Editing Cnidocyte (section)

== Structure and function ==
<!-- Deleted image removed: [[File:Nematocyst-discharged.png|thumb|right|A discharged nematocyst seen under a [[scanning electron microscope]]]] -->

Each cnidocyte contains an organelle called a cnidocyst,{{efn|Cnidocyst—also known as a cnida ({{plural abbr|cnidae}}), nematocyst, ptychocyst or spirocyst.}} which consists of a bulb-shaped capsule and a hollow, coiled tubule that is contained within. Immature cnidocytes are referred to as cnidoblasts or nematoblasts. The externally oriented side of the cell has a hair-like trigger called a cnidocil, a mechano-chemical receptor. When the trigger is activated, the tubule shaft of the cnidocyst is ejected and, in the case of the penetrant nematocyst, the forcefully ejected tubule penetrates the target organism. This discharge takes a few microseconds, and is able to reach accelerations of about 40,000&nbsp;''g''.<ref name="Holstein+1984">{{cite journal|author=Holstein T.|author2=Tardent P.|year=1984|title=An ultrahigh-speed analysis of exocytosis: nematocyst discharge|journal=Science|volume=223|issue=4638|pages=830–833|doi=10.1126/science.6695186|pmid=6695186|bibcode=1984Sci...223..830H}}<!--| access-date = 2012-11-11--></ref><ref name="Kass-Simon+2002">{{cite journal | author =Kass-Simon G. | author2 =Scappaticci A. A. Jr. | title =The behavioral and developmental physiology of nematocysts | journal =Canadian Journal of Zoology | volume =80 | issue = 10| pages =1772–1794 | year =2002 | url =http://www.biochem.uci.edu/steele/Kass-Simon.pdf | doi =10.1139/Z02-135 | access-date = 2012-10-25}}</ref> Research from 2006 suggests the process occurs in as little as 700&nbsp;nanoseconds, thus reaching an acceleration of up to 5,410,000&nbsp;''g''.<ref name="Timm+2006">{{cite journal | author =Nüchter Timm | author2 =Benoit Martin | author3 =Engel Ulrike | author4 =Özbek Suat | author5 =Holstein Thomas W | title =Nanosecond-scale kinetics of nematocyst discharge | journal =Current Biology | volume =16 | issue =9 | pages =R316–R318 | year =2006 | doi =10.1016/j.cub.2006.03.089 | pmid=16682335| doi-access =free }}</ref> After penetration, the toxic content of the nematocyst is injected into the target organism, allowing the sessile cnidarian to capture the immobilized prey. Recently, in two sea anemone species (''[[Nematostella vectensis]]'' and ''Anthopleura elegantissima''), the type I [[neurotoxin]] protein Nv1 was shown to be localized in ectodermal gland cells in the tentacles, next to but not in nematocysts. Upon encounter with a crustacean prey, nematocysts discharge and pierce the prey, and Nv1 is massively secreted into the extracellular medium by the nearby gland cells, thus suggesting another mode of entry for toxins.<ref>{{Cite journal|last1=Moran|first1=Yehu|last2=Genikhovich|first2=Grigory|last3=Gordon|first3=Dalia|last4=Wienkoop|first4=Stefanie|last5=Zenkert|first5=Claudia|last6=Ozbek|first6=Suat|last7=Technau|first7=Ulrich|last8=Gurevitz|first8=Michael|date=2012-04-07|title=Neurotoxin localization to ectodermal gland cells uncovers an alternative mechanism of venom delivery in sea anemones|journal=Proceedings. Biological Sciences|volume=279|issue=1732|pages=1351–1358|doi=10.1098/rspb.2011.1731|issn=1471-2954|pmc=3282367|pmid=22048953}}</ref>

=== Cnidocyte capsule composition ===
The cnidocyte capsule is made of novel Cnidaria-specific gene products which combine known protein domains. Minicollagen gene products (proteins) are one of the major structural components of the capsule. They are very short genes containing the characteristic collagen-triple helix sequence, as well as polyproline domains and cysteine-rich domains.<ref name="Beckmann">{{Cite journal|last1=Beckmann|first1=Anna|last2=Özbek|first2=Suat|date=2012-06-05|title=The Nematocyst: a molecular map of the Cnidarian stinging organelle|url=http://www.ijdb.ehu.es/web/paper/113472ab|journal=International Journal of Developmental Biology|language=en|volume=56|issue=6–7–8|pages=577–582|doi=10.1387/ijdb.113472ab|issn=0214-6282|pmid=22689365|doi-access=free}}</ref> Trimeres of mini collagen proteins assemble through their terminal cysteine-rich domain, forming highly organized and rigid supra-structures. Minicollagen 1 Ncol-1 polymers assemble on the inner shell while the outer capsule is composed of polymerized NOWA (Nematocyst Outer Wall Antigen) proteins. Nemato Galectin, minicollagen Ncol-15 and chondroitin are novel proteins used to build the tubule shaft. In piercing cnidocytes, the novel protein {{chem name|spinalin}} is used to make the spines present at the base of the shaft.<ref>{{Cite journal|last1=Shpirer|first1=Erez|last2=Chang|first2=E Sally|last3=Diamant|first3=Arik|last4=Rubinstein|first4=Nimrod|last5=Cartwright|first5=Paulyn|last6=Huchon|first6=Dorothée|date=2014-09-29|title=Diversity and evolution of myxozoan minicollagens and nematogalectins|journal=BMC Evolutionary Biology|volume=14|issue=1 |pages=205|doi=10.1186/s12862-014-0205-0|issn=1471-2148|pmc=4195985|pmid=25262812 |doi-access=free |bibcode=2014BMCEE..14..205S }}</ref><ref>{{Cite journal|last1=Balasubramanian|first1=Prakash G.|last2=Beckmann|first2=Anna|last3=Warnken|first3=Uwe|last4=Schnölzer|first4=Martina|last5=Schüler|first5=Andreas|last6=Bornberg-Bauer|first6=Erich|last7=Holstein|first7=Thomas W.|last8=Özbek|first8=Suat|date=2012-03-23|title=Proteome of Hydra Nematocyst|journal=The Journal of Biological Chemistry|volume=287|issue=13|pages=9672–9681|doi=10.1074/jbc.M111.328203|issn=0021-9258|pmc=3323026|pmid=22291027|doi-access=free}}</ref><ref name="David">{{Cite journal|last1=David|first1=Charles N.|last2=Özbek|first2=Suat|last3=Adamczyk|first3=Patrizia|last4=Meier|first4=Sebastian|last5=Pauly|first5=Barbara|last6=Chapman|first6=Jarrod|last7=Hwang|first7=Jung Shan|last8=Gojobori|first8=Takashi|last9=Holstein|first9=Thomas W.|date=2008-09-01|title=Evolution of complex structures: minicollagens shape the cnidarian nematocyst|journal=Trends in Genetics|volume=24|issue=9|pages=431–438|doi=10.1016/j.tig.2008.07.001|pmid=18676050|issn=0168-9525}}</ref>

