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  • 1
    ISSN: 1432-1106
    Keywords: Superior colliculus ; Synaptic patterns ; Primate
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary While the synaptic patterns studied with the electron microscope in the upper layers of the superior colliculus of the monkey are basically similar to those described for the rat, there are notable differences in the terminal patterns of the afferent pathways. The retinotectal pathway in monkeys terminates sparsely near the surface in the caudal part of the colliculus. Most of the terminals undergo an electron dense reaction in degeneration and are rapidly removed from their postsynaptic sites. The corticotectal pathway terminates heavily throughout superficial layers making both axodendritic and serial synaptic connections. The terminals undergo neurofilamentous and dense degenerative reactions. It is suggested from a comparison of the retinotectal and retinogeniculate pathways that terminals which undergo the neurofilamentous degenerative reaction belong to a different population of axons from those undergoing only the dense reaction.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Experimental brain research 45 (1982), S. 277-284 
    ISSN: 1432-1106
    Keywords: Development ; Visual system ; Retinotectal ; Laterality ; Chick
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The central retinal projections have been examined in normal chick embryos early in development to determine if a transient ipsilateral projection is present. Anterograde transport of HRP identified using tetramethyl benzidine as a chromogen and degeneration techniques were used on embryos of successive ages between 6 and 16 days of incubation to show the central distribution of the retinal axons from one eye. Besides the anticipated contralateral projection, a small projection was identified to primary visual nuclei on the side of the brain ipsilateral to the injected or lesioned eye in embryos between days 6 and 12 of incubation. A projection from the injected eye into the contralateral optic nerve was also identified in a number of embyos. By embryonic day 15 the retinal projection to the ipsilateral side of the brain and into the contralateral optic nerve had disappeared. This loss of the anomalous projections coincides with a period of substantial cell death in the retinal ganglion cell layer. It appears, therefore, that in the chick, like the rat, the ipsilateral retinofugal projection resulting from an embryonic enucleation may in part be due to retention of a normal ipsilateral projection.
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  • 3
    ISSN: 1432-1106
    Keywords: Parabigeminal nucleus ; Lateral geniculate nucleus ; Plasticity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Anatomical techniques are used to demonstrate a projection from the parabigeminal nucleus (PBG) to the contralateral dorsal lateral geniculate nucleus (dLGN) in adult pigmented rats which had been bilaterally enucleated at birth. No similar pathway is detected in normal animals. Ultrastructural examination of degeneration in dLGN of blinded rats after contralateral PBG lesion reveals a similarity of the PBG terminals to “replacement terminals” seen in dLGN of anophthalmic or neonatally enucleated mice (Cullen and Kaiserman-Abramof 1976)
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  • 4
    Electronic Resource
    Electronic Resource
    Springer
    Experimental brain research 47 (1982), S. 437-445 
    ISSN: 1432-1106
    Keywords: Rat ; Tectal transplants ; Host afferents ; Functional innervations
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Fetal tectal tissue was transplanted adjacent to the superior colliculus of neonatal rats. In mature recipient animals, ultrastructural studies provided evidence of host cortical synapses on cells within the transplants. To determine whether these synapses were functionally effective in driving transplant cells, tungsten in glass microelectrodes were used to record from single units in the transplants and the host cerebral cortex was electrically stimulated. Of the 214 tested units which were shown histologically to be within the transplants, 25 (11.7%) were orthodromically excited from host cortex at latencies of 15 ms or less (mean 7.3 ms). A further four transplant units were activated at relatively long latencies (40–300 ms). The presence of a functional host retinal input was also tested by electrically stimulating the host optic nerves. No transplant units were activated from these sites, apparently because very few electrode penetrations passed through the localized zones of optic axon distribution. The possible functional interrelationships between transplant and host are discussed.
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  • 5
    ISSN: 1432-1106
    Keywords: Retinotectal maps ; Visual system ; Transplants ; Topography
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary We report here that transplanted rat retinae do not project in a topographically organized fashion onto the host tectum. In fact, following the injection of two small non-overlapping regions of the host tectum, one with Fluorogold, the other with Rhodamine conjugated latex microspheres, groups of ganglion cells labelled with either dye are found interspersed throughout the ganglion cell layer of the retinal transplant. The absence of topography in the projections of transplanted retinae offers a new opportunity to examine the mechanisms which lead to topographically ordered projections in normal development.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Experimental brain research 40 (1980), S. 273-282 
    ISSN: 1432-1106
    Keywords: Transplants ; Development ; Visual system
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Retinae were taken from fetal rats and transplanted adjacent to the superior colliculus of neonatal rats. After 1 month survival, the transplants were surgically removed from the hosts, locally damaged or injected with horseradish peroxidase (HRP) to determine the distribution of the transplant efferents in the host brains. Histological examination of the transplants revealed cell and plexiform layers characteristic of normal retinae. Since the retinae were undifferentiated at the time of transplantation, this layering developed within the host. The only obvious differences from normal retina were that the layers were organized in rosettes or folded sheets and lacked well developed photoreceptor outer segments. In animals which had lesions or HRP injections confined to the retinal transplant, proper staining of sections of the host brain revealed transplant projections. These projections were confined to the optic tract and nuclei which are normally retinorecipient such as the superior colliculus and dorsal lateral geniculate nucleus. Projections were found along the border of non-retinorecipient nuclei such as the lateral posterior nucleus, but did not appear to enter these nuclei. It was observed that within the superior colliculus the host retinal input had an effect on the distribution of the transplant projection. In one-eyed hosts the transplant projection was distributed throughout the stratum (s.) zonale, s. griseum superficiale, and s. opticum; whereas in the two-eyed hosts, the transplant projection was confined to the s. zonale and the border between s. griseum superficiale and s. opticum. We suggest that a special affinity exists between the axons of the retinal transplants and host visual structures. Furthermore, factors, such as competition and timing may be important in determining the distribution of the transplant axons within the specific target nuclei. Transplantation appears to be a useful technique for further studies on the mechanisms underlying the development of specific neuronal connections.
