The question therefore arises as to whether the molecular identity of a cell is determined mainly by its developmental ancestry, by its local environment, or by its function. All three possibilities are plausible a priori: neurons with shared function (for example, long-range projecting neurons or neurons using a common principal neurotransmitter) might be expected show common gene expression states across brain regions. Alternatively, chemical cues arising from a local environment might impose constraints forcing neighboring cells of different functions to become molecularly similar. Finally, developmental origin, through shared gene regulatory circuits, might retain an imprint on cell types in the adult so that gene expression patterns would reflect developmental domains and borders.
