Age-dependent effects of a pure tone trauma in the chick basilar papilla: evidence for a development of the tonotopic organization.

58 chicks or chick embryos were continuously exposed during 12 h on either embryonic day 18 or 20 or on post-hatching days 1, 10, 20 or 30 to 1.5 kHz pure tone at an intensity of 125 dB SPL. After a 20- or 30-day survival time, audiograms were recorded and then the basilar papillae were prepared for scanning electron microscopy. The frequency of maximum threshold elevation was seen at about 4 kHz when the chicks were exposed to the traumatic tone at embryonic day 18. It was shifted toward lower frequencies when the exposure was done at later stages. This shifting ended when the animals were exposed one day after hatching. After this stage, the maximum threshold elevation stabilized about one octave above the frequency of the traumatic tone. The position of maximum anatomical damage was located at 29.14% of the total length of the basilar papilla measured from the base when the exposure was done at embryonic day 20. It was shifted to 37% when the chicks were exposed one day after hatching or later. These results are in good agreement with recent hypotheses on development of the place principle. This development change seems to end at post-natal day 1 which also corresponds to the end of the anatomical and functional maturation of the basilar papilla.