Abstract
Brain development begins with neurogenesis in progenitor zones and ends with expansive, intricately-patterned cellular diversity in the adult brain. We took advantage of bioelectric interactions between DNA and embryonic tissue to perform "stereo-tracking," a developmental targeting strategy that differentially labels cells at different depths within progenitor zones. This 3D labeling was achieved by delivery of plasmids with distinct electrokinetic mobilities in utero. We applied stereo-tracking with light sheet imaging in the cerebellum and identified that Purkinje cells follow embryonically committed developmental trajectories, linking distinct progenitor zone subfields to the mature topography of the cerebellar cortex. We additionally identified an unexpected subcellular structure on the axon initial segment of Purkinje cells that we termed "axon bubbles." These structures were revealed by glycosylphosphatidylinositol (GPI)-linked surface labeling and confirmed by electron microscopy. Our findings demonstrate organization of neural progenitor zones in three dimensions, exemplifying the potential of stereo-tracking to uncover new biology within developing systems.