FIMBRIN EXPRESSION IN THE DEVELOPING RAT COCHLEA

The expression of fimbrin in the developing rat cochlea was analyzed u sing an immunohistochemical technique with fimbrin antibody. The cochl ea displayed temporal and lateral-longitudinal gradients for fimbrin e xpression during development. Fimbrin immunoreactivity first appeared in the inner hair cell stereocilia of the basal turn on the first gest ational day studied (day 18). At birth, both inner (IHC) and outer hai r cell (OHC) stereocilia of the basal turn showed positive labeling wi th fimbrin antibody. The progression of appearance was always from IHC s to OHCs and fimbrin immunostaining appeared in the apical hair cells by postnatal day 6. Immunostaining was restricted to stereocilia and the cuticular plate, and no immunoreactivity was observed in neighbori ng structures of the epithelium. Double labeling using both fimbrin an tibody and phalloidin binding revealed similar chronological expressio n from the earliest stage studied. Increasing fimbrin immunoreactivity was observed in hair cells until late postnatal and adult stages. Thi s study suggests that fimbrin is expressed with F-actin during develop ment and fimbrin together with actin may constitute the two basic mole cules that participate in stereocilia formation. We speculate that fim brin may help maintain the parallel growth of actin filaments within t he stereocilia. These data additionally support previous findings that hair cell maturation occurs from the base to the apex and from IHCs t o OHCs.