Biomedical subjects

L D Beazley

Publications and source records attributed to L D Beazley.

At least 73 records · Page 4Linked to original sources

PubMed1988

Development of the optic tecta in the frog Limnodynastes dorsalis.

In Limnodynastes dorsalis neurogenesis of the optic tecta and the pattern of cellular lamination was determined by [3H]thymidine autoradiography. There was a rostral to caudal gradient of cell proliferation with peak neurogenesis in mid-larval life and by metamorphosis generation was complete. The cellular layers formed as each portion of tectum was generated. As evidenced by comparison of animals killed within 24 h of [3H]thymidine injection and those injected as larvae and killed at more mature stages, cells were generated in the ependymal and peri-ependymal layers and during development they migrated to form the peripheral layers. During late larval life, the soma diameter of cells in the most superficial layers, and the width of these layers increased. Tectal innervation during development was assessed by tracing axonal trajectories with horseradish peroxidase. Innervation recapitulated tectal growth with more caudal areas being progressively encompassed by primary optic fibres. By metamorphosis, the entire tectum was innervated. These results indicate that tectal generation, cellular lamination and innervation by optic fibres have a rostral to caudal gradient of development which is complete by metamorphic climax. Therefore to accommodate postmetamorphic retinal growth the developing retinotectal projection must remain labile beyond metamorphosis.

Aging↗

PubMed1988

The accessory optic system of the wallaby, Setonix brachyurus: anatomy in normal animals and after early unilateral eye removal.

We have traced primary visual projections to nuclei of the accessory optic system in the mature wallaby, Setonix brachyurus, the "quokka," following unilateral intraocular injections of horseradish peroxidase. The organization of pathways and nuclei is similar to that of other marsupials and to that of eutherian mammals. The dorsal, lateral and medial terminal nuclei receive bilateral input, though nuclei ipsilateral to the injected eye are weakly labelled in comparison with their contralateral counterparts. We also report on the accessory optic system in animals which were unilaterally enucleated neonatally or at postnatal day 35. At maturity in enucleated animals, ipsilateral projections to all nuclei of the accessory optic system are more densely labelled than normal. This exuberance is more pronounced in neonatally enucleated animals than in those enucleated at the later stage.

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PubMed1988

Retinal ganglion cell death is not prevented by application of tetrodotoxin during optic nerve regeneration in the frog Hyla moorei.

Following extracranial optic nerve crush in the adult frog Hyla moorei, regeneration takes place to restore topographically organised visual projections, despite partial depletion of the retinal ganglion cell population. In the present study, the right optic nerve was crushed and tetrodotoxin (TTX) repeatedly injected into the right eye to abolish electrical activity mediated by sodium channels in ganglion cell axons. At 70-78 days post-crush, the number and distribution of live cells in the ganglion cell layer were assessed from cresyl violet-stained wholemounts. After regeneration, cell numbers in TTX-treated animals fell by a mean of 32.6% in comparison with their unoperated partner retinae. This value was very similar to the 32.4% mean fall found after regeneration for animals receiving no injections of TTX. Furthermore, distributions of surviving cells were comparable for the two groups. We conclude that sodium-mediated electrical activity within retinal ganglion cells does not control the extent or pattern of their death during optic nerve regeneration.

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PubMed1988

Development of cell density gradients in the retinal ganglion cell layer of marsupials.

An area centralis and visual streak, specializations of high cell density in the mammalian retinal ganglion cell layer, develop from a more uniform cell distribution. In the wallaby and kangaroo, this transition is first seen 60 days postnatally. Total live cell numbers in this layer fall by approximately one-third as these specializations start to appear and dying cells are seen. However, dying cells have already become concentrated at the retinal rim prior to the emergence of live cell density gradients. Our results suggest that cell death may partially sculpt patterns of live cells, particularly by lowering densities around the entire far periphery. Studies of cell division demonstrate that ganglion cells are generated and enter the ganglion cell layer before the area centralis and visual streak are formed. However, cell addition to the inner and outer nuclear layers continues as cell density gradients become apparent in the ganglion cell layer. Furthermore, this late cell generation ceases first in areas adjacent to the presumptive area centralis. The differential distribution of such prolonged cell addition to the inner and outer nuclear layers may result in an asymmetric expansion of the retina. This process would partially explain the changing topography of cells in the ganglion cell layer. Mathematical analysis of patterns of overall retinal growth support this interpretation.

