Cerebral organoids differentiated from human-induced pluripotent stem cells (hiPSC) provide a unique opportunity to investigate brain development. However, organoids usually lack microglia, brain-resident immune cells, which are present in the early embryonic brain and participate in neuronal circuit development. Here, we find IBA1+ microglia-like cells alongside retinal cups between week 3 and 4 in 2.5D culture with an unguided retinal organoid differentiation protocol. Microglia do not infiltrate the neuroectoderm and instead enrich within non-pigmented, 3D-cystic compartments that develop in parallel to the 3D-retinal organoids. When we guide the retinal organoid differentiation with low-dosed BMP4, we prevent cup development and enhance microglia and 3D-cysts formation. Mass spectrometry identifies these 3D-cysts to express mesenchymal and epithelial markers. We confirmed this microglia-preferred environment also within the unguided protocol, providing insight into microglial behavior and migration and offer a model to study how they enter and distribute within the human brain.
We thank the scientific service units at ISTA, specifically the lab support facility and imaging & optics facility for their support; Nicolas Armel for performing the Mass Spectrometry. We thank Alexandra Lang and Tanja Peilnsteiner for their help in human brain tissue collection, Rouven Schulz for his insights into the functional assays We thank all members of the Siegert group for constant feedback on the project and Margaret Maes, Rouven Schulz, and Marco Benevento for feedback on the manuscript. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant No. 715571 to S.S.) and from the Gesellschaft für Forschungsförderung Niederösterreich (grant No. Sc19-017 to V.H.).
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