PubMed Health⌕ Search

Biomedical subjects

R W Guillery

Publications and source records attributed to R W Guillery.

At least 55 records · Page 3Linked to original sources
PubMed1995

Anatomical evidence concerning the role of the thalamus in corticocortical communication: a brief review.

Two distinct types of thalamic nucleus are proposed on the basis of the afferent fibres that they receive from ascending pathways and from the cerebral cortex. 'First order nuclei' receive primary afferent fibres, definable on the basis of their origin and their intrathalamic synaptic relationships, from ascending pathways. These nuclei receive corticothalamic afferents from pyramidal cells in cortical layer 6, which also send branches to the thalamic reticular nucleus and appear to have a modulatory function. 'Higher order nuclei' receive most or all of their 'primary afferents' from pyramidal cells in cortical layer 5. These resemble the ascending primary afferents in the first order nuclei in terms of fine structure, synaptic relationships and in lacking a branch to the thalamic reticular nucleus. The higher order nuclei also receive modulatory afferents from layer 6. It is proposed that the higher-order nuclei are largely concerned with transmitting information about the output of one cortical area to another cortical area, and that they are likely to play a key role in corticocortical communication and higher cortical functions.

Cerebral Cortex↗
PubMed1995

A comparison of eutherian and marsupial optic chiasms: a brief review.

There are four fiber components in the mammalian optic chiasm. Two crossed and two uncrossed components, one of each from each eye. The two crossed components meet at the midline, forming coarse interdigitating bundles in some species, and extremely fine crossing bundles in others. The two components from one eye mingle in the optic nerve. Then in some species they separate, before reaching the chiasm, but in others they do not separate until they reach the midline of the chiasm. Once past the chiasm, the two components that form the tract reunite and one more, the crossed and the uncrossed fibers mingle. The review considers that difference between the marsupial and the eutherian mammals that have been studied, and looks at the developmental changes that are likely to underlie these different types of chiasmatic organization.

Animals↗
PubMed1994

Early development of the optic chiasm in the gray short-tailed opossum, Monodelphis domestica.

We have studied the early development of the uncrossed retinofugal projection in the gray short-tailed opossum. Axons that form the adult uncrossed retinofugal projection arise from the temporal crescent of the retina and reach the optic chiasm on postnatal day 7. The sites at which the uncrossed fibres segregate from the crossed fibres and the pattern of this segregation are very different from those seen in eutherian mammals. In the opossum, the uncrossed fibres segregate from the crossed fibres within the juxtachiasmatic part of the optic nerve before they have encountered either the fibres of the other eye or midline structures of the ventral diencephalon. The uncrossed fibres turn perpendicular to the axis of the nerve and grow dorsoventrally through the crossed projection to gather as a discrete bundle at the ventral edge of the nerve. The abrupt divergence of the uncrossed fibres occurs at a border between two glial cell types: the interfascicular glia that characterise the main part of the optic nerve and the radial glia of the juxtachiasmatic part of the nerve. At the ventral part of the nerve, the bundle of uncrossed fibres turns caudally across the axis of the nerve and enters the ipsilateral optic tract. When retinofugal fibres encounter the border between the interfascicular and radial glia, a very specific axonal reorganisation occurs in marsupials, and this is strikingly different from the axonal reorganisation that occurs at the same site in eutherians, where essentially all retinofugal fibres reorganise, not just the uncrossed component. We believe this to be an important example of an identified cellular element that has quite distinct axon-guidance properties in different species.

Animals↗
PubMed1994

Changes in fiber order in the optic nerve and tract of rat embryos.

In order to define the extent to which retinotopic order in the optic pathways may contribute to fiber segregation at the chiasm or to the formation of central maps, the arrangement of fibers in the optic nerve and tract of rat embryos, on embryonic days 16.5 and 18.5, has been studied by placing a small granule of DiI in one of the four quadrants of the retina and tracing the filled fibers through transverse sections of the retinofugal pathway with confocal microscopy. There is a distinct quadrant-specific order in the optic stalk immediately behind the eye, with fibers from the ventral nasal, dorsal nasal, dorsal temporal, and ventral temporal retina arranged sequentially across the rostrocaudal axis of the cross section of the stalk. However, this distinct order is not maintained very far. There is a gradual increase in the degree of overlap between fibers from the different quadrants as the fibers pass towards the chiasm. The dorsal groups of fibers intermingle extensively along almost the entire length of the stalk, but the fibers from ventral sectors remain separate until they reach the prechiasmatic region, where the ventral temporal and the ventral nasal fibers spread throughout the rostrocaudal extent of the stalk and the chiasm. The initial quadrant-specific order is completely lost at the chiasm. However, beyond the optic chiasm, the fibers are reorganized into another distinct order. In the optic tract, there is a segregation of dorsal from ventral fibers, but the nasal and temporal groups remain intermingled. The results of this study indicate that the earliest fibers in the developing optic tract are arranged according to topographical rules that differ from those obtaining behind the eye. Since all topographical order is lost between these two levels, there must be an active sorting mechanism in the region where the chiasm joins the tract. Possibly this mechanism is related to the development of the dorsoventral axis of the topographic maps in the central visual targets.

