Morphological changes in the cochlea of the mouse after the onset of hearing

The cochleae of 5, 10, 12 and 15 day old mice and of adults (Mus musculus, strain NMRI) were studied by light and electron microscopy. In each case the same part of the organ of Corti (2.8-3.3 mm from the helicotrema) was examined. The results were correlated with the development of auditory thresholds (in mice of the same strain) obtained by Ehret (Ehret, G. (1971) J. Am. Audiol. Soc. 1, 179-184). It was demonstrated that morphological development of the organ of Corti is by no means complete at the onset of behavioural responses to acoustic stimuli. After this event the following morphological changes occur: (1) The basilar membrane filaments stain more intensely and the tympanic cover layer is greatly reduced in thickness and almost completely disappears. (2) The filaments of the pillar cells stain more intensely and apparently increase in number, and the angle between the outer and inner pillar cells increases so that the cross-sectional area of the tunnel of Corti expands. (3) Hook-shaped connections (marginal pillars) between the reticular membrane and the tectorial membrane disappear. The consequences of these morphological changes for the cochlear mechanics are discussed, especially in respect to the increase of sensitivity of hearing.