Morphometry of otoconia in the utricle and saccule of developing Japanese quail

A New Perspective to Interpret How the Vestibular Efferent System Correlates the Complexity of Routine Balance Maintenance with Management of Emergency Fall Prevention Strategies

Bipedalism is unique among mammals. Until modern times, a fall and resulting leg fracture could be fatal. Balance maintenance after a destabilizing event requires instantaneous decision making. The vestibular system plays an essential role in this process, initiating an emergency response. The afferent otolithic neural response is the first directionally oriented information to reach the cortex, and it can then be used to initiate an appropriate protective response. Some vestibular efferent axons feed directly into type I vestibular hair cells. This allows for rapid vestibular feedback via the striated organelle (STO), which has been largely ignored in most texts. We propose that this structure is essential in emergency fall prevention, and also that the system of sensory detection and resultant motor response works by having efferent movement information simultaneously transmitted to the maculae with the movement commands. This results in the otolithic membrane positioning itself precisely for the planned movement, and any error is due to an unexpected external cause. Error is fed back via the vestibular afferent system. The efferent system causes macular otolithic membrane movement through the STO, which occurs simultaneously with the initiating motor command. As a result, no vestibular afferent activity occurs unless an error must be dealt with.

Vestibular Injury After Low-Intensity Blast Exposure

The increased use of close range explosives has led to a higher incidence of exposure to blast-related head trauma. Exposure to primary blast waves is a significant cause of morbidity and mortality. Active service members and civilians who have experienced blast waves report high rates of vestibular dysfunction, such as vertigo, oscillopsia, imbalance, and dizziness. Accumulating evidence suggests that exposure to blast-wave trauma produces damage to both the peripheral and central vestibular system; similar to previous findings that blast exposure results in damage to auditory receptors. In this study, mice were exposed to a 63 kPa peak blast-wave over pressure and were examined for vestibular receptor damage as well as behavioral assays to identify vestibular dysfunction. We observed perforations to the tympanic membrane in all blast animals. We also observed significant loss of stereocilia on hair cells in the cristae and macule up to 1 month after blast-wave exposure; damage that is likely permanent. Significant reductions in the ability to perform the righting reflex and balance on a rotating rod that lasted several weeks after blast exposure were prominent behavioral effects. We also observed a significant reduction in horizontal vestibuloocular reflex gain and phase lags in the eye movement responses that lasted many weeks following a single blast exposure event. OKN responses were absent immediately following blast exposure, but began to return after several weeks’ recovery. These results show that blast-wave exposure can lead to peripheral vestibular damage (possibly central deficits as well) and provides some insight into causes of vestibular dysfunction in blast-trauma victims.

Morphology of the utricular otolith organ in the toadfish, Opsanus tau

The utricle provides the vestibular reflex pathways with the sensory codes of inertial acceleration of self-motion and head orientation with respect to gravity to control balance and equilibrium. Here we present an anatomical description of this structure in the adult oyster toadfish and establish a morphological basis for interpretation of subsequent functional studies. Light, scanning, and transmission electron microscopy techniques were applied to visualize the sensory epithelium at varying levels of detail, its neural innervation and its synaptic organization. Scanning electron microscopy was used to visualize otolith mass and morphological polarization patterns of hair cells. Afferent nerve fibers were visualized following labeling with biocytin, and light microscope images were used to make three-dimensional (3-D) reconstructions of individual labeled afferents to identify dendritic morphology with respect to epithelial location. Transmission electron micrographs were compiled to create a serial 3-D reconstruction of a labeled afferent over a segment of its dendritic field and to examine the cell-afferent synaptic contacts. Major observations are: a well-defined striola, medial and lateral extra-striolar regions with a zonal organization of hair bundles; prominent lacinia projecting laterally; dependence of hair cell density on macular location; narrow afferent dendritic fields that follow the hair bundle polarization; synaptic specializations issued by afferents are typically directed towards a limited number of 7-13 hair cells, but larger dendritic fields in the medial extra-striola can be associated with > 20 hair cells also; and hair cell synaptic bodies can be confined to only an individual afferent or can synapse upon several afferents.

