The early postnatal development of the cochlear vasculature in the gerbil

Neuroglobin Expression in the Mammalian Auditory System

The energy-yielding pathways that provide the large amounts of metabolic energy required by inner ear sensorineural cells are poorly understood. Neuroglobin (Ngb) is a neuron-specific hemoprotein of the globin family, which is suggested to be involved in oxidative energy metabolism. Here, we present quantitative real-time reverse transcription PCR, in situ hybridization, immunohistochemical, and Western blot evidence that neuroglobin is highly expressed in the mouse and rat cochlea. For primary cochlea neurons, Ngb expression is limited to the subpopulation of type I spiral ganglion cells, those which innervate inner hair cells, while the subpopulation of type II spiral ganglion cells which innervate the outer hair cells do not express Ngb. We further investigated Ngb distribution in rat, mouse, and human auditory brainstem centers, and found that the cochlear nuclei and superior olivary complex (SOC) also express considerable amounts of Ngb. Notably, the majority of olivocochlear neurons, those which provide efferent innervation of outer hair cells as identified by neuronal tract tracing, were Ngb-immunoreactive. We also observed that neuroglobin in the SOC frequently co-localized with neuronal nitric oxide synthase, the enzyme responsible for nitric oxide production. Our findings suggest that neuroglobin is well positioned to play an important physiologic role in the oxygen homeostasis of the peripheral and central auditory nervous system, and provides the first evidence that Ngb signal differentiates the central projections of the inner and outer hair cells.

Physiopathology of the cochlear microcirculation

Normal blood supply to the cochlea is critically important for establishing the endocochlear potential and sustaining production of endolymph. Abnormal cochlear microcirculation has long been considered an etiologic factor in noise-induced hearing loss, age-related hearing loss (presbycusis), sudden hearing loss or vestibular function, and Meniere's disease. Knowledge of the mechanisms underlying the pathophysiology of cochlear microcirculation is of fundamental clinical importance. A better understanding of cochlear blood flow (CoBF) will enable more effective management of hearing disorders resulting from aberrant blood flow. This review focuses on recent discoveries and findings related to the physiopathology of the cochlear microvasculature.

Disorders of cochlear blood flow

The cochlea is principally supplied from the inner ear artery (labyrinthine artery), which is usually a branch of the anterior inferior cerebellar artery. Cochlear blood flow is a function of cochlear perfusion pressure, which is calculated as the difference between mean arterial blood pressure and inner ear fluid pressure. Many otologic disorders such as noise-induced hearing loss, endolymphatic hydrops and presbycusis are suspected of being related to alterations in cochlear blood flow. However, the human cochlea is not easily accessible for investigation because this delicate sensory organ is hidden deep in the temporal bone. In patients with sensorineural hearing loss, magnetic resonance imaging, laser-Doppler flowmetry and ultrasonography have been used to investigate the status of cochlear blood flow. There have been many reports of hearing loss that were considered to be caused by blood flow disturbance in the cochlea. However, direct evidence of blood flow disturbance in the cochlea is still lacking in most of the cases.

Vascular variations of the inner ear

Vascular anomalies of the inner ear have been documented in only a few isolated case reports. The goal of our study was to describe, qualify and quantify vascular variations of the inner ear in 122 temporal bones from 64 pediatric subjects aged between 0 and 10 years. The average age was 11.6 months. Horizontal sections of the temporal bone, examined by light microscopy, revealed vessels coursing freely through the perilymphatic space of the cochlea, especially in the apical turn. Other findings included abnormally wide vessels in the stria vascularis as well as a vascular malformation of the internal auditory canal. Our study demonstrated more atypical vessels in the cochlea than in the vestibular labyrinth. We found a statistically significant positive correlation between vascular variations of the inner ear and concomitant cardiac anomalies or endolymphatic hydrops. We also discuss the possible etiology and potential significance of these findings in terms of disturbances of the function of the inner ear.

Development and regression of cochlear blood vessels in fetal and newborn mice

Various strains of mice have been used for hearing research, but there have been few reports regarding the cochlear vasculature in mice. In this study, the development of the cochlear vasculature was investigated in C57BL/6 mice from day 15 of gestation to day 15 after birth, and mature vessels were also observed in 3-month-old mice. Both India ink injection and the resin casting method were used. On gestational day 17, spiral vessels of the basilar membrane were developing and were elaborating communicating branches that ran toward the external wall and the spiral lamina. On day 18, the spiral vessels showed the largest diameter of all vessels in the cochlea, but these vessels subsequently regressed and finally disappeared by day 14 after birth. The external wall vessels formed a single-layer capillary network at birth and subsequently divided into two layers, which became the vessels of the stria vascularis and the spiral ligament vessels. This process occurred progressively from the basal turn toward the apical turn between days 5 and 8 after birth. A general tendency for the cochlear vasculature to mature from the basal turn towards the apex was observed.

