CONTINUOUS PROLIFERATION OF SUPPORTING CELLS AND EVIDENCE FOR DIFFERENTIATION OF HAIR-CELLS IN THE INNER-EAR OF AN ADULT SONG BIRD WITH GENETIC HEARING-LOSS

Our previous investigations demonstrated that the Belgian Waterslagers (BWS) canary (Serinus canarius) was affected by an inherited sensorin eural hearing loss. Compared to normal canaries of other strains, hair cell numbers in these birds were reduced on average by 30%. Since oth er 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 lac k the repair mechanisms known to operate in other birds. In the presen t study the S-phase marker bromodeoxyuridine (BrdU) was used to demons trate DNA synthesis and thus cell proliferation. We found on average s ix labelled nuclei per basilar papilla in BWS. This number of prolifer ating cells was in accordance with previous estimates of newly generat ed hair cells as based on the frequency of immature-appearing hair cel ls observed by scanning electron microscopy. We conclude that the divi sion of supporting cells in BWS precedes the differentiation of hair c ells. In contrast to BWS we found on average only one supporting cell division per day in normal canaries of other strains. However, this su pporting 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 p roliferation is increased above the normal low level (probably by the loss of hair cells). Since BWS do not repair their basilar papilla des pite a 30% hair cell loss (as compared to normal canaries) although th ey continuously produce new hair cells, we suggest that the regulation of the regeneration process is abnormal.