Structural changes in the vestibular epithelia in elderly monkeys and humans

Progress in protecting vestibular hair cells

Vestibular hair cells are mechanosensory receptors that are capable of detecting changes in head position and thereby allow animals to maintain their posture and coordinate their movement. Vestibular hair cells are susceptible to ototoxic drugs, aging, and genetic factors that can lead to permanent vestibular dysfunction. Vestibular dysfunction mainly results from the injury of hair cells, which are located in the vestibular sensory epithelium. This review summarizes the mechanisms of different factors causing vestibular hair cell damage and therapeutic strategies to protect vestibular hair cells.

Standard reference values of the postural control in healthy young female adults in Germany: an observational study

OBJECTIVE

Many people of all ages suffer from vertigo due to different reasons. The comparison of patient data with standard values can highlight deteriorations or changes in postural control and thus indicate, for example, an increased risk of falling. Our aim is to measure standard values for the postural control of young healthy women.

DESIGN

Observational study.

SETTING

Institute of Occupational Medicine, Social Medicine and Environmental Medicine, Goethe-University Frankfurt/Main.

PARTICIPANTS

106 healthy German female subjects aged between 21 and 30 years (25±2.7 years) were measured. Their average body mass index (BMI) was 21.1±2.61 kg/m².

OUTCOME MEASURES

A pressure measuring platform was used to measure the weight distribution and postural sway in habitual standing. Median, tolerance range and CI were calculated.

RESULTS

Height, weight and BMI are comparable to the average young German female population. The load distribution between right and left foot was 49.91%:50.09%. The forefoot was less loaded than the rear foot (33.3%:66.67%). The right rear foot carried most of the body weight (34.34%). The average body sway was 9.50 mm in the frontal and 13.00 mm in the sagittal plane.

CONCLUSIONS

Standard values for the postural control of the women aged 21-30 years correlate with the already collected data of healthy subjects and can therefore be described as representative. The standard values enable diagnosing and treating impaired balance.

Effects of short-term and prolonged bed rest on the vestibulosympathetic reflex

The mechanism(s) for post-bed rest (BR) orthostatic intolerance is equivocal. The vestibulosympathetic reflex contributes to postural blood pressure regulation. It was hypothesized that muscle sympathetic nerve responses to otolith stimulation would be attenuated by prolonged head-down BR. Arterial blood pressure, heart rate, muscle sympathetic nerve activity (MSNA), and peripheral vascular conductance were measured during head-down rotation (HDR; otolith organ stimulation) in the prone posture before and after short-duration (24 h; n = 22) and prolonged (36 ± 1 day; n = 8) BR. Head-up tilt at 80° was performed to assess orthostatic tolerance. After short-duration BR, MSNA responses to HDR were preserved (Δ5 ± 1 bursts/min, Δ53 ± 13% burst frequency, Δ65 ± 13% total activity; P < 0.001). After prolonged BR, MSNA responses to HDR were attenuated ∼50%. MSNA increased by Δ8 ± 2 vs. Δ3 ± 2 bursts/min and Δ83 ± 12 vs. Δ34 ± 22% total activity during HDR before and after prolonged BR, respectively. Moreover, these results were observed in three subjects tested again after 75 ± 1 days of BR. This reduction in MSNA responses to otolith organ stimulation at 5 wk occurred with reductions in head-up tilt duration. These results indicate that prolonged BR (∼5 wk) unlike short-term BR (24 h) attenuates the vestibulosympathetic reflex and possibly contributes to orthostatic intolerance following BR in humans. These results suggest a novel mechanism in the development of orthostatic intolerance in humans.

Aging, opioid-receptor agonists and antagonists, and the vestibulosympathetic reflex in humans

Animal studies indicate that opioids inhibit the firing rate of vestibular neurons, which are important in mediating the vestibulosympathetic reflex. Furthermore, this inhibition appears to be greater in more mature rats. In the present study, we tested the hypotheses that opioids inhibit the vestibulosympathetic reflex in humans and that endogenous opioids contribute to the age-related impairment of the vestibulosympathetic reflex. These hypotheses were tested by measuring muscle sympathetic nerve activity (MSNA), arterial blood pressure, and heart rate responses to otolith organ engagement during head-down rotation (HDR) in young (24 ± 2 yr old) and older (63 ± 2 yr) subjects before and after administration of either an opioid-receptor antagonist (16 mg naloxone in 9 young and 8 older subjects) or an opioid-receptor agonist (60 mg codeine in 7 young and 7 older subjects). Naloxone did not augment the reflex increase in MSNA during HDR in young (Δ7 ± 2 vs. Δ4 ± 2 bursts/min and Δ81 ± 23 vs. Δ60 ± 24% change in burst frequency and total MSNA before and after naloxone, respectively) or older subjects (Δ2 ± 2 vs. Δ1 ± 2 burst/min and Δ8 ± 7 vs. Δ8 ± 9% before and after naloxone). Similarly, codeine did not attenuate the increase in MSNA during HDR in young (Δ8 ± 1 vs. Δ7 ± 2 bursts/min and Δ53 ± 4 vs. Δ64 ± 16% before and after codeine) or older subjects (Δ6 ± 4 vs. Δ3 ± 3 bursts/min and Δ38 ± 21 vs. Δ33 ± 20%). Mean arterial blood pressure and heart rate responses to HDR were not altered by either naloxone or codeine. These data do not provide experimental support for the concept that opioids modulate the vestibulosympathetic reflex in humans. Moreover, endogenous opioids do not appear to contribute the age-associated impairment of the vestibulosympathetic reflex.

