Persistence of synaptic bodies in saccular hair cells of senescent mice.

The morphology of the afferent synapse in type I (HC I) and type II (HC II) 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 HC I than in HC II, they were as readily found in old saccules as in juveniles. The elongated rod was the most common form in HC I and the spheroid was the most common in HC II. Large clusters of SB (>5), found in both juvenile and old saccules, were unique to HC I. SB in HC II 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.