Persistence of synaptic bodies in saccular hair cells of senescent mice

Age-dependent structural reorganization of utricular ribbon synapses

In mammals, spatial orientation is synaptically-encoded by sensory hair cells of the vestibular labyrinth. Vestibular hair cells (VHCs) harbor synaptic ribbons at their presynaptic active zones (AZs), which play a critical role in molecular scaffolding and facilitate synaptic release and vesicular replenishment. With advancing age, the prevalence of vestibular deficits increases; yet, the underlying mechanisms are not well understood and the possible accompanying morphological changes in the VHC synapses have not yet been systematically examined. We investigated the effects of maturation and aging on the ultrastructure of the ribbon-type AZs in murine utricles using various electron microscopic techniques and combined them with confocal and super-resolution light microscopy as well as metabolic imaging up to 1 year of age. In older animals, we detected predominantly in type I VHCs the formation of floating ribbon clusters, mostly consisting of newly synthesized ribbon material. Our findings suggest that VHC ribbon-type AZs undergo dramatic structural alterations upon aging.

Morphology of Synapses at the Base of Hair Cells in the Organ of Corti of the Chimpanzee

The synaptic morphology of inner and outer hair cells of the organ of Corti of the chimpanzee was evaluated by serial section electron microscopy. The morphology of nerve terminals and synapses at both sites was very similar to that of human and other mammalian species. Two types of nerve terminals, nonvesiculated and vesiculated, with distinct synaptic morphology were found. In addition, between some nonvesiculated endings and outer hair cells, a reciprocal synaptic relationship was seen. In such terminals there was morphologic evidence for transmission from hair cell to neuron and from neuron to hair cell between a single neuron and an outer hair cell.

The correlated blanching of synaptic bodies and reduction in afferent firing rates caused by transmitter-depleting agents in the frog semicircular canal

Synaptic bodies (SBs) associated with rings of synaptic vesicles and well-defined, pre- and post-synaptic membrane structures are indicators of maturity in most hair cell-afferent nerve junctions. The role of the SBs remains elusive despite several experiments showing that they may be involved in storage of neurotransmitter. Our results demonstrate that SBs of the adult posterior semicircular canal (SCC) cristae hair cells become less electron dense following incubation of the SCC with the transmitter-depleting drug tetrabenazine (TBZ). Objective quantification and comparison of the densities of the SBs in untreated and TBZ-treated frog SCC demonstrated that TBZ significantly decreased the electron density of SBs. This reduction in electron density was accompanied by a reduction in firing rates of afferent fibers innervating the posterior SCC. A second transmitter-depleting drug, guanethidine, previously shown to reduce the electron density of hair cell SBs, also reduced the firing rates of afferent fibers innervating the posterior SCC. In contrast, the electron density of dense granules (DG), similar in size and shape to synaptic bodies (SB) in hair cells, did not change after incubation in TBZ, thus indicating that granules and SBs are not similar in regard to their electron density. The role of SBs in synaptic transmission and the transmitter, if any, stored in the SBs remain unknown. Nonetheless, the association of the lessening of electron density with a reduction in afferent firing rate provides impetus for the further investigation of the SB's role in neurotransmission.

An ultrastructural study on vestibular sensory cells in a new-mutant mouse

The ultrastructural characteristics of the vestibular epithelium and light microscopical study of the central nervous system of a strain of new-mutant mice were analyzed. For the vestibular study, we used 72 homozygotes with ages ranging from 10 days to 18 months. The most striking findings observed in these mice were the disarray of the stereocilia of the utricular and saccular maculae and disintegration of the saccular otoconia. Many hair cells displayed abnormality of the stereocilia such as reduced number, disorganized distribution, and giant cilia, although the hair cell cytoplasm, including the nerve terminals, became fully developed. Demineralization of the saccular otoconia was age dependent, and a complete loss of the saccular hair cells was demonstrated. In conjunction with the disarray of the outer hair cells of the cochlea, morphological manifestation of the gene abnormality of these mice was related to immaturation of the stereociliary tufts. Because no morphological abnormality was observed in the central nervous system, the abnormal behavior in these mice was primarily correlated with morphological abnormalities of the vestibule.

Volume fraction and ultrastructure of age pigment in the saccular epithelium of old mice

Age pigment in the sensory and supporting cells was a prominent characteristic distinguishing old saccules from young. However, the pigment was not distributed uniformly throughout the sensory epithelium but displayed cell-specific patterns of accumulation. The highest cytoplasmic volume density was in the old supporting cells followed by type I hair cells and then type II hair cells. The most common form of pigmented inclusion seen in old type I hair cells was a cluster of granules resembling melanin. This form was never seen in type II hair cells or supporting cells where a form containing a lipid-like droplet was prevalent. The differences in the amount of age pigment and the forms accumulated probably reflects metabolic differences between the three cell types.