Ballooning of myelin sheaths in normally aged macaques.

In aged animal brains, a variety of "holes" are formed in the neuropil. One type of hole, here designated as the myelin balloon, is an abnormality of the myelin sheath and is found in a number of diverse sites in the brain. Profiles of myelin balloons display rather smoothly rounded peripheral contours and typically range up to 10 microm in diameter, although exceptionally large examples may be twice this size. The balloons are bounded by lamellae of myelin, and to accommodate the contents of the balloon, the myelin sheath becomes split at the intraperiod line. Since the intraperiod line is formed by the apposition of the outer faces of the myelin-forming plasma membrane, the contents of the myelin balloons are, in effect, in continuity with the extracellular space, and it is suggested that the contents of the balloons are fluid, with the fluid exerting an outward pressure on the walls of the balloons to produce their spherical shapes. Myelin balloons are not only produced during aging but also occur in a number of genetic strains of mice and in a number of human disease states. They thus represent a non-specific, though distinctive and common, alteration of the myelin sheath and are a reflection of the fact that under a variety of conditions, including normal aging, oligodendrocytes are unable to maintain the integrity of their sheaths.

Cochlear spiral ganglion cell degeneration in wild-caught mice as a function of age.

Presbyacusis in humans is an age-related bilateral sensorineural hearing impairment generally associated with degeneration of cochlear hair cells and spiral ganglion cells (SGC) predominantly in the basal turn but present in the apical turn. Investigations of cochleas of aged rats and gerbils reveal a large loss of SGCs in the apical as well as the basal turns. Genetically inbred aged mice, on the other hand, seem to have variable amounts of SGC loss beginning in some strains very early in the life span of the animals and greatest in the basal turn. Three age groups of wild-caught, then laboratory-bred, mice were investigated to determine the pattern of SGC degeneration. In 18-19-month-old animals the main loss of SGCs occurred in the basal turn (49% loss compared to 2-3 months) followed by the apical turn (31%). The greatest SGC losses in the 28-31-month-old animals were in both the apical (76%) and basal turns (74%). Thus, this strain of mice is similar to other rodents in that both ends of the ganglion are affected by SGC degeneration associated with aging.

Purkinje-like cells in rat cochlear nucleus.

A unique class of cells, strongly immunopositive for anti-calbindin D-28 kDa was observed in and near the cochlear nucleus of young adult, male Sprague-Dawley rats. These cells are present in small numbers which are highly variable across animals and inconstant in position. They are preferentially located in the dorsal cochlear nucleus, with occasional examples being present in the ventral cochlear nucleus, as well as in adjacent brainstem locations. They have been referred to in other studies as displaced Purkinje cells or 'Purkinje cell-like cells', and are here designated 'Purkinje-like cells' (PLCs). PLCs have relatively large cell bodies, with thick, heavily spined dendrites, and are typically situated in an immediately subpial position. The dendritic arborization extends into the interior of the nucleus, away from the pial surface, a trajectory opposite in direction to that of the cerebellar Purkinje cells. The intense immunoreactivity exhibited by PLC somata and dendrites when treated with antiserum directed against calbindin is equivalent to that of cerebellar Purkinje cells, and markedly stronger than that of most other cell populations of the cochlear nucleus. However, in tissue treated with anti-parvalbumin, which also strongly labels cerebellar Purkinje cell somata and dendrites, PLC labeling, when present, is relatively weak, limited to the cell bodies and only the base of the dendrites of PLCs, indicating non-equivalence of the two cell types. In addition, the intensity of calbindin immunostaining in the PLCs appears to be more sensitive to glutaraldehyde in any of the fixative solutions than that seen in cerebellar Purkinje cells in the same sections. Of the cell types of the cochlear nucleus, the cartwheel cells would appear to be the most similar to the PLCs on morphological and immunocytochemical grounds. However, the subpial position and average somal dimensions of the PLCs, as well as the relatively modest immunoreactivity of the cartwheel cells for calbindin, rather clearly differentiate the PLCs from this class of neurons. The results of the present study suggest that the PLCs of the cochlear nucleus, although they may arise developmentally as ectopic cerebellar Purkinje cells and maintain certain Purkinje cell characteristics, represent a distinct neuronal cell type in the adult rat cochlear nucleus, exhibiting incomplete overlap of fixation, immunocytochemical and morphological characteristics with both cartwheel cells of the cochlear nucleus and cerebellar Purkinje cells.

