Loss of embryonic vestibular ganglion neurons in a chick model of congenital vestibular disorders.

People with syndromic, congenital vestibular disorders (CVDs) experience severe challenges to maintain posture and balance throughout life due to malformation of the inner ear into a sac-like structure with missing or truncated semicircular canals. How inner ear malformation affects the remaining vestibular neural circuitry is unknown, although decreased number of vestibular ganglion (VG) neurons is reported in CVD children. Our first step here was to determine whether VG neuron number is reduced before birth using our chick model for CVDs at embryonic day 13 (E13), a stage 2/3 of the way through gestation with hatching at E21. The lab implemented the ARO chick model by Anterior-posterior Rotation 180° of the Otocyst surgically on one side in E2 chick embryos which resulted in a sac-like inner ear, the ARO chick. Estimates of VG neuron numbers, ganglion volume, and 3-D ganglion configuration were made first with classical Nissl-stained, transverse tissue sections, and second on whole-mount preparations of biocytin Alexa Fluor-labeled VG neurons in E13 normal and ARO chicks. In both preparations, VG neuron number was reduced significantly on the rotated side of ARO chicks compared to normal chicks. VG neuron cell bodies on the rotated side failed to form compact anterior and posterior ganglionic masses typical of the normal VG. Reduced ganglion cell number and abnormal ganglion topography characterized this prenatal stage, laying down a foundation for defective central vestibular neural circuitry underlying the abnormal vestibular behaviors observed postnatally.