Biomedical subjects

J C Saunders

Publications and source records attributed to J C Saunders.

At least 37 records · Page 2Linked to original sources

PubMed1998

Low calcium abolishes tip links and alters relative stereocilia motion in chick cochlear hair cells.

The role of stereocilia tip links in controlling hair bundle motion on chick hair cells was examined in this study. Hair cells from the apical end of the basilar papilla were maintained in culture medium and oriented so that the sensory hair bundles were viewed in profile. A water-jet was used to stimulate the hair bundle and stroboscopic illumination allowed slow motion viewing of a sensory hair motion at the bundle edges. Motion of the tallest stereocilium in the bundle was set to a criterion angular deflection and the excursion of the shortest stereocilium was measured. These measurements were made in a sample of hair cells maintained in culture medium containing either near normal levels of calcium or very low calcium levels supplemented with EGTA. In low calcium the angular deflection of the shortest hair was significantly reduced from that observed in normal media. The resting inward tilt of the hairs in the bundle, however, did not change. Scanning electron microscopy verified an almost complete destruction of tip links after exposure to low calcium. These results suggest that tip links contribute significantly to the relative motion of stereocilia and exhibit the mechanical properties of a relatively stiff linkage.

Animals↗

PubMed1998

Effect of healed tympanic membrane perforations on umbo velocity in the rat.

Heterodyne laser interferometry was used to measure tympanic membrane (TM) velocity at the umbo during acoustic stimulation in 2 groups of rats. The control group had a normal TM, while in the other group the TM was perforated and then allowed to heal for at least 28 days. Umbo velocity functions for constant sound pressure level stimuli were obtained for test tones between 0.3 and 40.0 kHz in each animal. The results revealed that velocity was the same in the control and healed TMs below 3.0 kHz. Above 5.0 kHz, the velocity response in the healed ear was between 3 and 12 dB smaller than in the control ears. Histologic evaluation of the healed perforation revealed a thick fibrous reaction between the epidermal and lamina propria layers. The results indicated that the added mass of the scar tissue changed the middle- and high-frequency TM responses.

Acoustic Stimulation↗

PubMed1998

How body size affects middle-ear structure and function and auditory sensitivity in gekkonoid lizards.

Gekkonoid lizards increase in body size throughout life, and the present study investigates whether changes in auditory function accompany these increases. Middle-ear structures in four groups of animals, adults and juveniles of two gekkonoid species (Eublepharis macularius and Oedura marmorata), were examined. Tympanic membrane velocity and phase were also measured in all four groups. An indication of peripheral auditory function was obtained for each group by measuring compound action potentials (CAPs) from the round window membrane. The middle-ear contribution to CAP thresholds was obtained by comparing threshold levels of the CAP response with and without an intact middle-ear system. The results from these studies indicated that significant changes occurred in middle-ear structure, tympanic membrane velocity and CAP threshold between the younger and older animals. In addition, the adults of both species exhibited better auditory function when the acoustic stimulus was delivered to the tympanic membrane than when it was delivered to the columella footplate. The findings show clearly that increased body size (or age) is accompanied by functional changes in the auditory periphery.

Action Potentials↗

PubMed1997

Maturation of tympanic membrane layers and collagen in the embryonic and post-hatch chick (Gallus domesticus).

The surface area of the chick (Gallus domesticus) tympanic membrane (TM) increases by as much as 400% from hatching to 70 days of age (Cohen et al. [1992] J. Morphol. 212:187-193). The present study is concerned with the processes that contribute to this remarkable size increase. Middle-ear specimens were harvested in embryos aged between E10 and E18, and in post-hatch animals between days P1 and P57. Specimens were embedded in paraffin, cut in serial sections, stained for collagen fibers, and examined with light microscopy. Four locations were examined in each specimen: the anterior and posterior perimeter of the TM, the TM, over the extra-stapedius, and the TM at the tip of the extra-columella. The thickness of the epithelial, respiratory, and lamina propria layers was measured at each location. The radial collagen fibers in the lamina propria were also counted at each location, and fiber density per square micrometer was determined at each age. Thickness of the epithelial and respiratory layers remains relatively constant throughout development in all areas of the TM, whereas the lamina propria at the extra-columella and extra-stapedius continues to thicken with increasing age. Collagen density also increases during development, and this is attributed to an increase in fiber number and a reduction in the space between fibers. The results suggest that collagen may be synthesized first in the central regions of the TM and then later in more peripheral areas of the TM.

