| The transduction apparatus in hair cells consists of the transduction channel, the tip link, which conveys force to the channel, myosin motors, which regulate the tension in the tip link, and a plasma membrane Ca 2+-ATPase, which modulates the microdomain Ca2+ concentration to optimize transduction currents. This dissertation describes the embryonic development of the transduction apparatus in the chicken basilar papilla (cochlea), where the morphological progression of hair-bundle maturation has been described. On day 10.5, the stereocilia initiate elongation and begin to form the staircase. Microphonic responses to sound can be elicited in chicks as young as 10-11 days.; This study was aimed to: (1) identify the molecules constituting the transduction apparatus, (2) assess their expression during development, and (3) investigate how the components are regulated to produce a functional transduction apparatus. In vitro assays of chicken basilar papilla explants were used to examine the mechanisms of transduction-apparatus assembly. Hair cell labeling with FM1-43, which is thought to permeate the transduction channels, whole-cell recording of mechanotransduction currents, the hair bundle's structure by scanning electron microscopy and immunofluorescence labeling for myosin 1c, myosin 7a and PMCA2 of the embryonic chick papilla were performed to define mechanisms and sequences of transduction apparatus assembly. These experiments demonstrate that the functional transduction channels were present in embryonic chicken inner ear as early as E12 demonstrated as hair cell loading of FM 1-43 and whole-cell recording of transduction currents. Immunofluorescence labeling also demonstrates a precise temporal juxtaposition of myosin 1c and PMCA2 with the transduction currents. However, marked refinement of the transduction machinery was observed beyond E16. For example, myosin 1c appeared diffusely in hair bundles from E12-E16, but subsequently consolidated into punctate pattern at the tips of the hair bundle.; Recent molecular identification of the tip link and transduction channel will extend this knowledge of the precisely defined onset of sensory transduction in the inner ear. These results presented here draw attention to a critical period (E16) in the normal development of sensory transduction, which may lead to a better understanding of the mechanisms that regulate transduction and adaptation in a mature bundle. |
