Nematocysts are the nonliving secretions of specialized cells, the nematocytes, which develop from multipotent stem cells. Nematocysts are the means by which coelenterates capture prey and defend against predation. The 25 or more known types of nematocysts can be divided into to four functional categories: those that pierce, ensnare, or adhere to prey, and those that adhere to the substrate. During development a collagenous cyst, which may contain toxins, forms; a hollow thread, which becomes coiled as it invaginates, develops. Maturing nematocytenematocyst complexes migrate to their discharge sites and are deployed in specific patterns. The mechanisms of pattern determination are not clear. Discharge of nematocysts appears to involve increases in intracapsular osmotic pressure consequent upon release of bound calcium within the capsule; the eversion of the filament may depend upon release of structural tension consequent upon a loss of zinc from the thread. Evidence exists that discharge is initiated as a calcium-dependent exocytosis, triggered by an electrical signal resulting from the transduction of mechanical stimuli received at the nematocyte's cnidocil. Chemical signals transduced in adjacent sensory cells alter the frequency response of the nematocyte. In opposition to the nematocyte-nematocyst independent effector hypothesis, excitatory and inhibitory neuronal input appears to regulate discharge.Résumé : Les nématocystes sont des sécrétions non vivantes produites par des cellules spécialisées, les nématocytes, qui proviennent de cellules souches multipotentes. Chez les coelentérés, les nématocystes servent à la capture des proies et sont des armes de défense contre les prédateurs. Les quelque 25 types connus de nématocystes peuvent se diviser en quatre catégories fonctionnelles : ceux qui percent, ceux qui servent de pièges, ceux qui adhèrent aux proies et ceux qui adhèrent au substrat. Au cours du développement, il se forme un kyste de collagène qui peut contenir des toxines; un fil creux se déploie en spirale en s'invaginant. Les complexes nématocytes-nématocystes migrent vers leur site de vidange et ils sont disposés selon un arrangement particulier. Les mécanismes qui régissent cet arrangement sont mal connus. L'expulsion des nématocystes semble se faire par augmentation de la pression osmotique intra-capsulaire consécutive à la libération du calcium lié dans la capsule; l'évagination du filament peut dépendre du relâchement de la tension structurale qui se produit lors de la perte du zinc du filament. Il semble, d'après certains indices, que l'expulsion des nématocystes soit d'abord une exocytose dépendant du calcium, un processus déclenché par un signal électrique résultant de la transduction de stimulus mécaniques perçus au niveau du cnidocil du nématocyte. Les stimulus chimiques modifiés par transduction dans des cellules sensorielles adjacentes modifient la fréquence de la réponse du nématocyte. En contrepartie de l'hypothèse d'un effecteur indépendant nématocyte-nématocyste, il faut envisager c...

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We have previously reported immunocytochemical, biochemical, behavioral, and electrophysiological evidence for glutamatergic transmission through (+/-)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)/kainate receptors in hydra. We now report specific localization of the N-Methyl-D-aspartic acid receptor subunit 1 (NMDAR1) in epithelial, nerve, nematocytes, and interstitial cells of hydra. Macerates of tentacle/hypostome pieces of Hydra vulgaris were prepared on agar-coated slides, fixed with buffered formaldehyde/glutaraldehyde, and fluorescently labeled with monoclonal antibodies against mammalian NMDAR1. Negative controls omitted primary antibody. Digital images were recorded and analyzed. Specific localized and intense labeling was found in ectodermal battery cells, other epithelial cells, nematocytes, interstitial cells, and sensory and ganglionic nerve cells, and in battery cells was associated with enclosed nematocytes and neurons. The labeling of myonemes was more diffuse and less intense. In nerve and sensory cells, punctate labeling was prominent on cell bodies. These results are consistent with our earlier evidence for glutamatergic neurotransmission and kainate/NMDA regulation of stenotele discharge. They support other behavioral and biochemical evidence for a D-serine-sensitive, strychnine-insensitive, glycine receptor in hydra and suggest that the glutamatergic AMPA/kainate-NMDA system is an early evolved, phylogenetically old, behavioral control mechanism.

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