Dynamic roles of embryonic glial cells in octopus brain development

Coleoid cephalopods, such as octopus, squid, and cuttlefish, have evolved remarkably large nervous systems with neuron numbers comparable to those of small mammals. Despite this, little is known about how these nervous systems develop, which genetic programs drive the formation and diversification of their neural cells, and what factors contribute to evolutionary brain expansion. This study combined single-cell multi-omics with spatial expression analysis to uncover the cell types and the regulatory mechanisms underlying nervous system development and evolution in Octopus vulgaris. We discovered an early embryonic glial cell population that is distinct from neurogenic progenitors and defined by orphan nuclear receptor 4a (nr4a), distal-less homeobox (dlx), and prospero homeobox (pros) regulons. Their spatial distribution and dynamic transcriptional profile during embryogenesis indicated that these cells might orchestrate cell migration, axon guidance, and brain patterning before expressing neuronal markers or differentiating into mature glia. Comparison of embryonic glial transcriptional signatures across two cephalopods, a gastropod, and Drosophila revealed shared expression of nr4a, dlx, and pros in mollusks, while embryonic glia in Drosophila expressed pros, dlx, repo, ttk, and gcm but not nr4a. Intriguingly, the overall gene expression profile of cephalopod embryonic glia has diverged from that of Pomacea canaliculata (golden apple snail), whose glia do not express guidance or patterning genes. Together, these findings identified a conserved embryonic glial cell type in mollusks that might have acquired new functions in coleoid cephalopods to organize neuronal migration and wiring in their expanded brain.