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G Rager

Publications and source records attributed to G Rager.

At least 37 records · Page 2Linked to original sources
PubMed1992

Postnatal development of area 17 callosal connections in Tupaia.

The goal of the present study was to investigate the pattern of maturation of callosal projecting neurons in a well-studied mammalian visual system with unique structural and functional properties. Studies of the distribution pattern of interhemispheric connections in the adult tree shrew primary visual cortex reveal not only a high concentration of labeled neurons along the area 17/18 border, as in standard experimental animals such as the cat and monkey, but also numerous callosal projecting neurons in the adjacent dorsal part of area 17, which largely corresponds to the binocular visual field (Kretz and Rager, Exp. Brain Res. 82:271, '90). Callosal projections were anatomically traced in 11 tree shrews (Tupaia belangeri) at various ages between postnatal day 7 (7, 9, 10, 13, 15, 17, 19, and 26 days old) and adulthood (107 days old). In each animal, four injections of wheat germ agglutinin conjugated to horseradish peroxidase were made in a standard configuration into the striate cortex of one hemisphere. In young tree shrews only 7 and 9 days old, heavily labeled terminal axon structures could be seen in the white matter and in layer VI of the opposite hemisphere. Only a few labeled neurons, however, were detected in layer III. The small number of labeled neurons indicated that early in postnatal development, only a few callosal axons had invaded the upper cortical layers. By 10 days of age, the number of supragranular neurons was increasing and the maximal value was counted in a 13-day-old tree shrew. A sharp decline in the number of labeled supragranular neurons was noticed--about 94% in our case--between days 13 and 15. In animals more than 15 days old, the distribution pattern and the density of the neurons looked like the pattern seen in the adult Tupaia brain. The labeled cells were mostly concentrated in layers II and III. The majority of neurons resembled typical pyramidal cells. However, some of the neurons in sublayer IIIc had elongated cell bodies oriented parallel to the laminar boundaries. In contrast to the supragranular cells found in all stages investigated, small populations of labeled cells in layer VI were observed in 9- to 17-day-old tree shrews only. In young postnatal animals 7 to 13 days old, a peculiar cell type was labeled on the ipsilateral side. In coronal sections these cell bodies formed a continuous band that extended from the ventricular wall to the subcortical white matter. These cells might belong to a population of cells still in migration.

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PubMed1992

Organization of the optic chiasm in the hatched chick.

In the hatched chick the fibers of the two optic nerves segregate into clearly defined bundles when they cross to the other side. These bundles run in horizontally oriented tiers. The tiers are demarcated by blood vessels and pial tissue. The organization of these tiers was investigated qualitatively and quantitatively using light and electron microscopy as well as tracer techniques. The fibers within the tiers cross to the contralateral optic tract without leaving their respective tier. The mean total number of tiers is 34 with a great individual variation. A preference in the superposition of one side over the other could not be observed. Comparing these data with our earlier study (Rager et al.: Anat. Embryol., 179:135-148, 1988) it can be concluded that neither the segregation of fibers into discrete bundles nor the variability in the number of alternating tiers seem to disturb the topography of fibers as it is achieved in the optic nerve. The pattern of vascularization correlates with the order of crossing axon bundles and contributes to the demarcation of the tiers. The chiasm is vascularized by the Aa. preopticae and the A. infundibularis.

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PubMed1991

Synaptogenesis in the primary visual cortex of the tree shrew (Tupaia belangeri).

The primary visual cortex of the tree shrew is characterized by the lack of ocular dominance columns. The two eyes are represented in sublayers of laminae 3 and 4. In an earlier study using the transneuronal transport we observed that the geniculate afferents from the two eyes do not initially overlap and then segregate into their appropriate sublaminae. The final distribution pattern can already be observed during the early postnatal period. Since segregation and elimination of afferent terminal branches do not seem to take place, we wanted to investigate whether or not an overproduction of synapses can be observed as in several other animals. We examined layers 3B, 3C, 4A, and 4B, which receive afferents from the retina via the lateral geniculate nucleus, from P5 to maturity by using the electron microscope. The brain tissue was excised in the region where the central vision is represented in adult animals. Then we determined the density of synapses per 100 microns 2 neuropil for each of the four sublayers at the ages P5, P15, P19, P23, P31, and P42 and in the adult animal (AD). In determining the neuropil we measured the size of two additional compartments, i.e., the compartments consisting of perikarya and of blood vessels. At a higher resolution we determined the fraction of Gray type I and type II synapses in each sublamina and in each developmental stage. The size of the neuropil increases from 57% at P5 to 81% in AD whereas the compartment of perikarya decreases from 42% to 15% and the compartment of blood vessels increases from 1.3% to 3.9%. The synaptic density starts with very low values (3.5/100 microns 2) at P5. Then it increases rapidly and attains a maximal rate of increase during the period of eyelid opening. After this period the increase is slowed down and approaches the adult value (12.5/100 microns 2) slowly. An overproduction of synapses could not be observed. The percentage of type I and type II synapses also changes during this period. The fraction of type I synapses amounts to 73% at P5 and increases to 92% in AD. The increase in density of type I synapses is continuous and does not show any sign of overproduction. The density of type II synapses rapidly reaches it final value and then remains constant. Possibly there is a slight overproduction during the period of eyelid opening.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1991

[The visual cortex of Tupaia: an alternative model?].

