Progenitor cell cycling during hair cell regeneration in the vestibular and auditory epithelia of the chick

We investigated nucleotide-labeling patterns during ongoing hair cell regen eration in the avian vestibular epithelium and during drug-induced regenera tion in the avian auditory epithelium. For utricle experiments, post-hatch chicks received an injection of bromodeoxyuridine (BrdU) and were allowed t o survive from 2 hours to 110 days after the injection. Utricles were fixed and immunoreacted to detect BrdU. The number of BrdU-labeled nuclei in the hair cell and support cell layers of the utricular sensory epithelium chan ges significantly between 2 hours and 110 days post-BrdU. At 2 hours, most labeled cells are isolated, while by 5-10 days, the majority of labeled cel ls are organized in pairs that are most frequently composed of a hair cell and a support cell. Pairs of labeled cells are seen as late as 110 days. Cl usters of more than 3 labeled cells are uncommon at all time-points. The to tal number of labeled cells increases approximately 1.5-fold between 5 and 60 days post-BrdU. This increase is due primarily to a rise in the number o f labeled support cells, and it is likely that it represents additional rou nds of division by a subset of cells that were labeled at the time of the B rdU injection. There is a significant decrease in labeled nuclei in the hai r cell layer between 60 and 110 days post-BrdU, suggesting that hair cells die during this period. To investigate support cell recycling in the drug-d amaged auditory epithelium, we examined nucleotide double labeling after se parate injections of BrdU and tritiated thymidine. A small number of suppor t cells that incorporate BrdU administered at 3 days post-gentamicin treatm ent also label with tritiated thymidine administered between 17 and 38 hour s later. We conclude that a small population of support cells recycles duri ng regeneration in both the normal utricle and the drug-damaged basilar pap illa.