Until now, it has been difficult to maintain retinal ganglion cells deep inside organoids over extended periods. The supply of nutrients and oxygen in the densely packed tissues is limited, leading to cell death. The international team led by Prof. Volker Busskamp at the University Hospital Bonn (UKB), the University of Bonn and the Institute of Molecular and Clinical Ophthalmology Basel solved this problem by combining human stem cell-derived retinal organoids with endothelial cells, which integrate into the organoids and create lumen-like networks that transport nutrients and oxygen – a crucial requirement for preserving sensitive retinal ganglion cells. In vivo, axons of retinal ganglion cells are forming the optic nerve and relay visual information from the retina to higher brain areas.

The scientists tested several methods for integrating the vascular cells and found that pre-cultured endothelial cells integrate best into already formed organoid spheres. This approach preserves developmental processes while significantly increasing the number of surviving ganglion cells. Analyses show that cell types in the vROs differentiate normally, while optic nerve cells survive longer and achieve higher functional maturity.
To examine ganglion cell activity, the researchers used microelectrodes and microfluidic devices that allow the axons of ganglion cells to grow stably. In these vROs, the retinal ganglion cells are more active: they send electrical signals more frequently, more synchronously, and with higher intensity than cells in non-vascularized organoids. Using optogenetic techniques, the cells could be precisely stimulated with light, resulting in markedly stronger and more reliable responses. After several weeks of maturation, the vascularized organoids formed functional light-signal pathways: photoreceptors responded to light stimuli, and the signals were correctly transmitted to the ganglion cells, including the typical ON, OFF, and ON-OFF response patterns.

“Incorporation of vascular cells dramatically improves ganglion cell survival and function, enabling the first comprehensive in vitro demonstration of vertical signal transmission from photoreceptors to ganglion cells. This advance establishes retinal organoids as a functional in vitro platform for studying human retinal development and disease,” explains Prof. Volker Busskamp, corresponding author of the study and head of the Degenerative Retinal Diseases research group at UKB. He is also a member of the Transdisciplinary Research Area (TRA) “Life and Health” of the University of Bonn.

Furthermore, the vascularized organoids demonstrated the ability to respond to hypoxia. Under low-oxygen conditions, the artificial vessels formed new networks, similar to changes seen in certain retinal diseases. This opens up possibilities for modeling conditions such as retinopathy of prematurity and for testing new therapeutic approaches.

The method is simple to apply and can be adapted to other organoid models. Vascularized retinal organoids now provide more sophisticated human retinal models that contain functional retinal ganglion cells and allow the development of light-signal pathways in the lab. These advances offer new perspectives for studying retinal diseases, testing drugs, and developing future therapies.

Funding: Volkswagen Foundation, German Research Foundation, Pro Retina Foundation, TRA “Life and Health” of the University Bonn, Cluster of Excellence ImmunoSensation³, Federal Ministry for Economic Affairs and Climate Action.