Animals↗
PubMed1993

Differential action of the albino mutation on two components of the rat's uncrossed retinofugal pathway.

The development of the uncrossed retinofugal pathways in normally pigmented and albino rats, aged from embryonic day (E) 14.5 to E18.5, was investigated. DiI was placed into one optic tract and the retinal origin of the uncrossed component, as well as its course in the optic stalk, was studied. The results show that, as in the mouse, the uncrossed retinal projection has two components. The first component is seen at E15.5 in normally pigmented animals. It develops exclusively in the central parts of the retina and is normal in albino littermates. The second component, which arises from the peripheral parts of the ventrotemporal retina, is seen two days later at E17.5 in all animals but is significantly smaller in albinos than in their pigmented littermates. Studies of axons in the optic stalk labelled retrogradely with DiI placed in the optic tract indicate that the uncrossed axons have no preference for any position in the stalk except when they approach the chiasm, where they tend to accumulate at the caudal region of the stalk. The uncrossed axons intermingle with the crossed axons along the entire length of the stalk. In albino embryos, no obvious difference in the prechiasmatic course of uncrossed axons was seen at any age examined. It is concluded that the albino mutation in rats affects the late ventrotemporal component of the uncrossed pathway selectively. It does not act on the early central component. Further, the intermingling of crossed and uncrossed axons in the stalk and the apparently unaffected prechiasmatic course of uncrossed axons in albinos indicate that the albino gene has its primary action in the retina.

Albinism↗
PubMed1993

Different rates of axonal degeneration in the crossed and uncrossed retinofugal pathways of Monodelphis domestica.

The uncrossed retinofugal fibres in the marsupial Monodelphis domestica form a separate bundle as they pass through the optic chiasm. The uncrossed fibres segregate from the crossed fibres a short distance before they reach the chiasm, gathering as an essentially exclusive bundle in the ventral part of the optic nerve. This bundle then passes laterally through the optic chiasm and into the optic tract. The distinctive position of the uncrossed fibres has allowed us to recognise that, surprisingly, the uncrossed fibres degenerate more rapidly than the rest. Seven days after a monocular enucleation approximately 60-80% of the fibres of the crossed component in the main part of the optic nerve near the chiasm have a normal cross sectional appearance in electron micrographs whereas less than 20% of the fibres in the uncrossed bundle look normal. The rapid degeneration of the uncrossed fibres cannot be related to any morphological parameter of the axons. Their fibre diameters are mainly medium to thick, lying within the range of axon diameters found in the rest of the nerve. The axon-myelin ratios of the uncrossed fibres are also no different from those of the crossed optic fibres. There are no structural peculiarities identifiable with light or electron microscopical methods in either the axons or in the glia of the uncrossed bundle that might account for the more rapid degeneration. There is evidence that the degenerative change in the main part of the optic nerve progresses from the lesion towards the chiasm, and that for the crossed fibres it may progress slightly faster for the thicker than for the thinner fibres. The degeneration in the uncrossed bundle does not fit any of the rules that have been proposed for relating rate of degeneration to fibre diameter. We conclude that the rate of Wallerian degeneration is determined by factors that yet remain to be defined.

Animals↗
PubMed1993

New views of the thalamic reticular nucleus in the adult and the developing brain.

The thalamic reticular nucleus plays a crucial role in modifying the patterns of activity that can reach the cerebral cortex from the thalamus. Although the nucleus is generally viewed as a cell group with widespread and nonspecific thalamic and cortical connections, recent evidence has begun to stress the extent to which at least some of the reticular pathways transmit well-defined maps with a clear local sign from the cortex and the thalamus. Further, evidence from the adult structure of the nucleus and ongoing developmental studies suggest that the reticular nucleus plays an important part in organizing the earliest connections between cortex and thalamus and that the developmental sequence may explain the complex connections formed in the adult.

Aging↗
PubMed1993

Developmental changes produced in the retinofugal pathways of rats and ferrets by early monocular enucleations: the effects of age and the differences between normal and albino animals.