Magnetic resonance imaging diagnosis of discoid medial meniscus.

A case of discoid medial meniscus of the massive type diagnosed preoperatively by magnetic resonance imaging is presented. Additionally, the preoperative studies and arthroscopic findings are presented.

Cochlear degeneration in aged rats of four strains.

Animals with various degrees of inbreeding, some of which are albino, are frequently used in biological research. Albinos do not produce melanin and it is therefore absent from the cochlea. While the function of melanin is unknown, it has been hypothesized that it is involved in cochlear homeostasis. It is possible then, that age-related degeneration may be affected by the presence or absence of melanin. We therefore evaluated young (2-6 months old) and aged (24-36 months old) cochleas in 4 different rat strains: albino Fischer 344 and Lewis rats and pigmented Lewis-Brown Norway F1 rats and Brown Norway rats. Cochlear morphology was the same across all strains of young adult animals with the exception that the pigmented animals had small, darkly stained granules in the stria vascularis. The aged pigmented animals all had large granules as well as small ones. Degeneration of spiral ganglion cells in the apical region of the ganglion had occurred in the old animals of all strains. Strial degeneration at the apex was also present in aged animals. There was no correlation between the presence or absence of melanin and the magnitude of cochlear degenerative changes in the aged animals. The presence or absence of melanin therefore, appears to have no effect on cochlear degeneration in the aged rat cochlea.

Morphology of the cochlear nerve in Sprague-Dawley and brown Norway rats.

Cochlear nerve morphology was studied in young adult albino (Sprague-Dawley) and pigmented (Brown Norway) rats. Analysis of the material included counts of normal and degenerating fibers and of glial cell nuclei, and measurements of vascularity and of the nerves' cross-sectional areas. The median number of normal fibers in the Sprague-Dawley rats was 21,216, and, in the Brown Norway rats, it was 20,186. There were no statistically significant differences between the two strains in numbers of normal fibers, degenerating myelin sheaths, or glial cell nuclei, or in the cross-sectional areas of the nerves. The area density of blood vessels was significantly higher in nerves from the Sprague-Dawley rats. The median area density in that strain was 0.0149, while in the Brown Norway rats the median area density was 0.0105.

Changes with age in the morphology of the cochlear nerve in rats: light microscopy.

Cochlear nerve morphology was examined in a series of rats ranging in age from young adulthood to advanced age in order to assess the extent of fiber loss and the nature of degenerative changes with age. The animals were perfused via the aorta with mixed aldehydes. Blocks including the cochlear nerves were removed, embedded in Araldite, and sectioned in a plane transverse to the longitudinal axis of the nerve. Analysis of the material included counts of normal and degenerating fibers and of glial cells, maps of fiber packing densities, and measurements of the cross-sectional area of the nerve. The median number of normal fibers in the young adult animals (2-3 months) was 21,218. This number was reduced by 21% at 26.5 months and by 24% in the oldest group (35-36 months). The number of degenerating myelin sheaths was first seen to be significantly increased at 6 months, reached a peak at 26.5 months, and declined at 35-36 months. There was an age-related increase in the cross-sectional area of the nerve, amounting to about 60% at 26.5 months and to about 50% at 35-36 months. Fiber packing density decreased evenly with age over the area of the nerve. The increased cross-sectional area and decreased fiber packing density appeared to be related to increases in the thickness of myelin sheaths and in the area occupied by interneural elements.