Age-related sensitivity to cisplatin ototoxicity in gerbils

This study was undertaken to define the developmental period of maximal sensitivity to cisplatin ototoxicity in gerbils. Five groups were established based upon post-natal age (P) at exposure to cisplatin, P10 (n = 8), P14 (n = 8), P18 (n = 6), P22 (n = 7) and P42 (n = 7). Animals were given cisplatin, 1 mg/kg/day intraperitoneal for 4 days. In the first four groups, P10, P14, P18 and P22, distortion product otoacoustic emissions were measured at 45 days of age, when responses were expected to be developmentally stable. Distortion product grams and input-output functions were measured. There was a statistically significant difference only between P14 and P42 (P<0.01). There was a significant interaction of age and frequency in the P14 group only (P<0.01). A secondary analysis compared distortion product grams of P14 animals, exposed to cisplatin, and age-matched saline-treated animals (n = 6). There was a significant treatment effect. In summary, there was an effect of age on the cisplatin ototoxicity in gerbils. Also, there was an effect of the frequency on DPOAE levels in P14 gerbils. These data support the presence of a 'sensitive' period to cisplatin ototoxicity in gerbils.

Postnatal vascular development in the lateral wall of the cochlear duct of gerbils: quantitative analysis by electron microscopy and confocal laser microscopy

The development of the capillary network in the stria vascularis and in the underlying spiral ligament of gerbils was systematically and quantitatively investigated by conventional electron microscopy and confocal laser microscopy in association with vascular labeling with fluorescent gelatin. The developmental changes of capillaries in the lateral wall were observed as the following series of events. (i) At 0 days after birth (DAB) capillaries already existed in the spiral ligament as a network. (ii) At 3-9 DAB the capillary network developed into two layers starting from the scala vestibuli side to the scala tympani side; one layer was located in the stria and the other in the spiral ligament. (iii) At 9 DAB capillaries in the stria became separated from the spiral ligament, and the capillary network consisting of a two-layered structure was complete. (iv) Total capillary length and capillary density in the lateral wall increased until 9 DAB and leveled off thereafter, but changes in the relative position of capillaries in the stria toward the luminal surface of marginal cells continued until 31 DAB. On the basis of the above observations, we propose two possible mechanisms underlying the vascular development in the lateral wall: (i) the formation of new vasculature (angiogenesis), and (ii) changes in the position of cellular components relative to capillaries in association with the differentiation and maturation of marginal cells and intermediate cells.

In vitro induction of microcyst-like structures in the superior olivary complex

To investigate the etiology of hole formation in the gerbil and rat central auditory system, organotypic cultures were grown in control and veratridine-containing media. The latter condition is known to increase neuronal activity. Tissue was obtained at postnatal day 6 and grown for 6-9 days in vitro, a period prior to the formation of holes in vivo. In both rats and gerbils, veratridine led to the appearance of large numbers of holes, and these were phenotypically similar to those found in vivo. These results support the idea that hole formation is an activity-dependent phenomenon, and suggest that it is not restricted to the mature gerbil auditory system.

Sensitive developmental periods for kanamycin ototoxic effects on distortion-product otoacoustic emissions

The developing rat is hypersensitive to aminoglycoside toxicity, which is expressed early on as a destruction of outer hair cells (OHC). In the current study, distortion-product otoacoustic emissions (DPOAE), which specifically measure the micromechanical activity of OHCs, were used to assess functional effects of administering a regimen of kanamycin to three groups of neonatal rats representing discrete postnatal developmental periods. In this manner, pigmented rats were treated at postnatal days 1-10, 11-20, and 21-30. A series of input-output (I/O) functions obtained for the 2f1-f2 DPOAE during the post-treatment period indicated that detection thresholds were significantly elevated for the animals treated on postnatal days 1-10 and 11-20, with the greatest elevations observed at the higher test frequencies.

Structural and ultrastructural aspects of isolated immature cochlear outer hair cells maintained in short-term culture

Immature outer hair cells (OHCs), isolated from developing rat cochlea without using proteolytic enzymes, were maintained in short-term culture in a clot of coagulated plasma. Cell viability was assessed by a laser scanning image cytometer, using double-fluorescent labeling. Light and transmission electron microscopy was used to study the morphology of isolated cells. Ten to 60 healthy OHCs were obtained from one cochlea, either as single isolated cells or clusters containing 2-10 cells from the same row. Although dead cells were observed only 1 h after dissociation, there were still viable cells after 6 h. Isolated OHCs were not perfectly cylindrical, due to the immaturity of their cortical structures. One hour after dissociation the ultrastructural organization of the isolated cells was generally well preserved, but this was followed by dilatation of the Golgi apparatus and endoplasmic reticulum. Specific changes in isolated OHCs were also observed at the subsurface cisternae and cuticular plate. Although degenerating OHCs generally showed a classic pattern of necrosis, certain morphological features reminiscent of apoptosis were also observed. This study emphasises the difficulty involved in investigating isolated immature OHCs in vitro and provides a basis for future research into the physiological requirements of isolated immature OHCs.