The tree shrew has become an interesting animal for research in neurobiology because of its unique structural and functional properties, in particular in its visual system. Some of these properties are described and discussed here as far as they are related to the functional architecture of the primary visual cortex: The representation of the ocularity, the ON and OFF responses, the development of the retino-geniculo-cortical projection, and synaptogenesis. From these data it appears that the tree shrew forms an important exception to the functional organization as it is found in standard experimental animals like cats and monkeys. It challenges a deeper thought on a common basis for both types of organization and development.

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PubMed1990

Reciprocal heterotopic callosal connections between the two striate areas in Tupaia.

WGA-HRP injections were placed into area 17 close to the border with area 18 of Tupaia belangeri in order to study the callosal connections of the striate area in this animal. Most callosal neurons were found in the striate cortex (57.6-86.9%), some in the extrastriate area 18 (10.6-28.1%), and a few in even more temporal regions (2.5-14.3%). Concerning only the area 17, reciprocal homotopic connections could be observed as a strip along the area 17/18 border. Additionally, heterotopic callosal connections could be seen in regions representing the binocular visual field, especially the lower part. The area 17 cells were mostly located in the supragranular layers II and III (94.1-97.2%). But neurons could also be found in the infragranular layers, especially layer VI (2.6-5.2%) and in layer IV (0.2-1.1%). Homotopic projections were mostly seen in layers IIIc and V. The majority of the supragranular and infragranular neurons are pyramidal cells. However, a newly defined subpopulation of neurons, most probably stellate cells, were discovered forming a band in sublayer IIIc, very close to the layer III/IV border.

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PubMed1990

Projection of topographic maps from curved brain surfaces into a plane.

The environment is represented in the brain by topographic maps. These maps and their interrelationship form a dominant feature in the brain, but unfortunately, they are often projected onto surfaces which are variously curved. It would be useful to be able to compare the topography on these surfaces quantitatively with the origin of the map. This comparison would be possible if the curved brain surfaces could be represented in planes. Therefore, it was necessary to develop a procedure for unfolding maps on convex structures like the retina and the optic tectum of the chick. For this purpose a unit sphere was constructed whose origin was close to the centre of gravity of the curved brain structure. The surface of this structure was then projected into the sphere. From its representation on the spherical surface it was unfolded into the plane using a polar azimuthal radially equidistant projection. Our method can safely be applied to convex structures only. However, it is not limited to the visual system. The mathematical procedure is formulated in such a way as to be easily implemented. It is exemplified by experiments on the optic tectum and the developing retina of the chick.

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PubMed1988

Callosal projections between areas 17 in the adult tree shrew (Tupaia belangeri).

In the primary visual cortex (area 17) of the tree shrew (Tupaia belangeri) neurons projecting to the contralateral area 17 via the corpus callosum were identified by horseradish peroxidase histochemistry (HRP, WGA-HRP). The distribution of homotopic and heterotopic connections was studied. We found that a narrow stripe of area 17 close to the dorsal area 17/18 border - which corresponds to the visual field along the vertical meridian - is connected via homotopic callosal projections. The adjacent dorsal part of area 17, which largely corresponds to the binocular visual field, is connected via homotopic as well as heterotopic projections. Heterotopic projections originate in the cortical stripe along the area 17/18 border and their contralateral targets are displaced medially. Callosal neurons are located mostly in supragranular but also occur in infragranular layers. The supragranular neurons in general are pyramidal cells. In addition to these findings, we confirmed earlier reports on ipsilateral projections of the primary visual area to the dLGN, the claustrum, area 18 and other visual areas.

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PubMed1988

Transformations of the retinal topography along the visual pathway of the chicken.