Degeneration in the cochlear nerve of the rat following cochlear lesions.

Left unilateral cochlear lesions were performed on 26 albino rats at 1.5 months of age. After survival times ranging from 1 h to 6 months, the animals were perfused via the aorta with mixed aldehydes. Blocks including the cochlear nerves were removed, embedded in Araldite, sectioned in a plane transverse to the longitudinal axis of the nerve, and analyzed in the light microscope. Degenerating fiber profiles were grouped into 4 categories, and their relative frequencies were counted, as were numbers of normal fibers and glial cell nuclei. The cross-sectional areas of the nerves were measured. Lesion extent was evaluated by means of sections through operated cochleas from short and long survival times, and right cochlear nerves from 11 of the animals were used as controls. In the left nerves, segmental swelling of fibers occurred as early as 16 h survival, followed by collapse of fibers and breakdown of myelin sheaths. Starting at 36 h survival, increased numbers of glial cells were seen in the nerve. At longer survival times there were decreases in the cross-sectional area of the nerve and in the packing density of degenerating fiber profiles. At the longest survival times, a substantial amount of debris remained which resembled that seen in early stages. Finally, there was evidence of continued loss of nerve fibers occurring over a period of weeks to months.

Age-related loss of synaptic terminals in the rat medial nucleus of the trapezoid body.

The effect of aging on axosomatic synaptic terminals in the rat medial nucleus of the trapezoid body was studied using quantitative electron microscopy. In young adult rats (3 months of age), the mean percentage of the surface area of principal cells covered by synaptic terminals is 61.7% (S.E.M. = 4.1) while in aged animals (27-33 months of age) the per cent coverage is 43.7% (S.E.M. = 3.3). Likewise, between 3 and 27-33 months of age, the average number of synaptic terminals present along a 100 micron length of principal cell surface decreases significantly (P less than 0.001) from 28.3 (S.E.M. = 1.3) to 18.9 (S.E.M. = 1.3). Only terminals derived from calyces of Held are lost in the aged animals, displaying a 37% reduction between 3 and 27-33 months of age. The length of apposition by synaptic terminals in the medial nucleus of the trapezoid body does not change significantly with aging. We conclude that because of a significant loss of calycine synaptic endings, the structure of calyces of Held becomes less complex with advancing age in rats. This would presumably result in an age-related partial deafferentation of principal cells, causing significant alterations in the processing of auditory information in the medial nucleus of the trapezoid body.

Control of responding by the location of sound: role of binaural cues.

In auditory localization experiments, where the subject observes from a fixed position, both relative sound intensity and arrival time at the two ears determine the extent of localization performance. The present experiment investigated the role of binaural cues in a different context, the sound-position discrimination task, where the subject is free to move and interact with the sound source. The role of binaural cues was investigated in rats by producing an interaural imbalance through unilateral removal of the middle auditory ossicle (incus) prior to discrimination training. Discrete trial go-right/go-left sound-position discrimination of unilaterally incudectomised rats was then compared with that of normal rats and of rats with the incus of both sides removed. While bilateral incus removal affected binaural intensity and arrival times, the symmetry of sound input between the two ears was preserved. Percentage of correct responses and videotaped observations of sound approach and exploration showed that the unilateral rats failed to localize the sounding speaker. Rats with symmetrical binaural input (normal and bilaterally incudectomised rats) accurately discriminated sound position for the duration of the experiment. Previously reported monaural localization based upon following the intensity gradient to the sound source was not observed in the unilaterally incudectomised rats of the present experiment. It is concluded that sound-position discrimination depends upon the use of binaural cues.

Aging in the rat medial nucleus of the trapezoid body. II. Electron microscopy.

