What is Trichocyst?

As per the Trichocyst definition, it is a structure that is generally found in ciliates such as Paramecium. It functions as a secretory granule and has a shape that is structurally complex and constrained to a great extent. Additionally, a trichocyst can also occur in tetrahymena and along cilia pathways of a number of metabolic systems. 

The shape of a Trichocyst resembles that of a bottle or that of an inverted golf tree. Its body is spindle-shaped, often bearing a tip at its wide end. 


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Trichocyst in Paramecium

Usually seen in ciliates, such as Paramecium and flagellate protozoans, trichocysts can be widely dispersed in an organism or remain confined to certain areas such as the tentacles around the mouth, the papillae etc. Trichocysts in Paramecium and other ciliates are usually found confined to the cortical sites. Each cell bears approximately 1000 trichocysts. In Paramecium and other ciliates, trichocysts are often found tethered at right angles to the cell surface. 


Types of Trichocysts

Trichocysts found in organisms can be of several types. These include:

Filamentous Trichocysts: Such trichocysts are usually found in Paramecium and a number of other ciliates. These trichocysts comprise a cross-striated shaft with a tip and are discharged as filaments 

Mucoid Trichocysts: These types of trichocysts are inclusions of elongated nature. They are likely to be ejected as visible bodies on being stimulated artificially. 

Toxicysts 

These resemble trichocysts and are usually seen located around the mouth areas of certain carnivorous protozoans including Dileptus. 


Trichocyst Structure

As seen under a light electron microscope, a trichocyst in its undischarged state comprises 10 components. These components are: 

  • A mesh-like sheath that envelops the body of the organelle.

  • An inner and an outer covering that encloses the tip - the inner sheath is comprised of 4 spiralling envelopes that have a square net substructure while the outer sheath is formed of a dense amorphous matrix that contains longitudinal microtubules as well as scattered fine filaments.

  • A surface boundary to the outer sheath

  • A membranous sac in the apical region surrounded by a cylinder consisting of microtubules that are joined to each other with dense material.

  • At last, a crystalline matrix of the body and tip of the trichocyst. This crystalline appearance, apparently, is related to the presence of a complex of fine filaments that are loosely interwoven. These form a pattern of unit structures that are highly regular and repeat at 16-nm intervals.


Trichocyst Function

As already stated above, trichocysts are cortical structures seen in certain flagellate and ciliate protozoans that are ejected in response to a certain kind of stimuli. They are ejected in the form of long thin threads. The function of trichocysts is largely being studied and is thought to be of defensive nature against predators. This is especially true in the case of toxicysts. A toxicyst when discharged expels a non-striated long filament that bears a rodlike tip. This tip is used in the killing or paralysing of other microorganisms. The filament, on the other hand, is more likely used to capture food and also in defence. In the case of Paramecium, however, trichocysts are also extruded as a means of anchorage during feeding. 

Action of Trichocysts in Response to External Stimuli. 

In the event of an external stimulus, a synchronous and massive exocytosis is triggered in the organism bearing trichocyst. Once the membrane fuses during exocytosis, contact with calcium ions and water present in the external medium causes an irreversible and extremely rapid (less than 50ms) expansion of the contents of the trichocysts to generate a secondary needle-shaped form that is insoluble in water. 


Conclusion

Here we have seen trichocyst definition, types and function. While certain trichocysts are explosive in nature that discharge a thread-like shaft crowned by a barb by means of pore a pore to the exterior, either to capture prey or provide anchorage during feeding, other trichocysts are filled with fluid and discharge toxins and mucus.

FAQs (Frequently Asked Questions)

1. What is the Function of Trichocysts in Paramecium?

Ans. After a series of experiments, it was concluded that the function of trichocyst in Paramecium is mainly of a defensive nature. The trichocysts defend Paramecium against  D. margaritifer. Thus, trichocysts in Paramecium serve as defensive organelles against several predators. 

2. What Leads to the Release of Trichocysts?

Ans. Trichocysts are made of protein and are often attached and barbed to the surface of the cell by means of microscopic threads. They are basically dart-like structures that are shot out from certain cells in response to physical and/or chemical structures. They are either used for defence or attachment. 

3. What are Toxicysts?

Ans. Toxicysts are organelles that are found in certain protozoa. They resemble trichocysts but are distinct from them in that their filament carries a poison that is capable of killing other protozoa. Toxicysts are mainly used to capture prey.