Aging and the human vestibular nuclei: morphometric analysis

The data concerning the effects of age on the brainstem are scarce and few works are devoted to the human vestibular nuclear complex. The study of the effects of aging in the vestibular nuclei could have clinical interest due to the high prevalence of balance control and gait problems in the elderly. We have used in this work eight human brainstems of different ages sectioned and stained by the formaldehyde-thionin technique. The neuron's profiles were drawn with a camera lucida and Abercrombie's method was used to estimate the total number of neurons. The test of Kolmogorov-Smirnov with the correction of Lilliefors was used to evaluate the fit of our data to a normal distribution and a regression analysis was done to determine if the variation of our data with age was statistically significant. Aging does not affect the volume or length of the vestibular nuclear complex. Our results clearly show that neuronal loss occurs with aging in the descending (DVN), medial (MVN), and lateral (LVN) vestibular nuclei, but not in the superior (SVN). There are changes in the proportions of neurons of different sizes but they are not statistically significant. The neuronal loss could be related with the problems that elderly people have to compensate unilateral vestibular lesions and the alterations of the vestibulospinal reflexes. The preservation of SVN neurons can explain why vestibulo-ocular reflexes are compensated after unilateral vestibular injuries.

Light and electron microscopic study of vestibular sensory epithelia in 17 cases with acoustic neurinoma

Vestibular sensory epithelia were studied histologically and ultrastructurally in 17 cases with acoustic neurinoma (AN). The superior vestibular nerve (SVN) near the fundus was also histologically studied in 5 of these 17 cases. Histologically, severe fibrotic change of the vestibular sensory epithelia was found in 1 case, and severe fibrotic change of the SVN was also found in this case. Intra-epithelial cysts were found at the edge of the utricular sensory epithelia in 2 cases. The cysts consisted of the transitional epithelium and were filled with the darkly stained substances. Ultrastructurally, abnormal accumulation of darkly stained masses within the nerve ending and abnormal accumulation of the fibrillar material below the normal basal lamina were frequently observed. These morphological changes described above were regarded as the pathological changes due to AN. In addition, the relationship among the histologic changes, ultrastructural changes and clinical data were fully investigated, and some histologic and ultrastructural changes were regarded as artifacts or age-related changes.

Schwann cells associated with vestibular ganglion cells in the aging rat

The purpose of this study was to quantify any age-related change in the components of Schwann cells associated with vestibular ganglion cells: adperikaryonal Schwann cell cytoplasm, myelin bubbles, compact myelin, Schwann cell lipofuscin, or mitochondria in addition to extracellular intraganglionic space. Studies were carried out in young (Y, 3 to 5 months of age, N = 6), old (0, 24 to 26 months of age, N = 3), and very old (VO, 28 to 31 months of age, N = 6) female Wistar rats using point counting stereologic techniques. Recent reports have demonstrated age-related increases in the peripheral nerve counterparts to the following entities: adaxonal Schwann cell cytoplasm, myelin bubble, endoneurial space, and a decrease in Schwann cell mitochondria. Our results indicate no age-related change in the volume fraction of extracellular intraganglionic space (Y, 0.179 +/- 0.04; O, 0.174 +/- 0.049; VO, 0.205 +/- 0.043), adperikaryonal Schwann cell cytoplasm (Y, 0.026 +/- 0.008; O, 0.019 +/- 0.003; VO; 0.028 +/- 0.007), myelin bubble (Y, 0.004 +/- 0.003; O, 0.011 +/- 0.009; VO, 0.006 +/- 0.002), Schwann cell lipofuscin (Y, 0.002 +/- 0.001; O, 0.004 +/- 0.003; VO, 0.002 +/- 0.001), or mitochondria (Y, 0.194 +/- 0.019; O, 0.208 +/- 0.024; VO, 0.205 +/- 0.04). The only age-related change was an increase in compact myelin (Y, 0.006 +/- 0.001; O, 0.008 +/- 0.006; VO, 0.011 +/- 0.004). These findings differ from those of the peripheral nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructural aspects of the human peripheral vestibular system

The sensory epithelia from the membranous labyrinths of 4 patients were examined by transmission electron microscopy. The distribution ratio of type 1 to type 2 cells was recorded, compared for each sensory area and correlated with age. An ultrastructural assessment confirmed generalized patterns consistent with autolytic and preparation artefact, viz. calyceal dilatation, cytoplasmic protrusions and some sensory hair loss. More specific features, i.e. lipofuscin accumulation, membrane-bound inclusions and neural degeneration were consistent with pre-mortem pathological change. In 2 patients, total nerve fibre counts of 15,766 and 19,741 were obtained. Total fibre counts correlated with the sensory cell density of the innervated areas. Differential counts of the superior and inferior vestibular nerves suggested that there was a reduced number of fibres in the superior division of both patients, in comparison with established normative data. Morphometric analysis of the constituent fibre diameters revealed a skewed distribution with a modal value of 6 microns for both patients.

