Neurons have a highly compartmentalized morphology, consisting of an axon, a cell body, and multiple dendrites. It has long been described as a common paradigm that the axon and dendrites of a neuron emerge directly from the cell body, and that this morphological set-up forms the basis of neuronal transmission.

Notably, already in the 19th century, neurons with axons emanating from a basal dendrite were observed, suggesting that the axon onset of neurons can occur in different subcellular regions. However, these neurons with dendritic axon origin were omitted for decades, and they only gained scientific prominence recently when being discovered to participate in specific neuronal circuits. In our research, we are interested in understanding the cellular organization of neurons with a dendritic axon origin, as our knowledge regarding the basic cell biology of these cells remains lacking.

By using advanced imaging techniques, we thoroughly characterized their cellular features, with a specific focus on the microtubule cytoskeleton and the axon initial segment (AIS), structures essential for the establishment of neuronal polarity. We also elucidated the in vitro developmental sequence of these neurons to illustrate how the axon can be carried by a basal dendrite. Moreover, to further investigate the ultrastructure of cellular contents at the AIS in native state, we established a workflow based on cryo-correlative light and electron tomography (cryo-CLEM). This cryo CLEM workflow has enabled us to precisely reconstruct the native cellular ultrastructure of the AIS in 3D.