Human-specific progenitor sub-domain contributes to extended neurogenesis and increased motor neuron production

Neurogenesis lasts ~10 times longer in developing humans compared to mice, resulting in >1000-fold more neurons in the human central nervous system. Expansion of human neocortex has been in part attributed to the population of outer radial glia and amplifying progenitors that increase the output of neurogenic lineages. However, outer radial glia appear to be absent in many regions of the developing human nervous system, prompting us to search for alternative populations of progenitors that contribute to the expansion of human neurogenesis in one such region - the developing spinal cord. To this end, we performed high-temporal resolution single-cell expression analysis of human and mouse motor neuron progenitors generated from pluripotent stem cells in vitro. Alignment of human and mouse data using canonical correlation analysis identified "human-specific" progenitor clusters characterized by early co-expression of NKX2-2 and OLIG2 that lacked an orthologous murine counterpart. A matching progenitor population has been previously described in the human embryonic spinal cord1, but its function remained unknown. Our lineage tracing analysis demonstrates that these cells function as ventral motor neuron progenitors (vpMNs), but in contrast to classical pMNs, vpMNs exhibit increased Notch activity and generate motor neurons in a delayed and protracted manner. Concomitantly, vpMNs undergo more rounds of cell division before undergoing neurogenesis, leading to ~2-fold increase in total motor neuron output, and contributing preferentially to later-born, limb-innervating motor neuron subtypes. Thus, instead of relying on transit-amplifying progenitors, human spinal cord evolved a novel progenitor subdomain that extends timescales and expands output of human motor neurogenesis.