Ultrastructural study of calycine synaptic endings of colossal vestibular fibers in the cristae ampullares of the developing chick

Bipolar vestibular ganglion cells give rise to the colossal vestibular fibers in the chicken. These fibers form the largest calycine endings in the cristae ampullares and also the spoon endings in the tangential vestibular nucleus of the medulla oblongata. Because these synaptic endings are two of the largest and most distinctive in the vertebrate nervous system, they are especially suitable for comparisons of the development of synapses and synaptic endings of a specific cell type. An ultrastructural study of the spoon endings and quantitative data on their synapses were available from material of 15-day-old chick embryos, hatchlings, and 3-yr-old chickens. Here we provide similar data on the large calyces. Briefly, large calyces exhibited no ultrastructural changes corresponding to the changes in the spoon endings apparent when they retract from their target cell surfaces around hatching time. However, the concentration of the ribbon synapses at the large calyces decreased around hatching, when the concentration of the chemical synapses at the spoon endings declined. Moreover, the concentration of the ribbon synapses at the large calyces corresponded closely to the concentration of the chemical synapses at the spoon endings at the same age. Thus at the developmental ages studied, there were similar concentrations in the peripheral and central synapses formed at two different synaptic endings, both derived from one cell type and participating in the same neural pathway. These findings raise the issue of how synapses are regulated locally, but also suggest the possibility for central-peripheral interactions to produce correlative changes in parallel.

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.

Morphometric analysis and fine structure of the vestibular epithelium of aged C57BL/6NNia mice

The vestibular organs of young and very old C57BL/6NNia (B6) mice were compared by light and electron microscopy. Hair cell density decreased an average of 14% in the utricle, 19% in the saccule and posterior crista, 23% in the horizontal crista, and 24% in the anterior crista. Hair cell size remained the same throughout the mouse's life span as did the ratio of Type I to Type II hair cells. The most apparent sign of advanced age was dense inclusions found in sensory and supporting cells. Although small inclusions were present at five weeks, by 29 months, additional, larger forms appeared. An unusual melanin-like form was characteristic of old Type I hair cells. Synaptic morphology and synaptic bodies were well preserved even in very old B6 mice. Elongated bars were common in Type I hair cells and spheroid synaptic bodies were the most common form in Type II hair cells. Large clusters of synaptic bodies occurring in both young and old mice were seen only in Type I hair cells. Although the B6 strain suffers from genetically determined early cochlear degeneration, it does not experience early degeneration of the peripheral vestibular organs.

Selected pathological findings in the human cochlea

Out of a material of 45 patients with known audiograms where the inner ears had been fixed with an aldehyde within 7 h after death, 4 cases were chosen for detailed morphological examination. The general findings in the ageing human cochlea are presented as well as the findings in the 4 specific cases.

A comparative study of the effect of age on the human cochlear and vestibular neuroepithelia

A semi-quantitative and semi-qualitative comparative analysis was performed on the maculae and cochleas of 3 patients. The cochleas were examined by SEM and the maculae by TEM and light microscopy. The surface features of the cochleas were minimally affected by autolysis. Hearing loss and increasing age correlated well with inner and outer hair cell counts. In the labyrinth, the sacculi were more resistant to autolysis than were the utriculi and the type 2 cells better preserved than the type 1 cells. The pattern of cellular degeneration in the utriculus and sacculus varied with both age and functional deficit. Lipofuscin was present in the sensory cell of all 3 patients but was most pronounced in the oldest. Long-spaced collagen, laminated bodies and membrane-bound inclusions were seen in all maculae.

Microtubules and laminated structures in inner ear hair cells

Rat vestibular hair cells and adder (a snake) auditory and vestibular hair cells were examined by electron microscopy after various fixations. An albumin/unbuffered osmic acid/unbuffered glutaraldehyde fixation was found to preserve the microtubules well. After this fixation a narrow connection was observed between stereocilia rootlets or rootlet-like structures and apical microtubules. A laminated structure formed by rootlet filament or rootlet-like fibres was consistently found in the lower part of the cuticular structure. Similar but isolated laminated bodies (Friedmann bodies) were occasionally observed in the snake auditory organ. Bundles of microtubules from the area below the cuticular structure as well as from the dense reticular lamina stretch down in the hair cells. In the dense reticular lamina stretch down in the hair cells. In the basal part of the cells' microtubules very frequently connect with afferent synapses. A microtubular connection between the stereocilia rootlets or laminated part of the cuticular structure and the synapses is proposed. Vesicles adherent to microtubules may suggest a transport or guidance function for synaptic vesicles.