Myelination of the human spiral ganglion.

Temporal bones from adolescents and adults with healthy ears were fixed immediately after death with glutaraldehyde and processed for the electron microscopic examination without decalcification. No myelinated ganglion cells could be identified by light microscopy. Electron microscopically, we did not find any myelinated ganglion cells in the region of the spiral ganglion of 4 neonatal infants. In the temporal bones of 6 young adults (aged between 18 and 25 years), 0.3% to 0.5% of all ganglion cells were enveloped in a loose myelin coat. The spiral ganglion of two temporal bones from elderly persons (aged 61 and 63 years) contained up to 2% ganglion cells with a thin but regular myelination (up to five myelin layers). Large ganglion cells (type I) are situated in the centre of the ganglion cell complex with a palisade arrangement. Small ganglion cells (type II) are found almost exclusively in the periphery of the ganglion cell complex. There is no difference in the frequency of myelination of large and small ganglion cells. The small ganglion cells amount to about 3% of the entire ganglion cell population. Since myelination is to be observed more frequently in elderly people than in adolescents or neonates, it is possible that myelination of ganglion cells of the spiral ganglion does not play an important physiological role in humans.