Table 1.

Features of signaling filopodia described and characterized in vertebrate species

Species	Location and function of filopodia	Length and diameter (µm) Dynamics	Characteristics of dynamic and stable filopodia	References
Zebrafish embryo	Epiblast cells in blastula Function Delta-notch signaling	Average length 215 µm (range 60–380 µm) Diameter: 1 µm	Stable filopodia: “intercellular bridges” Delta-positive filopodia  Actin positive  Tubulin only at the bases  Cargo transport: v = 55 nm/s	[29]
Zebrafish embryo	A subset of migrating NCCs		CXCR4-GFP localizes to fscn1a-dependent filopodia  Actin positive	[50]
Zebrafish embryo	Epithelial cells in the presumptive neuroectoderm		Wnt8a-venus positive “puncta” at filopodial tip is released as the filopodium retracts Formation of Wnt8a/Fzd9b clusters	[37]
Zebrafish embryo	Transfected epiblast cells Function Neural plate patterning	Average length: 16.6 µm (up to 50 µm) Diameter max 1.5 µm	Wnt8a-eGFP-positive filopodial tips give rise to extracellular puncta, which are attached to target cells On stable filopodia formation of “signalosomes”: clusters of Wnt8a and Lrp6-GFP, Dvl-mCherry (or Axin1) on the target cell	[17]
Zebrafish embryo	HEK293 cells, fish fibroblasts PAC2, mouse 3T3	Length in PAC2 cells: up to 70 µm Elongation: average v = 110 nm/s Max. v = 240 nm/s	Wnt8a-eGFP-positive, stable filopodia  Actin positive  Tubulin at the base  Myosin10 positive Cdc42 determines the length of Wnt-positive filopodia	[17]
Xenopus	Blastocyst Function Connecting blastomeres	Length Short dynamic filopodia 10 µm Long blastocoel traversing filopodia up to 250 µm Diameter: 0.2–0.7 µm	Dynamic filopodia  PFA stable Stable filopodia spanning the blastocoel cavity  Exchange of cytoplasm and membrane  Caveolar endocytosis at proximal base  Actin positive  Tubulin negative (α-tubulin staining)  Retro- and anterograde vesicle transport (v = 1–3 µm/s)  PFA stable	[31]
Xenopus fibroblasts			Stable filopodia  Actin positive  Tubulin-positive Fluorophore spots: (v = 763 nm/s) colocalize only with mCherry tagged tubulin Active Wnt2a-eGFP is transported along microtubules to the plasma membrane, located at the very tips of filopodia and handed over to a Wnt receiving cell where it is rapidly internalized	[18]
Chick embryo	Limb bud, mesenchymal cells Function Limb patterning	Average length 34.27 ± 9.6 µm Max. length 150 µm Diameter: 0.2–0.8 µm Elongation v = 150 nm/s	ShhN-eGFP-positive filopodia Shh-eGFP particles: anterograde movement with a maximal net v = 120 nm/s  Subpopulation positive for Shh co-receptors Cdo and Boc contact formation between Cdo/Boc- and Shh-positive filopodia  PFA sensitive  Cdc42 independent  Actin positive  Tubulin (Tau, EB3) only at the proximal base of a subset of mesenchymal filopodia  myosin10 positive	[19]
Chick embryo	Between dermomyotome and ectoderm, spanning the subectodermal space, penetrate through the basal lamina Function Wnt-signaling (?)	Length limited by the overlying ectoderm about 20 µm Diameter: 0.2–0.5 µm v (extending) 53.8 nm/s v (retracting) 68.8 nm/s	Dynamic filopodia  Actin positive  Tubulin positive  PFA stable Stable filopodia 82 % stable protrusions  Actin positive  Tubulin positive  PFA stable	[20]
Mouse embryo	Between cells of the inner cell mass and the mural trophoectoderm (mTE) Function Signal transduction to induce cell proliferation of TE cells	Short filopodia Length 3 – 8 µm Diameter: 0.2–1 µm Long, traversing filopodia Length up to 34.6 µm Diameter at narrowest point: 0.2–0.4 µm	Short filopodia  Actin positive  Microtubule negative  PFA stable TE-derived filopodia: FGFR2 positive/ErbB3 negative ICM-derived filopodia: FGFR2 negative/ErbB3 negative Long, traversing filopodia  Actin positive  Microtubule negative (as deduced due to their colchicine resistance)  Retrograde vesicle transport  PFA stable TE-derived filopodia: FGFR2 positive/ErbB3 positive ICM-derived filopodia: FGFR2 negative Junctional filopodia (see also Fleming et al. [73])  Actin positive  Tubulin positive  PFA stable	[30]
Mouse embryo	Between cells of the inner cell mass and the mural trophoectoderm (mTE)	Junctional filopodia Length: 0.1–3 µm	Junctional filopodia  Microtubule positive (as visualized electron microscopically)  PFA stable	[73]
Mouse embryo	Between cells of the late morula stage Function Providing tension, stabilizing mechanical force necessary for compaction of preimplantation embryo	Length 10.9 ± 0.8 µm	Dynamic filopodia  Actin positive  Microtubule negative Stable filopodia  Filopodia to neighboring cells  Dependent on E-cadherin, a- and b-catenin  F-actin positive  myosin10 positive	[74]
Mouse mesenchymal cells C3H/10T1/2		Length <10 µm Length <20 µm Long: length >60 µm	Dynamic, short filopodia Cdc42-induced filopodia  Actin positive  Tubulin positive  Fascin positive RhoD-induced filopodia Short filopodia  Actin positive  Tubulin negative  Fascin positive and negative  PFA stable Stable filopodia  Actin positive  Tubulin negative  PFA sensitive  Net retrograde transport of fluorescent nodules and of FGF/FGFRs  Coexpression of Rab5 (early endosome-like vesicle)	[41]
Human cell lines A431 HeLa MCF7			ErbB1 receptor positive  Actin positive  Tubulin negative  Retrograde transport of EGF/ErbB1 complex (v = 22 nm/s)  Activation of the receptor tyrosine kinase essential for retrograde transport  Endocytosis of the EGF/ErbB1 complex at the filopodial base	[42]
Human cell lines HEK cells	Mammalian stem cells Lgr4- and Lgr5-induced filopodia	Length 80 µm Diameter 0.53 µm	Dynamic filopodia  Actin positive  myosin10 positive  PFA stable Stable filopodia Mature Lgr5-induced cytonemes	[32]