![Tracheal cytonemes](https://upload.wikimedia.org/wikipedia/commons/9/90/Tracheal_cytonemes.jpg)
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
Filopodia (singular filopodium) are slender pseudopodia, cytoplasmic projections that extend beyond the leading edge of lamellipodia in cell migration, migrating cells. Within the lamellipodium, actin ribs are known as ''microspikes'', and when ...
- 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 over ...
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 form, ...
.
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. N ...
,
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 response. ...
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 splinte ...
,
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 ce ...
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's 1 ...
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 species ...
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 chemica ...
in
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 as ...
,
Delta-Notch signaling,
vasculogenesis
Vasculogenesis is the process of blood vessel formation, occurring by a '' de novo'' production of endothelial cells. It is sometimes paired with angiogenesis, as the first stage of the formation of the vascular network, closely followed by angio ...
,
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
Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, then, ...
. Filopodia have been given various names: microspikes,
pseudopods
A pseudopod or pseudopodium (plural: pseudopods or pseudopodia) is a temporary arm-like projection of a eukaryotic cell membrane that is emerged in the direction of movement. Filled with cytoplasm, pseudopodia primarily consist of actin filamen ...
, thin filopodia,
thick filopodia,
gliopodia,
myopodia,
invadopodia
Invadopodia are actin-rich protrusions of the plasma membrane that are associated with degradation of the extracellular matrix in cancer invasiveness and metastasis. Very similar to podosomes, invadopodia are found in invasive cancer cells and are ...
,
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
A tunneling nanotube (TNT) or membrane nanotube is a term that has been applied to protrusions that extend from the plasma membrane which enable different animal cells to touch over long distances, sometimes over 100 μm between T cells. Two t ...
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 (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 early '' ...
. 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 Cartilage, 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 ...
,
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 par ...
,
B-lymphocytes
B cells, also known as B lymphocytes, are a type of white blood cell of the lymphocyte subtype. They function in the humoral immunity component of the adaptive immune system. B cells produce antibody molecules which may be either secreted o ...
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 in ...
.
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