The ultrastructure of the extrusomes in Pseudourostyla cristata, a hypotrichous ciliated protozoan.

By using scanning and transmission electron microscopy, the present study demonstrates a great number of trichocyst-like extrusomes distributed in the cortical cytoplasm of the protozoan Pseudourostyla cristata, a hypotrichous ciliate. Of these, the mature organelles are rod-shaped with a cap consisting of tubular structures, a tip located at the apex of the cap, a body consisting of strateform structures of uneven electron density and an elongated shaft located along the longitudinal central axis of the body. The electron microscopic observations suggest that the extrusive organelles in P. cristata might undergo a morphogenetic process including the following sequential events: the occurrence of the vesicles in the cytoplasm, the condensation of the fibrous substances within the vesicles, the appearance of the electron-dense shaft, and the formation of the cap. In contrast with a large quantity of extrusomes in trophozoit P. cristata, there are no such extrusive organelles in the encysted cells of the ciliate. The phenomena that P. cristata ciliates can readily enter physiological reorganization or encysting phases and discharge a great number of their extrusomes when prepared for SEM and TEM observation suggest that the extrusive process of the extrusomes in P. cristata might have an important influence on the life activity of the ciliate and could be one of the causes leading to the physiological reorganization and the encysting of the ciliate. These reactions of P. cristata might be a protective or defensive response to the environmental changes.