Aging↗

PubMed1996

The effect of tympanic membrane perforation size on umbo velocity in the rat.

The tympanic membrane (TM) in adult rats was surgically exposed and laser interferometry was used to measure TM velocity at the umbo for frequencies between 1.0 and 40.0 kHz. Velocity measures were obtained for five conditions: TM intact, and four progressively larger holes cut into the posterior region of the membrane. Photomicrographs of each condition were used to calculate the percentage of pars tensa lost to the perforation. The relation between TM velocity and stimulus sound pressure level (SPL) was also examined for each of the conditions. The results revealed a systematic loss in low-frequency velocity as perforation size increased. These observations were consistent with clinical reports of low-frequency hearing loss in the perforated human TM. The rat appears to be a successful model for studying this form of conductive pathology.

Animals↗

PubMed1996

Middle-ear development VII: umbo velocity in the neonatal rat.

Laser interferometry was used to measure umbo velocity in the developing rat. The tympanic membrane was stimulated with pure tones between 0.4 and 40.0 kHz, at intensity levels between 50 and 130 dB SPL. The corresponding umbo velocity response was measured. Umbo velocity responded linearly with respect to sound pressure throughout development. When the stimulus level was held constant at 100 dB SPL, all animals displayed a velocity response that increased with frequency until a peak response was reached at about 20.0 kHz. Above this frequency the response decreased in all age groups. Umbo velocity increased with age at all frequencies, and at 1.0, 2.0, 4.0, 8.0, 16.0, and 32.0 kHz the velocity reached 90% of its mature value by 68, 24, 24, 15, 19, and 50 days after birth, respectively. These age-related increases in tympanic membrane velocity coincided with improvements in compound action potential (CAP) thresholds (as measured by other investigators) at similar frequencies. Both umbo velocity and CAP thresholds showed substantial growth after 10 days of age. The role of middle-ear functional development with respect to overall auditory sensitivity is discussed.

Acoustic Stimulation↗

PubMed1996

Cochlear nerve activity after intense sound exposure in neonatal chicks.

1. Single-neuron behavior in the cochlear nerve of neonatal (3-day-old) chicks was examined after exposure to a 120-dB SPL pure tone (0.9 kHz) for 48 h. Exposed animals were tested after 0 days or 12 days of recovery. Nonexposed chicks, age-matched to the exposed animals, formed two control groups. 2. Spectral response plots were obtained from each cell. These plots described the neuron discharge rates in response to 1,767 tone burst stimuli, each with a unique frequency-intensity combination. The tone bursts were presented at frequencies between 0.1 and 4.5 kHz and for intensities between 0 and 100 dB SPL. From these plots the characteristic frequency (CF), CF threshold, and sharpness of tuning (Q10 dB) were derived for each cell. Frequency response-area functions at selected stimulus levels and rate-intensity functions at the CF were also constructed from the spectral response plots. In addition, spontaneous activity was determined. Data were obtained from 903 cells. 3. Neuron activity in the control cells revealed no differences between CF thresholds, Q10 dB, or spontaneous activity in the two age groups. However, age differences at all frequencies were noted in the rate-intensity functions. 4. A frequency-dependent loss in CF threshold was observed in the 0-day recovered cells. The threshold shift (relative to age-matched control cells) was 55-65 dB between 0.8 and 1.5 kHz, but only 10-15 dB between 0.1-0.4 kHz and 2.5-3.5 kHz. The exposed cells showed no loss in frequency selectivity (Q10 dB) at < 0.5 kHz, whereas above this frequency an increasing deterioration in tuning was noted. Spontaneous activity in the 0-day cells was suppressed across the entire range of CFs. The rate-intensity function of exposed cells had a steeper growth rate than that of control cells. 5. At 12 days of recovery, CF threshold, Q10 dB, and spontaneous activity all recovered to the levels exhibited by age-matched control cells. However, the rate-intensity function for cells with CFs between 0.8 and 1.0 kHz showed abnormal growth and higher discharge rates at saturation than the control cells. Outside of this frequency range the rate-intensity functions of control and exposed cells were similar to each other. 6. Recovery of function in the sound-damaged chick ear is accompanied by almost complete repair of the basilar papilla. The tectorial membrane, however, retains a major defect and only the lower layer of this membrane regenerates. An important observation in this presentation was the abnormal rate-intensity functions (in the 12-day recovered cells) reported for frequencies served by that region of the sensory epithelium where the tectorial membrane defect was found. This observation may be related to sustained structural damage to the short hair cell region of the papilla and/or alterations in the efferent control of papilla function mediated by the short hair cells.