The medial nucleus of the trapezoid body (MTB), the largest cell group of the rat superior olivary complex, was studied ultrastructurally in five groups of rats ranging in age from 2 to 33 months. Relatively little aging pigment (lipofuscin) accumulates intraneuronally, even in rats aged 33 months. The heaviest accumulation of age pigment is in glial and perivascular cells of the MTB. In rats aged 24-33 months, there is evidence of axonal and dendritic degeneration in the MTB neuropil. Aged rats also show evidence of degeneration of nerve endings. This includes the loss of calycine axosomatic terminals synapsing with MTB principal cells. All of the ultrastructural changes observed in the MTB gradually increase in frequency with advancing age.

Aging in the rat medial nucleus of the trapezoid body. III. Alterations in capillaries.

The medial nucleus of the trapezoid body (MTB), a large cell group in the rat brainstem auditory pathway, undergoes significant cell loss and loss of synapses with advancing age [5,6]. The purpose of the present study was to examine the microvasculature of the MTB in rats of the following ages: 3 months (MO), 6 MO, 24 MO, 27 MO, and 33 MO. In rats aged 24 to 33 MO, the following ultrastructural changes were observed in MTB capillaries: (1) large cavitations or spaces within capillary basal laminae, and (2) membranous debris, indicative of cellular degeneration within leaflets of capillary basal lamina. The volume density ratio (VDR) of capillaries decreases significantly between 6 and 33 MO of age.

The spiral ganglion and hair cells of Bronx waltzer mice.

Inner hair cells and spiral ganglion cells were counted in a mutant mouse strain (Bronx waltzer) reported to have very few of these cells (6) in order to determine if the remaining ganglion cells would be predominantly type II cells. These cell counts indicate a 50% reduction of spiral ganglion cells in Bronx waltzer cochleas compared to normal mice. Averaged throughout the cochlea about 11% of the remaining cells are type II cells while in normal mice this percentage is 5%. In some regions however, as many as 20% of the remaining cells are type II cells. Counts of IHC in surface preparations reveal 37 normal looking IHC (about 5% of the normal population) in each of two Bronx waltzer mouse cochleas. There were also about 120 shrunken IHC in each cochlea, representing 17% of the normal cell population. While there appears to be an increased proportion of type II cells in the Bronx waltzer spiral ganglion there are also many more type I cells than might be expected from the small number of IHC.

The effect of aging on the neuronal population within area 17 of adult rat cerebral cortex.

The brains of Sprague-Dawley rats in various age groups from 3 to 33 months were fixed by perfusion with standard aldehyde solutions in order to determine the effects of aging on neuronal numbers. Several indices of cortical volume were then measured to determine whether neuronal packing densities were affected by age-related change in cortical volume. The lengths, heights and widths of individual hemispheres for 160 animals ranging in age from 1 day to 36 months were first determined, after which blocks of tissue were removed from area 17 of some of the brains. These blocks were osmicated, embedded in Araldite and sectioned at 1 micrometer to ascertain, in the vertical plane, the thickness of area 17 and, in the tangential plane, the packing density of the clusters of apical dendrites extending from layer V pyramidal neurons. Results indicate the overall dimensions of the cerebral hemispheres increased until 3 months of age, after which there was no further increase in size. Between 3 and 33 months of age there was no age-related change in either the thickness of area 17 or in the separation between dendritic clusters, indicating the volume of area 17 did not change after 3 months of age. Within individual age groups the amount of variation present is greater than that among age groups. Since the number of nucleus-containing neuronal profiles per unit area of layers II/III, IV, V, VIa and VIb was similar in two groups of three animals at 3 and 33 months of age and the diameters of neuronal nuclei were unchanged, there seems to be no significant change in the number of neurons contained in these layers of rat visual cortex between 3 and 33 months of age. It is therefore concluded that no neurons are lost from area 17 as the mature cerebral cortex ages.

Hair cell counts in an age-graded series of rat cochleas.

