Deconstructing Retinal Organoids: single cell RNA-Seq reveals the cellular components of human pluripotent stem cell-derived retina

The rapid improvements in single cell sequencing technologies and analyses methods afford greater scope for dissecting organoid cultures composed of multiple cell types and create an opportunity to interrogate these models to understand tissue biology, cellular behaviour and interactions. To this end, retinal organoids generated from human embryonic stem cells (hESCs) were analysed by single cell RNA-Sequencing at three time points of differentiation. Combinatorial data from all time points revealed the presence of nine clusters, five of which corresponded to key retinal cell types, namely retinal pigment epithelium (RPE), retinal ganglion cells (RGCs), cone and rod photoreceptors and Müller glia cells. The remaining four clusters expressed genes typical of mitotic cells, extracellular matrix (ECM) components and those involved in retinal homeostasis. The cell clustering analysis revealed the decreasing presence of mitotic cells and RGCs, formation of a distinct RPE cluster, the emergence of cone and rod photoreceptors from photoreceptor precursors and an increasing number of Müller Glia cells over time. The pseudotime analysis resembled the order of cell birth during retinal development, with the mitotic cluster commencing the trajectory and the large majority of Müller glia being the latest. Together, these data demonstrate the feasibility and potential of single cell RNA-Seq to dissect the inherent complexity of the organoids and the orderly birth of key retinal cell types.