Ivan Lopez

Internal auditory artery infarction Clinicopathologic correlation

Objective: To study the pathophysiology of labyrinthine infarction. Background: The syndrome of sudden onset vertigo or hearing loss is commonly attributed to inner ear vascular disease, yet histologic studies of isolated labyrinthine infarction in humans have been rare and have not included a complete examination of the vertebrobasilar vascular system. Methods: Temporal bones, brainstem, cerebellum, and the supplying blood vessels were subjected to gross and microscopic postmortem examinations in a 92-year-old woman who had a sudden onset of vertigo and hearing loss in the right ear 7 years before death. Results: There were prominent atherosclerotic changes at the vertebrobasilar junction, but the internal auditory artery and its branches were patent on both sides. Histologic studies showed degenerative changes in the cochlea and vestibular labyrinth on the right. The posterior canal ampulla and saccular macule were relatively preserved showing partial areas of intact sensory epithelium with underlying nerve fibers. The right vestibulocochlear nerve showed a fibrotic scar and multiple patchy areas of degeneration. These findings are most consistent with a transient period of reduced perfusion of the internal auditory artery. Conclusion: The partial sparing of the inferior vestibular labyrinth may indicate a decreased vulnerability to ischemia because of its better collateral blood supply.

Blindness and auditory impairment caused by loss of the sodium bicarbonate cotransporter NBC3

Normal sensory transduction requires the efficient disposal of acid (H+) generated by neuronal and sensory receptor activity. Multiple highly sensitive transport mechanisms have evolved in prokaryotic and eukaryotic organisms to maintain acidity within strict limits. It is currently assumed that the multiplicity of these processes provides a biological robustness. Here we report that the visual and auditory systems have a specific requirement for H+ disposal mediated by the sodium bicarbonate cotransporter NBC3 (refs. 7,8). Mice lacking NBC3 develop blindness and auditory impairment because of degeneration of sensory receptors in the eye and inner ear as in Usher syndrome. Our results indicate that in certain sensory organs, in which the requirement to transduce specific environmental signals with speed, sensitivity and reliability is paramount, the choice of the H+ disposal mechanism used is limited.

Aging and the human vestibular nucleus.

Degenerative changes during aging have been identified in the inner ear and in the vestibular nerve, but not in the human vestibular nuclear complex (VNC). Therefore, the purpose of this study was to document quantitative morphometric changes within the VNC in humans as a function of age. The VNC of normal human subjects was examined for age-related changes using computer-based microscopy. Neuronal counts, nuclear volume, neuronal density, and nuclear length of the 4 vestibular nuclei were determined in 15 normal people, age 40 to 93 years. Based on a linear model, there was approximately a 3% neuronal loss per decade from age forty to ninety. VNC volume and neuronal density also decreased significantly with age, although to a lesser degree than did the number of neurons. Neuronal loss as a percentage of the total number of neurons was greatest in the superior vestibular nucleus and least in the medial vestibular nucleus. Despite the overall loss of neurons, the number of giant neurons (> 500 microns2) increased in older people. This increase in giant neurons could be traced to the accumulation of lipofuscin deposits in the cell somata. The overall rate of neuronal loss with aging in the VNC is comparable to that previously observed in hair cells, primary vestibular neurons, and cerebellar Purkinje cells, but is in contrast to prior reports of no age-related loss of neurons in other brain stem nuclei.

Time course of inner ear degeneration and deafness in mice lacking the Kir4.1 potassium channel subunit

The Kir4.1 gene (KCNJ10) encodes an inwardly rectifying K(+) channel subunit abundantly expressed in the CNS. Its expression in the mammalian inner ear has been suggested but its function in vivo in the inner ear is unknown. Because diverse human hereditary deafness syndromes are associated with mutations in K(+) channels, we examined auditory function and inner ear structure in mice with a genetically inactivated Kir4.1 K(+) channel subunit. Startle response experiments suggest that Kir4.1-/- mice are profoundly deaf, whereas Kir4.1+/- mice react like wild-type mice to acoustic stimuli. In Kir4.1-/- mice, the Reissner membrane is collapsed, the tectorial membrane is swollen, and type I hair cells and spiral ganglion neurons as well as their central processes degenerate over the first postnatal weeks. In the vestibular ganglia, neuronal cell death with apoptotic features is also observed. Immunostaining reveals that Kir4.1 is strongly expressed in stria vascularis of wild-type but not Kir4.1-/- mice. Within the spiral ganglion, Kir4.1 labeling was detected on satellite cells surrounding spiral ganglion neurons and axons. We conclude that Kir4.1 is crucial for normal development of the cochlea and hearing, via two distinct aspects of extracellular K(+) homeostasis: (1). in stria vascularis, Kir4.1 helps to generate the cochlear endolymph; and (2). in spiral and vestibular ganglia, Kir4.1 in surrounding glial cells helps to support the spiral and vestibular ganglion neurons and their projecting axons.

Quantification of the process of hair cell loss and recovery in the chinchilla crista ampullaris after gentamicin treatment.

