trichocyst

Trichocysts are small, elongated secretory organelles found in many ciliates. They belong to the larger group of secretory structures called extrusomes and lie just beneath the cell cortex in apical regions. Each trichocyst consists of a capsule embedded in the cytoskeleton and an associated thread precursor that remains folded inside until discharge.

Discharge occurs when the cell experiences appropriate stimulation, such as mechanical contact or certain chemical cues. The capsule opens and a thread evaginates explosively, extending many times the length of the organelle within milliseconds. The filament produced can be rigid or slightly adhesive, forming a defensive barrier, an anchoring filament, or a net that can entangle prey, depending on the species. The discharge is a rapid form of exocytosis powered by osmotic and cytoskeletal forces within the cell.

Functionally, trichocysts serve defensive and predatory roles. They deter predators by creating a sudden, physical barrier and can aid in prey capture or immobilization. In some ciliates, discharged threads may also help the organism anchor to substrates or alter local microenvironments to its advantage.

In research and taxonomy, trichocysts are valuable distinguishing features because their size, shape, distribution, and discharge patterns vary among species. They are studied using light and electron microscopy to understand exocytosis, organelle eversion, and the cortical architecture of ciliates.