Extrusome

Extrusomes are membrane-bound organelles found in eukaryotic cells that are specialised for the rapid discharge of their contents to the exterior of the cell. They represent a diverse group of cellular structures involved primarily in defence, prey capture, locomotion, and protective secretion. Owing to their wide variation in morphology, molecular composition, and function, it is generally considered unlikely that all extrusomes are homologous in an evolutionary sense. Instead, they appear to represent multiple independent solutions to similar ecological and cellular challenges.
Extrusomes are particularly characteristic of unicellular eukaryotes, especially protists, although analogous discharge structures are also found in some multicellular organisms. Despite extensive observation, the precise functions and evolutionary origins of many extrusomes remain incompletely understood, and several proposed roles are still debated in cell biology.

General Characteristics

Extrusomes are typically enclosed by a membrane and positioned close to the cell surface, allowing rapid interaction with the external environment. They contain pre-assembled material that can be explosively or forcefully expelled in response to specific stimuli, such as mechanical contact, chemical signals, or changes in light conditions.
Common features of extrusomes include:

• Rapid discharge triggered by external stimuli
• Highly specialised internal structures
• One-time or limited-use deployment per organelle
• Regeneration or replacement following discharge

The expelled material may take the form of fibres, rods, mucus, toxins, or enzymes, depending on the extrusome type and the ecological role of the organism.

Types of Extrusomes

Several distinct types of extrusomes have been identified across different eukaryotic lineages.
TrichocystsTrichocysts are among the most extensively studied extrusomes and are especially common in ciliates. They discharge long, fibrous or rod-like structures that can function in defence, anchorage, or prey deterrence. Upon activation, structural proteins within the trichocyst undergo a rapid conformational change that dramatically increases the length of the expelled shaft.
MucocystsMucocysts secrete mucous material onto the cell surface. This mucus may contribute to cyst formation, surface protection, or adhesion. In some protists, the secreted mucus can trap organic particles, which may later be reingested along with nutrients.
ToxicystsToxicysts are specialised extrusomes found in predatory ciliates, particularly within the classes Prostomatea and Litostomatea. When discharged, they release toxic substances that paralyse or kill prey, facilitating ingestion.
HaptocystsHaptocysts occur in ciliates of the class Phyllopharyngea and are typically located at the tips of feeding tentacles. They inject adhesive or enzymatic material into prey, anchoring it in place and initiating external digestion.
EjectosomesEjectosomes are found in cryptomonads, a group of unicellular algae. They consist of paired spiral ribbon-like structures held under tension. When triggered by mechanical, chemical, or light stress, ejectosomes rapidly discharge, propelling the cell away from potential threats.
AncoracystsAncoracysts are highly specialised extrusomes described in the provoran eukaryote Ancoracysta twista. They are thought to play a role in immobilising prey, although their precise mechanism of action is still under investigation.

Extrusomes in Ciliates

Ciliates display some of the highest diversity and abundance of extrusomes. In Paramecium, for example, approximately 8,000 extrusomes may be present in a single cell, distributed across the entire cell surface.
In defensive contexts, trichocysts respond rapidly to mechanical or physical stimulation, ejecting sharp projectiles that may deter predators. The discharge mechanism involves a rapid structural transformation of internal proteins, resulting in sudden elongation and expulsion.
Predatory ciliates employ toxicysts and haptocysts to subdue prey, while mucocysts contribute to protection and cyst formation. This diversity highlights the central role of extrusomes in ciliate ecology and survival.

Extrusomes in Other Eukaryotic Groups

Beyond ciliates, extrusomes are found in a variety of other protist and animal lineages.
Many dinoflagellates possess trichocyst-like extrusomes capable of ejecting sharp spindles. Although dinoflagellate trichocysts were once thought to be unrelated to those of ciliates, molecular studies of ribosomal DNA suggest closer evolutionary relationships than previously assumed, despite marked physiological differences.
Cryptomonads rely on ejectosomes primarily for escape responses, enabling rapid movement away from unfavourable conditions. These extrusomes play a role in cellular behaviour rather than defence or feeding.
In multicellular animals, the cnidocyte of cnidarians such as jellyfish, sea anemones, and corals contains an organelle known as the cnidocyst. While cnidocytes are specialised cells rather than free organelles, the cnidocyst itself functions analogously to an extrusome. When the hair-like cnidocil is stimulated, a tightly coiled, hollow tube is explosively ejected, piercing the target and injecting toxin. This discharge occurs within microseconds and can reach accelerations of approximately 40,000 g, making it one of the fastest known cellular processes.

Functional Roles and Biological Significance

Extrusomes serve a range of ecological and physiological functions, including:

• Defence against predators
• Prey capture and immobilisation
• Escape responses
• Surface protection and cyst formation
• Facilitation of feeding