The degree of ototoxic drug sensitivity and hair cell repair was determined in the chinchilla horizontal crista ampullaris after intraotic administration of gentamicin. Histological evaluation was made of 22 cristae ampullaris from one normal and six post‐treatment (PT) animal groups killed at 1, 4, 7, 14, 28, and 56 days. New hair cell production was quantified, using the dissector technique. Transmission electron microscopy was used to investigate the ultrastructural characteristics of the hair cells in the regenerated epithelium. At I day PT, type I and II hair cells presented cytoplasmic vacuolization, swollen nerve calyces and 20% of type I and 18% of type II hair cells were lost. At 4days PT, 95% of type I hair cells and 14% of type II hair cells had disappeared. In addition, most of the type II hair cells showed clumping of nuclear material. Nerve fibers were not found in the sensory epithelium, but were still observed below the basal lamina. Supporting cells appeared unaffected, maintaining their location in the crista. At 1 and 4 days PT, the damage to hair cells was more pronounced in the central region of the crista ampullaris. The degree of ototoxic damage at 7 days was similar to that of 14 days: no type I hair cells were present and most of the type II hair cells had disappeared; supporting cell nuclei began to occupy the apical part of the sensory epithelium and most of the nerve fibers had retracted. Quantitatively, 87 and 93% of type II hair cells were lost at 7 and 14 days PT, respectively. Initial signs of hair cell recovery began at 28 days PT; immature type II‐like hair cells appeared, supporting cell nuclei began to align at the base of the sensory epithelium and nerve fibers penetrating the basal lamina were observed. No type I hair cells were found, but 40% of the normal number of type II hair cells were present. Hair cells appeared to regenerate in the peripheral areas of the cristae ampullaris first. At 56 days PT, an increase in the number of mature type II hair cells was present, supporting cells were aligned at the base of the epithelium, and more nerve fibers appeared to penetrate the basal lamina to the sensory epithelium. Although type I hair cells were absent from the epithelium, 55% of the normal number of type II hair cells were present. At this time, more regenerated hair cells were located in the center of the cristae ampullaris as compared to the periphery. At the transmission electron microscopic level, type II hair cells at different stages of maturation were observed. Some exhibited mature stereocilia, a cuticular plate, and terminal endings with synaptic specialization opposing these hair cells. In conclusion, type I hair cells were more sensitive than type II hair cells to gentamicin intoxication (as they disappeared as early as 4days PT). After 56 days PT, the number of type II hair cells reached 55% of normal. No type I hair cells had regenerated at this time. These results demonstrate quantitatively the differential ototoxic sensitivity and regenerative capacity of hair cells.

Brain volume changes on longitudinal magnetic resonance imaging in normal older people.

Background and Purpose. The purpose of this study is to investigate the longitudinal age‐related changes in human brain volume using stereological methods. Methods. Sixty‐six older participants (34 men, 32 women, age [mean ± SD] 78.9 ± 3.3 years, range 74–87 years) with normal baseline and follow‐up examinations underwent 2 MRIs (magnetic resonance imaging) of the brain on average 4.4 years apart. The volumes of the cerebrum (defined as cortex, basal ganglia, thalamus, and white matter), lateral ventricles, and cerebellum were estimated on the 2 MRIs using an unbiased stereological method (Cavalieri principle). Results. The annual decrease (mean ± SD) of the cerebral volume was 2.1%± 1.6% (P < .001). The average volume of the lateral ventricles on the second MRI was increased by 5.6%± 3.6% per year (P < .001). The average volume of the cerebellum on the second MRI was decreased by 1.2%± 2.2% per year (P < .001). Even though the average cerebral volume was significantly different between men and women on initial MRI and second MRI, the percentage change of the age‐related cerebral volume decrease in male and female brains between initial MRI and second MRI were identical. Conclusions. The findings showed that there was age‐related atrophy of cerebrum and cerebellum and age‐related disproportional enlargement of lateral ventricles in normal older men and women.

Histological evidence for hair cell regeneration after ototoxic cell destruction with local application of gentamicin in the chinchilla crista ampullaris.

