The nettle cells , also known as nematocytes or cnidocytes , are a special type of cell that can only be found in the cnidarians (Cnidaria). These are cells that are embedded in the outer layer ( epidermis ) of the animals and can be used to catch prey or to ward off enemies or competitors. If irritated, a nettle tube is ejected, which often injects a potent poison into the victim. Although they usually only irritate the skin and cause minor burns in humans, the nettle poisons of some types are so effective that they can lead to the breakdown of the cardiovascular system and thus death.
The arrangement of the cells on the animal's body is not uniform. In the outer skin, the epidermis, stinging cells are found on all structures that are used to capture prey or for defense, in a particularly high density - these accumulations are then also called batteries . They sit primarily on the tentacles, but also on special structures: These include, for example, the acontia of the sea anemones (actinaria), which normally lie as fine threads in the body cavity, the coelenteron , but if there is danger through small openings in the same, the cinclids , be thrown outwards. Sea anemones also have acrorhagi to defend themselves against genetically alien conspecifics , thread-like structures that lie below the real tentacles and are used as weapons - after an acrorhagi duel, the inferior anemone usually retreats with considerable tissue damage. Stony corals (Scleractinia) have neither acontia nor acrorhagi, but they can turn out ribbons that carry nettle cells and sit on the septa of the coelenteron and use them to catch prey and for external digestion. The Hydrozoa (Hydrozoa) have specialized polyps without mouth or tentacles , but which are densely covered with nettle cells and only serve for defense. They are called dactylozooids or nematophores , depending on the taxon they belong to .
In all cnidarians, nettle cells are also found locally in the inner skin, the gastrodermis. There they serve to keep prey devoured, but not yet completely killed, in a paralyzed state during the digestive process.
Some cnidarians have up to six million nettle cells, which are arranged in over 1000 batteries.
Nettle cells contain a nettle capsule or nematocyst as their main component, which takes up almost the entire cell space. It is surrounded by a capsule shell that is stiffened by an additional layer of collagen . The cell nucleus and other cell compartments lie on the edge of the cell between this capsule and the cell membrane . The capsule itself contains a more or less coiled and 5 to 100 micrometer long nettle tube, which is to be regarded as an indentation (invagination) in the capsule membrane and, depending on the type of cell, is equipped with different structures such as stylets, spines or adhesive elements. The nettle cell is filled with fluid and contains individual amino acids, some proteins, often with poisonous effects, and high concentrations of acidic peptides such as γ-polyglutamic acid .
At the upper end, exposed to the outside world, the cell has a receptor pole that reacts to mechanical stimuli ( mechanoreceptor ). In the flower animals (Anthozoa) one finds the probably more original type as a receptor. It consists of a normal sensory scourge, the cilia , with an associated basal body and cilia root as well as an accessory centriole . This basic structure corresponds to that of a number of other mechanoreceptors such as the skin sensors of various animal groups or the receptors in the inner ear of vertebrates . The other taxa of the cnidarians on the other hand have a so-called cnidocil , which consists of a specially transformed, stiffened cilia with microvilli (stereocilia) arranged around it. This structure also has a basal body, the root and the accessory centriole are missing.
The nettle cell is embedded in the outer cell layer of the cnidarians, the epidermis, whereby several nettle cells are always connected to an epithelial muscle cell. At the contact points in both cells, fine tubes made of proteins run vertically to the cell surface, the microtubules , which apparently represent a mechanical connection between the two cell types. Nerve cells , the ends of which can contain vesicles with chemical neurotransmitters , also end in this cell complex . Both the epithelial muscle cell and the nerve endings probably play a role in activating the stinging cell: For a long time it was assumed that stinging cells were autonomous systems that, in response to a corresponding stimulus from the environment, trigger the discharge independently of all other cells. For this reason they were also referred to as independent receptor effectors to indicate that they both receive stimuli from the environment (receptor function) and subsequently trigger a corresponding action (effector function). On the other hand, it is now considered very likely that the triggering of the cell is controlled by more extensive receptor-effector complexes in which the individual stinging cell is embedded, but which also includes other auxiliary cells and possibly sensory nerve cells.