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PubMed1987

Cell death in the developing retinal ganglion cell layer of the wallaby Setonix brachyurus.

The distribution and number of dying cells in the developing retinal ganglion cell layer of the wallaby Setonix brachyurus were assessed by using cresyl violet stained tissue. The density of dying cells has been expressed per 100 live cells for the entire retinal surface, data being presented as a grid of 500 micron squares. For statistical analysis, retinae were divided into 8 regions; dorsal, ventral, nasal, and temporal quadrants, each further divided into center and periphery. This method allowed comparison of the extent of cell death at different retinal locations as the high density area centralis of live cells developed temporal to the optic disk from 60 days onward. Between 30 and 70 days, dying cells were seen across the entire retina; beyond 100 days very few were seen. Initially, there was a significantly higher incidence of dying cells in the central retina compared to the periphery, whereas from 50 days this situation was reversed. Analysis of the central retina before and during area centralis formation consistently indicated a significantly lower number of dying cells per 100 live cells in temporal compared to other retinal quadrants. This differential pattern suggests that cell death lowers live cell densities less in the emerging area centralis than elsewhere, and therefore must play a part in establishing live cell density gradients. However, we cannot exclude the possibility that other factors are also instrumental. Indeed, factors such as areal growth (Beazley et al., in press) presumably operate at later stages since live cell density gradients continue to be accentuated even after cell death is complete. Numbers of dying cells peaked by 50 days, reaching approximately 1% of the live cell population. At this stage, counts were also maximal for live cells with values up to 30% above the adult range.

Aging↗

PubMed1987

Patterns of cytogenesis in the developing retina of the wallaby Setonix brachyurus.

Patterns of mitosis were examined during development from embryonic day (E) 19, 9 days before birth, for retinae of the wallaby Setonix brachyurus, using cresyl violet stained material. For neural retina, mitosis took place at the ventricular surface from the earliest stage of eye formation until postnatal day (P) 100. Numbers of mitotic figures reached a peak of approximately 12,000 by P43. Average densities ranged between 300/mm2 and 600/mm2 up to P12 and then fell to below 50/mm2 by P25 before reaching a second peak of over 400/mm2 at P43. Up to P50, mitoses were present across the entire retina. By P60, a 'cold spot' lacking mitotic activity had formed in temporal retina and progressively extended to reach peripheral regions by P100; no mitoses were seen at P150. The timing and location of the 'cold spot' coincided with our previous description of the appearance of an area centralis in the ganglion cell layer (Dunlop and Beazley 1985). Inner and outer plexiform layers (IPL and OPL) formed between P24-40 and P50-100 respectively and were seen first in temporal retina. Furthermore, the extent of the OPL matched the mitotic 'cold spot'. By contrast to neural retina, mitosis in the pigment epithelium was panretinal and was largely complete by P3. The data suggest that cell addition to the inner or outer nuclear layers contribute to differential retinal expansion and the establishment of cell density gradients in the ganglion cell layer.

Age Factors↗

PubMed1987

Development of the area centralis and visual streak in the grey kangaroo Macropus fuliginosus.