Two experiments were conducted to study the ototoxic effects of local gentamicin (GM) administration and the subsequent hair cell (HC) regeneration process in the chinchilla cristae ampullares (CA). In the first experiment, 3 different doses of GM (0.1, 0.2 and 1.2 mg) were administered by surgical implantation of GM-soaked Gelfoam pledgets in the perilymphatic space in the otic capsule of the left superior semicircular canal. The CA was histologically processed for light-microscopic examination. In the second experiment, 6 groups of 2 chinchillas each were treated with 0.1 mg of GM. To document cell proliferation and HC regeneration, Alzet micro-osmotic pumps were implanted in each chinchilla to deliver bromodeoxyuridine (BrdU) at 125 micrograms/h for 1 week. Chinchillas were subsequently killed at 1 and 4 days and 1, 2, 4 and 8 weeks post-treatment (PT). The CA was processed for light microscopy and BrdU immunocytochemistry. In the first experiment the smallest dose produced damage restricted to HCs alone, while the medium and large doses produced severe damage in the sensory epithelium, including supporting cells and HCs. Results in the second experiment demonstrated that at 1 and 4 days PT the HCs showed extensive damage, including clumping of nuclear material. By 4 days PT the supporting cell nuclei lost their monolayer configuration. Calyceal terminals appeared empty, and vacuolized remnants of nerve calyces were evident in the basal portion. At 1 week PT complete disappearance of HCs from the sensory epithelium was evident, and there was cytoplasmic extrusion into the endolymphatic space. At 2 weeks PT there was complete HC loss, the supporting cell nuclei were scattered randomly in the crista, and the nerve fibers were retracted from the sensory epithelium. At 4 weeks PT there was evidence of sensory epithelium repair and HC regeneration. Short cells resembling type-II HCs were evident in the surface of the sensory epithelium. At 8 weeks PT the number of HCs increased in a uniform fashion on the surface of the sensory epithelium, and the supporting cell nuclei were realigned on the basal membrane. Nerve fibers with growth cones penetrated the basal membrane. Supporting cell proliferation was evident by the presence of mitotic figures and BrdU immunoreactivity in the chromatin material of dividing cells at 2 weeks PT. The labeling was more evident in newly formed cells at 4 and 8 weeks PT. These results demonstrate that in chinchillas the vestibular organs have the capacity of self-repair and the process includes HC regeneration after local administration of GM. The overall process involves changes in different cells in the sensory epithelium and neural elements, all of which show modifications with an orderly pattern.

Age-related change in the number of neurons in the human vestibular ganglion

Dysequilibrium of aging in humans has been speculated to arise from progressive deterioration within anatomical components of the vestibular system. An integral part of this system is vestibular ganglions, which are bipolar neurons that relay peripheral vestibular information to the central nervous system. To assess the effect of aging on the number of human vestibular ganglion neurons, assumption‐free stereology in the form of the optical fractionator was used on 20 serially sectioned archival human temporal bone specimens. Donors had no history of vestibular pathology and ranged in age from 2 to 88 years. An average of 25,812 (coefficient of variation = 0.13) vestibular ganglion neurons was found throughout this age range, a significant departure from the results of past studies. Logistics‐based regression analysis pointed to a nonlinear pattern of decline in the neuronal population: the number of cells remained roughly constant at about 28,952 cells in youth and then declined gradually between 30 and 60 years of age before leveling off at approximately 23,349 cells in older individuals. This study confirmed the existence of an age‐related decline in the primary neurons of the human vestibular system, thus providing one anatomical basis for the increased incidence of imbalance seen with age. J. Comp. Neurol. 431:437–443, 2001.

MUSCARINIC ACETYLCHOLINE RECEPTOR SUBTYPE mRNAs IN THE HUMAN AND RAT VESTIBULAR PERIPHERY

The expression of the five muscarinic acetylcholine receptor (mAChR) subtypes (m1–m5) in the vestibular end‐organs and in the primary afferent vestibular ganglia of the human and rat was studied using RT‐PCR from the two tissue populations from both species. In the human, although all five mAChR subtypes were expressed in brain, only the m1, m2, and m5 mAChR subtypes were amplified from both the vestibular ganglia and the vestibular end‐organs, while in the rat, all five mAChR subtypes were expressed. These data suggest that the efferent cholinergic axo‐dendritic and axo‐somatic synapses have a muscarinic component and that there are pharmacologic implications for patients with vestibular dysfunction.

Slc4a11 gene disruption in mice: Cellular targets of sensorineuronal abnormalities

NaBC1 (the SLC4A11 gene) belongs to the SLC4 family of sodium-coupled bicarbonate (carbonate) transporter proteins and functions as an electrogenic sodium borate cotransporter. Mutations in SLC4A11 cause either corneal abnormalities (corneal hereditary dystrophy type 2) or a combined auditory and visual impairment (Harboyan syndrome). The role of NaBC1 in sensory systems is poorly understood, given the difficulty of studying patients with NaBC1 mutations. We report our findings in Slc4a11−/− mice generated to investigate the role of NaBC1 in sensorineural systems. In wild-type mice, specific NaBC1 immunoreactivity was detected in fibrocytes of the spiral ligament, from the basal to the apical portion of the cochlea. NaBC1 immunoreactivity was present in the vestibular labyrinth, in stromal cells underneath the non-immunoreactive sensory epithelia of the macula utricle, sacule, and crista ampullaris, and the membranous vestibular labyrinth was collapsed. Both auditory brain response and vestibular evoked potential waveforms were significantly abnormal in Slc4a11−/− mice. In the cornea, NaBC1 was highly expressed in the endothelial cell layer with less staining in epithelial cells. However, unlike humans, the corneal phenotype was mild with a normal slit lamp evaluation. Corneal endothelial cells were morphologically normal; however, both the absolute height of the corneal basal epithelial cells and the relative basal epithelial cell/total corneal thickness were significantly increased in Slc4a11−/− mice. Our results demonstrate for the first time the importance of NaBC1 in the audio-vestibular system and provide support for the hypothesis that SLC4A11 should be considered a potential candidate gene in patients with isolated sensorineural vestibular hearing abnormalities.