=== Discharge mechanism ===
[[File:Fluids-05-00020-g002.png|thumb|Discharge mechanism of a nematocyst.]]

The cnidocyst capsule stores a large concentration of [[calcium]] [[ion]]s, which are released from the capsule into the [[cytoplasm]] of the ''cnidocyte'' when the trigger is activated. This causes a large concentration gradient of calcium across the cnidocyte plasma membrane. The resulting [[osmotic pressure]] causes a rapid influx of water into the cell. This increase in water volume in the cytoplasm forces the coiled cnidae tubule to eject rapidly. Prior to discharge the coiled cnidae tubule exists inside the cell in an "inside out" condition. The back pressure resulting from the influx of water into the cnidocyte together with the opening of the capsule tip structure or operculum, triggers the forceful eversion of the cnidae tubule causing it to right itself as it comes rushing out of the cell with enough force to impale a prey organism.

That force is to be calculated as the mass of the mechanism's stylet multiplied by its acceleration. The pressure that is generated by this impact into its prey is to be calculated by the stylet's force divided by its area. Researchers have calculated an ejected mass of 1&nbsp;nanogram, an acceleration of 5,410,000&nbsp;g and a stylet tip radius of 15&nbsp;±&nbsp;8&nbsp;nm.<ref name="Timm+2006" /> Therefore, a pressure of more than 7&nbsp;[[Pascal (unit)|GPa]] was estimated at the stylet tip which they write is in the range of technical bullets.<ref name="Timm+2006" />

=== Fluid dynamics in nematocyst discharge ===
[[File:Model Overview.png|thumb|Computational fluid dynamics model parameters of nematocyst discharge.]]
Few papers have modeled the discharge aside from direct observation. Observational studies typically used a tentacle solution assay with a chemical stimulant to create discharge and cameras to record them. One in 1984<ref name="Holstein+1984" /> and another in 2006<ref name="Timm+2006" /> as imaging technology improved. One study involved [[computational fluid dynamics]] where variables such as barb plate size, prey cylindrical diameter and fluid medium Reynolds number were manipulated.<ref>{{Cite journal |last1=Hamlet |first1=Christina |last2=Strychalski |first2=Wanda |last3=Miller |first3=Laura |date=March 2020 |title=Fluid Dynamics of Ballistic Strategies in Nematocyst Firing |journal=Fluids |language=en |volume=5 |issue=1 |pages=20 |doi=10.3390/fluids5010020 |bibcode=2020Fluid...5...20H |issn=2311-5521 |doi-access=free }}</ref>

Observational studies indicate that velocities of the barb/stylet decrease throughout the discharge. As such, the incredible maximum acceleration is achieved at the beginning. Dynamic traits such as maximum discharge velocities and trajectory patterns may not correspond to static traits such as tubule lengths and capsule volumes.<ref name=":0">{{Cite journal |last1=Colin |first1=Sean P. |last2=Costello |first2=John H. |date=2007-11-23 |title=Functional characteristics of nematocysts found on the scyphomedusa Cyanea capillata |url=https://www.sciencedirect.com/science/article/pii/S0022098107003206 |journal=Journal of Experimental Marine Biology and Ecology |language=en |volume=351 |issue=1 |pages=114–120 |doi=10.1016/j.jembe.2007.06.033 |s2cid=51791589 |issn=0022-0981}}</ref> Therefore, caution is appropriate when using medusan nematocyst assemblages as indicators of prey selection and trophic role.<ref name=":0" /> This is possibly the case for other jelly species and hence one cannot generally infer nematocyst static traits to prey size.

=== Prey detection ===

Cnidae are "single use" cells, and thus represent a large expenditure of energy to produce. In [[Hydrozoa]]ns, in order to regulate discharge, cnidocytes are connected as "batteries", containing several types of cnidocytes connected to supporting cells and neurons. The supporting cells contain [[Chemoreceptor|chemosensor]]s, which, together with the [[mechanoreceptor]] on the cnidocyte (cnidocil), allow only the right combination of stimuli to cause discharge, such as prey swimming, and chemicals found in prey [[cuticle]] or cutaneous tissue. This prevents the cnidarian from stinging itself although sloughed off cnidae can be induced to fire independently.