Prevalence and ultrastructural morphology of axosomatic synapses on spiralganglion cells in humans of different ages

Axosomatic synapses were found on human spiral ganglion cells (HSGCs). Ultr astructural characterization and calculation of the prevalence of these syn apses were performed by electron microscopic semi-serial sections of both t ype I and type II HSGCs, in specimens from subjects of ages 1 day, 14 days, 21 years and 51 years. Synapses on type I HSGCs were extremely rare. In co ntrast, axosomatic synapses were present on approximately 50% of type II HS GCs of a young adult. This prevalence seemed to vary by age. Thus, no synap ses were found in a I-day old neonate, few in a 14-day old, and on approxim ately 15% of the type II SGCs from a 51-year old specimen. The origin of th e nerve fibers synapsing on the type II HSGCs could not be determined. In v iew of the fact that some of the fibers projected from the intraganglionic spiral bundle, which is known to contain olivocochlear efferents, these fib ers may represent an efferent pathway to the spiral ganglion. However, sinc e there was morphological evidence of more than one type of nerve fiber syn apsing on type II HSGCs, other neural origins must be considered. Although the physiological function of these synapses is unknown, they may mediate p re-synaptic neural modulation of the type II HSGCs at the level of the spir al ganglion. (C) 2000 Elsevier Science B.V. All rights reserved.