The behavioral and developmental physiology of nematocysts

Abstract

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...

Functional characteristics of nematocysts found on the scyphomedusa Cyanea capillata

“…A substantial amount of progress has been made on understanding the nature of nematocyst toxins (summarized in Williamson et al, 1996) and the stimuli involved in initiating nematocyst discharge (e.g. Thorington andHessinger, 1998, reviewed in Kass-Simon andScappaticci, 2002). Less is understood about how nematocysts function in capturing prey.…”

Formation and discharge of nematocysts is controlled by a proton gradient across the cyst membrane

“…Stenoteles of Hydra are the best-studied nematocysts (for review, see Kass-Simon andScappaticci 2002, Thurm et al 2004). The whole process of tubule extrusion is completed within 3 ms. After an electric triggering the volume of the capsule instantaneously increases by more than 10%.…”

“…The mechanisms and driving forces of discharge have been studied by a large number of authors for more than 100 years (for review see Ku¨hn 1916;Bouillon 1993;Kass-Simon and Scappaticci 2002). In general, a high internal osmotic pressure and mechanical energy stored in the walls of cysts and/or tubules have been suggested as causing the discharge when an external cue reaches the cnidocil.…”

The evolutionary emergence of cell type-specific genes inferred from the gene expression analysis of Hydra

“…Hydra contains four types of nematocytes (stenotele, holotrichous isorhiza, atrichous isorhiza, and desmoneme) (20). In general, the differentiation of nematocytes proceeds as follows: growth (capsule wall and tubule formation), maturation (tubule invagination, spine assembly, capsule wall hardening and poly-␥-glutamate polymerization), migration (cnidocil apparatus formation), and deployment (septate junction formation and cyst positioning) (21,22). Using the array technique and in situ hybridization, we have identified 51 nematocyte-specific genes that are expressed at distinct differentiation stages or in a particular type of nematocyte (Table 1).…”