Prevalence and ultrastructural morphology of axosomatic synapses on spiral ganglion cells in humans of different ages.
Axosomatic synapses were found on human spiral ganglion cells (HSGCs). Ultrastructural characterization and calculation of the prevalence of these synapses were performed by electron microscopic semi-serial sections of both type 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 contrast, axosomatic synapses were present on approximately 50% of type II HSGCs of a young adult. This prevalence seemed to vary by age. Thus, no synapses were found in a 1-day old neonate, few in a 14-day old, and on approximately 15% of the type II SGCs from a 51-year old specimen. The origin of the nerve fibers synapsing on the type II HSGCs could not be determined. In view of the fact that some of the fibers projected from the intraganglionic spiral bundle, which is known to contain olivocochlear efferents, these fibers may represent an efferent pathway to the spiral ganglion. However, since there was morphological evidence of more than one type of nerve fiber synapsing on type II HSGCs, other neural origins must be considered. Although the physiological function of these synapses is unknown, they may mediate pre-synaptic neural modulation of the type II HSGCs at the level of the spiral ganglion.
Thiers
F A
FA
Department of Otolaryngology, Massachusetts Eye and Ear Infirmary, Boston, MA 02114, USA.
Burgess
B J
BJ
Nadol
J B
JB
D000328:Adult; D000375:Aging / Q000502:physiology*; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D008854:Microscopy, Electron; D008875:Middle Aged; D013136:Spiral Ganglion / Q000166:cytology*; D013569:Synapses / Q000502:physiology* / Q000648:ultrastructure*