Microsporidian spore: new electron microscope appearance

Abstract

The phylum Microsporidia comprises a great number of species of parasitic protozoa which are ubiquitous in nature. Some species have medical importante as causative agents of opportunistic infections in AlDS patients. Since the first electron microscope description of the microsporidian spore (1), they have been shown as in figures 1-2. The spore wall shows two layers: the endospore and the exospore. The endospore appears transparent. The internal content of the spore (sporoplasm) is not easily seen because it looks dark and the spores are often broken after thin sectioning. Moreover, sections of the polar tube show a dark central core surrounded by a clear ring. Microsporidian spores of any species are observed as described above when fixed with the conventional osmium tetroxide (OsO4) fixation method. These spore features are very useful for the taxonomy of the Microsporidia (2). A new methodological procedure for electron microscopy makes other ultrastructural details of the spore wall visible (3). Figures 3-4 show the first images of whole spores of Microsporidia as seen by ruthenium tetroxide (Ru04) fixation. Besides revealing the content of the endospore, the internal structure of the sporoplasm looks well preserved. The appearance of the polar tube in section differs from that of the polar tube when fixed with osmium tetroxide. The central core is clear and the surrounding ring is electron-dense. This feature was also observed in spores of Polydispyrenia simulii (3). Differences in electron density of the layers of the polar tube could be explained by different lipid composition in accordance with the fixative properties of OsO4 and RuO4 (3). These images complement the description of the microsporidian spore presented in a previous paper (3).