Development of otolith receptors in Japanese quail

This study examined the morphological development of the otolith vestibular receptors in quail. Here, we describe epithelial growth, hair cell density, stereocilia polarization, and afferent nerve innervation during development. The otolith maculae epithelial areas increased exponentially throughout embryonic development reaching asymptotic values near posthatch day P7. Increases in hair cell density were dependent upon macular location; striolar hair cells developed first followed by hair cells in extrastriola regions. Stereocilia polarization was initiated early, with defining reversal zones forming at E8. Less than half of all immature hair cells observed had nonpolarized internal kinocilia with the remaining exhibiting planar polarity. Immunohistochemistry and neural tracing techniques were employed to examine the shape and location of the striolar regions. Initial innervation of the maculae was by small fibers with terminal growth cones at E6, followed by collateral branches with apparent bouton terminals at E8. Calyceal terminal formation began at E10; however, no mature calyces were observed until E12, when all fibers appeared to be dimorphs. Calyx afferents innervating only Type I hair cells did not develop until E14. Finally, the topographic organization of afferent macular innervation in the adult quail utricle was quantified. Calyx and dimorph afferents were primarily confined to the striolar regions, while bouton fibers were located in the extrastriola and Type II band. Calyx fibers were the least complex, followed by dimorph units. Bouton fibers had large innervation fields, with arborous branches and many terminal boutons.

Morphometry of otoliths in chicken macula lagena

The macula lagena located at the apical end of the cochlea in birds is characterized by the presence of numerous otoliths with unclear sensory functions. These otoliths are reported to be similar to those in the vestibular system but their detailed features in morphology are unknown. In the present study, we examined the number, size and shape of otoliths from the macula lagena in Chinese domestic chickens (Gallus Ling Nan) with a scanning electron microscope for morphometry. For chickens aged 10-15 post-hatch days, the otoliths in each macula lagena were counted to be 16,055 +/- 4038 (mean +/- S.D., n = 4). The average length and width were 12.98 +/- 3.70 microm and 5.10 +/- 1.48 microm (n = 526 otoliths), respectively. The ratio of length to width for the otolith was 2.58 +/- 0.39 (n = 526 otoliths) and remained relatively constant despite their variations in physical size. Almost all the otoliths were in regular shape and appeared like isolated cylinders with smooth facets at each end, but a few of them (0.025% of 64,221 otoliths screened) were found to be in odd shapes, such as T-shape and cross-shape. The results suggest that otoliths in the macula lagena and those in the vestibular system of bird's inner ear have similar physical properties and may play a similar role in sensing gravitational and acceleration signals.

Age-related changes on the morphology of the otoconia

OBJECTIVES/HYPOTHESIS

Although there are numerous reports on otoconial morphology using field-emission scanning electron microscopy (FESEM), there are few reports regarding the changes of otoconial morphology with aging. The aim of the current study was to investigate changes in otoconial morphology in rats according to age, using FESEM.

STUDY DESIGN

Laboratory study using experimental animals.

METHODS

We investigated age-related changes in otoconial morphology using FESEM in three groups of rats: young (1 wk old), middle-aged (6 mo old), and aged (23 mo old).

RESULTS

There was great size variation in utricular otoconia in the young and aged rats, but we found no clear regional separation of saccular otoconia in all groups based on size. In the oldest rats, the bodies of many otoconia in both maculae were pitted, fissured, penetrated, and eventually broken into several fragments. However, the terminal facets were smooth and the lines of intersection of facets were sharp, despite the degenerated bodies of the otoconia in this group. Giant otoconia were discovered frequently on the outer margin of the utricular maculae in aged rats. We directly observed weakened or broken linking filaments and otoconial fragments in the aged group.

CONCLUSION

The oldest rats showed the most degeneration of otoconia and linking filaments with otoconial fragments. This study of age-related morphologic changes in otoconia might help us understand the origin of idiopathic benign paroxysmal positional vertigo.

Age effects and size effects in the ears of gekkonomorph lizards: inner ear

Audiograms have indicated greater auditory sensitivity in larger than in smaller geckos; part of this difference, interspecifically and intraspecifically, is explained by middle-ear proportions. To investigate the contribution of the inner ear to the variation in sensitivity, we examined it in museum specimens representing 11 species and three subfamilies. We measured papilla basilaris length, and, when intact, the saccular otoconial mass. Papilla length approximated 1% of rostrum-anus length in large geckos but 2% in small geckos; in some species some inter-aural difference was indicated. Over the lumped material, relative papilla length varied as a function of body length, with highly significant correlation. Similar relations prevailed within each subfamily. However, intraspecifically the correlation of papilla basilaris length with animal size was usually nonsignificant. Hair cell populations assessed from SEM photographs were larger in the larger species but intraspecifically did not relate to an individual's size. Hence interspecifically, the dependence of auditory sensitivity on animal size seems supported by inner-ear differences but intraspecifically this relation derives only from the middle ear. Otoconial mass, as measured by its volume, was correlated with animal length both interspecifically and intraspecifically.