Transient invagination of flagella by Tritrichomonas foetus.

We describe an experimental system for the study of rapid and reversible formation of pseudocysts, which are spherical forms that lack a true cyst wall, by Tritrichomonas foetus, a trichomonad parasite of the bovine genitourinary tract. It highlights the dynamics of the plasma membrane and cytoskeleton of this parasite, which is perpetually devoid of any sort of protective cell wall, and can reflect a responsive survival mechanism. We have found that cooling of axenic cultures of T. foetus from their normal 37 degrees C to below about 16 degrees C can trigger pseudocyst formation. The three anterior flagella and the single recurrent flagellum can be fully internalized within 1 3 min at 37 degrees C, with the axonemes and flagellar membranes remaining intact within the cell body. Electron microscopy confirms that the internalized flagella are surrounded by a separate membrane. At 37 degrees C the internalized flagella can resume beating movements and become externalized as quickly as within 10 min. We have begun to elucidate the mechanisms of this unusual phenomenon, characterizing its temperature dependence and exploring the effects of agents that interfere with various aspects of the cytoskeleton, phagocytosis, endocytosis, and exocytosis.