[Continuous proliferation of supporting cells and indications for hair cell differentiation in the inner ear of adult song birds with genetic cochlear hearing loss].

Our previous investigations demonstrated that the Belgian Waterslagers (BWS) canary (Serinus canarius) was affected by an inherited sensorineural hearing loss. Compared to normal canaries of others strains, hair cell numbers in these birds were reduced on average by 30%. Since other birds are able to replace similar hair cell numbers after cochlear trauma, we investigated if BWS have the potential for supporting cell proliferation with subsequent hair cell differentiation or if they lack the repair mechanisms known to operate in other birds. In the present study the S-phase marker bromodeoxyuridine (BrdU) was used to demonstrate DNA synthesis and thus cell proliferation. We found on average six labelled nuclei per basilar papilla in BWS. This number of proliferating cells was in accordance with previous estimates of newly generated hair cells as based on the frequency of immature-appearing hair cells observed by scanning electron microscopy. We conclude that the division of supporting cells in BWS precedes the differentiation of hair cells. In contrast to BWS we found on average only one supporting cell division per day in normal canaries of other strains. However, this supporting cell proliferation in normal birds is probably not related to a loss of hair cells and does not lead to the differentiation of new hair cells. Our data indicate that differentiation of hair cells after supporting cell division occurs only if the rate of supporting cell proliferation is increased above the normal low level (probably by the loss of hair cells). Since BWS do not repair their basilar papilla despite a 30% hair cell loss (as compared to normal canaries) although they continuously produce new hair cells, we suggest that the regulation of the regeneration process is abnormal.