Of Terms in Biology: Extrusomes

by Elio

Extrusomes are nasty membrane-bound structures released by ciliates and flagellates. They come in a number of varieties, the best studied being the trichocysts of paramecia. These can be dis­char­ged by the thousands per cell in response to physical stimuli, including when attacked by pre­dators. Instead of being used for defense, some extrusomes are offensive weapons that rapacious ciliates deploy to capture their prey. The function of yet others remains to be figured out.

Trichocysts of Paramecium. mt, mi­cro­tu­bules. Bar = 0.25µm. Magnification = 14,750X. TEM by R. Allen. Source

Trichocysts are pointed projectiles resembling small arrows that can pierce the covering of predators. Other kinds of extrusomes are amorphous material that, once outside the cells, changes into capsules or cysts. "Ex­tru­some" is a catchall term for a variety of structures that have in common their electability and, besides tri­cho­cysts, go by names such as toxicysts, trichites, cortical ampules, mucocysts, cortical granules, and others.

The stinging nematocysts of some jellyfish (sea nettles) that cause pain to unsuspecting swimmers may be con­si­dered a kind of extrusomes. However, the ciliate kind can be regenerated in the same cell, the ones of the small animals cannot.

How are extrusomes extruded? Under appropriate stimuli, the trichocyst membrane fuses with the cell membrane and forms an opening, a form of exocytosis. In the extrusion process, the trichocysts elongate about tenfold, which involves re­ar­range­ments of filaments within them. Thistakesplaceinmilliseconds. For a review, see here and here.

The theme of extrusion of body parts extends itself to the prokaryotes. Bacteria on the surface of certain ciliates (e.g., Euplotidium) can, in suicidal fashion, themselves function as projectiles. They open up and extrude a coiled tubulin ribbon into the exterior, carrying the bacterial DNA at its tip. We discussed this earlier. Such structures have been given a different name, epixenosomes. Are these the origin of the extrusomes? Could be…