Early monocular enucleations were done in rats, either at embryonic day 16 (E16) or on the day of birth, and the surviving uncrossed pathway was studied either at birth for some of the animals enucleated prenatally, or in the adult for all of the other animals. The uncrossed pathways were studied by using HRP as a retrograde tracer. The neonatal enucleations showed the increase of the surviving uncrossed component previously documented by others. In contrast to this, a prenatal enucleation produced a significant reduction in the surviving uncrossed pathway at birth. If these animals survived to be adults, then the surviving uncrossed pathway was slightly increased relative to normal. We conclude that two quite distinct mechanisms have been exposed by these experiments, one acting prenatally and producing a reduction in the uncrossed pathway, and the other acting postnatally and producing an increase. The postnatal effect, which is due to a decrease of the normally occurring ganglion cell death, thus neutralizes the prenatal effect, so that the most effective demonstration of the prenatal effect is to be seen before the period of cell death (early postnatal in rats and ferrets). The same methods were applied to prenatal ferrets at E26-E28 and, in order to see the maximum prenatal effects, the uncrossed pathways were studied at birth in all of these animals. There was a severe reduction of the uncrossed pathway throughout, and this was greatest in the animals with the earliest enucleations. Since the uncrossed pathway in normally reared albino animals is abnormally small, the effects of an early prenatal enucleation in albino rats and ferrets were compared with the effects in normally pigmented animals in order to determine whether the early enucleation was producing an abnormality comparable to the albino abnormality. Prenatal enucleations reduced the uncrossed pathway not only in normally pigmented but also in albino neonatal rats and ferrets. Further, the characteristic position of the nasal border of the temporal retina, which is abnormal in albino animals, was unaffected by the enucleation in either the albino or the pigmented animals, except where, in ferrets, enucleations produced a complete loss of the temporal concentration of ipsilaterally projecting ganglion cells (the temporal crescent). The earlier enucleations showed a greater tendency to produce such a complete loss of the temporal crescent. We conclude that the developmental mechanisms affected by the early enucleations are distinct from those that act to produce the albino abnormality even though both produce an abnormally small uncrossed pathway.(ABSTRACT TRUNCATED AT 400 WORDS)

Albinism↗
PubMed1992

Time of ganglion cell genesis in relation to the chiasmatic pathway choice of retinofugal axons.

The time of generation of retinal ganglion cells in fetal cats has been related to the course taken later by their axons in the optic chiasm. The ganglion cells were labelled with tritiated thymidine either on embryonic day (E) 26 or on E-30. When the cats were mature, ganglion cells were retrogradely labelled with horseradish peroxidase injected into one optic tract. The distribution of double-labelled cells showed that cells in the temporal retina generated on E-26 all have axons that take an uncrossed course in the chiasm, whereas, of the cells generated on E-30 in the temporal retina, some take a crossed course and others take an uncrossed course. The uncrossed axons of the E-26 cohort come from cells having a central distribution on the retina. For the E-30 cohort, the uncrossed axons come from cells having a relatively peripheral distribution, whereas the crossed axons come from more central cells. The present results suggest that the mechanism which serves to direct temporal retinal axons into the ipsilateral optic tract weakens as development proceeds. In principle, the change may occur in either a chiasmatic signal, read by temporal but not nasal optic axons, or in a retinal label, carried by temporal but not nasal cells and their processes. Since temporal retinal cells born concurrently at different places can project to opposite optic tracts, a retinal signal that deteriorates with time in a centroperipheral fashion is favored by the present results.

Animals↗
PubMed1992

Observations on the early development of the optic nerve and tract of the mouse.

The early development of the retinofugal pathway of mice has been studied by light and electron microscopic methods in order to define the spatial distribution and the structure of the growth cones as they advance from the eye to the brain. We have studied the relationships of the growth cones to each other, to the glia and, in the older individuals, to the nerve fibers that are already terminating in the brain. We have looked at the rate of advance of the growth cones and have paid particular attention to the changing relationships of the growth cones as they approach the optic chiasm. We have also looked to see whether, at early stages, it is possible to recognise any characteristic features distinguishing the fibers destined to be the thickest in the adult, which come from ganglion cells that are generated among the earliest ganglion cells. In transverse sections through the optic stalk about 50-100 microns behind the eye, the first bundles of fibers are seen on embryonic day 12.5 (E12.5) as a mixture of thin (less than 0.5 micron) axons, thicker growth cones, and fine filopodial and foliopodial extensions. During the next two days, as these bundles in the intraorbital nerve increase in size and number, growth cones can be seen in all of the bundles and in all parts of the bundles. They show only a slight preference for one part of the nerve relative to another, and our material provides no evidence for the view that axons are particularly inclined to follow pre-existing bundles. The structure of the pathway changes significantly as it is traced towards the chiasm, and no section or small stretch of sections can be regarded as representative of the nerve as a whole. As the fibers approach the optic chiasm the growth cones come to lie predominantly close to the pial surface, with the deeper regions occupied almost entirely by fine axons. The change occurs in a region where the glial environment also changes, and where a characteristic neural tube-like organization first becomes recognizable. Here the glial cells lie in a periventricular position and send slender radial processes out towards the subpial surface. The newly invading axons in the early optic nerve taper from a broad growth cone back to an extremely slender axon, less than 0.5 micron in diameter. The tapered region is of the order of 100-300 microns in length and advances through the nerve at approximately 60 microns per hour.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1991

