Reappraisal of vesicular types in the syncytial tegument of the Echinococcus granulosus protoscolex.

The structure, abundance, and distribution of tegumentary vesicles was compared among Echinococcus granulosus protoscoleces that had been prepared for electron microscopy using four processing schedules: a conventional method, alternative fixations using uranyl acetate or osmium tetroxide-potassium ferricyanide, and a freeze-substitution method. Four morphologically distinct types of vesicles were found in the somal region. The morphology of the first form, with moderately electron-opaque contents, and the second form, with similar size and shape but containing an electron-opaque core, varied little among the preparation methods. Two additional forms of vesicles, with characteristic intensely electron-opaque contents, were revealed only after freeze-substitution. These elongate vesicles were also found in the scolex tegument where they were most conspicuous, and appeared markedly increased in number after freeze-substitution. Large, spherical vesicles with an electron-lucent core embedded in a dense matrix of fibrillar strands were the dominant vesicle forms in the scolex region after all methods of preparation. Fixation by osmium tetroxide-potassium ferricyanide revealed the presence of spherical vesicles with amorphous electron-opaque contents and a few inclusions. This form of vesicle was also observed after freeze-substitution, but the inclusions in the vesicular lumen were more numerous. The variation in the distribution of vesicle forms among the body regions strongly implies a variety of vesicle functions. In addition, our observations suggest that comparative studies of different fixative methods are necessary to demonstrate the detailed vesicular morphology of the tegument of E. granulosus and other cestodes.