is extended the innermost layers of the organoid containing the RGCs begin to degenerate ( Fathi et al, 2021 ). This is likely due to the fact that there is no vasculature present in the organoid, so as it grows bigger the amount of oxygen and nutrients that can diffuse through to the inner layers is not sufficient to maintain viability ( Fathi et al, 2021 ), while spatial constraints imposed by the culture vessels in which organoids are grown mean that RGC axons cannot extend to the length they normally would in the native retina ( Ohlemacher et al, 2019 ).…”

“…Despite being able to produce all of the major retinal cell types ( Chichagova et al, 2019 ), as time in culture is extended the innermost layers of the organoid containing the RGCs begin to degenerate ( Fathi et al, 2021 ). This is likely due to the fact that there is no vasculature present in the organoid, so as it grows bigger the amount of oxygen and nutrients that can diffuse through to the inner layers is not sufficient to maintain viability ( Fathi et al, 2021 ), while spatial constraints imposed by the culture vessels in which organoids are grown mean that RGC axons cannot extend to the length they normally would in the native retina ( Ohlemacher et al, 2019 ). In addition to this, microglial cells, which would ordinarily interact with Müller glial cells to regulate neuronal development ( Li et al, 2019 ) are absent ( Fathi et al, 2021 ), while mature structures such as an optic nerve are also not present.…”

“…This is likely due to the fact that there is no vasculature present in the organoid, so as it grows bigger the amount of oxygen and nutrients that can diffuse through to the inner layers is not sufficient to maintain viability ( Fathi et al, 2021 ), while spatial constraints imposed by the culture vessels in which organoids are grown mean that RGC axons cannot extend to the length they normally would in the native retina ( Ohlemacher et al, 2019 ). In addition to this, microglial cells, which would ordinarily interact with Müller glial cells to regulate neuronal development ( Li et al, 2019 ) are absent ( Fathi et al, 2021 ), while mature structures such as an optic nerve are also not present. Retinal organoids have already proven to be a valuable tool for studying development and disease, and hold huge therapeutic potential from the perspective of regenerative medicine ( Kobayashi et al, 2018 ; Eastlake et al, 2019 ).…”