Development and innervation of the paratympanic organ (Vitali organ) in chick embryos.

The paratympanic organ (Vitali organ) is a small sensory organ in the middle ear of birds. It possesses a sensory epithelium with hair cells similar to those of the inner ear. Injections of fluorescent carbocyanine tracers into the paratympanic organ of 9- to 11-day-old chick embryos labeled ganglion cells in the facial ganglia. Paratympanic nerve fibers enter the brainstem with the facial nerve but proceed to vestibular brainstem nuclei. A dorsal branch terminates in ventral areas of the cerebellum, while a ventral component projects to the descending vestibular nucleus, with some fibers turning medially into lateral parts of the medial vestibular nucleus. No fibers were labeled in the motor or sensory facial nucleus or in auditory brainstem nuclei. This projection pattern suggests a function of the paratympanic organ in equilibrium rather than audition. Projections similar to those of the paratympanic nerve have been reported for the lagenar nerve. Immunocytochemical techniques using an antiserum to gamma-aminobutyric acid (GABA) demonstrate that hair cells in the paratympanic organ develop GABA immunoreactivity at 5 days of incubation (E5), 2-4 days earlier than GABA immunoreactivity can be detected in hair cells of the inner ear, i.e. in the saccule (E6.5-7.0), the utricle (E7), the cristae (E8-9) and the cochlea (E9-9.5). Afferent fibers that are transiently GABAergic are rare in the paratympanic organ (1-2 fibers), though present from E6 to E7.5. The early onset of GABA immunoreactivity in the paratympanic organ may indicate that this organ matures (and possibly functions) earlier in ontogenetic development than its counterparts located in the inner ear. The present findings are consistent with the hypothesis that the paratympanic organ is homologous with the spiracular sense organ of fishes. The paratympanic organ of birds may represent a sense organ that is derived phylogenetically and ontogenetically from the lateral-line system.