Aging↗

PubMed1996

The ontogeny of hepatic growth hormone receptor and insulin-like growth factor I gene expression in the sheep fetus during late gestation: developmental regulation by cortisol.

The effects of cortisol on hepatic GH receptor and insulin-like growth factor-I (IGF-I) gene expression were investigated in sheep fetuses during late gestation and after experimental manipulation of plasma cortisol levels by fetal adrenalectomy and exogenous infusion of cortisol. Hepatic GH receptor and IGF-I messenger RNA (mRNA) levels increased with increasing gestational age in parallel with the normal rise in fetal cortisol levels toward term (145 +/- 2 days). These increases in mRNA abundance toward term were prevented when the prepartum cortisol surge was abolished by fetal adrenalectomy and were stimulated prematurely in fetuses younger than 130 days by exogenous infusion of cortisol. Both the class 1 and class 2 transcripts of the IGF-I gene were increased when cortisol levels were elevated either endogenously or exogenously. However, there were no significant changes in fetal plasma IGF-I levels either with increasing gestational age or in response to experimental manipulation of the fetal cortisol level. When the data from all the fetuses were combined irrespective of treatment or gestational age, there were significant positive correlations between the log plasma cortisol concentration in utero and the abundance of GH receptor and IGF-I mRNA in the fetal liver. There was also a significant inverse relationship between log plasma cortisol and the ratio of class 1 to class 2 transcript abundance in the fetal liver. These findings show that cortisol is a physiological regulator of hepatic GH receptor and IGF-I gene expression in fetal sheep during late gestation and indicate that it preferentially increases the class 2 transcript of the IGF-I gene. The prepartum cortisol surge therefore appears to have an important maturational role in initiating the perinatal switch from the fetal to adult modes of somatotrophic regulation.

Adrenalectomy↗

PubMed1996

Effects of growth hormone administration and dietary protein intake on insulin-like growth factor I and growth hormone receptor mRNA Expression in porcine liver, skeletal muscle, and adipose tissue.

The effects of growth hormone (GH) and dietary protein on expression of IGF-I and GH receptor (GHR) genes in liver, muscle, and fat of pigs were investigated. Forty-eight intact male Large White x Landrace pigs were allotted to eight treatment groups (four diets with or without GH). The pigs were restriction-fed one of four diets, which differed only in their protein content (9.9, 13.1, 16.2, and 19.4%, as-fed basis), for a total of 3 wk. The pigs were then injected intramuscularly with either porcine GH (50 micrograms.kg-1.d-1 of rpST) or vehicle for the last 7 d. Pigs were slaughtered 4 h after the final injection. Total RNA was extracted from all tissues and then RNase protection assays were performed to measure expression of IGF-I and GHR genes. Expression of IGF-I mRNA was found to be GH responsive in the liver, semitendinosus (ST), and adipose tissue (P < .01) but not in longissimus muscle (LD). Dietary protein increased IGF-I expression only in the adipose tissue (P < .01). Expression of class 2 transcripts of IGF-I were observed only in the livers of GH-treated pigs, with no effect of dietary protein. Expression of GHR mRNA was found to increase with GH administration in liver and skeletal muscle (LD and ST, P < .05) but not in adipose tissue. There were diet x GH interactions on GHR expression in liver, ST, and adipose tissue, resulting in the highest GHR expression being in the high protein-fed, GH-treated group for liver, but in the low protein-fed, GH-treated group for muscle and adipose tissue. This study demonstrates tissue-specific control of expression of the two genes and also tissue-specific promoter usage (IGF-I exon 2 in liver) in response to GH administration.

