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 o ...

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 An imaginal disc is one of the parts of a holometabolous insect larva that will become a portion of the outside of the adult insect during the pupal transformation. Contained within the body of the larva, there are pairs of discs that will fo ...

. Many cell types have filopodia. The functions of filopodia have been attributed to pathfinding of

neurons A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa. ...

, early stages of

synapse In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...

formation,

antigen In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respon ...

presentation by

dendritic cells Dendritic cells (DCs) are antigen-presenting cells (also known as ''accessory cells'') of the mammalian immune system. Their main function is to process antigen material and present it on the cell surface to the T cells of the immune system. The ...

of the

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splint ...

, force generation by

macrophages Macrophages (abbreviated as M φ, MΦ or MP) ( el, large eaters, from Greek ''μακρός'' (') = large, ''φαγεῖν'' (') = to eat) are a type of white blood cell of the immune system that engulfs and digests pathogens, such as cancer cell ...

and

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsky' ...

transmission. They have been associated with wound closure, dorsal closure of

Drosophila ''Drosophila'' () is a genus of flies, belonging to the family Drosophilidae, whose members are often called "small fruit flies" or (less frequently) pomace flies, vinegar flies, or wine flies, a reference to the characteristic of many specie ...

embryos,

chemotaxis Chemotaxis (from '' chemo-'' + ''taxis'') is the movement of an organism or entity in response to a chemical stimulus. Somatic cells, bacteria, and other single-cell or multicellular organisms direct their movements according to certain chemical ...

Dictyostelium ''Dictyostelium'' is a genus of single- and multi-celled eukaryotic, phagotrophic bacterivores. Though they are Protista and in no way fungal, they traditionally are known as " slime molds". They are present in most terrestrial ecosystems ...

, Delta-Notch signaling, vasculogenesis,

cell adhesion Cell adhesion is the process by which cells interact and attach to neighbouring cells through specialised molecules of the cell surface. This process can occur either through direct contact between cell surfaces such as cell junctions or indir ...

cell migration Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular dire ...

, and cancer metastasis. Filopodia have been given various names: microspikes, pseudopods, thin filopodia, thick filopodia, gliopodia, myopodia, invadopodia,

podosomes Podosomes are conical, actin-rich structures found on the outer surface of the plasma membrane of animal cells. Their size ranges from approximately 0.5 µm to 2.0 µm in diameter. While usually situated on the periphery of the cellular ...

, telopodes, tunneling nanotubes and dendrites. 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 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, 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 air-breathing animals with lungs. The trachea extends from the l ...

, 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 p ...

, B-lymphocytes and

neutrophils Neutrophils (also known as neutrocytes or heterophils) are the most abundant type of granulocytes and make up 40% to 70% of all white blood cells in humans. They form an essential part of the innate immune system, with their functions varying ...

. 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