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L D Beazley

Publications and source records attributed to L D Beazley.

At least 91 records · Page 5Linked to original sources
PubMed1983

An electrophysiological study of early retinotectal projection patterns during regeneration following optic nerve crush inside the cranium in Hyla moorei.

The sequence of regeneration following intracranial optic nerve crush has been studied using electrophysiological visual mapping in the frog Hyla moorei. Compared to our earlier series with extracranial crush, the time course was slower and the intermediate projections more disorganized. It is suggested that the apparent discrepancy between the early patterns of regeneration in Xenopus and Rana compared to Hyla, fish and newt, is not due to species differences but to the location of the lesion site.

Animals↗
PubMed1983

The evolution of an area centralis and visual streak in the marsupial Setonix brachyurus.

The distribution, morphology, size, and number of cells in the retinal ganglion layer of the marsupial Setonix brachyurus, "quokka," was studied from 25 days postnatal to adulthood using Nissl-stained wholemounts. The total cell population was evenly distributed up to 50 days, but by 75 days highest densities were generally observed in a broad band extending across the nasotemporal axis. At 87 days, a temporally situated area centralis was seen for the first time. This was embedded in a horizontally aligned visual streak, the nasal arm of which contained areas of high density. By 106 days, densities in the area centralis had stabilized while peripheral values were higher than adult levels even at 180 days. In the adult, the area centralis was surrounded by a weak visual streak. Retinal area increased steadily during development to reach 168 mm2 at 180 days, the adult range being 225-250 mm2. All cells in the ganglion layer appeared undifferentiated and rounded at 33 days with soma diameters of 3-6 micrometers; by 70 days diameters had increased to 4-12 micrometers and some cells had axon hillocks containing Nissl substance. From 87 days we distinguished ganglion cells, which constituted 54-63% of the total. These were identified by deeply stained Nissl substance and had diameters of 7-18 micrometers, compared to 7-23 micrometers at 143 days and 7-24 micrometers in the adult; the remaining cells, termed glia/interneurons, were 5-8 micrometers throughout. Only ganglion cells were organized into an area centralis and visual streak. Glia/interneurons were evenly distributed except at the extreme periphery, where their density increased. In sectioned material, the ganglion layer was distinct from 25 days while the neuroblastic layer separated only between 48 and 85 days. From 25 to 250 days the total number of cells in the ganglion layer remained similar to the adult range of 336,000-393,000. At both 87 days and in adults optic axon counts fell between 180,000 and 224,000, close to ganglionic cell estimates. At 25 and 34 days, respectively, optic axon numbers were 75,000 and 172,000. Myelination was absent at 25 and 34 days, 3% at 87 days, and almost 100% in adults. Mechanisms are discussed whereby the area centralis and visual streak may evolve from an even distribution of cells while their number remains constant; migration is considered likely to be important.

Animals↗
PubMed1982

The development of connections between the isthmic nucleus and the tectum in Xenopus and Limnodynastes tadpoles.

In adult Anura reciprocal connections exist between the isthmic nucleus and ipsilateral tectum. These pathways, with a projection from the isthmic nucleus to the opposite tectum, constitute the binocular visual connections. Isthmic nuclei were examined histologically after injection of horseradish peroxidase into the tectum of Xenopus laevis and Limnodynastes dorsalis from midlarval stages to the completion of metamorphosis and in adults. The isthmic nuclei were present throughout, their cell number reaching the adult complement by metamorphic climax. At all stages each isthmic nucleus projected to both tecta. Late onset of electrical activity in the intertectal pathway cannot therefore be explained by the time at which isthmic fibres reach the tecta.

Animals↗
PubMed1982

An electrophysiological study of early retinotectal projection patterns during optic nerve regeneration in Hyla moorei.

The sequence of regeneration of the optic nerve into the tectum has been studied using electrophysiological visual mapping in the frog. Hyla moorei. Some individuals were remapped to confirm the sequential nature of the changes described. The projection was retinotopic throughout regeneration. Multiunit receptive field sizes were initially large but progressively decreased to normal over approximately 30 days. The differences between these results and earlier studies are discussed.

Animals↗
PubMed1982

Adult levels of meridional anisotropy and contrast threshold in 5-year olds.

This study, using a forced-choice technique, shows that 5-yr-old children have greater sensitivity for vertical and horizontal sinusoidal gratings than for obliques. Contrast thresholds for all orientations were similar to those of adults as was the magnitude of the meridional anisotropy. These results suggest that meridional anisotropy does not develop over an extended period of childhood as was previously reported (Mayer, 1977). Reasons for the discrepancy between the two studies are discussed.