To further our understanding of area centralis and visual streak formation we have investigated the distribution and number of the total cell population in the retinal ganglion cell layer of the western grey kangaroo Macropus fuliginosus using adults and young from 7-200 days postnatal. This species was chosen since, as we describe, the adult possesses a particularly prominent area centralis and visual streak. By studying the total cell population we were able to compare cell distributions in immature retinae, in which cell types could not be distinguished, with topography in the adult. By 57 days postnatal a weak visual streak was apparent; a pronounced area centralis and visual streak were seen by 84 days although densities across the retina, particularly in the far periphery, were still considerably higher than in the adult. Dying cells were also observed up to 160 days, but not at 188 days or in the adult. The fall of approximately one third in the total cell number as the area centralis and visual streak developed is presumably related to cell death. Furthermore, at 57 days, dying cells were seen preferentially in retinal regions outside the immature visual streak and may therefore play a part in refining density gradients to their mature form. As the area centralis and visual streak developed, the relative lengths of the dorso-ventral and naso-temporal axes remained similar, suggesting that differential radial growth cannot underlie the changing live cell density gradients.

Age Factors↗

PubMed1987

Displaced retinal ganglion cells in the wallaby Setonix brachyurus.

Displaced ganglion cells have been examined in wholemounted and sectioned retinae following bilateral injection of horseradish peroxidase into optic tracts of the wallaby, Setonix brachyurus, "quokka". Such cells, which lie in the vitread part of the inner nuclear layer, are located mainly in superior retina as a streak-like band dorsal to the area centralis and visual streak of orthotopic ganglion cells. Only between 1 and 2% of the total ganglion cell population were displaced, but an analysis of cell morphology and soma diameter suggested that displaced ganglion cells represented several cell types.

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PubMed1987

An investigation into the role of ganglion cells in the regulation of division and death of other retinal cells.

The patterns of cell death and division are described in the normal postnatal rat retina and following transection of the optic nerve on the day of birth. Optic nerve transection on the day of birth results in the rapid degeneration of the ganglion cells. Mitosis at the outer retinal surface ceases first in the temporal retina, then in the nasal retina and becomes progressively more restricted to peripheral regions. Mitotic activity was not affected by the loss of ganglion cells. Cell death takes place in a wave passing from the ganglion cell to the inner nuclear to the outer nuclear layer. The time course of cell death is not affected by the loss of ganglion cells following optic nerve transection, and there is no significant increase in the number of cells which degenerate in the inner nuclear layer. The effects of removing a major postsynaptic target of local circuit neurones appears to be less pronounced than has been reported for relay neurones.

Age Factors↗

PubMed1986

Development of visual projections in the marsupial, Setonix brachyurus.

Retinal projections to the primary visual centres were studied following injection of tritiated proline into one eye in the Marsupial, Setonix brachyurus between 10 and 100 days postnatal and in adults. Initially, projections from the two eyes overlapped extensively, particularly between 20 and 50 days. There was a gradual refinement thereafter, including a segregation of inputs from the two eyes within both the lateral geniculate nucleus (LGN) and superior colliculus (SC) by 70 days. Such refinement in visual centres is discussed in relation to the concurrent emergence of retinal ganglion cell density gradients, a decrease in ganglion cell numbers, cell death in the ganglion cell layer and loss of optic axonal profiles.

Aging↗

PubMed1986

Cell death in the retinal ganglion cell layer during optic nerve regeneration for the frog Rana pipiens.

Cell number in the retinal ganglion cell layer of adult Rana pipiens was estimated from cresyl stained wholemounts. Values for normal animals ranged from 466,000 to 643,000 but differences between sides of individual animals varied by 9% or less. During optic nerve regeneration, following unilateral extracranial optic nerve crush, cell numbers in experimental retinae fell compared to their unoperated partners with the majority of the loss taking place between 56 and 84 days; by 200 day only half the cell complement remained. Since retrograde transport of horseradish peroxidase labelled 87% of cells in the normal ganglion cell layer, most of the loss during regeneration must have been from the ganglion cell population.

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PubMed1986

Numbers of axons in the optic nerve and of retinal ganglion cells during development in the marsupial Setonix brachyurus.