Position of axons in the cat's optic tract in relation to their retinal origin and chiasmatic pathway.

The positions of the crossed and uncrossed optic axons of distinct diameter classes has been examined in the optic tract of the adult cat. In addition, the retinal origin of axons occupying different positions within the tract has been studied. Since the position of a fibre within the optic tract reflects its time of arrival during development, we have used axonal position as an indicator of age and have related this to the chiasmatic pathway choice of the axons. Cats were either monocularly enucleated, to reveal the position and diameter of surviving crossed and uncrossed optic axons in semithin and thin sections, or implants of horseradish peroxidase (HRP) were placed so as to retrogradely label the ganglion cells giving rise to axons within the deep (early arriving), or superficial (later arriving) parts of the tract selectively. This was accomplished by either 1) surgically implanting HRP into the superficial portion of the optic tract, via a transbuccal approach, or 2) making such a transbuccal transection of the superficial fibres, followed by intracerebral injections of HRP to retrogradely label the surviving, deeper, optic axons from their target nuclei. The deep parts of the optic tract contain fine and medium, crossed and uncrossed axons arising from mainly medium sized cells in the contralateral nasal and the ipsilateral temporal retina; there is a clear line of decussation. In contrast, the superficial parts of the tract contain mainly fine diameter axons arising from small cells in the whole contralateral retina, and a small proportion of large diameter axons arising from large, alpha cells in the whole contralateral retina and in the ipsilateral temporal retina. The likelihood that axons from the temporal retina will project contralaterally therefore increases as development proceeds, since these axons are found in the superficial parts of the tract only. This suggests that a time-dependent signal that weakens with age is responsible for directing early arriving optic axons from the temporal retina to take an exclusively uncrossed course.

Animals↗
PubMed1990

The early development of retinal ganglion cells with uncrossed axons in the mouse: retinal position and axonal course.

The carbocyanine dye, DiI, has been used to study the retinal origin of the uncrossed retinofugal component of the mouse and to show the course taken by these fibres through the optic nerve and chiasm during development. Optic axons first arrive at the chiasm at embryonic day 13 (E13) but do not cross the midline until E14. After this stage, fibres taking an uncrossed course can be selectively labelled by unilateral tract implants of DiI. The earliest ipsilaterally projecting ganglion cells are located in the dorsal central retina. The first sign of the adult pattern of distribution of ganglion cells with uncrossed axons located mainly in the ventrotemporal retina is seen on embryonic day 16.5, thus showing that the adult line of decussation forms early in development. A small number of labelled cells continue to be found in nasal and dorsal retina at all later stages. At early stages (E14-15), retrogradely labelled uncrossed fibres are found in virtually all fascicles of the developing nerve, intermingling with crossed axons throughout the length of the nerve. At later stages of development (E16-17), although uncrossed fibres pass predominantly within the temporal part of the stalk, they remain intermingled with crossed axons. A significant number of uncrossed axons also lie within the nasal part of the optic stalk. The position of uncrossed fibres throughout the nerve in the later developmental stages is comparable to that seen in the adult rodent (Baker and Jeffery, 1989). The distribution of uncrossed axons thus indicates that positional cues are not sufficient to account for the choice made by axons when they reach the optic chiasm.

Animals↗
PubMed1990

Normal and abnormal visual field maps in albinos. Central effects of non-matching maps.

