Expression of cytochrome oxidase in hair cells of the teleost utricle.

Ultrastructural variation in some cytoplasmic organelles and synaptic structures is one characteristic distinguishing the types of hair cells in the teleost ear. In this study, we explored differences in mitochondria by analyzing mitochondrial reactivity for cytochrome oxidase (COX) in hair cells of the teleost utricle. The reactivity for COX within mitochondria in the subcuticular compartment directly beneath the cuticular plate differentiated among hair cells in utricles of three teleost species, Carassius auratus, Pantodon buchholzi, and Astronotus ocellatus. Mitochondria in the subcuticular region of hair cells in the striola reacted intensely. Within juxtastriola and extrastriolar hair cells near the striola, mitochondria reacted at a lowered intensity than in striolar hair cells. Subcuticular mitochondria of extrastriolar hair cells located distant from the striola reacted negligibly. The reactivity of mitochondria in other cytoplasmic compartments did not provide similar evidence for distinguishing among teleost hair cells. Mitochondria within intraepithelial branches of the eighth nerve terminals in the different utricular regions reacted to COX histochemistry commensurate with their respective presynaptic hair cells. Branches of sensory afferent neurons innervating striolar hair cells displayed a dense COX reaction. Sensory afferents innervating the extrastriolar hair cells did not display many mitochondria at synapses nor, when present, was the staining as dense. The presynaptic side of the hair cell-afferent nerve synapse usually, but not always, contained reactive mitochondria. The presynaptic side of the efferent nerve-hair cell synapse did not necessarily contain mitochondria. Mitochondria filling the cytoplasm in a type of juxtamacula cell revealed uniformly dense COX reactivity.