At the top level, neurons were separated from non-neuronal types regardless of tissue, reflecting a split between major classes of cells that express thousands of genes differentially. Notably, this first split does not correspond to any shared developmental or anatomical origins, as it groups neurons from both the CNS and PNS on one side and the corresponding central glia (e.g., astrocytes) and peripheral glia (e.g., Schwann cells) on the other, along with developmentally unrelated vascular and immune cells.
