Neuron-receptor cell interaction during development of the inner ear a heterochronic ganglion study

This experiment tests the ability of the neurons of a staged series of statoacoustic ganglia to extend neuritic processes in response to the attractant fields produced by differentiating inner ear sensory epithelia. A previous study of the developing mouse inner ear (Sher, Acta Otolaryngol. Suppl. 285: 1-77, 1971) has shown that neuritic elements of the statoacoustic ganglion begin to actively extend their processes to peripheral target sites on the 12th day of gestation (12 GD). Therefore, an aneural 12 GD otocyst was used as the target tissue source for all of the heterochronic ganglia implants. Statoacoustic ganglia were removed from 12,13,14 and 15 GD mouse inner ears and then implanted into the aganglionic 12 GD otic explants prior to explantation to in vitro. All cultures were grown for a period of 7 days in vitro. At the end of the culture period, the 12 GD otocyst explants with their implanted homo and heterochronic SAG were fixed and processed for nerve fiber staining by the Bodian technique. Specimens were examined by light microscopy and evaluated for the presence of SAG nerve fibers projecting from the implanted ganglia to both vestibular and auditory sensory receptors that developed within the explants. The results show that SAG neurons of the implanted homochronic (i.e. 12 GD SAG/12 GD OTOCYST) and all heterochronic ganglia (i.e. 13, 14 or 15 GD SAG/12 GD OTOCYST) extended neuritic processes to the areas of developing sensory epithelium of both vestibular and auditory character. Nerve fibers were present in association with sensory hair cells of cristae, maculae and organ of Corti sensory receptors. These fibers made contact with the bases of the sensory hair cells in all of the sensory structures studied. Ultrastructural studies are in progress that will determine the nature of the sensory cell/nerve fiber association in both the homo and heterochronic SAG implants of these cultures and compare them to an equivalent level of in utero development (i.e. 19 GD) of the mouse inner ear. The results of this study demonstrate that the neurons which compose the murine statoacoustic ganglion are capable of extending neuritic processes to attractant fields produced by areas of differentiating otic sensory epithelia for an extended period (i.e. 12-15 GD) during the development of this ganglion. These findings support the hypothesis that the onset and duration of the receptor generated attractant fields may act as a controlling factor in establishing patterns of innervation within the developing inner ear.