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ADDED TO THE CNIDOCYTE ARTICLE

in the structure and function sectionHowever, in two sea anemones species (Nematostella vectensis and Anthopleura elegantissima), a 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, thus suggesting a different mode of entry for toxins ^[1].

Cnidocyte capsule composition

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 ^[2]. 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 spinalin is used to make the spines present at the base of the shaft ^{[3] [4] [5]}.

In the types of cnidocytes section

The diversity of cnidocytes types correlates with the expansion and diversification of structural cnidocyst genes like minicollagen genes ^[6]. Minicollagen genes form compact gene clusters in Cnidarian genomes, suggesting a diversification through gene duplication and subfunctionalization. Anthozoans display less capsule diversity and a reduced number of minicollagen genes, and meduzosoans have more capsule diversity (about 25 types) and a vastly expanded minicollagen genes repertoire^[6]. In the sea anemone Nematostella vectensis, some minicollagens display a differential expression pattern in different cnidocytes subtypes ^{[7] [8]}.

Cnidocyte subtypes can be differentially localized in the animal. In the sea anemone Nematostella vectensis, the majority of its non-penetrant sticky cnidocytes, the spirocytes, are found in the tentacles, and are thought to help with prey capture by sticking to the prey. By contrast, the two penetrant types of cnidocytes present in this species display a much broader localization, on the outer epithelial layer of the tentacles and body column, as well as on the pharynx epithelium and within mesenteries ^[9].

Cnidocyte development

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

Modes of renewal

In Hydra polyps, cnidocytes differentiate from a specific population of stem cells, the interstitial cells (I-cells) located within the body column. They 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 capsule. Nests separate into single nematocytes when the formation of the capsule is complete. Most of them migrate to the tentacles where they are incorporated into battery cells, with hold several nematocytes, and neurons ^[10].

In the hydrozoan jellyfish Clytia hemispherica, nematogenesis takes place at the base of the tentacles, as well as in the manubrium. Nematoblasts proliferates then differentiate along a proximal-distal gradient, giving rise to mature nematocytes in the tentacles through a conveyor belt system^[11].

In the Anthozoan sea anemone Nematostella vectensis, nematocytes are thought to develop throughout the animal from epithelial progenitors ^[12].

Cnidocyst maturation

The nematocyst forms through a multi-step assembly process from a giant post-Golgi vacuole. Vesicles from the Golgi apparatus first fuse onto a primary vesicle: the capsule primordium. Subsequent vesicle fusion enables the formation of a tubule outside of the capsule, which then invaginates into the capsule. Then, an early maturation phase enables the formation of long arrays of barbed spines onto the invaginated tubule through the condensation of spinalin proteins. Finally, a late maturation stage gives rise to undischarged capsules under high osmotic pressure through the synthesis of poly-γ-glutamate into the matrix of the capsule. This trapped osmotic pressure enables rapid thread discharge upon triggering through a massive osmotic shock. ^[13]

Myxozoans

Myxozoans are highly derived Cnidarian parasites which infect fish species through their polar capsule, thought to be critical to anchor the myxozoan to the host ^[14]. These polar capsule were recently identified as nematocytes.