Determinants of ganglion-receptor cell interaction during development of the inner ear. A heterochronic ganglia study.

It has been suggested that inner ear sensory receptors produce attractant fields that guide neurite outgrowth from statoacoustic ganglion (SAG) neurons to appropriate target sites within the developing labyrinth. This experiment tested the temporal limitations of SAG neurons in their ability to respond to these attractant fields. Statoacoustic ganglia were excised from 12, 13, 14 and 15 gestation day (GD) mouse embryos. This temporal series of SAG was implanted into aganglionic 12 GD otocysts. All cultures were grown for 7 days in vitro, then fixed and processed for nerve fiber staining. Specimens were evaluated for the presence of neurites associated with the inner ear sensory receptors that developed within the otic explants. All of the implanted heterochronic ganglia (i.e. 13, 14 or 15 GD) as well as the homochronic (i.e. 12 GD) ganglion controls extended neurites to sensory epithelium of both vestibular and auditory character. Neurites made contact with the base of hair cells in all of the sensory structures. These findings demonstrate that SAG neurons are capable of extending processes in response to otic attractant fields for an extended period during the embryonic development of this ganglion. This observation supports the hypothesis that the onset and duration of receptor generated attractant fields may act as a controlling factor in establishing patterns of innervation within the developing inner ear.