Generation and assembly of human brain region–specific three-dimensional cultures

The ability to generate region-specific three-dimensional (3D) models to study human brain development offers great promise for understanding the nervous system in both healthy individuals and patients. In this protocol, we describe how to generate and assemble subdomain-specific forebrain spheroids, also known as brain region–specific organoids, from human pluripotent stem cells (hPSCs). We describe how to pattern the neural spheroids toward either a dorsal forebrain or a ventral forebrain fate, establishing human cortical spheroids (hCSs) and human subpallial spheroids (hSSs), respectively. We also describe how to combine the neural spheroids in vitro to assemble forebrain assembloids that recapitulate the interactions of glutamatergic and GABAergic neurons seen in vivo. Astrocytes are also present in the human forebrain–specific spheroids, and these undergo maturation when the forebrain spheroids are cultured long term. The initial generation of neural spheroids from hPSCs occurs in <1 week, with regional patterning occurring over the subsequent 5 weeks. After the maturation stage, brain region–specific spheroids are amenable to a variety of assays, including live-cell imaging, calcium dynamics, electrophysiology, cell purification, single-cell transcriptomics, and immunohistochemistry studies. Once generated, forebrain spheroids can also be matured for >24 months in culture. Neural differentiation and self-organization of hPSCs are induced by culture in suspension. Neural spheroids are then differentiated into dorsal or ventral forebrain spheroids that can be combined to obtain functionally integrated human assembloids.