cnidocyten

Cnidocyten, commonly called cnidocytes in English, are specialized cells unique to the phylum Cnidaria. They house cnidae, a family of intracellular organelles that include nematocysts. Cnidocyten are distributed throughout the body surface and tentacles, with high density in areas used for prey capture and defense.

Structure: Each cnidocyte contains a capsule called a nematocyst, which encloses a coiled tubule armed with barbs or spines and loaded with venom. The cell surface bears a trigger, the cnidocil, that detects mechanical and chemical stimuli. When stimulated, the capsule opens and the tubule everts rapidly, delivering venom or helping to entangle prey.

Discharge mechanism: Discharge is triggered by stimuli that cause water to enter the capsule, raising internal pressure. The operculum opens and the tubule everts in a fraction of a millisecond. The process can penetrate tissue and deliver venom, or entangle prey, depending on nematocyst type. Most nematocysts fire once; after discharge the cnidocyte is inactivated and replaced later by new cells.

Function and significance: Cnidocyten play a central role in prey capture and defense for all major cnidarian groups, including jellyfish, sea anemones, corals, and hydroids. The venom content varies widely among species, producing effects ranging from mild irritation to intense pain or systemic reactions in humans.

Development and renewal: Cnidocytes arise from stem-cell lineages, including interstitial cells, and are continually produced during an organism’s life. After discharge, replacement cnidocytes are formed to maintain the animal’s functional capability.