It is still unclear how the retinotectal map of the chick is formed during development. In particular, it is not yet known whether or not the organization of fibres plays a role in the formation of this map. In order to contribute to the solution of this problem, we analysed the representation of the retinal topography at closely spaced intervals along the fibre pathway. We injected HRP into various sites of the tectal surface and traced the labelled fibre bundles back to the retina. The retinal topography was reconstructed at ten different levels, i.e. in the retina, the optic nerve head, the middle of the optic nerve, the chiasm (three levels), the optic tract (three levels), and the optic tectum. We obtained the following results: (1) The labelled fibre bundles as well as the fields of labelled retinal ganglion cells were always well delimited and coherent. (2) The reconstructions show that transformations of the retinal topography occur in the fibre pathway. The first and most important transformation is found in the optic nerve head where the retinal image is mirrored across an axis extending from dorsotemporal to ventronasal retina. In addition, the retinal representation is split in its temporal periphery. Thus, central and centrotemporal fibres are no longer in the centre of the image but close to the dorsal border of the nerve. Peripheral fibres are found along the medial, ventral and lateral circumference of the nerve. In the optic tract a second transformation occurs. The retinal topography is rotated clockwise by about 90 degrees and flattened to a band. The flattening is accompanied by a segregation of fibre bundles so that eventually central and centrotemporal retinal fibres are located centrally, ventral fibres dorsally and dorsal retinal fibres ventrally in the tract. By these two transformations an organization of fibres is produced in the optic tract which can be projected onto the tectal surface without major changes given that dorsal and ventral fibres remain in their relative positions, and that deep lying fibres project to the rostral and central tectum, superficial fibres to the caudal tectum. The transformations which we have observed follow specific rules and thus maintain order in the pathway although retinotopy is lost. In conjunction with our earlier studies on the development of the retinotectal system we conclude that fibres are laid down in a chronotopic order. The transformations take place under particular structural constraints.(ABSTRACT TRUNCATED AT 400 WORDS)

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PubMed1987

Classes of neurons in relation to the laminar organization of the lateral geniculate nucleus in the tree shrew, Tupaia belangeri.

We used the rapid Golgi and horseradish peroxidase (HRP) techniques to study the dendritic spread of relay neurons in functionally distinct laminae of the tree shrew dorsal lateral geniculate nucleus (LGNd). On the basis of their dendritic spread in relation to laminar and interlaminar zones, we describe three classes of relay neurons. Unilaminar neurons with multipolar radiate, bitufted, and intermediate types of dendrites. Dendrites of these neurons are confined to one lamina only, but also can have some of their segments in adjacent interlaminar zones. Multilaminar neurons with multipolar radiate, bitufted, and intermediate types of dendrites. Independent of the site of their cell bodies in a laminar or interlaminar zone, these neurons spread their dendrites over two or more laminae. Interlaminar neurons whose cell bodies and dendrites are confined to a single interlaminar zone. Unilaminar neurons are found in all the laminae. In the medial three laminae, they are more of the radiate type, whereas in laminae 4 and 5 their dendrites tend to be more of a tufted nature. Lamina 6 shows a preponderance of the elongated bitufted type. Multilaminar neurons, although less common as compared to the unilaminar, are also observed in all the laminae. Some neurons have their dendrites confined to an interlaminar zone. By retrograde transport of HRP injected into the visual cortex, we have shown that these neurons are, in fact, relay neurons. In addition to relay neurons, there are small interneurons with "axoniform" dendrites and an unmyelinated axon whose arborization is confined within the limits of the neuron's dendritic spread. Neurons of this type are not labeled with HRP injected into the visual cortex. We conclude that although each lamina is functionally specialized by input from ipsilateral or contralateral retina and by segregation of neurons responding to on or off stimuli, some multilaminar neurons can be found in each lamina. Thus, laminar as well as interlaminar zones contain a class of neurons that could provide a cross-talk between the functionally specialized laminae. Most relay neurons in all the laminae, however, confine their dendrites to their home lamina. Thus, the dendritic architecture of relay neurons allows for processing of information both within channels and between channels.

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PubMed1987

Structure and postnatal development of photoreceptors and their synapses in the retina of the tree shrew (Tupaia belangeri).

The "all cone" retina of the tree shrew (Tupaia belangeri) was examined in the adult and early postnatal stages by light and electron microscopy. Rods are not as rare as previously thought, but make up about 4% of the photoreceptors. They are relatively short and narrow cells, which stain (toluidine blue) more intensively and lie more proximal than cones. Among the cones three morphological varieties could be distinguished. Most cones stain lightly but have a light or a dark giant mitochondrion in their inner segment; a third type stains darker but occurs only rarely. All cones possess extensive radial processes ("lateral fins") around the basal part of their inner segments. Such fins are well known from reptiles and birds, but have only once been described in a mammal (gray squirrel). The maturation of the retina in Tupaia belangeri proceeds centrifugally, i.e., from the vitreal to the scleral side, as in most mammals. A few synapses are already present at birth in the outer and inner plexiform layers, but seem to be more advanced in the latter. Such early synapses are small and have only few synaptic vesicles; they appear almost mature by day 14. The light-sensitive outer segments develop last. The first disks are seen by day 10, but regular membrane stacks are only present by day 18. Thus, it seems that the retina is functional when the young first open their eyes, which occurs around day 18.

