Thalamus–cortex interactions drive cell type-specific cortical development in human pluripotent stem cell-derived assembloids

The thalamus plays a pivotal role in the development and function of neural circuits in the cerebral cortex. However, how thalamus–cortex interactions influence human cortical development remains unknown primarily because of the inaccessibility of the human embryonic brain. Here, we demonstrate thalamus-dependent gene expression, circuit organization, and neural activity during corticogenesis using human thalamocortical assembloids (hThCAs). Human cortical and thalamic organoids (hCOs and hThOs) derived from induced pluripotent stem cells exhibited region-specific gene expression and spontaneous neuronal activity. Upon the fusion of these organoids, hThCAs reconstructed reciprocal thalamus–cortex axonal projections and synaptic connections. Transcriptomic analysis revealed thalamus-dependent acceleration of cortical maturation, with upregulation of programs linked to axon development, subplate/cortical plate identity, and activity-regulated genes. Histological analysis showed expanded progenitor pools and increased deep-layer neurons within hThCAs. Wide-field Ca2+ imaging demonstrated that wave-like activity originated in the thalamic region and propagated to the cortical region. Furthermore, two-photon Ca2+ imaging of cortical neurons revealed that synchronous activity emerged exclusively in pyramidal tract (PT) and corticothalamic (CT) neurons, whereas intratelencephalic (IT) neurons remain asynchronous, highlighting cell type–specific circuit integration within hThCAs. These synchronized events were absent in isolated hCOs or in cortico–cortical assembloids, underscoring the specificity of thalamic input. Our findings suggest that diffusible thalamic cues broadly enhance progenitor expansion, while long-range thalamic input organizes cell type–specific synchronous activity. This study demonstrates the thalamus-dependent acquisition of mature cortical phenotypes in a cell type-specific manner in hThCAs, establishing developmental mechanisms linking regional interactions and cell type–specific circuit specification. Thus, hThCAs provide a tractable human platform for dissecting human-specific developmental processes and modeling neurodevelopmental disorders with disrupted thalamocortical communication at the molecular, cellular, and circuit levels.