RT Journal Article SR Electronic T1 Actin-based protrusions lead microtubules during stereotyped axon initiation in spinal neurons in vivo JF bioRxiv FD Cold Spring Harbor Laboratory SP 2020.10.24.353284 DO 10.1101/2020.10.24.353284 A1 Rachel E. Moore A1 Sînziana Pop A1 Caché Alleyne A1 Jonathan DW Clarke YR 2020 UL http://biorxiv.org/content/early/2020/10/24/2020.10.24.353284.abstract AB In vitro, developing neurons progress through well-defined stages to form an axon and multiple dendrites. In vivo, neurons are derived from progenitors within a polarised neuroepithelium and it is not clear how axon initiation observed in vitro relates to what occurs in a complex, three-dimensional in vivo environment. Here we show that the position of axon initiation in embryonic zebrafish spinal neurons is extremely consistent across neuronal sub-types. We investigated what mechanisms may regulate axon positioning in vivo and found that microtubule organising centres are located distant from the site of axon initiation in contrast to that observed in vitro, and that microtubule plus-ends are not enriched in the axon during axon initiation. F-actin accumulation precedes axon formation and nascent axons form but are not stabilised in the absence of microtubules. Laminin depletion removes a spatial cue for axon initiation but axon initiation remains robust.Competing Interest StatementThe authors have declared no competing interest.