Membrane Specializations in the Form of Regular Membrane-to-Membrane Attachment Sites in Paramecium. A Correlated Freeze-Etching and Ultrathin-Sectioning Analysis
The outer membrane complex of Paramecium was investigated by ultrathin-sectioning techniques and by freeze-etching of unfixed cells without cryoprotectants. Granules were found in the freeze-etched cortical membrane complex in a highly ordered arrangement; morphometric analyses and partial disruption of this membrane complex showed that some of these granular membrane specializations (which we call types a, b, e, f, g) represent membrane-to-membrane attachment sites. Type a: granules arranged in rings (single or double), 300 nm in diameter, connect plasmalemma and alveolar membranes around trichocysts. Type b: within these a-type rings, a concentric zone (180 nm diameter) of granules connects plasma membrane and trichocyst membrane. Type c: the trichocyst membrane further contains a central ring or patch, 80 nm in diameter, of rather large granules where it is in contact with the crystalline trichocyst matrix. Type d: ‘ciliary necklaces’ are formed by groups of triple rows of 3–6, most frequently 5 granules, with 21-nm periodicity. Rows of granules also connect the alveolar membranes to the apical portion of the trichocyst membrane (type e) and the alveolar membrane to the plasmalemma around cilia (type f). Type g: the inner alveolar membrane displays an intense granularity and contains double rows of granules along alveolar septa at attachment sites between 2 alveolar membranes. Upon experimental discharge of the majority of trichocysts only the innermost concentric circles of membrane-bound granules of this region (b- and c-type) disappeared from the plasmalemma, while the outermost a-type rings of granules persisted for a longer time. Among other possible functions, these regular membrane-to-membrane attachments are likely to maintain the specific cellular shape.