Adult↗
PubMed1982

Visuotectal projections following temporary transplantation of embryonic eyes to the body in Xenopus laevis.

Eye anlagen of Xenopus laevis at stages 22-26 were transplanted, 180 degrees rotated, into the flank of similarly staged embryos and 24-36 h later were placed into the orbits of final hosts at stages 32-36. After metamorphosis visuotectal projections were mapped electrophysiologically. In one series, eyes were normally oriented in the final host and visuotectal projections were found to be normal. A second series, in which eyes were placed in a rotated orientation in the final host, had developed rotated projections. The results suggest that at stage 22 the eye already contained positional information in terms of the organization of central connections which was not overridden by cues from surrounding tissue on the flank or in the final host orbit. Earlier experiments by Hunt and Jacobson (1972), which indicated that body tissues impose positional information on the developing eye between stages 29 and 31, are discussed in relation to the present study.

Animals↗
PubMed1980

Contrast sensitivity in children and adults.

Contrast sensitivity functions to vertical sinusoidal gratings have been measured in 135 subjects ranging from 3 to 29 years of age. The reported contrast sensitivities for all spatial frequencies increased steadily with age, reaching adult levels during adolescence. The usefulness of these functions as a clinical test is discussed.

Adolescent↗
PubMed1977

Abnormalities in the visual system of Xenopus after larval optic nerve section.

One optic nerve was cut in a series of larval Xenopus laevis; in approximately half of these the corresponding eye was also rotated. The resultant primary visual projections were mapped in the adult. Although in most cases the projections were retinotopically normal, some of them were not. These abnormal projections were consistently associated with a very thin optic nerve which reached the tectum via the oculomotor route. Animals with rotated eyes which developed retinotopically organised primary projections had a normal intertectal ipsilateral projection from the rotated eye and a rotated projection from the normal eye, thus confirming earlier findings that a process of binocular interaction determines the formation of intertectal connections. In all frogs with retinotopically abnormal primary projections the intertectal connections from the normal eye were of the type found after enucleation and those from the abnormal side were fragmentary or absent. Since the development of abnormal retinotopic patterns preceded that of intertectal connections the mechanisms of binocular interaction must have broken down in the presence of the abnormal primary projections.

Animals↗
PubMed1975

Factors determining decussation at the optic chiasma by developing retinotectal fibres in Xenopus.

In Amphibia all primary retinotectal nerve fibres cross at the chiasma. To investigate why this decussation takes place, a series of embryological operations were performed in which one host eye was replaced by an eye from the opposite side of a donor in Xenopus laevis. The visual projections to the optic tecta were mapped electrophysiologically in these animals when adult to reveal to which side of the brain the nerve fibres had become connected. In the majority of cases the contralaterally grafted eyes had developed primary projections to both tecta, in others all the fibres had innervated the contralateral tectum and in 2 animals optic axons were detected only from the ipsilateral side. In 2 other animals optic nerve fibres from both the normal and operated eyes had innervated both tecta. Control experiments showed that section of the optic nerve and stalk without any misalignment of the eye and stalk did not disturb the normal decussation of the fibres. Neither did enucleation in the embryo affect the decussation of the fibres from the remaining eye. It is concluded that no "side specificity" has been shown to exist to determine the side of the brain into which retinotectal fibres develop; rather it seems that complete decussation depends on the developing optic nerve fibres being able to enter a normally aligned optic stalk as they leave the eye.

Animals↗
PubMed1975

Development of intertectal neuronal connections in xenopus: the effects of contralateral transposition of the eye and of eye removal.

The development of intertectal neruonal connections has been investigated in Xenopus laevis. Contralateral eye grafts and enucleations were performed in embryos and the resultant visual projections to the optic tecta were mapped electrophysiologically after metamorphosis. In enucleated animals the ipsilateral projections were found to be normally organised retinotopically but consisted of visual units with abnormally large multi-unit receptive fields. In 10 animals with contralaterally grafted eyes a normal ipsilateral projection had developed from the abnormal eye and an abnormal projection from the normal eye, to produce congruent maps via the two eyes to one tectum. All the maps in these animals were retinotopically organised. In another 11 animals the ipsilateral projection from the operated eye was fragmentary or absent, while that from the unoperated eye resembled the pattern found after enucleation. Retinotopically abnormal contralateral projections had developed in 5 animals of this group. These results suggest that prefunctional specification determines the initial development of diffuse intertectal visual connections but these may be modified by a process of binocular interaction in the presence of a normal primary contralateral input.