The function of the nettle capsule is to eject the nettle tube it contains. Some types of capsules (e.g. Volventen) form a sticky, wrapped tube that is expelled within several seconds. Other types of capsules (e.g. penetrants) form a stylet in addition to the nettle tube, which is ejected extremely quickly and punches a hole in the body wall of the prey. The nettle tube is then turned out through this hole and the nettle poison is injected into the prey (Fig.). The two processes take place on very different time scales. The stiletto is ejected extremely quickly in order to be able to puncture a hard shell on impact. Measurements on the nettle cell of a Hydra with a time resolution of 700 nanoseconds (1.4 million images per second ) resulted in one or two images for the duration of the ejection (an intermediate state was not shown in all sequences). Together with the observed distance of 13 micrometers, this results in an average speed of 9.3 to 18.6 meters per second - the maximum must be greater - and accordingly a minimum acceleration value of 1.4 to 5.4 million g . The authors found a broad distribution of the latency period (from stimulation to discharge) of a few tenths of a millisecond, depending on the Ca 2+ concentration.
There was the idea that a high internal osmotic pressure of approx. 140 bar in combination with elastic collagen-like proteins would cause the rapid discharge and the high penetration power. The pressure should be generated by the γ-polyglutamic acid (approx. 20 subunits per molecule) detected in the capsule (the penetrant of Hydra ). However, measurements inside the capsule showed a very low resting pH value. At this pH value, the γ-polyglutamic acid is not dissociated, it forms aggregates via hydrogen bonds and in this state hardly contributes to the osmotic value. So there is probably no overpressure in the ready-to-fire capsule. Cnidoin was identified and characterized as an elastic protein .
The authors take the view that the extremely fast discharge z. B. the steno Thelen by a kind of Coulomb explosion is caused. Triggering the discharge causes the proton gradient to collapse to the cytoplasm of the nettle cell. The very mobile oxonium ions (protonated water molecules) then leave the capsule. As a result, the acid groups of γ-polyglutamic acid are charged at the same time and suddenly, they repel each other. This should lead directly to the bursting of the capsule lid and the rapid expulsion of the stylet, without any osmotic contribution.
As soon as the nettle capsule is discharged, the nettle cell degenerates, dies and is replaced within 48 hours. The Hydrozoen -Polyp Hydra littoralis loses in this way when catching a Saline cancer ( Artemia salina ) is about a quarter of its seated on the tentacles of stinging cells. In order to avoid unnecessary loss of nettle cells, for example when touching harmless objects, cnidarians have various adaptations. On the one hand, two independent stimuli, for example mechanical and chemical, are usually necessary to trigger a large-area discharge of stinging cells. However, a small number "fires" with the slightest touch, while chemical irritation alone does not generally lead to triggering. The mechanical receptors are also particularly sensitive to the vibration frequencies and amplitudes with which the limbs of their prey move. The presence of special amino acids and sugars or small peptides such as glutathione , which could indicate slimy secretions from prey or predators, also reduces the trigger threshold.
There are indications that both pieces of information are integrated: a chemical stimulus characteristic of certain prey sets the mechanical receptors to the perception of a suitable vibration frequency. For example, the normal trigger frequency for freshwater polyps of the Hydra genus is between 50 and 75 hertz , in sugar-containing solutions it is only 5 to 30 hertz, and in the presence of large amounts of the amino acid proline it is 90 to 100 hertz. These data are interpreted roughly as follows:
Smaller prey such as crustaceans from the plankton with their slimy secretions always give smaller amounts of sugars into its environment. These initially lower the stimulus threshold of the stinging cells, so that mechanical contact leads to a discharge of numerous cells into the prey. Wounded, it releases additional sugar and smaller peptides from the wound and begins at the same time with violent kicking movements, which have their highest amplitude in the low-frequency range at less than 30 Hertz. Since this corresponds to the trigger profile in the presence of sugars, further nettle cell batteries are then discharged into the prey. This is now badly wounded; Amino acids such as proline are increasingly emerging from the body cavity. This in turn increases the trigger frequency of the nettle cells to such a high level that it is outside the prey's range of motion - with the result that no more nettle cells are discharged. The use of nettle cells is therefore economically limited to fresh and wounded, but still fighting prey.
Last but not least, the sensitivity of the stinging cells also depends on the nutritional status of their carriers; it is known that the trigger threshold is considerably reduced in starved animals; this information is thought to be conveyed through the nervous system.
The nettle cells are formed from the interstitial cells (I cells) at the base of the epidermis. These are embryonic cells that can develop into all cell types of the cnidarians with the exception of the germ cells and the epithelial cells. The I cells divide, and the formation of the nettle cells, the cells of which are then called cnido or nematoblasts, begins with a spatial enlargement of the Golgi apparatus and the rough endoplasmic reticulum in the interstitial cell. In these cell compartments the material for the later nettle capsule is formed, which initially appears as a homogeneous matrix and increases in size through fusion with vesicles of the Golgi apparatus. Strictly speaking, it is a secretion product of the stinging cell that forms. Apparently, as transport structures for the vesicles, microtubules are formed between the Golgi apparatus and the matrix.
