Cytonemes are thin, cellular projections that are specialized for exchange of signaling proteins between cells. Cytonemes emanate from cells that make signaling proteins, extending directly to cells that receive signaling proteins. Cytonemes also extend directly from cells that receive signaling proteins to cells that make them. A cytoneme is a type of filopodium - a thin, tubular extension of a cell’s plasma membrane that has a core composed of tightly bundled, parallel

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ...

filaments. Filopodia can extend more than 100 μm and have been measured as thin as 0.1 μm and as thick as 0.5 μm. Cytonemes with a diameter of approximately 0.2 μm and as long as 80 μm have been observed in the Drosophila wing imaginal disc. The term cytoneme was coined to denote the presence of cytoplasm in their interior (cyto-) and their finger-like appearance (-neme), and to distinguish their role as signaling, rather than structural or force-generating, organelles. Filopodia with behaviors suggestive of roles in sensing patterning information were first observed in sea urchin embryos, and subsequent characterizations support the idea that they convey patterning signals between cells. The discovery of cytonemes in Drosophila imaginal discs correlated for the first time the presence and behavior of filopodia with a known

morphogen A morphogen is a substance whose non-uniform distribution governs the pattern of tissue development in the process of morphogenesis or pattern formation, one of the core processes of developmental biology, establishing positions of the various ...

signaling protein -

decapentaplegic ''Decapentaplegic'' (''Dpp'') is a key morphogen involved in the development of the fruit fly ''Drosophila melanogaster'' and is the first validated secreted morphogen. It is known to be necessary for the correct patterning and development of the ...

. Decapentaplegic is expressed in the wing disc by cells that function as a developmental organizer, and cytonemes that are responsive to decapentaplegic orient toward this developmental organizer. Receptors for signaling proteins are present in motile vesicles in cytonemes, and receptors for different signaling proteins segregate specifically to different types of cytonemes. In Drosophila, cytonemes have been found in wing and eye imaginal discs,

trachea The trachea (: tracheae or tracheas), also known as the windpipe, is a cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all animals' lungs. The trachea extends from ...

, lymph glands and ovaries. They have also been described in spider embryos, earwig ovaries, Rhodnius, Calpodes, earthworms, retroviral-infected cells,

mast cells A mast cell (also known as a mastocyte or a labrocyte) is a resident cell of connective tissue that contains many granules rich in histamine and heparin. Specifically, it is a type of granulocyte derived from the myeloid stem cell that is a ...

, B-lymphocytes and

neutrophils Neutrophils are a type of phagocytic white blood cell and part of innate immunity. More specifically, they form the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. Their functions vary in different ...

. Recent observations suggest that cytonemes have also an important role during vertebrate development. Recent observations suggest that cytonemes also have an important role during development of the zebrafish neural plate where they transport Wnt8a and of the chick limb where they transport Sonic hedgehog.

References

{{Reflist, 32em Cell biology