Origin of Neuroblasts in the Avian Otic Placode and Their Distributions in the Acoustic and Vestibular Ganglia

SIMPLE SUMMARY: A key question in embryonic development is to determine the molecular and cellular mechanisms involved in the early specification and differentiation of neuroblast populations and the final disposition of mature neurons in the central and peripheral nervous system. These embryonic developmental events are governed by genetic factors intrinsic to each neuroblast subpopulation and by diffusible signals from the tissue environment. The inner ear is an attractive model for building a knowledge base in this field, as it is accessible to manipulation and undergoes dynamic self-organization, with significant morphogenetic changes. Using the chimeric chick/quail model, which provides stable cell markers of quail cells grafted into a chick tissue environment, long after transplantation experiments, our results clearly show that, in birds, there does not seem to be a strict segregation of acoustic and vestibular ganglion neurons in the otic placode. Segregation of both otic neuroblast populations occurs from the otic cup stage onwards, especially in the otic vesicle stage, once the clonal restriction compartments separating the distinct cell lineages have been defined. Further descriptive and experimental studies are needed to better characterize the gene expression mosaic of transcription factors and the signaling pathway governing neuronal fate acquisition in the early otic placode. ABSTRACT: The inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. This intricate sensory organ originates from the otic placode, which generates the sensory elements of the membranous labyrinth, as well as all the ganglionic neuronal precursors. How auditory and vestibular neurons establish their fate identities remains to be determined. Their topological origin in the incipient otic placode could provide positional information before they migrate, to later segregate in specific portions of the acoustic and vestibular ganglia. To address this question, transplants of small portions of the avian otic placode were performed according to our previous fate map study, using the quail/chick chimeric graft model. All grafts taking small areas of the neurogenic placodal domain contributed neuroblasts to both acoustic and vestibular ganglia. A differential distribution of otic neurons in the anterior and posterior lobes of the vestibular ganglion, as well as in the proximal, intermediate, and distal portions of the acoustic ganglion, was found. Our results clearly show that, in birds, there does not seem to be a strict segregation of acoustic and vestibular neurons in the incipient otic placode.