In the matrix, a peripheral area is formed, which later forms the capsule, as well as a central area in which (depending on the type of nettle cell) stilettos or other structures develop. The nettle thread also forms inside the cell. At the upper edge of the capsule, a lid ( operculum ) is set off from the rest of the capsule. At the end of the formation of the nettle cells, the size of the Golgi apparatus is reduced again and the endoplasmic reticulum breaks down into free ribosomes and individual vesicles. The cell now moves to its final position within the epidermis and the internal pressure of the capsule is built up. The formation of these cells is irreversible.
A total of about 30 different morphological main types of nettle cells are known to date, which were found in different representatives of the nettle animals and thus play a major role in the systematic classification of the animals. However, up to now there is no uniform system of naming and none of the naming systems is uniformly recognized. The 30 main types can be subdivided into 50 to 60 capsule shapes by further minor differences. Some of the main types are limited to certain large groups. A species usually has several types at the same time. For example, the freshwater polyps of the Hydra genus have four different types of nettle cells. The totality of the nettle cells of a species, i.e. the arrangement and distribution of the nettle cells as well as the basic types of nettle cells and their distribution over the body is called a cnidoma.
A distinction is first made between three different basic types of nettle capsules i. w. S., which are called nematocysts in the narrower sense, spirocysts and ptychocysts . The terms presented below are often synonyms or the same type can be named differently in different naming systems.
The nematocysts have been researched best so far; Here two groups can be distinguished based on the structure of the nettle tube:
- The Astomocniden have no opening at the end of the nettle tube . In this group belong the
- Desmonemen or volvents (winding capsules), which have a long thread wrapped in the capsule, with which prey animals or parts of them are wrapped and held.
- The rhopalonemes with a club-ending tube are also assigned to this group.
- In the stomocnid there is always an opening at the end of the nettle thread . Belong to this group
- the Haplonemen with an evenly structured hose including the glutinants (adhesive capsules), which, for example, as adhesive capsules in the case of Hydra, also support locomotion. In their mode of action, these capsules are very similar to the
- Heteronemen whose hives can be divided into a shaft and a thread. A distinction is made between different sub-types, such as
- the rhabdoid with an evenly thick shaft,
- the rhopaloids have a shaft of unequal diameter. This includes the
- the Euryteles with a club-shaped extended shaft,
- the birhopaloids, with two swellings and regularly arranged thorns, and the
- Trirhopaloids, with three swellings, the middle swelling is covered with thorns. In principle, the Stenoteles and Tumiteles also belong to this large group.
- the mastigophores. A distinction is made between B and P mastigophores. In the case of the B mastigophores, the shaft gradually merges into the tube, while in the P mastigophores the tube attaches somewhat abruptly to the shaft (clearly visible after the cell has exploded). P-mastigophores can be recognized in the non-triggered state by a pronounced, V-shaped indentation at the distal end of the shaft. The P-mastigophores are often subdivided into four other types, which are denoted by the Roman numerals I to IV. Type I: all thorns are arranged at right angles to the longitudinal axis of the capsule, Type II: all thorns point towards the capsule, Type III: shorter thorns point towards the capsule, longer thorns towards the front, Type IV: all thorns are long and point away from the capsule
- Isorhizos. a nettle cell without a clearly recognizable shaft, d. H. the tube has approximately the same thickness over its entire length, or is only slightly thinner at the distal end. Nettle cells with a tube that is slightly thicker at the bottom are called anisorhizas.
- the complexly structured stenoteles, also penetrants or penetration capsules, with a stylet made of three thorns. The capsule also contains highly effective nettle poisons, which the animals can use to paralyze or kill their victims.
- Tumiteles are characterized by a shaft that is somewhat thicker median; the swelling bears thorns. Most editors do not recognize this type, but rather summarize it with the Euryteles. Due to the backward-pointing thorns, they have a certain resemblance to P-mastigophores.
- Euryteles are characterized by a single swelling at the distal end of the shaft (they also belong to the rhopaloids). They occur in all classes of cnidarians, with the exception of the flower animals (Anthozoa).
For further characterization, adjectives are often put in front of the different types.
