THE MECHANISM OF THE WATER EXPULSION VESICLE OF THE CILIATE TETRAHYMENA PYRIFORMIS

Analysis of high-speed (150 frames/sec) cinematographs of the filling and expulsion of the water expulsion vesicle of Tetrahymena pyriformis shows that the vesicle fills as water is pumped into it by contractions of at least four ampullary sacs which are continuous with the endoplasmic reticulum. When filled, the vesicle is pressed against its two excretory pores by cyclotic movements of the cytoplasm. This pressure closes the apertures of the ampullae, preventing backflow from the vesicle into them, and also spreads the pellicle of and at the pore, thereby stretching and rupturing the pore-sealing membrane. The vesicle is then invaginated by the cytoplasmic pressure, driving fluid out of the pore. The pore-sealing membrane then reforms, apparently by constriction, and the vesicle is again filled. Electron micrographs show that crisscrossed pore-microtubules extend from the pore to the openings of the ampullae, anchoring the vesicle in place. Each pore is surrounded by a stack of at least 11 ring-microtubules, to which the anchoring pore-microtubules are attached. The pore-microtubules appear to exert tension which assists in spreading the pore, aiding cyclotic pressures in rupturing the pore-sealing membrane. A possible mechanism for the cyclotic pressure and ampullary contraction is proposed.