ULTRASTRUCTURAL ANALYSIS OF THE CORTICAL MODIFICATIONS DURING TRICHOCYST SECRETORY GRANULE DOCKING IN THE CILIATE PSEUDOMICROTHORAX-DUBIUS

To elucidate the morphological modifications that occur in the cell co rtex during trichocyst docking in the ciliated protist Pseudomicrothor ax dubius, the cortical ultrastructure of cells with or without docked trichocysts was compared. Additionally, the ultrastructure of the doc king sequence was followed in cells fixed when many trichocyst docking events were induced. The P. dubius cortex is not composed of discrete alveolar sacs and alveolar septa as in many other ciliates. Instead, one finds a continuous alveolar space that is interrupted only at cert ain, specific points such as at the tips of docked trichocysts. Not on ly is the alveolus absent at the trichocyst tips, but so is the rigid epiplasmic layer. Thus, the docked trichocyst is positioned in pores t hat penetrate these cortical layers and is attached to the plasma memb rane in an exocytosis-ready position. In cells without docked trichocy sts, one observes unoccupied docking sites in the cortex; however, the alveolar space and the epiplasmic layer are continuous and intact abo ve the unoccupied sites. Therefore, a docking trichocyst must penetrat e the epiplasm and the alveolus before the trichocyst can become attac hed to the plasma membrane. The localized dissolution of the epiplasm and the membrane fusion during alveolar penetration were followed step by step in serial sections of docking trichocysts, revealing that the cortical modifications at the docking site are induced by a series of direct, sequential interactions between the membrane of the trichocys t tip and specific cortical components.