Like mitochondria and hydrogenosomes, mitosomes are morphologically heterogeneous but share the one defining characteristic of all endosymbiosis-derived organelles: they are surrounded by two closely apposed limiting membranes. Trachipleistophora and Giardia mitosomes are abundant organelles (average of 28 and 55 organelles per cell, respectively), and are the smallest mitochondrion-related organelles thus far reported. In immunoelec-tron microscopy micrographs they appear elongated in profile and measure an average of 50 mm x 90 nm and 60 mm x 140 nm, respectively (Tovar et al. 2003; Williams et al. 2002). No invaginations of the inner membrane (cristae) are present in these organelles. Entamoeba mitosomes appear in electron microscopy micrographs of enriched preparations as ovoid double-membrane-bounded organelles lacking cristae and measuring an estimated 0.5-1.0 ^m in diameter (Ghosh et al. 2000). Size-restricted confocal imaging of fixed trophozoites has revealed the presence of over 150 mitosomes per cell and suggests that most of these organelles are smaller than half a micron in size (León-Avila and Tovar 2004). In contrast to the mitosomes of other organisms, the Entamoeba organelles have not been unequivocally identified by immunoelectron microscopy as a good mitosome-specific antibody suitable for this purpose has yet to be identified. In Cryptosporidum, a single oval mitosome estimated in different studies at around 200 and 500 nm in diameter has been identified by electron microscopy (Putignani et al. 2004; Riordan et al. 2003). This mitosome is invariably located between the crystalloid body and the nucleus and appears surrounded by the endoplasmic reticulum. Electron tomographic reconstruction has documented the existence of atrophic cristae-like membrane networks within the organelle but their functional significance is unclear as no oxidative phosphorylation occurs in this parasite (Keithly et al. 2005). The mitosomes of Blastocystis are variable in number, from just a few in rapidly dividing cells to well over 100 in older giant cells, and display unusually short goblet-shaped cristae that can also be filamentous and branched (Nasirudeen and Tan 2004; Zierdt 1991). Such patent diversity in mitosome morphology, akin to the morphological variability observed amongst mitochondria and amongst hydrogensomes of unrelated taxa, may result from niche-specific environmental pressures driving organelle evolution in diverse taxonomic groups.

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