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PubMed1986

Laminar organization of ON and OFF regions and ocular dominance in the striate cortex of the tree shrew (Tupaia belangeri).

The organization of ON and OFF responses and ocular dominance in the striate cortex of the tree shrew was electrophysiologically investigated by using flashed, stationary visual stimuli presented monocularly to either the ipsilateral or contralateral eye. We measured cortical multi-unit activity at 25-micron intervals with glass-insulated platinum-plated tungsten microelectrodes. Penetrations were made perpendicular to the cortical layers and the responses were quantitatively analyzed in layers IIIc to V. In sublayers IIIb, IIIc, and upper V, phasic responses of approximately equal magnitude occurred to both light ON and light OFF (ON-OFF regions). In layer IV, tonic as well as phasic responses were often evoked by the flashed spot of light. In sublayer IVa stronger responses occurred to light ON than to light OFF (ON region) while in sublayer IVb stronger responses occurred to light OFF than to light ON (OFF region). In an ON region, the increased neural activity that occurred at light ON was often accompanied by a decrease in activity below baseline level at light OFF. A similar decrease often occurred in an OFF region at light ON. Recordings from the region of the cell-sparse cleft in layer IV were characterized by ON-OFF responses, signalling a transition zone between sublayers IVa and IVb. In addition, the responses to stimulation of the ipsilateral eye typically were very weak in the cleft region. In the other regions examined, the multi-unit activity generally was driven binocularly with slightly greater responses being elicited by the contralateral eye. We conclude that the ON-center and OFF-center afferent pathways that are organized at the retinal level remain generally segregated in the tree shrew through the first synapse in the striate cortex. In addition, our recordings confirm that a horizontal organization of ocular dominance occurs in layer IV of the striate cortex in tree shrews.

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PubMed1985

ON and OFF regions in layer IV of striate cortex.

In vertical penetrations through the striate cortex in the tree shrew (Tupaia belangeri), we found regions where neural activity was evoked predominantly by light ON. These were followed by regions where responses were evoked predominantly by light OFF. Histological reconstructions indicated that the ON regions were correlated with layer IVa and the OFF regions were correlated with layer IVb. Local application of cobalt chloride produced a transient cessation of visually evoked activity, suggesting that the electrodes sampled cortical activity rather than lateral geniculate nucleus afferents. These data demonstrate that separate ON and OFF regions are present in the tree shrew striate cortex and suggest that spatially separate, parallel ON and OFF afferent channels extend, in this species, at least through the first synapse in the striate cortex.

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PubMed1982

Specificity of neural connections in the retinotectal system.

The existence of a retinotopic map raises the question of how such a map is formed during development. Since little is known about molecular mechanisms which may be involved in the formation of retinotopy, the structural aspects of this developmental process were studied to a greater detail in the chick. In the retina the following patterns of growth can be observed: (1) Ganglion cell formation starts in the centre and spreads towards the periphery. Thus, each cell is labelled by both its position and its time of origin. (2) Axons tend to grow towards the optic fissure by the shortest possible route, and more peripheral axons whose perikarya are born later than that of central ones tend to lie next to the inner limiting membrane. A regular change in the pattern of fibres, which we call transformation, occurs at the optic fissure. The incremental rings are transformed into crescents lying ventrally in the optic stalk at early stages of development. In the middle of the optic nerve retinotopy is re-established. Another transformation occurs shortly behind the chiasm. Except for these transformations, fibres run parallel to each other even across the chiasm. Thus, fibres arriving at the optic tectum are well organized. It is concluded that a multiplicity of factors contribute to the formation of the retinotectal map at any site of the visual pathway. The analysis of these factors may indicate how topographically organized projections develop.

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PubMed1980

[Ontogeny of retinal projections: observation and reflection].

Recent evidence indicates that the retinotectal projection has a field-to-field rather than a point-to-point precision. Therefore, individual fibres can vary their relative position in the fibre pathway to some extent. In addition it is not necessary that retinotopy is maintained throughout the whole pathway; transformations may also occur. In fact, in the chick embryo outgrowing retinal fibres maintain not an absolute but only a high degree of order. Transformations occur at the entrance to the optic nerve and probably also right behind the chiasm. The origin of the map is determined by the fact that central retinal fibres which are formed first connect to tectal neurons near the centre of the optic tectum where neurons mature first. There they immediately invade the cellular tectal layers and form functional synapses soon after. Supernumerary fibres degenerate.

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