Two parallel neural ectoderm populations contribute to the developing brain [scRNA-seq]

Stem cell research strives to create diverse types of human brain cells. Given plentiful successes in converting human pluripotent stem cells (hPSCs) into forebrain and midbrain cells, here we focus on generating cells of the human hindbrain, a life-sustaining brain region. This is, in turn, predicated on a developmental roadmap of when and how brain progenitors diversify from one another. In one model, a common brain progenitor can generate all brain regions. Here our studies of mouse embryos and hPSCs support a different model that as early as gastrulation, two parallel brain progenitors emerge simultaneously: anterior neural ectoderm (forebrain/midbrain progenitor) and posterior neural ectoderm (which forms most of the hindbrain). Expanding on classical studies, we suggest anterior and posterior neural ectoderm are already lineage-committed in vitro and cannot transdifferentiate. Anterior and posterior neural ectoderm have divergent chromatin landscapes, respectively foreshadowing their forebrain versus hindbrain potentials. We differentiated hPSCs into posterior neural ectoderm, and subsequently, hindbrain rhombomere 5/6-specific motor neurons. Anterior and posterior neural ectoderm thus represent two parallel developmental routes to generate forebrain/midbrain vs. hindbrain neurons. Finally, anterior and posterior ectoderm are likely conserved from vertebrates to hemichordates, intimating an ancient neural lineage bifurcation that predated chordates, with implications for nervous system evolution. Overall design: Single cell gene expression was profiled using the Chromium Single Cell Gene Expression Flex kit (10x Genomics) for all progenitor and neuronal cell-types generated in this manuscript. Two separate rounds of experiment were performed, one for D0-D2 differentiated cell populations, and one for D4-D14 cell populations.