Table 1.

Cellular extensions contributing to cell–cell communication

	Cytonemes	Tunneling nanotubes	Intercellular bridges	Airinemes
Diameter	~ 200 nm	50–700 nm	200 nm- 10 μm	< 1 μm
Length	4–700 μm Average 10–25 μm	10–200 μm	Up to 350 μm	~ 10–250 μm
Cell/tissue type	Developing and adult tissues Stem cell niche	Migrating neural crest cells Immune cells: macrophages, T cells, natural killer (NK) cells, dendritic cells	Germ cells Developing tissues	Extend from Xanthophores to melanophores
Formation	Extension toward cell signaling partners	Extension toward neighboring cell Contact maintained between cells migrating apart	Incomplete cytokinesis/ syncytia	Macrophage engulfs membrane bleb, migrates to deposit on signaling cell
Function	Morphogen and growth factor transport Maintenance of stem cell niche Tissue homeostasis	Exchange of organelles Ca2+ signaling Electrical synchronization NK-mediated cell lysis, inflammation	Spermatogenesis and fertility Molecular/organelle transport Ca2+ signaling	Zebrafish stripe patterning
References	43, 56, 59–67, 75, 76, 104, 105, 106	77–87	90–98	99–101, 107