Extremely elongated mitochondria in ionocytes of the saccular epithelium of a teleost, Oreochromis mossambicus (Cichlidae).

Unusually large mitochondria are a rather scarce feature in normal biological tissue and string-like giant mitochondria have hitherto not been reported in animals. Investigating the role of inner ear ionocytes for otolith growth, large ionocytes of the saccular epithelium of the cichlid fish Oreochromis mossambicus were analyzed by imaging of thick sections with energy-filtering transmission electron microscopy. We report here that ionocytes do not contain numerous small-sized mitochondria as has been suggested earlier but rather few, extremely elongated megamitochondria. Since the particular mitochondrial structure is important for normal cell function, such megamitochondria possibly reflect a functional advantage in the context of the presumed role of teleostean ionocytes in regulating the composition of the endolymphatic fluid.