Persistence of synaptic bodies in saccular hair cells of senescent mice

The morphology of the afferent synapse in type I (HCI) and type II (HCII) saccular hair cells was compared in juvenile and very old C57BL/6NNia mice. Normal membrane specializations and normal synaptic body (SB) form and organization were found to continue well beyond the expected life span. Although SB were found less frequently in HCI than in HCII, they were as readily found in old saccules as in juveniles. The elongated rod was the most common form in HCI and the spheroid was the most common in HCII. Large clusters of SB (greater than 5), found in both juvenile and old saccules, were unique to HCI. SB in HCII were generally found singly. A range of forms, including hollow and striated dense bodies, also occurred in both juvenile and very old saccules. Singular or clustered SB were found free in the cytoplasm as well as engaged at the membrane. No SB were seen at sites other than nerve terminal appositions. In rare instances a SB was observed engaged at a site that appeared to be opposite an efferent fiber.