Development of Microglia sufficient Vascularized Brain Organoids to study three-way Neuro-Immune-Vascular Interactions

Abstract

Most of our understanding of human embryonic development is limited by technical and ethical difficulties in acquiring samples and mainly relies on comparative studies in mice and other higher mammals. Although this approach works for very early developmental events, soon fundamental evolutionary differences begin to add a layer of complexity which simply does not exist in mice. Recently, human induced pluripotent stem cells (hiPSC) or human embryonic stem cell (hESC) based organoids have emerged as a strong model to recapitulate essential aspects of human development in vitro. The developing human brain, unquestionably the crowing achievement of evolution, is the most difficult to study in mice due to significant species-specific differences necessitating a model able to recapitulate cellular diversity and organizational features during human development in vitro. In this work, we build on the complexity of brain organoids by introducing a vascular network as well as the microglial resident immune compartment. In these immune-sufficient vascularized organoids, we observe a close apposition of blood vessels with neural progenitors and neurons. This emphasises the strength of our model for future explorations of vascular involvement in neurodevelopment. Incorporation of the microglial compartment affects not only the neurodevelopmental processes but appears to show a trending increase in the proportion of endothelial cells giving a closer insight into the early vascularisation of the human brain. Overall, the development of this model provides a unique platform for further transcriptomic characterization of microglial heterogeneity and contributes to the long-term goal of incorporating a perfusable circulatory network into organoids.