- heterotrich / holotrich / homotrich / basitrich / atrich: these terms denote the different stocking of the nettle tube with thorns (atrich = completely without thorns, homotrich = continuously occupied with the same thorns, holotrich = continuously occupied with thorns, heterotrich = occupied with different thorns, basitrich = Thorns only at the base of the hose)
- macrobasic / microbasic: refers to the length of the shaft of a triggered nettle capsule. Traditionally, microbasic was defined as having three times (or less) the length of the capsule, while macrobasic was four times (or more) the length of the shaft. In more recent works the terms are used in a somewhat simplified sense; microbasic = the shaft fits into the released capsule without twisting or folding, macrobasic = the shaft is too long for the capsule and folds or twists.
The second basic type of nettle capsules are the spirocysts , which are only found in the flower animals (anthozoa). In contrast to the nematocysts, they only have a thin capsule wall. The nettle tube is rolled up like a spiral spring and is not studded with thorns, but instead has fine adhesive threads on the side. Cilia or cnidocils as trigger mechanisms do not occur in the spirocysts.
Finally, ptychocysts , which are only found in cylinder roses (Ceriantharia), are highly modified nettle capsules that are not used for defense. The sticky nettle tube is not spiral here, but folded in a zigzag shape, but unlike the spirocysts, it does not have any side threads. Triggered by cilia, the sticky fibers become matted and thus form a firm, finely woven covering around the polyp, which serves as protection and a living tube.
The nettle poisons of the cnidarians are, on the one hand, animal poisons that have a toxic effect on the nervous system, i.e. are assigned to neurotoxins . They lead to a blockage of the sodium ion transport on the cell membrane of the nerves and thus prevent the formation of action potentials . The result is paralysis .
Another effect can arise in heart muscle cells . Here are calcium ions released, with cramps through to heart failure can be the result and a failure of the entire cardiovascular system. Poisons that work in this way are called cardiotoxins .
Nettle cells and man
Most nettle poisons cause skin irritation or minor burns in humans. More serious burns, necrosis and deep wounds in the skin are relatively rare. However, contact with the nettle cells of some species such as the Portuguese galley ( Physalia physalis ) or the sea wasps ( Chironex fleckeri and Chiropsalmus quadrigatus ), which are among the generally highly dangerous box jellyfish, can be fatal; their poison is more potent than cobra toxin . In Australia, more people die each year from sea wasps than from shark attacks. Death usually occurs within minutes and is brought about by the action of the toxins on the cardiovascular system. The extent to which the effect can be used in the manufacture of drugs to increase the pumping power in patients with heart failure is still being researched.
In the case of poisoning by nettle poisons of the sea wasps, rinsing with vinegar or soda is recommended, since nettle cells hardly trigger in either an acidic or a basic environment. The acidic pH value of human urine can also be used in emergency situations . Rinsing with cola and other lemonades should still be avoided despite the low pH value, as nettle cells trigger more frequently in a solution containing sugar.
Although the cnidarians are the only animals that develop nettle cells themselves, they can also be found in some other marine animals. This is the case when the animals feed on the polyps or medusa of the cnidarians and have developed mechanisms to take up the stinging cells. Nettle cells are then not digested, but stored in their own epidermis, where they serve to protect the animals from predators or from catching prey. Nettle cells that occur in this way in non-cnidarians are referred to as kleptocnids , or “stolen nettle cells”.
A new idea (see section 'Function of the nettle cells' above) provides an explanation: For the formation and function of the nettle capsule, acidification of the capsule matrix is of central importance. Thread snails , vortex worms of the order Macrostomida and rib jellyfish of the genus Haeckelia , which eat cnidarians, also try to digest the (unfinished) nettle capsules. The capsules are acidified and made ready to be fired, so to speak, "involuntarily". The acidification of the nettle capsules was shown in 2012 in the Aeolidiella snail . How these capsules get to the launch site is unclear.
The nettle capsules found in the capillary jellyfish of the genus Haeckelia led to the assumption that the taxon of the capillary jellyfish was closely related to the cnidarians until the origin of the capsules in Haeckelia from eaten cnidarians was discovered.
Tribal origin of the stinging cells
The origin of the nettle cells is still unknown today. A symbiogenic origin was proposed, i.e. a symbiotic relationship between two partners in which both ultimately lost their own identity.
According to this, a simply constructed tissue animal , for example in the manner of Trichoplax adhaerens, would have entered into a symbiosis with a protist partner, for example from the ranks of the (today) parasitic sporozoa or microsporidia , which already had a cnidal-like structure. As long as this had increased the evolutionary fitness of both partners, an ever closer relationship could have developed through coevolution , which would ultimately have led to the merging of two different lineages and the integration of the genetic information in one cell nucleus .
