Human brain development involves complex interactions between different areas, including long distance neuronal migration or formation of major axonal tracts. 3D cerebral organoids allow the growth of diverse brain regions in vitro, but the random arrangement of regional identities limits the reliable analysis of complex phenotypes. Here, we describe a co-culture method combining various brain regions of choice within one organoid tissue. By fusing organoids specified toward dorsal and ventral forebrain, we generate a dorsal-ventral axis. Using fluorescent reporters, we demonstrate robust directional GABAergic interneuron migration from ventral into dorsal forebrain. We describe methodology for time-lapse imaging of human interneuron migration that is inhibited by the CXCR4 antagonist AMD3100. Our results demonstrate that cerebral organoid fusion cultures can model complex interactions between different brain regions. Combined with reprogramming technology, fusions offer the possibility to analyze complex neurodevelopmental defects using cells from neurological disease patients, and to test potential therapeutic compounds.

The prevalence of diabetes is increasing constantly, resulting in a global epidemic 1 . Diabetes is a major cause of blindness, kidney failure, heart attacks, stroke or lower limb amputation; in large parts because of marked changes in blood vessels, defined by expansion of the basement membrane and a loss of vascular cells [2][3][4] . Diabetes also impairs endothelial cell (EC) function 5 and disturbs EC-pericyte communication 6 . How endothelial/pericyte dysfunction leads to diabetic vasculopathy remains largely elusive. Here we report the development of self-organizing 3D human blood vessel organoids from pluripotent stem cells. These human blood vessel organoids contain endothelial cells and pericytes that self-assemble into capillary networks enveloped by a basement membrane. Human blood vessel organoids transplanted into mice form a stable, perfused vascular tree, including arteries, arterioles and venules. Exposure of blood vessel organoids to hyperglycemia and inflammatory cytokines in vitro induced thickening of the vascular basement membrane. Human blood vessels, exposed in vivo to a diabetic milieu in mice, also mimick the microvascular changes in diabetic patients. Dll4-Notch3 were identified as key

Brain tumors are among the most lethal and devastating cancers. Their study is limited by genetic heterogeneity and the incompleteness of available laboratory models. Three-dimensional organoid culture models offer innovative possibilities for the modeling of human disease. Here we establish a 3D in vitro model called a neoplastic cerebral organoid (neoCOR), in which we recapitulate brain tumorigenesis by introducing oncogenic mutations in cerebral organoids via transposon- and CRISPR-Cas9-mediated mutagenesis. By screening clinically relevant mutations identified in cancer genome projects, we defined mutation combinations that result in glioblastoma-like and central nervous system primitive neuroectodermal tumor (CNS-PNET)-like neoplasms. We demonstrate that neoCORs are suitable for use in investigations of aspects of tumor biology such as invasiveness, and for evaluation of drug effects in the context of specific DNA aberrations. NeoCORs will provide a valuable complement to the current basic and preclinical models used to study brain tumor biology.