Light microscopy (Figures 11.1, 11.2) shows T. vaginalis to be pear-shaped, approximately 10-13x8-10 ^ (when living; fixed and stained organisms are about 25% smaller), with four anterior flagella and a fold of cytoplasm; the undulating membrane, running along one side of the body for about two-thirds of its length. The latter is supported by a third rod called the costa; its wave-like motion is produced by a fifth (recurrent) flagellum attached to it. In T. vaginalis and T. tenax this does not extend beyond the end of the undulating membrane to form a free
Principles and Practice of Clinical Parasitology
Edited by Stephen Gillespie and Richard D. Pearson © 2001 John Wiley & Sons Ltd
flagellum, while in P. hominis it does (Figure 11.1). A rigid microtubular rod, the axostyle, runs through the body of the organism and appears to project from its posterior end; the prominent nucleus is enfolded by the anterior end of the axostyle. Electron microscopy (Honigberg and Brugerolle, 1989) reveals the deeply staining parabasal body to consist of an elaborate Golgi complex supported by filaments; anterior to this the basal bodies (one orthogonal to the other four), from which the flagella arise, comprise the kinetosomal complex. An intricate system of microtubular organelles presumably maintains the shape of the organism. A considerable number of electron-dense granules are also present, arranged alongside the costa and the axostyle; these are now identified as hydrogeno-somes (see below).
The description given here refers to T. vaginalis in clinical specimens or free in culture; it has long
been known, however, that it will adhere to certain cultured cells and some non-living surfaces, becoming much more amoeboid in the process (Arroyo et al., 1993). In contact with vaginal epithelial cells in vitro, the organism became extremely flattened and adherent and it seems likely that this is their normal morphological form in infected females.
Classically, T. vaginalis is placed in a well-characterized Order Trichomonadida within the Superorder Parabasalidea; as a non-photosynthetic flagellate, the higher classification is Class Zoomastigophorea, Subphylum Masti-gophora, Phylum Sarcomastigophora. Although the first two taxa are almost certainly valid, the larger scheme probably relies too much on features (such as modes of locomotion) much subject to convergent evolution to give an accurate phylogenetic tree. Molecular taxonomy suggests that trichomonads branched very early from the main eukaryotic line of descent, although the suggestion that this occurred before the acquisition of the pre-mitochondrial symbiont appear to have been disproved (Roger et al., 1996).
Hydrogenosomes (the electron-dense granules) are membrane-bound organelles, functionally equivalent to but metabolically very different from mitochondria. Within them pyruvate is oxidised and further ATP is produced; under strictly anaerobic conditions, protons act as terminal electron acceptors and molecular hydrogen is evolved. Whether hydrogenosomes are highly evolved mitochondria, arose following a separate endosymbiotic event or have some different origin is not clear, although evidence supporting the first suggestion is increasing.
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