TABLE 1.

Summary of protrusion mediated signaling in the nervous system.

Organism/cell type	Protrusion type	Structural components	Length and lifetime	Known cargo/signaling pathway	Function
Zebrafish embryo/neural plate cells (Stanganello et al., 2015)	Cytoneme – multidirectional	Actin Tubulin is present at the base	10–50 μm Those carrying Wnt8a measure 16.6 μm on average.	Wnt8a	Mediate long-range Wnt signaling and pattern the neural plate
Zebrafish embryo/neuronal precursors in the spinal cord (Hadjivasiliou et al., 2019)	Basal protrusion – bidirectional along the A/P axis	Microtubules (Hadjivasiliou et al., 2019)	Average 42.6 μm length (4-cell diameters) Remain elongated for 6.8 h on average	Delta	Mediate long-distance Delta Notch signaling pathway activation – pattern neuronal differentiation along the zebrafish spinal cord
Drosophila notum/SOP cells (De Joussineau et al., 2003; Cohen et al., 2010)	Basal filopodia – multidirectional	Actin	Filopodia formed by small bristles precursors (microchaetes) measure on average 11 μm (spanning 1.4-cell diameters) and last <10 min, while in macrochaetes filopodia can span 120 μm (12- to 21-cell diameters)	Delta	Mediate long-distance Delta-Notch signaling pathway activation – pattern mechanosensory bristles precursors in Drosophila notum
Adult zebrafish brain/neural stem cells and progenitors (Obermann et al., 2019)	Apical and basal filopodia-like protrusions – multidirectional	Some filopodia have F-actin	The longest filopodia span 4-cell diameters	Delta	Unknown
Rodent cortex/intermediate progenitors (Noctor et al., 2004; Nelson et al., 2013)	Long and short cellular protrusions. Long protrusion is directed toward the apical surface while short protrusions are multidirectional	Unknown	Unknown	Delta	Suggested to mediate long-distance Delta-Notch signaling pathway activation and maintain radial glia cells in proliferation
Neuronal cocultures (reviewed in Victoria and Zurzolo, 2017)	Tunneling nanotubes	May contain microtubules or F-actin	Up to 100 μm	α-Synuclein, amyloid-β, huntingtin, tau, and prion	Transport components that have been associated with neurodegenerative diseases. May mediate the propagation of disease components to healthy cells or healthy components to diseased cells
Rat/hippocampal neurons cocultured with astrocytes (Wang et al., 2012)	Tunneling nanotubes – directed toward astrocytes	Contain microtubules and some also contain F-actin	Up to 30-μm length and 15-min lifetime	Can contain connexin43	Regulate electrical coupling between immature neurons and astrocytes