But this scenario is highly speculative. The following observations are made to substantiate it:
- Between the ordinary epithelial cells and the interstitial cells (I cells), from which the nettle cells arise, there are great differences in terms of morphology , cell differentiation and cell division, so that one can speak of two cell populations that share a common body.
- The cell populations can exist separately from one another: While I cells in the test tube can exist outside of their normal cellular environment under certain conditions, hydra polyps as well as their planula larvae , from which all I cells and their differentiation products such as nettle cells have been taken, are viable as long as they are supplied with nutrients. Cell divisions also continue to take place.
- When the two cell types are recombined by "vaccination" of the animals reduced to epithelial cells with I cells, an intact polyp is formed again.
- The predecessor cells of the stinging cells, the cnidoblasts, show a form of cell division that is similar to the merogony of parasitic protists.
- Numerous symbioses of cnidarians with protists, primarily various algae, are known.
Nevertheless, the above model is only to be understood as a possible scenario - the actual formation of the stinging cells must remain unclear for the time being.
A new idea (see above, section "Function of the nettle cells") also provides explanations for the formation of the nettle capsules. According to the new model, acidification of the capsule matrix is of central importance for the formation and function of the capsule. It is therefore assumed that the nettle capsules have their origin in lysosomes or similar cell organelles. However, the pH value in the capsule must be lower than that in lysosomes.
- DA Hessinger, HM Lenhoff (Ed.): The Biology of Nematocysts. Academic Press, San Diego 1988.
- in particular: T. Holstein, K. Hausmann: The Cnidocil Apparatus of hydrozoans: A progenitor of higher metacoans mechanoreceptors?
- T. Holstein: The morphogenesis of nematocysts in Hydra and Forskalia : An ultrastructural study. In: J. Ultrastruct. Res. 75, 1981, pp. 276-290.
- T. Holstein, P. Tardent: An ultrahigh-speed analysis of exocytosis: Nematocyst discharge. In: Science. 223, 1984, pp. 830-833.
- W. Schäfer: Cnidarians, cnidarians. In: W. Westheide, R. Rieger: Special Zoology. Part 1: Protozoa and invertebrates. Gustav Fischer Verlag, Stuttgart / Jena 1996.
- S. Shostak: A symbiogenetic theory for the origins of cnidocysts in Cnidaria. In: Biosystems. 29, 1993, pp. 49-58.
- S. Shostak, V. Kollavi: Symbiogenetic origins of cnidarian cnidocysts. In: Symbiosis. 19, pp. 1-29.
- GM Watson, DA Hessinger: Cnidocyte mechanoreceptors are tuned to the movements of swimming prey by chemoreceptors. In: Science. 243, 1989, pp. 1589-1591.
- GM Watson, P. Mire-Thibodeaux: The cell biology of nematocysts. In: International Review of Cytology. 156, 1994, pp. 275-300.
- U. Welsch, V. Storch: Introduction to cytology and histology of animals. Gustav Fischer Verlag, Stuttgart 1973.
- T. Nüchter, M. Benoit, U. Engel, S. Özbek, TW Holstein: Nanosecond-scale kinetics of nematocyst discharge. In: Current Biology. 16, 2006, pp. R316-R318 ( PDF ).
- S. Berking, K. Herrmann: Formation and discharge of nematocysts is controlled by a proton gradient across the cyst membrane. In: Helgoland Marine Research. 60, 2006, pp. 180-188 ( PDF ).
- A. Beckmann, S. Xiao, JP Müller, D. Mercadante, T. Nüchter, N. Kröger, F. Langhojer, W. Petrich, TW Holstein, M. Benoit, F. Gräter, S. Özbek: A fast recoiling silk-like elastomer facilitates nanosecond nematocyst discharge. In: BMC biology. Number 1, January 2015, doi: 10.1186 / s12915-014-0113-1 , PMID 25592740 . ,
- AH lattice, U. Thurm: Starvation increases the electrically induced exocytosis of nematocysts in Hydra vulgaris. In: N. Elsner, G. Roth (Ed.): Gene brain behavior. Proceedings of the 21st Göttingen Neurobiology Conference. Thieme, Stuttgart 1993, p. 154.
- Fautin (2009, p. 1054)
- D. Obermann, U. Bickmeyer, H. Wägele Incorporated nematocysts in Aeolidiella stephanieae (Gastropoda, Opisthobranchia, Aeolidoidea) mature by acidification shown by the pH sensitive fluorescing alkaloid Ageladine A In: Toxicon 60, 2012, pp. 1108-1116