The abnormal chiasmatic crossing characteristic of all albino mammals brings two discordant representations of the visual field to the central visual relays. The representation from the nasal retina is normal whereas the one from the temporal retina is disrupted, a part representing the contralateral visual field as is normal and a part coming from the ipsilateral visual field as a mirror image of a part of the normal representation. Experiments that were designed to define the rules on the basis of which the abnormal representations can be established in the lateral geniculate nucleus and visual cortex are described. These experiments are related to more recent studies of albino cats, and the observations of the visual pathways are related speculatively to abnormalities seen in the auditory pathways of albinos. The possibility is raised that the auditory abnormalities are secondary to the visual abnormalities, produced by a failure of the two systems to establish normally matching maps of sensory space.

Albinism↗
PubMed1989

Evidence for the delayed expression of a brainstem abnormality in albino ferrets.

Previous reports have suggested that neurons of the medial superior olivary nucleus in albino cats and rabbits are smaller than those in normally pigmented strains. In this investigation, the mean cross-sectional areas of neuronal perikarya in the medial superior olivary nucleus of pigmented and albino ferrets were compared at juvenile (14 weeks) and adult (greater than six months) ages. The mean cross-sectional area of the perikarya in adult albino ferrets was found to be 45% smaller than that of adult pigmented ferrets, confirming observations in cat and rabbit. The same comparison in juvenile ferrets, however, showed no significant differences. These results suggest that the mechanisms producing the abnormality of the auditory pathways differ from those responsible for the production of abnormal retinofugal pathways in albinos.

Albinism↗
PubMed1989

Early monocular enucleations in fetal ferrets produce a decrease of uncrossed and an increase of crossed retinofugal components: a possible model for the albino abnormality.

The terminal distributions of retinofugal axons to geniculate laminae or cell groups have been studied in monocular ferrets that had had one eye removed on the 28th or 29th day of intrauterine life and survived until the end of the fourth postnatal week. Normally pigmented and albino animals were studied and the patterns of retinogeniculate terminations in these were compared with earlier accounts of the patterns that develop normally or after a monocular enucleation on the day of birth. Birth normally occurs after 41 days of gestation. In albino animals the neonatal and prenatal enucleations produce essentially the same result. The abnormally large crossed retinogeniculate component, which is also characteristic of normal adult albinos, innervates the major (A) laminae and these fuse medially and caudally as in normal albinos. These represent geniculate Layers A and A1. The abnormally small uncrossed component resembles the abnormally small uncrossed component of normal albinos in innervating several separate terminal islands within the geniculate region. These are larger than in a normal albino animal and are surrounded by a zone of sparser termination not seen in a normal albino. In normally pigmented animals the prenatal enucleation produces a result essentially like that produced by the enucleation in albinos, whereas the postnatal enucleation produces a relatively more symmetrical retinogeniculate pathway in which the crossed component innervates an abnormally enlarged Lamina A and the uncrossed component innervates an enlarged Lamina A1. These results can be most readily explained by assuming that between embryonic Day 28 and the day of birth there is an interaction between the two retinofugal pathways that produces an increase in the uncrossed component from the levels characteristic of albinos and early monocular enucleates to normal levels. This interaction must then be absent in albinos.

Albinism↗
PubMed1988

The beta sector of the rabbit's dorsal lateral geniculate nucleus.

The beta sector of the rabbit's dorsal lateral geniculate nucleus is a small region of nerve cells scattered among the fibres of the geniculocortical pathway. In its topographical relations it resembles the perigeniculate nucleus of carnivores, which contains neurons driven by geniculate and visual cortical neurons and which sends inhibitory fibres back into the geniculate relay. We have traced retinogeniculate, geniculocortical and corticogeniculate pathways in rabbits by using horseradish peroxidase or radioactively labelled proline and have found that the beta sector resembles the perigeniculate nucleus in receiving no direct retinal afferents, sending no efferents to the visual cortex (V-I), and receiving afferents from the visual cortex. The corticogeniculate afferents are organized so that the visual field map in the beta sector and the main part of the lateral geniculate relays are aligned, as are the maps in the cat's perigeniculate nucleus and the main part of the geniculate relay of carnivores. Electron microscopical studies show similar types of axon terminals in the rabbit and the cat for the main part of the geniculate relay on the one hand and for the beta sector and the perigeniculate nucleus on the other. Earlier observations that the proportion of putative inhibitory terminals (F-type terminals) is lower in the rabbit's than the cat's geniculate region are confirmed. A major difference between the beta sector and the perigeniculate nucleus has been revealed by immunohistochemical staining for GABA. Whereas almost all of the cat's perigeniculate cells appear to be GABAergic, the proportion in the beta sector is much lower, and not significantly different from that found in the main part of the rabbit's geniculate relay. It is concluded that the beta sector shares many of the organizational features of the perigeniculate nucleus. A common developmental origin seems probable, but the functional differences remain to be explored.

Animals↗