ELECTRON MICROSCOPICAL AND HISTOCHEMICAL STUDIES OF OUTER HAIR CELLS IN NORMAL AND ACOUSTICALLY EXPOSED RABBITS

Abstract

Recently the fine structures of outer hair cells in guinea pigs, rats, rhesus, monkies, cats, mice and humen have been made clear by electron microscopy. But we have very little information on these in rabbits at the ultrastructural level.I examined electron microscopical and histochemical features of normal outer hair cells, in rabbits and changes of those cells after acoustic exposure.The outer hair cells in rabbits were essentially similar to those of other animals. However, the distribution of organelles in the infranuclear region of these cells differed from that of others. In this region of rabbits, many small vesicles, coated vesicles, and multivesicular body were present, but there were few or no mitochondria. The hyaloplasm of this region had higher electron density than did the supranuclear region. This infranuclear region showed a marked lactatic dehydrogenase enzymatic activity but there was no succinic dehydrogenase activity.The outer hair cells of exposed rabbits showed distortions or swellings, and the sensory hairs showed blebs. Numerous lysosomes were seen in the subcuticular region of the outer hair cell, and below this region the organelles had a tendency to be sparsely distributed and the cytoplasmic matrix between them had a tendency to appear lighter than normal. In the infranuclear region, small vesicles, free ribosomes and coated vesicles decreased in number and the cytoplasmic matrix between them became lower in electron density. In this region, the lactic dehydrogenase activity decreased. These electron microscopical and histochemical changes were seen mainly in the lower half of the second turn. In the basal turn, the outer hair cells were almost normal, however occasionally it was found that in the infranuclear region, electron microscopically small vesicles were very densely packed and the cytoplasmic matrix between them appeared darker than normal, and histochemically the lactate dehydrogenase activity increased more than normal. Small vesicles were found to be preferentially distributed in the presynaptic side of the afferent nerve endings after acoustic exposure. Movement of these small vesicles led to speculations that these small vesicles have a close relationship to the afferent nerve endings.