CHANGING SPATIAL PATTERNS OF DNA-REPLICATION IN THE NOISE-DAMAGED CHICK COCHLEA

The purpose of the present study was to examine the spatio-temporal pa ttern of cell proliferation in the chick cochlea in response to the se nsory hair cell loss induced by a 1.5 kHz pure tone at 120 dB SPL (1 d B=20 muPa) for 48 h. DNA replication was evaluated with the bromodeoxy uridine (BrdU) pulse-fix technique. One group of birds was given multi ple injections of BrdU (50 mg/kg) over a period of 8 h at various star ting times during or after the exposure. Afterwards, their cochleas we re removed and processed as whole mounts for BrdU immunohistochemistry . The cochleas of a second group of acoustically traumatized chicks we re evaluated by scanning electron microscopy in order to determine the spatio-temporal pattern of hair cell loss. Hair cell loss was first o bserved 12 h after the start of the exposure and DNA replication start ed near the inferior edge of the hair cell lesion 24-32 h after the st art of the exposure, i.e. 12-20 h after the first sign of hair cell lo ss. The site of hair cell loss and DNA replication shifted toward the superior edge of the basilar papilla as the exposure continued. The ra te of DNA replication accelerated and reached its peak near the end of the 48 h exposure. The estimated latency of cell proliferation after hair cell loss was faster and the duration of DNA replication shorter than that observed in other sensory systems. The spatio-temporal patte rn of DNA replication follows the spatio-temporal gradient of hair cel l loss, suggesting that cell proliferation is triggered by hair cell l oss itself rather than by intrinsic positional cues or gradients.