Effects of removal of the statoacoustic ganglion complex upon the growing otocyst.

An experiment was designed to answer the question as to whether or not the neural elements of the statoacoustic ganglion complex have a trophic effect upon the histodifferentiation of the sensory structures of the embryonic mouse inner ear anlage as it develops in vitro. The embryonic inner ear anlage with associated otic mesenchyme and statoacoustic ganglion complex was excised from 11, 12, and 13-day CBA/C57 mouse embryos. The inner ear explants of each gestational age group were further divided into two groups: the first group "A" (with) statoacoustic ganglion was explanted to the organ culture system without further surgical intervention; the second group "B" (without) statoacoustic ganglion underwent further surgical manipulation during which their statoacoustic ganglion complexes were dissected away prior to explantation to in vitro. The explanted embryonic inner ears were allowed to develope in organ culture until the equivalent of gestation day 21 in vivo was reached for each group; then all cultures were fixed and histologically processed and stained by a nerve fiber stain, in combination with a stain for glucoprotein membranes. Each specimen was code labeled and scored for histodifferentiation of sensory structures. Light microscopic observations confirmed that in group "A" cultures, statoacoustic ganglion neurons and their nerve fibers were present in association with the developed sensory structures; neither ganglion cell neurons nor their nerve fibers were found to be present in the sensory structures that developed in the group "B" organ culture specimens. Quantification revealed no consistent trend of greater occurrence of any sensory structure in the groups of explants analyzed. The presence of such a trend would have signified the probable existence of a trophic effect of the statoacoustic ganglion neural elements upon development of inner ear sensory structures in the group "A" explants of the 11, 12, and 13-day embryo inner ear organ culture specimens when compared to the aganglionic group "B" cultures. Microscopic comparison of the sensory structures and their sensory hair cells that developed in the organ cultures revealed no differences in the quality of the histodifferentiation of eithergroup "A" or goup "B" explants. A base to apex pattern of histodifferentiation of the organ of Corti sensory structures, which has been described to occur in vivo, was noted to occur in the in vitro developed cochlear ducts of all of the explanted inner ears without respect to whether neural elements were present ("A") or absent ("B") during development. It was concluded from the quantification of histodifferentiation data and the above observation on the pattern of differentiation of Corti's organ that no trophic effect of neural elements of the statoacoustic ganglion complex influencing the histodifferentiation of sensory structures of 11, 12, and 13-gestation day mouse embryo inner ear explants as they differentiate in vitro could be demonstrated.