Alterations in microvasculature are associated with atrophy of the stria vascularis in quiet-aged gerbils

Age-related changes in the integrity of the stria vascularis and its microvasculature were assessed in whole mount preparations of the gerbil cochlear lateral wall. Small focal regions containing few or no capillaries were present at the extreme ends of the stria vascularis in most 5-9 month-old gerbils. A few ears in this age range also contained small regions devoid of capillaries at the extreme basal end of the stria. These degenerate foci expanded in a systematic fashion toward the middle turn of the cochlea with increasing age. Gerbils aged 33 months or older exhibited a normal strial vasculature pattern only in portions of the middle and upper basal turns. The remainder of the stria in these older gerbils contained degenerate regions which showed both loss of capillaries and atrophy of strial marginal cells. Quantification via computer-aided image analysis confirmed that the areas of strial atrophy correlated well with the loss of strial capillaries at all ages. However, regions containing capillaries with decreased diameter were not necessarily accompanied by atrophic changes in marginal cells. The results suggest that degeneration of the stria vascularis begins prior to onset of auditory threshold shift and is preceded and possibly initiated by changes in the strial microvasculature.

Ototoxicity in developing mammals

Developing mammals are more sensitive to noise, chemical and drug-induced ototoxicity than adults, with maximum sensitivity occurring during periods of anatomical and functional maturation of the cochlea. Normal physiological development of resting potentials (the endocochlear potential) and sound-evoked potentials including cochlear microphonics, summating potentials, compound action potentials, auditory brainstem responses and more recently distortion-product otoacoustic emissions have been characterized in several species including rats, mice, kittens, gerbils and guinea pigs. All of these responses are significantly impaired following acoustic trauma and/or exposure to a variety of ototoxic agents including aminoglycoside antibiotics, loop diuretics, antithyroid and antitumor drugs (alpha-difluoromethylornithine) and excitatory amino acids. Coupled with physiological and anatomical development is the maturation of specific biochemical pathways, which may be vulnerable targets of environmental noise and chemicals, excitatory amino acids and therapeutic drugs with ototoxic potentials.

Development of endocochlear potential and compound action potential in the rat

The present study was designed to investigate the developmental changes of the endocochlear potential and compound action potential simultaneously from rat pups of various ages. Animals were anesthetized with ketamine/xylazine, and the endocochlear potential was measured with a glass microelectrode. At the same time, a wire electrode was placed on the round window to record the click-evoked compound action potential. The endocochlear potential was found to be very low during the first few days of postnatal life. A rapid increase in the value of the endocochlear potential was noted between eleven and thirteen days of age, and adult-like values were recorded by seventeen days of age. Compound action potential responses were recorded at thirteen days of age to high intensity clicks, followed by a progressive improvement of thresholds and reduction of latencies. The development of the endocochlear potential and compound action potential was found to be reciprocally related - as the magnitude of the endocochlear potential increased, the compound action potential threshold declined with increasing age. The development of the endocochlear potential was found to closely approximate the development of enzymatic activity of sodium, potassium-ATPase in the stria vascularis reported by Kuijpers (1974).

The development of melanin in the stria vascularis of the gerbil

One factor that influences noise susceptibility is pigmentation. The aim of this study was to investigate the development of melanocytes, other melanin-containing cells and the amount of melanin in stria vascularis from birth to adult age in the gerbil which has a uniform pigmentation of the fur and eyes, is born without hearing but establishes hearing function at 14-18 days after birth. Changes in the melanin morphology, concentration and distribution have been correlated to the development of the inner ear and to the time period during which hearing function is established, which indicates that the melanocytes in stria vascularis are of importance for the hearing function.

Comparative anatomy of cochlear blood vessels

The vascular anatomy of most mammalian cochleas is similar. Of those examined to date, the human cochlea is the most unusual. The blood flow is segmentally, centrifugally, and centripetally arranged, with many spirally perpendicular vessels shunted in, suggesting good possibilities for variations of circulation. There is a pronounced apical simplification of the vascular pattern probably more than corresponding to the decreasing volume of the turns. The blood supply of the scala vestibuli is mainly arterial; that of the scala media is capillary or by-pass, and that of the scala tympani is venous. The vessel of the basilar membrane shows great variations among species, from presence in all turns to complete non-existence. Its importance and size is greater during embryonic life.

The surface morphology of the endolymphatic sac of the Mongolian gerbil (Meriones unguiculatus) (a scanning electron microscopic study)

A scanning electron microscopic study of the endolymphatic sac of the mongolian gerbil is presented. As described in other animal species and in man, three rather distinct regions on the epithelial surface can also be recognized in this rodent. Light and dark cells are seen to line the sac epithelium throughout. At the level of the intermediate portion, however, a different cell type--the granular cell--is present, with its luminal surface covered by large amounts of solid material. On the basis of light and transmission electron microscopic studies presented elsewhere, a higher degree of functional specialization in this portion of the gerbilline sac is proposed.