Animals↗
PubMedDate not supplied

Repeated prenatal corticosteroids delay myelination in the ovine central nervous system.

Maternal administration of corticosteroids is used to promote lung maturation in human infants considered at risk of preterm delivery [1]. Randomised trials of a single course of corticosteroid treatment have indicated no adverse long-term neurological or cognitive sequelae [2-5]. However, the current trend in many obstetric centres is to use repeated courses in cases where preterm birth has not eventuated, but the risk persists 7 days beyond administration of the original course [6-7]. This practice has not yet been subject to randomised trials of outcome. We have examined the effect of repeated injections of corticosteroids on the development of the optic nerve in prenatal fetal sheep and report a significant delay in the myelination of optic axons. Our results, together with those from other animal studies [8], show that repeated courses of corticosteroids may be detrimental to central nervous system (CNS) development. Clinical practice should balance the known beneficial effects on lung maturation of a single course of corticosteroid against the potential damage to the CNS of repeated courses.

Animals↗
PubMedDate not supplied

A second episode of ganglion cell death takes place when an optic nerve regenerates for a second time in the frog.

We have previously reported that during optic nerve regeneration in the frog, 30-40% of retinal ganglion cells die, the loss being complete within 10 weeks. In the present study, we crushed the optic nerve, waited 10 weeks, and then recrushed the nerve at the same site. Retinae were examined 10 weeks later. We estimated ganglion cell numbers from cresyl-violet-stained wholemounts and found a fall of 53% compared to normals. The loss was significantly greater than the losses of 36% and 35%, respectively, in frogs which received a single optic nerve crush and were examined 10 or 20-24 weeks later. The results indicate that a second episode of ganglion cell death took place when the optic nerve regenerated a second time. We conclude that ganglion cells in the frog are not comprised of two subpopulations, only one of which intrinsically possesses the ability to regenerate.

Animals↗
PubMedDate not supplied

Survival of ganglion cells which form the retino-retinal projection during optic nerve regeneration in the frog.

During optic nerve regeneration in the frog, axons transiently grow along the opposite optic nerve forming a retino-retinal projection. In the present study, we crushed the left optic nerve in the frog Litoria (Hyla) moorei and later applied horseradish peroxidase (HRP) or diamidino yellow (DY) to the right optic nerve. In one series, retinae were examined 3 days after application of the tracer. The retino-retinal projection was found to be maximal at 5 weeks, fell significantly by 7 weeks, and returned to close-to-normal levels by 24 weeks. In a second series, we applied DY at 5 weeks as before but did not sacrifice the frogs until 7 weeks. Numbers of labeled ganglion cells were not significantly different from those frogs in the first series labeled and examined at 5 weeks. We conclude that ganglion cells giving rise to the retino-retinal projection had not died in appreciable numbers, presumably being sustained by collateral axons in the brain.

Amidines↗
PubMedDate not supplied

Changing topography of the RPE resulting from experimentally induced rapid eye growth.

The retinal pigment epithelium (RPE) of the quokka wallaby. Setonix brachyurus, grows and changes throughout life. To investigate factors that determine changes in the quokka RPE, we have examined topography of this tissue in experimentally enlarged eyes. Unilateral eyelid suture was conducted at the time of normal eye opening, postnatal day (P) 110, and animals were examined at 1 or 1 1/2 years of age. The numbers and densities of RPE cells and the extent of multinucleation were compared with those in normal animals. Eyelid suture resulted in a 9.8% and 17.4% increase in retinal area at 1 and 1 1/2 years, respectively; a significant degree of myopia was associated with this enlargement. Cell density topography in experimental eyes was not the same as in controls. Cells from central retina were disproportionately larger in the experimental than control eyes. However, the RPE cell topography in sutured eyes was not the same as that of aged retinae of a similar size. Notably, in sutured eyes there was no development of the high or highest cell densities seen in equatorial and temporal central RPE in aged retinae, respectively. Furthermore, the degree of cell enlargement in peripheral regions was slight compared with that observed in similar-sized, aged retinae. There was no increase in RPE cell number; rather, average cell area increased accompanied by no change or a slight decrease in RPE thickness. Consequently, overall volume of cells did not change significantly. The large number of multinucleate cells normally seen in aged animals was not observed in experimentally enlarged eyes, implying that an increase in cell volume may be the trigger for multinucleation.

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