Adipose Tissue↗

PubMed1996

Functional changes in the aging mouse middle ear.

Laser interferometry was used to measure sound-induced umbo velocity in the aging mouse middle ear. Velocity reductions of as much as 8 dB were seen as the mice aged. These functional differences suggest a variety of structural changes that may occur in the aging middle ear.

Acoustic Stimulation↗

PubMed1996

Hair bundle morphology on surviving hair cells of the chick basilar papilla exposed to intense sound.

Exposure to intense sound produces a well-defined "patch" lesion on the chick basilar papilla in which 30-35% of the short hair cells are lost. The present study compares various aspects of sensory hair bundle morphology on surviving hair cells in the patch lesion with hair bundles from matched locations on nonexposed control papilla immediately after removal from the exposure and 12-days post exposure. The height and thickness of the hairs, the total number of hairs in the bundle, the width of the bundle, and the area and perimeter of the apical surface of the hair cell were quantified from scanning electron microscope photomicrographs. An attempt was also made to determine if there was a consistent microstructure to the pattern of hair cell loss within the lesion area. Similar observations in 12-day recovered ears are also presented. The results indicated that stereocilia height increased and width decreased on surviving hair cells in the exposed ear. The width of the hair bundle, the hair cell surface area, and perimeter also decreased. However, the number of hairs per cell remained unchanged, and there was no evidence of any consistent organization to the hair cell loss within the patch across a number of specimens. These observations indicated that the hair bundles on short hair cells underwent changes as a consequence of intense sound exposure. The results after 12 days of recovery were complicated by developmental changes on the papilla and incomplete maturation of the newly regenerated hair cells. It remains to be seen whether these changes were the result of cell sampling in the sound-damaged ear or were due to true structural alterations within the sensory hairs themselves.

Animals↗

PubMed1995

Stiffness changes in chick hair bundles following in vitro overstimulation.

1. The in vitro effect of intense stimulation on the micromechanical stiffness of hair cell sensory hair bundles was studied at three locations on the chick basilar papilla. Threshold levels of hair bundle motion, produced by a water jet stimulus, were examined before and after exposure to a 300 Hz water jet stimulus set at 25 dB above the pre-exposure threshold level. 2. Threshold levels of motion were systematically examined in 8 unexposed control cells. The level of water jet stimulus needed to achieve the detection threshold of motion remained constant in these cells when periodically tested over a 36.5-min interval. 3. Post-exposure changes in the motion detection threshold of hair bundles were examined in 82 hair bundles, and a number of effects were identified: 2.4% of the hair bundles showed no threshold changes; 31.7% of the hair bundles had threshold shifts which indicated an increase in stiffness; 18.3% exhibited a threshold shift that indicated a decrease in hair bundle stiffness, but with no recovery; and 47.6% had thresholds that indicated a decrease in hair bundle stiffness with recovery to pre-exposure levels within 16-18 min. 4. The results suggest that chick hair bundles exhibit complex and varied responses to overstimulation which are very different from that seen in the mammal.

Animals↗

PubMed1995

Hair cell replacement in the avian inner ear following two exposures to intense sound.

