Thalamic NRXN1-Mediated Input to Human Cortical Progenitors Drives Upper Layer Neurogenesis

According to the protocortex hypothesis, extrinsic thalamic signaling is necessary for refining cortical areas and cell types, but the mechanism by which these inputs shape the development and expansion of the human cortex remains largely unexplored. We fuse cortical and thalamic organoids to study this process. Using single-nuclei RNA-sequencing and cellular imaging, we discover that thalamic signals during a critical period promote human cortical upper-layer neurogenesis. In assembloid models and human primary cortex, we find NRXN1 mediates thalamic axon contact with primate-enriched outer radial glia, driving developmental gene expression changes. Genetic perturbation of NRXN1 in thalamic neurons reduces these contacts and attenuates cortical upper-layer neurogenesis. These findings in human developmental models suggest a novel role for thalamic regulation of primate outer radial glia cell fate.