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  • 7
    ISSN: 1432-1106
    Keywords: Plasticity ; Corticotectal pathway ; Visual system ; Rats
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary An aberrant crossed corticotectal pathway can be generated by removal of one visual cortex and the contralateral superior colliculus from newborn rats. This aberrant crossed corticotectal projection arises from the pyramidal neurons located in layer V of the visual cortex and terminates in a spatially orderly manner in the appropriate laminae of the cortically deafferented contralateral colliculus. Comparable results cannot be reproduced by unilateral collicular lesions alone. The significance of these findings and the possible mechanisms involved in the formation of the aberrant pathway are discussed and compared with the retinotectal system.
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  • 8
    ISSN: 1432-1106
    Keywords: Transplant ; Cerebral cortex (rat) ; Neurogenesis ; Differentiation ; Autoradiography
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The neurogenesis of immature cerebral cortex transplants was investigated using tritiated thymidine (3HT) autoradiography. Cortical tissue taken from rat fetuses during their last week of gestation (E15-E21) was transplanted to the tectum or cerebral cortex of newborn rat hosts. At different times after transplantation, a single injection of 3HT was given to the host. Most of the experimental animals were killed after the transplants had grown to maturity (5–12 weeks), and some were studied shortly after the tracer had been given. In other experiments, donor tissue was used that was labeled in utero up to 1 day before being transplanted on E16, E17, E18, or E19. It was found that neurons labeled before transplantation survived and differentiated in the graft. Removal of the graft from its natural context did not prevent 3HT incorporation into surviving precursor neurons, indicating continuation of neurogenesis in the cortical transplants. In transplants from E16 donors neurons continued to be generated for 5–6 days after transplantation. Termination of neurogenesis occurred at successively earlier times in transplants taken from correspondingly older embryos. Independent of size and position of the transplant, application of 3HT after “projected” transplant ages of E23 and older labeled only non-neuronal cells. This suggests a time schedule of neuron generation in the cortical transplants similar to that observed during normal development of the cerebral cortex, which is not disturbed by the transplanting procedure.
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  • 9
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The relationship between the development of the pupilloconstriction response to changes in light levels and retinal maturation was studied in normal rats and rats that had received intracranial retinal transplants at birth. A pupillary response to light was first observed between postnatal days 7 and 9 in normal rats, and was typically of small amplitude and sluggish. By the time the eyelids first open, 2 weeks after birth, the pupillary response had improved to near adult levels. The inception of the pupillary response correlates with the first appearance of conventional synaptic contacts in the inner and outer plexiform layers of the retina, while improved responses correlate with maturation of photoreceptor outer segments and formation of synaptic ribbons in the inner plexiform layer. When embryonic retinae were transplanted to intracranial locations in newborn hosts and the transplants later illuminated as the host matured, the onset of a pupillary response to transplant illumination was delayed in proportion to the developmental disparity between the transplant and the host. The pattern of anatomical development in transplanted retinae was also similar, but delayed in time, compared to normal retinae. This indicates that the limiting factors for expression of light-activated pupilloconstriction exist within the retina, rather than being intrinsic to the central nuclei or to the output pathway subserving the response.
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  • 10
    ISSN: 1460-9568
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The developmental time-course and growth characteristics of efferent graft-to-host projections were studied from mouse fetal striatal grafts (E13–14) implanted as a cell suspension into the ibotenate-lesioned striatum of immunosuppressed adult rats. A cell surface monoclonal antibody specific for mouse neurons (M6) was used to identify the donor cells and their projections into the host brain. At 3–5 days after implantation, sparse fascicles of M6-positive graft-derived fibres extended for ∼0.3–0.4 mm across the graft-host border into the surrounding host striatum. From the beginning they were selectively orientated in one direction, i.e. caudally along the myelinated fibre bundles of the internal capsule. At 8 days, the graft-derived fibres were more numerous and more densely labelled. They ran in dense fascicles inside the myelinated bundles of the host internal capsule and reached the rostral host globus pallidus, a distance of ∼ 1.2 mm from the caudal tip of the graft. Two weeks after grafting, the M6-positive fibre fascicles were clearly seen to branch within the globus pallidus to form terminal-like networks. From this time onwards, the immunoreactivity of the outgrowing fibre fascicles gradually diminished, although small but dense terminal-like networks could be found in the host globus pallidus in most, but not all, of the rats at longer survival times (3–15 weeks). This is consistent with previous work showing that outgrowing axons lose their M6 immunoreactivity as they mature and become myelinated. Control grafts of fetal neocortical and fetal cerebellar tissue were used to assess the tissue-type specificity of the efferent fibre growth. The neocortical implants projected densely up to about 3 mm into the host brain, along the internal capsule and the corpus callosum and into the overlying cortex. By contrast, although the cerebellar grafts survived well, they showed very little efferent fibre growth. Double immunostaining for DARPP-32 and M6 revealed that all M6-positive fibre fascicles extending from the striatal (but not neocortical) grafts also showed DARPP-32 positivity, and thus that it was the DARPP-32-positive regions of the striatal grafts that projected to the host brain. It is concluded that graft-to-host projections, running along and inside host myelinated bundles, are formed from intrastriatal striatal grafts within 1–2 weeks of implantation. Grafts of neocortical tissue grew well along the same trajectory, whereas neurons of a type not normally projecting along the internal capsule, i.e. cerebellum, failed to extend axons over any significant distance along this trajectory.
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