The present study is concerned with the degree to which the avian cochlea retains the capacity to regenerate hair cells following repeated exposures to intense sound. Two groups of chicks were exposed once to an intense pure tone for 48 h at either 2 or 16 days of age. In a third group, they were exposed to the same stimulus at both ages. Structural alterations of the auditory epithelium were assessed both qualitatively and quantitatively at 0, 12 or 26 days following the single exposure at 2 days of age, and at 0 or 12 days following the single or second exposure at 16 days of age. The numbers of hair cells lost in the twice-exposed birds and those exposed once at 2 days of age were approximately 24 and 36%, respectively, and were not significantly different. Interestingly, the single exposure at 16 days of age caused greater hair cell loss (61%). Twelve days after overstimulation, the hair cell population in all experimental groups returned to near normal levels due to the emergence of new hair cells. This observation of hair cell replacement extends the early findings that birds are able to repair their acoustically damaged ears after either a single or repeated overexposure.

Animals↗

PubMed1995

The effects of exposure to intense sound on the DC endocochlear potential in the chick.

Chicks were exposed to an intense pure tone (0.9 kHz, 120 dB SPL) for 48 h. The DC endocochlear potential was measured with a microelectrode inserted into scala media in six separate groups of animals between 0 and 12 days post exposure. Similar data were obtained from seven groups of unexposed control chicks between 2 and 15 days of age. One to nine animals were tested in each control or exposed group. The results in the control chicks revealed that the EP at 2 days of age (6.7 mV) was 36% of the mature value (15.2 mV) noted at 6 days of age. In the exposed animals, at 0-days recovery, the EP showed a 63% reduction relative to that measured in age matched control chicks. The level of EP in the exposed animals quickly recovered, and by 3 and 6 days post exposure exhibited a loss of only 7 and 3 percent relative to the age-matched controls. The recovery of EP was plotted against the recovery of evoked potential thresholds reported by others, and the time-course of the recovery functions were very similar. This relationship suggested that recovery of auditory function in the chick was highly correlated with the restoration of the EP. Damage to the tegmentum vasculosum and its capacity to secrete potassium, as well as the shunting of current through the damaged basilar papilla, are considered as mechanisms for the reduction of EP loss in the exposed ear.

Acoustic Stimulation↗

PubMed1995

Hormonal control of insulin-like growth factor-I and growth hormone receptor mRNA expression by porcine hepatocytes in culture.

The effects of various hormones commonly added to hepatocyte culture media upon the expression of the GH receptor (GHR) and insulin-like growth factor-I (IGF-I) genes in cultured porcine hepatocytes were investigated. Preliminary investigations indicated that there was an absolute requirement only for insulin, with high losses of cell viability upon long term exclusion of insulin from the culture medium. The decline in GHR expression with time in culture was found to be less when high levels of glucose were included in the medium. Therefore the basal culture medium used in these studies was Williams' medium E supplemented with 0.2% (w/v) BSA, 5000 mg glucose/l and 100 nmol porcine insulin/l. The addition of dexamethasone (10 nmol/l) increased the expression of both GHR and IGF-I (class 1 transcripts only) mRNA (p < 0.001 and P < 0.05 respectively), and resulted in an increased responsiveness of IGF-I mRNA expression to GH (1 microgram/ml), when the two were added in combination (although only class 1 transcripts were shown to be statistically significant, P < 0.01). The addition of either thyroid hormone (1 nmol/l T3 or T4) alone also increased the expression of GHR mRNA (P < 0.01) in addition to the dexamethasone stimulated expression, with T4 appearing to decrease IGF-I expression slightly (P < 0.05) (either on its own or with T3). As with dexamethasone, the thyroid hormones increased the response of IGF-I mRNA expression to GH (1 microgram/ml) when added in combination with GH (P < 0.001). These observations demonstrate one possible mechanism for the interactions of glucocorticoids and thyroid hormones with the GH-IGF axis.

Animals↗

PubMed1994

Intra- and extracellular calcium modulates stereocilia stiffness on chick cochlear hair cells.

