AXIAL SPECIFICATION FOR SENSORY ORGANS VERSUS NONSENSORY STRUCTURES OF THE CHICKEN INNER-EAR

A mature inner ear is a complex labyrinth containing multiple sensory organs and nonsensory structures in a fixed configuration. Any perturb ation in the structure of the labyrinth will undoubtedly lead to funct ional deficits. Therefore, it is important to understand molecularly h ow and when the position of each inner ear component is determined dur ing development, To address this issue, each axis of the otocyst (embr yonic day 2.5, E2.5, stage 16-17) was changed systematically at an age when axial information of the inner ear is predicted to be fixed base d on gene expression patterns, Transplanted inner ears were analyzed a t E4.5 for gene expression of BMP4 (bone morphogenetic protein), SOHo- 1 (sensory organ homeobox-l), Otx1 (cognate of Drosophila orthodenticl e gene),p75NGFR (nerve growth factor receptor) and Msx1 (muscle segmen t homeobox), or at E9 for their gross anatomy and sensory organ format ion, Our results showed that axial specification in the chick inner ea r occurs later than expected and patterning of sensory organs in the i nner ear was first specified along the anterior/posterior (A/P) axis, followed by the dorsal/ventral (D/V) axis, Whereas the A/P axis of the sensory organs was fixed at the time of transplantation, the A/P axis for most non-sensory structures was not and was able to be re-specifi ed according to the new axial information from the host. The D/V axis for the inner ear was not fixed at the time of transplantation. The as ynchronous specification of the A/P and D/V axes of the chick inner ea r suggests that sensory organ formation is a multi-step phenomenon, ra ther than a single inductive event.