Numbers of axons during postnatal development in the optic nerve of the marsupial Setonix brachyurus were estimated from electron micrographs. Between 14 and 45 days axon totals rose 10-fold from 95,000 to 961,000. Thereafter, counts decreased gradually by almost 5-fold stabilising at the adult level of 180,000-224,000 between 130-150 days. Myelinated axons were first seen at 85 days and constituted 76% of all axons by 150 days, compared to almost 100% in the adult. At comparable stages of development, estimates of cell number in the ganglion cell layer were made from sectioned and whole-mounted retinae. Ganglion cells, labelled with horseradish peroxidase (HRP) injected into the optic tract and visual brain centres, reached a maximum of 304,000 by 50 days. This value was only one-third higher than the adult ganglion cell estimate of 202,000, to which counts fell by 70 days. Such a fall of ca. 100,000 ganglion cells was matched by a reduction from 443,000 to 361,000 in the total number of cells in the retinal ganglion cell layer, that is, cells labelled by HRP plus those which remained unlabelled. Thus, during development axon counts exceeded adult values 4-fold whereas ganglion cell totals were only one-third higher than the mature value. The shortfall of ganglion cells to optic axons indicates that during development the transiently high axon number cannot be explained by a correspondingly raised count of ganglion cells. It is therefore necessary to hypothesise a substantial fibre population which temporarily projects into the nerve from either the brain or the opposite eye. Alternatively, optic axons may branch extensively during development with subsequent loss of collaterals.

Age Factors↗

PubMed1985

Retinal ganglion cell death during optic nerve regeneration in the frog Hyla moorei.

In the frog Hyla moorei we have estimated there to be between approximately 450,000 and 750,000 cells in the retinal ganglion cell layer. Optic axon counts and retrograde transport of horseradish peroxidase (HRP) indicated that 72-76% of these were ganglion cells. Cells of this type were distributed as a temporally situated area centralis within a horizontal visual streak. Cell and optic axon counts showed that there was an approximately 40% loss of ganglion cells during optic nerve regeneration. Ganglion cells appeared chromatolysed by 6-8 days after an extracranial nerve crush but there was no indication of cell death until 15 days. By this stage anterograde transport of HRP indicated that axons had reached the chiasma. Death was first seen in the area centralis, extended along the streak, and finally was observed in the periphery by 65 days; cell counts demonstrated that at this time the wave of death was almost complete. We have previously shown by electrophysiological visual mapping (Humphrey and Beazley, '82) and confirmed in this study that visuotectal projections were retinotopically organized during regeneration. Multiunit receptive fields were initially large but progressively refined starting in nasal field (temporal retina) to restore a normal projection. The similar sequences whereby the visuotectal projection became refined and death took place in the retinal ganglion cell layer suggested that death may be related to a process of organization within the regenerating projection. In normal animals primary visual pathways revealed by anterograde transport of HRP were essentially similar to those of Rana pipiens and R. esculenta. Regenerating axons generally remained within optic pathways. Exceptions were a retinoretinal projection which was not completely withdrawn even after 1,028 days and a direct projection to the ipsilateral tectum via an inappropriate part of the optic tract.

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PubMed1985

Changing distribution of retinal ganglion cells during area centralis and visual streak formation in the marsupial Setonix brachyurus.

Injections of the axonal marker horseradish peroxidase (HRP) were made into optic tracts and/or visual centres of Setonix brachyurus, a small wallaby (quokka) during development and in adults. Distributions of HRP-labelled and unlabelled cells in the retinal ganglion cell layer were estimated from sections or wholemounts counterstained with cresyl violet. Between 20 and 40 days postnatal we did not observe an area centralis or visual streak in either the labelled or unlabelled cell populations. These regional specialisations arose between 60 and 80 days within the labelled cell population, while unlabelled cells remained approximately evenly distributed. Our findings suggest there is no 'hidden' area centralis and visual streak within the evenly distributed total cell population of the retinal ganglion cell layer at stages before cells can be classified on morphological grounds.

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PubMed1985

Pattern formation in the retinal ganglion cell layer and visual brain centres.

