Order of Microspora
Microsporidia are widespread small unicellular, obligate intracellular parasites which are transmitted via resistant double-walled (chitin containing) spores normally ingested by their hosts ( , Amblyospora/Fig. 1, Encephalitozoon/Fig. 1, Table 1). When these spores hatch under suitable stimuli, a hollow polar tube (polar filament) is everted, enabling the tip to penetrate a host cell. The sporoplasm passes through the tube and enters the host cell cytoplasm, inside which asexual reproduction (schizogony = merogony, sporogony) is initiated ( ). Microsporidia lack mitochondria, but are typically eukaryotic. Nuclear division occurs in the absence of centrioles with spindles being anchored to dense plaques along the inner nuclear membrane. The systematic position of the Microsporidia is under discussion, since several authors consider them as fungi. Just recently a large number of Microsporidia turned out to be opportunistic agents and they are found in many AIDS patients.
Host Cell Invasion
Although rather poorly known, the mechanism of cell invasion by Microsporidia has very peculiar characteristics. Microsporidia self-inject into their host cell by devaginating a membranous organelle (polar tube) which forces its way through the host cell plasmalemma and through which the parasite cytoplasm moves into the recipient cell (Host Cell Invasion/Fig. 1). Invasion is thus intrusive: this is the only case known among intracellular parasitic protozoa. Parasite development may occur either in the cytoplasm of the cell or within a parasitophorous vacuole; but whether this vacuole forms at invasion or later is not known. No recognition mechanisms have been described so far in this group. However, the signalling for polar tube extrusion is likely to be driven by recognition of a suitable target in the vicinity of the spore, and the detailed study of this phenomenon will certainly lead to identifying receptor-ligand interaction in this process.