Взрывная клетка книдоцит: строение и механизм действия

Explosive cell containing one giant secretory organelle (cnida)

A cnidocyte (also known as a cnidoblast) is an explosive cell containing one large secretory organelle called a cnidocyst (also known as a cnida (: cnidae)) that can deliver a sting to other organisms. The presence of this cell defines the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.). Cnidae are used to capture prey and as a defense against predators. A cnidocyte fires a structure that contains a toxin within the cnidocyst; this is responsible for the stings delivered by a cnidarian. Cnidocytes are single use cells that need to be continuously replaced.

Each cnidocyte contains an organelle called a cnida, cnidocyst, nematocyst, ptychocyst or spirocyst. This organelle consists of a bulb shaped capsule containing a coiled hollow tubule structure attached to it. An immature cnidocyte is referred to as a cnidoblast or nematoblast. The externally oriented side of the cell has a hair like trigger called a cnidocil, which is a mechano and chemo 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 g. Research from 2006 suggests the process occurs in as little as 700 nanoseconds, thus reaching an acceleration of up to 5,410,000 g. 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 nematocytes. Upon encounter with a crustacean prey, nematocytes 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.

The cnidocyte capsule is made of novel Cnidaria specific genes which combine known protein domains. Minicollagen genes 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 cystein rich domains. Trimers of minicollagen proteins assemble through their terminal cystein 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. Nematogalectin, minicollagen Ncol 15 and chondroitin are novel proteins used to build the tubule shaft. In piercing cnidocytes, the novel protein is used to make the spines present at the base of the shaft.

Cnidocytes are single use cells that need to be continuously replaced throughout the life of the animal with different mode of renewal across species.

In Hydra polyps, cnidocytes differentiate from a specific population of stem cells, the interstitial cells (I cells) located within the body column. Developing nematocytes first undergo multiple rounds of mitosis without cytokinesis, giving rise to nematoblast nests with 8, 16, 32 or 64 cells. After this expansion phase, nematoblasts develop their capsules. Nests separate into single nematocytes when the formation of the capsule is complete. In the Anthozoan sea anemone Nematostella vectensis, nematocytes are thought to develop throughout the animal from epithelial progenitors. Furthermore, a single regulatory gene that codes for the transcription factor ZNF845 also called CnZNF1 promotes the development of a cnidocyte and inhibits the development of a RFamide producing neuron cell. This gene evolved in the stem cnidarian through domain shuffling. One member of this family, the sea wasp, Chironex fleckeri, is "claimed to be the most venomous marine animal known," according to the Australian Institute of Marine Science. It can cause excruciating pain to humans, sometimes followed by death. Other cnidarians, such as the jellyfish Cyanea capillata (the "Lion's Mane" made famous by Sherlock Holmes) or the siphonophore Physalia physalis (Portuguese man o' war, "Bluebottle") can cause extremely painful and sometimes fatal stings. On the other hand, aggregating sea anemones may have the lowest sting intensity, perhaps due to the inability of the nematocysts to penetrate the skin, creating a feeling similar to touching sticky candies. Besides feeding and defense, sea anemone and coral colonies use cnidocytes to sting one another in order to defend or win space. Despite their effectiveness in prey predator interactions, there is an evolutionary tradeoff as cnidarian venom systems are known to reduce the cnidarian's reproductive fitness and overall growth. Venom from animals such as cnidarians, scorpions and spiders may be species specific. A substance that is weakly toxic for humans or other mammals may be strongly toxic to the natural prey or predators of the venomous animal. Such specificity has been used to create new medicines and bioinsecticides, and biopesticides. Animals in the phylum Ctenophora ("sea gooseberries" or "comb jellies") are transparent and jelly like but have no nematocysts, and are harmless to humans. Certain types of sea slugs, such as the nudibranch aeolids, are known to undergo kleptocnidy (in addition to kleptoplasty), whereby the organisms store nematocysts of digested prey at the tips of their cerata.