Epixenosomes: peculiar epibionts of the ciliate Euplotidium itoi: morphological and functional cell compartmentalization.

As already reported, epixenosomes have a complex cell organization although they lack membranous organelles and a nuclear envelope. Different morphological compartments have been evidenced: (1) a space (IM) between the 2 external membranes in which acid phosphatase has been observed; (2) an apical, electrondense zone (DZ) not delimited by a membrane or thin envelope, containing DNA and basic proteins; (3) a roundish body (RE), 200 nm in diameter, containing reserve polysaccharides and enzymes, in particular, peroxidase; (4) an extrusive apparatus (EA) consisting of a ribbon with smooth surfaces in which lectin binding sites are present, and immersed in a complex matrix different from the remaining cytoplasm. Along its internal and external limits, active adenylate cyclase has been evidenced. At its top, a 'ring' of peroxidase is present; (5) a basket surrounding the extrusive apparatus probably consisting of tubulin microtubules, The cytochemical data, especially the precise localization of the enzymes shown here, indicate that in epixenosomes, a functional compartmentalization corresponds to a structural complexity, in spite of the absence of internal true membranes.