Pre-Existing Serotonergic Pathways Guide the Navigation of Developing Serotonergic Axons
- 1University of Pisa
Abstract
The serotonergic system originates from a small population of brainstem neurons whose axons form one of the most extensive projection networks in the vertebrate nervous system. Although a number of molecular regulators of serotonergic development have been identified, the core principles that organize this widespread axonal architecture remain poorly understood. Classical neuroanatomical studies have proposed that serotonergic axons may navigate by growing along pre-existing fiber tracts, a process termed epiphytic guidance, but this hypothesis has remained largely untested. Here, we used organotypic transplantation assays to investigate how embryonic serotonergic axons navigate within developing and adult neural tissue. Rostral raphe explants from Tph2-GFP embryos were grafted onto embryonic hindbrain flat-mounts or adult brain slices, allowing donor-derived axons to be visualized in relation to genetically labelled endogenous serotonergic pathways and host tissue architecture. Across different grafting configurations, developmental stages, and both homotopic and heterotopic host territories, donor-derived axons consistently aligned with pre-existing serotonergic pathways adopting local trajectories rather than growing independently of them. This substrate-dependent behavior extended to adult tissue, where transplanted embryonic axons preferentially followed pre-existing serotonergic axon directions. These observations reveal substantial navigational plasticity and indicate that axonal and tissue architectures can provide permissive and potentially instructive substrates for the dispersal of serotonergic fibers. Together, our findings provide experimental support for epiphytic guidance and suggest that serotonergic pathway assembly relies, at least in part, on a pioneer-follower mechanism of progressive self-scaffolding. Such a strategy may help explain how a small population of raphe neurons generates an extensive and spatially coherent neuromodulatory system.
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BibTeX
@misc{picchi2026preexisting,
title = {{Pre-Existing Serotonergic Pathways Guide the Navigation of Developing Serotonergic Axons}},
author = {M. Picchi and S. Migliarini and S. Nazzi and G. Gianni and S. Janusonis and N. Barsotti and M. Pasqualetti},
howpublished = {bioRxiv},
year = {2026},
month = sep,
doi = {10.64898/2026.09.04.749432},
url = {https://www.biorxiv.org/content/10.64898/2026.09.04.749432v1},
note = {CC BY-NC-ND},
}
RIS
TY - UNPB
TI - Pre-Existing Serotonergic Pathways Guide the Navigation of Developing Serotonergic Axons
AU - M. Picchi
AU - S. Migliarini
AU - S. Nazzi
AU - G. Gianni
AU - S. Janusonis
AU - N. Barsotti
AU - M. Pasqualetti
PY - 2026
DA - 2026/09/04
PB - bioRxiv
DO - 10.64898/2026.09.04.749432
UR - https://www.biorxiv.org/content/10.64898/2026.09.04.749432v1
AB - The serotonergic system originates from a small population of brainstem neurons whose axons form one of the most extensive projection networks in the vertebrate nervous system. Although a number of molecular regulators of serotonergic development have been identified, the core principles that organize this widespread axonal architecture remain poorly understood. Classical neuroanatomical studies have proposed that serotonergic axons may navigate by growing along pre-existing fiber tracts, a process termed epiphytic guidance, but this hypothesis has remained largely untested. Here, we used organotypic transplantation assays to investigate how embryonic serotonergic axons navigate within developing and adult neural tissue. Rostral raphe explants from Tph2-GFP embryos were grafted onto embryonic hindbrain flat-mounts or adult brain slices, allowing donor-derived axons to be visualized in relation to genetically labelled endogenous serotonergic pathways and host tissue architecture. Across different grafting configurations, developmental stages, and both homotopic and heterotopic host territories, donor-derived axons consistently aligned with pre-existing serotonergic pathways adopting local trajectories rather than growing independently of them. This substrate-dependent behavior extended to adult tissue, where transplanted embryonic axons preferentially followed pre-existing serotonergic axon directions. These observations reveal substantial navigational plasticity and indicate that axonal and tissue architectures can provide permissive and potentially instructive substrates for the dispersal of serotonergic fibers. Together, our findings provide experimental support for epiphytic guidance and suggest that serotonergic pathway assembly relies, at least in part, on a pioneer-follower mechanism of progressive self-scaffolding. Such a strategy may help explain how a small population of raphe neurons generates an extensive and spatially coherent neuromodulatory system.
ER -