Segments of the chick basilar papilla were isolated and maintained in culture medium. The sensory hair bundle of individual hair cells was observed with light microscopy and stimulated with a water microjet at 600 Hz. Hair bundle motion was slowed by illuminating the microscope with stroboscopic light, and water jet intensity was systematically varied in decibel (dB) steps until a visual detection level (VDL) threshold of hair bundle motion was achieved. The VDL threshold of many hair cells was measured in each isolated papilla. However, only one of eight extracellular calcium concentrations (0.0, 0.0001, 0.001, 0.01, 0.1, 1.25, 6.0, and 12.0 mM) was used with each papilla. In a second series, a calcium ionophore (ionomycin) was added to the culture medium, and VDL thresholds were again measured at seven of these extracellular calcium concentrations. With extracellular calcium alone, the stimulus level needed to achieve threshold was reduced by 2.73 dB between 0.1 and 0.01 mM. This change in threshold represented a 1.37-fold decrease in hair bundle stiffness. When ionomycin was added to the culture medium, a progressively greater stimulus intensity was needed to achieve threshold as calcium concentration increased. The 11.7-dB increase in threshold, with the addition of ionomycin, between 0.0001 and 6.0 mM extracellular calcium was equivalent to a 3.85-fold increase in bundle stiffness. These large changes in hair-bundle stiffness, as a function of the extra- or intracellular calcium environment, may play an important role in the micromechanical behavior of the hair cell during sound simulation.

Animals↗

PubMed1994

Middle-ear development. VI: Structural maturation of the rat conducting apparatus.

BACKGROUND: The contribution of middle-ear development to the overall development of hearing has not been explored in great detail. This presentation describes the maturation of conductive elements in the rat middle ear, and provides the basis on which future studies of middle-ear functional development will follow. METHODS: The middle-ear apparatus was examined at nine different ages (between 1 and 80 days postpartum) in Long Evans rats. At each age elements of the conducting apparatus was observed with either light or scanning electron microscopy (SEM), and quantitative measurements were made from video enhanced photomicrographs. Tympanic membrane area and cone depth, the length of the malleus and incus arms, ossicular weight, stapes foot plate and oval window areas, and bulla volume were all measured. Development of the area and lever ratios were derived from these measurements. The data were fitted to exponential equations and the time in days required to reach 90% of the adult level determined. RESULTS: The pars tensa achieved 90% of total area by 17 days. The oval window achieved the 90% criterion by 13 days, while the area ratio was within 10% of its adult size by 8 days. The ossicles took between 26 and 34 days, while bulla volume took 59 days to reach the 90% level. CONCLUSIONS: Middle-ear growth was very orderly and systematic in the data reported. When maturation of the area ratio was considered against development of the endocochlear potential or the round window compound action potential, it was clear that the growth of this important aspect of the middle ear preceded the onset of cochlear function.

Aging↗

PubMed1994

Middle-ear development. IV. Umbo motion in neonatal mice.

Laser interferometry was used to measure umbo velocity in the developing BALB/c mouse middle ear at 133 pure-tone frequencies between 2.0 kHz and 40.0 kHz, all at a constant 100 dB sound pressure level. Umbo velocities increased with age across the entire frequency range, and reached adult-like levels by about 19 days between 2.0 and 22.0 kHz. Velocities at 28.0 and 34.0 kHz took 27 and 52 days respectively to reach adult-like levels. A simple middle-ear model utilizing compliance, resistance, and inertia elements matched the general trends of our velocity results and provided an indication of the anatomical basis for the growth in umbo velocity. The model suggested that velocity development at the lowest frequencies may be attributed to increases in tympanic membrane compliance. The model also indicated that both the frictional resistance of the middle ear and the inertia of the tympanic membrane and ossicles decreased during the growth period. At frequencies below 20.0 kHz, age-related increases in umbo velocity coincided with improvements in N1 thresholds recorded from the round window and evoked potential thresholds obtained from the cochlear nucleus. These results indicated that the functional development of the middle-ear plays a major role in the development of hearing in the mouse.

Acoustic Stimulation↗