Hair cells of Sprague-Dawley rats aged 2-33 months were counted in order to assess the magnitude, location and time course of cell degeneration. The mean number of hair cell places (hair cells plus phalangeal scars) was approximately 4700: 960 inner hair cell places and 3470 outer hair cell places. These numbers do not vary systematically with age. Hair cell degeneration was observed in all animals. At 31-33 months of age, animals had inner hair cell losses ranging from 1.6 to 4.2% and outer hair cell losses ranging from 2.1 to 23.3%. The loss of hair cells was greatest in the upper apex, where the 31-33-month-old animals had 3.1-9.2% inner hair cell losses and 7.4-46.8% outer hair cell losses. Outer hair cell losses were also large in the basal end, where inner hair cell losses were small. In the older animals, hair cell losses were consistently most prominent in the third row of outer hair cells. Following examination of the hair cell population, the ganglion cells in the apical region were evaluated in a number of cochleas. No significant correlation was found between the magnitude of inner hair cell and ganglion cell losses.

Aging in the rat medial nucleus of the trapezoid body. I. Light microscopy.

The medial nucleus of the trapezoid body (MTB), a cell group in the superior olivary complex, was examined in an age-graded series of rats for neuron loss, changes in the giant synaptic endings (chalices of Held) on MTB neurons, and accumulation of age pigment. Neuron counts were done on protargol-stained paraffin sections of MTBs from a series of 17 rats aged 2-3, 6, 18, and 24 months. Between 2-3 and 24 months a 34% decrease in the mean number of MTB neurons was observed. Significant loss (p less than 0.05) was first evident in the early portion of the life span, between 2-3 and 6 months. In thionin-stained sections, there was no change with aging in the proportions of three MTB neuron types: principal cells (approximately 82%), elongate cells (approximately 15%), and stellate cells (approximately 3%). In young adult rats, 25-26% of all MTB neurons were associated with identifiable chalices of Held in protargol stained sections. This ratio did not vary significantly with aging. Age pigment accumulation in the MTB was examined in 2 micrometers Araldite sections stained with toluidine blue. Age pigment deposits were larger and more numerous in the MTBs of old animals, but not as extensive as has been described previously in many other parts of the nervous system. This study is the first to report neuron loss in an animal brainstem nucleus.

Ultrastructural changes in atrial myocardium of the ageing rat.

Ageing changes in the fine structure of atrial myocardial cells were studied in rats ranging from 2-32 months in age. The most striking change observed was the increasingly frequent appearance, from about 6 months onwards, of helically aggregated strands containing filaments which in respects other than arrangement bore resemblance to thick myofilaments. Other ageing changes included the accumulation of dense bodies and various types of mitochondrial degradation.

A technique for estimating total spine numbers on Golgi-impregnated dendrites.

The functional significance of dendritic spines and their morphological sensitivity to a wide spectrum of experimental manipulations and pathological states have led to a number of studies in which counts of dendritic spine number have been carried out. These studies have, for the most part, involved the enumeration of only those spines which protrude from the opaque shafts of Golgi-impregnated dendrites into the clear zones flanking the dendrite. Such counts, limited to only those spines which are visible, underrepresent the true total number of spines borne by the dendrites. The magnitude of underrepresentation correlates positively with dendritic shaft diameter and negatively with spine length. This seriously restricts the usefulness of comparisons of spine density between dendrites, or even between segments of the same dendrite. In the present report, a geometrically based method is presented whereby total dendritic spine numbers can be estimated with reasonable accuracy, taking into account factors such as dendrite diameter and spine length. The technique entails the following principal steps: a determination, for a given length of dendrite over which spines are to be enumerated, of the volume of the flanking zones in which spines are visible and can be counted; a determination of the volume of the entire zone which encircles the dendritic shaft and which contains all spines, both visible and not visible; and a proportional extrapolation from the number of visible spines to obtain an estimate of the true total spine number. Tests of the predictive accuracy of the technique using dendrites of known total spine number suggest that estimates which deviate from true total spine numbers by less than 10% can be achieved.