hESC-derived thalamic organoids form reciprocal projections when fused with cortical organoids, Xiang et al.

Human brain organoid techniques have rapidly advanced to facilitate investigating human brain development and diseases. Recent reports in generation and fusion of regionally defined brain organoids provide further opportunities to investigate the specific brain domains and their interactions. Focus of brain organoids has been to generate telencephalon due to its direct relevance in a variety of forebrain disorders. Despite its importance as a relay hub between cortex and peripheral tissues, the investigation of three-dimensional (3D) organoid model for the human thalamus has not yet been explored. Here, we describe a method to differentiate human embryonic stem cells (hESCs) to the thalamic organoids (hThOs) that specifically recapitulate the development of thalamus. Single-cell RNA sequencing (scRNA-seq) revealed a formation of distinct thalamic lineage cells, a lineage divergence from telencephalic lineage. Importantly, we developed a 3D system to create the reciprocal projections between thalamus and cortex by fusing the two distinct region-specific organoids representing the developing thalamus or cortex. Our study provides a platform for understanding human thalamic development and modeling circuit organizations and related disorders in the brain.