Synchronous exocytosis in Paramecium cells. II. Intramembranous changes analysed by freeze-fracturing

Since Paramecium tetraurelia cells were found to discharge synchronously most of their secretory organelles ('trichocysts') when exposed to 10(-6) M aminoethyldextran (AED) [17], this was now used for a freeze-fracture and -etching analysis of intramembranous changes during exocytosis performance, in conjunction with a rapid freezing method. In controls the potential exocytosis sites of the cell membrane revealed a 'rosette' of approximately 8 membrane-intercalated particles (MIPs) within a 300 nm large double 'ring' of MIPs (see [18]). During exocytosis we found the following changes: (a) Membrane fusion starts as a focal event, the smallest recognizable openings measuring 20-30 nm in diameter. (b) The exocytotic opening always forms in the center of the rosette. (c) Rosette MIPs may stay very close to the exocytotic opening, or they may partly be dispersed as the exocytotic opening is formed. (d) No diaphragm is formed during exocytotic membrane fusion. (e) The exocytotic opening is increasing to a size where it fills the total fusogenic zone contained within a ring, but not any further. (f) Rosette MIPs become further dispersed through the rings. (g) Resealing involves the transformation of rings into a collapsed form ('parenthesis'). (h) A resealed exocytosis site contains no conspicuous MIP aggregates, such as rosettes or 'annulus' structures from the trichocyst membrane, indicating a clear separation of both components.