Fig. 1 |. Generation and characterization of fusion brain organoids.

a, Schematic outlining the generation, patterning, and fusion of dorsal cortex (Cx) and ventral ganglionic eminence (GE) organoids. b, Immunohistochemical analysis of H9 hESC or non-mutant hiPSC-derived Cx and GE organoids prior to fusion at the indicated days (D) of differentiation in vitro. c, Prior to fusion, D56 Cx or GE organoids were infected with AAV1 CAG:tdTomato virus, allowing for tracking of cells emanating from each compartment. Two weeks after fusion, labeled Cx cells showed limited migration into adjacent Cx or GE structures (left and middle images) while labeled GE progenitors display robust migration and colonization of their Cx partner (right image). d, Immunohistochemical analysis showing intermingling of SATB2⁺ cortical neurons with DLX5⁺ GAD65⁺ inhibitory interneurons in the cortical compartment of D106 Cx+GE but not Cx+Cx fusion organoids. e, At day 84, Cx+Cx fusions (left panels) contain numerous excitatory synapses reflected by prominent colocalization of the pre- and post-synaptic markers VGLUT1 and PSD95, yet sparse numbers of inhibitory synapses detected by VGAT/GEPHYRIN costaining. Cx+GE fusions (right panels) on the other hand contain numerous VGLUT1⁺/PSD95⁺ excitatory and VGAT⁺/GEPHYRIN⁺ inhibitory synapses (right panels). All images in b-e are representative images from multiple experiments and represent one of at least 3 or more imaged sections. For specific details refer to Supplementary Table 4.