| Organ/tissue induction has been a central topic of developmental biology. To date, little is known about the signaling mechanisms that instruct the specification of different fields of organ primordia. In a manner similar to the formation of other sensory organs, the vertebrate inner ear arises from the surface ectoderm on either side of the developing hindbrain or neural ectoderm. The overall goal of this study was to investigate the signaling inductive mechanisms used during the specification of otic epithelium and test the adjacent neural ectoderm for its ability to act as an ear inducer.; Micro-surgeries of H-H stage 10 (Hamburger & Hamilton, 1951) neural ectoderm, either by: (1) 180 degree rotation of hindbrain rhombomeres 4 to 6 or (2) transplantation of rhombomere 4 to the developing midbrain, induced the formation of ectopic vesicles. These vesicles were identified as otic vesicles by the expression of an otic tissue specific gene Nkx5.1. During normal inner ear initiation, a group of otic genes, Pax-2, Lmx-1, Sox-3, Bmp-7, and Notch-1 are expressed in the presumptive otic primordial. These genes are expressed throughout the formation of otic vesicles with an altered temporal sequence. Their expression domains are restricted to the ectoderm at the level of the otic placode but not in the underlying mesoderm or endoderm. All these genes were also detected in the ectopic vesicles resulting from both kinds of operations, but not in the same temporal pattern.; The otic field has been reported to become restricted to the ectoderm of the hindbrain region by the late neurula stage. In this study, the transplantation of rhombomere 4 at stage 10 into midbrain induced the re-specification of otic epithelium at the midbrain level beyond the previously described developmental stage. This suggests the existence of a broader field of otic competent ectoderm at late neurula stage. This study also showed all otic genes of interest could be induced in an indifferent site, the midbrain, by a neural inducer, rhombomere 4. It further demonstrated that neural ectoderm plays a major role in the signaling sequence of inner ear induction. |