Development of adult patterns of cell distribution in the retinal ganglion cell layer from an approximately even initial topography are described for frogs and mammals. An area centralis and visual streak form in the frog only after metamorphosis, probably by the greater addition of cells to the ciliary margin nasally and temporally than dorsally and ventrally. By contrast, in mammals all cells destined to occupy the ganglion cell layer are generated before the area centralis and visual streak are formed. Patterns of cell death seem unlikely to explain changing cells distributions within the ganglion cell population and we are currently documenting patterns of cell migration to address this issue. The formation of specific connections between retinal ganglion cells and their partner cells in the brains of lower vertebrates is also discussed. At stages before optic axon outgrowth the eye already contains positional information to determine formation of appropriate central connections. Optic nerve regeneration studies in frogs indicate that positional cues are available in visual centres. However, interactions between fibres can overwhelm any such cues, allowing an orderly shift in connections to accommodate changes in the relative numbers of ganglion cells and their partner cells in the brain. Thus in the frog, retinotopicity is maintained during the postmetamorphic increase in ganglion cell number associated with area centralis and visual streak formation, despite an absence of central cell division. Similarly, an entire visual field in time becomes represented in an orderly fashion within the remaining portion of a partially ablated mature visual centre. Interactions between fibres are also considered responsible for the formation of ocular dominance columns, such as those found in binocular regions of the mammalian visual cortex. The role of visual experience is usually to refine and maintain the precise patterns of connectivity established by ontogenetic mechanisms.

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PubMed1985

Cell distributions in the retinal ganglion cell layer of adult Leptodactylid frogs after premetamorphic eye rotation.

In adult Limnodynastes dorsalis and Heleioporus eyrei regions of high cell density in the retinal ganglion cell layer are normally found along the nasotemporal axis and become apparent only after metamorphosis (Dunlop & Beazley, 1981; Coleman, Dunlop & Beazley, 1984). Eye rotations were performed from embryonic to mid-larval life and cell topography mapped after metamorphosis using cresyl violet-stained wholemounts. Mature cell distributions indicated that from stages equivalent to 30/31 in Xenopus (Nieuwkoop & Faber, 1956) alignment of high cell density regions had already been determined and developed to reflect the degree of eye rotation. We conclude that cell topography in the adult ganglion cell layer is determined from the time at which invagination of the eye cup nears completion. Furthermore, the correspondence in adults between alignment of high cell density regions and the orientation of the visuotectal projection suggests that these features could not be dissociated by manipulation of the developing eye.

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PubMed1984

A morphometric study of the retinal ganglion cell layer and optic nerve from metamorphosis in Xenopus laevis.

The number and distribution of cells in the retinal ganglion layer have been examined in Nissl-stained wholemounts during postmetamorphic life in Xenopus laevis. Total cell number at metamorphic climax, in the 3 and 4 month juveniles and in 5 year adults were 30,000-41,000, 45,000-51,000 and 71,000-82,000 respectively. Optic axon counts of 29,000, 39,000 and 58,000 at these stages suggested that most cells counted were ganglion cells and that their number doubled during this period. Cell distribution changed after metamorphosis producing nasal and temporal regions of higher density in juveniles and adults. These results are discussed in relation to retinal growth and plasticity of retinotectal connections.

Animals↗

PubMed1984

Patterns of cell division during visual streak formation in the frog Limnodynastes dorsalis.

The site and extent of cell division were determined in midlarval tadpoles, animals at metamorphic climax and 2-month juvenile Limnodynastes dorsalis using untreated animals and those injected with colchicine or [3H]thymidine shortly before sacrifice. Mitosis was restricted to the ciliary margin at all stages and there were significantly more dividing cells nasally and temporally than dorsally and ventrally. In animals injected with [3H]thymidine and killed at a subsequent stage, labelled cells were grouped at a distance from the ciliary margin and were more frequent in nasal and temporal than in dorsal and ventral retina. These results suggest that differing extents of mitosis around the ciliary margin, reflected in the number of cells entering the ganglion cell layer, may largely underlie the postmetamorphic formation of a visual streak observed using wholemounts.

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