Glen Jeffery

Retinal ganglion cell death and terminal field retraction in the developing rodent visual system

The anterograde and retrograde transport of HRP has been employed in neonatal rats and adult rats which were unilaterally enucleated at various stages during the first week after birth. In neonatal animals given unilateral thalamic implants of horseradish peroxidase, the number of labelled retinal ganglion cells in the ipsilateral eye declines over the first week. This is considered to be a consequence of cell death. At the same time unilateral intraocular injections of the same tracer reveals that the terminal field of ipsilaterally projecting retinal axons in the dorsal lateral geniculate nucleus is retracting to form the adult pattern. It is proposed that retraction and ganglion cell death are related. In the monocular adult animals it is shown that fewer ipsilaterally projecting ganglion cells are found the later enucleation takes place. But the number of ipsilaterally projecting cells found in the adult animal enucleated at birth is not as great as the number found in the newborn rat. In spite of this the proportion of the dorsal lateral geniculate nucleus occupied by ipsilaterally projecting ganglion cells is similar in neonates of a given age and adults that were enucleated at that age.

Evidence for ganglion cell death during development of the Ipsilateral retinal projection in the rat

When one eye of a rat is removed at birth an increased projection to the brain is found from the remaining eye. This projection arises from the whole retina and mainly from ganglion cells which project to only one hemisphere of the brain. In the normal course of development many of these cells die and the uncrossed projection diminishes to form the normal adult pattern.

The albino retina: an abnormality that provides insight into normal retinal development

Albino mammals have specific deficits in their retinae and in the pattern of decussation at their optic chiasm, demonstrating that a melanin-related agent is crucial for normal development of the visual system. Although much attention has been paid to chiasmatic abnormality over the past 30 years, little progress has been made in understanding the abnormality. There has now been a shift of attention towards an analysis of the developing albino retina, which is providing significant advances in our understanding of the role played by this melanin-related agent. It is now possible to identify one candidate substance that emanates from the synthetic pathway of melanin that plays a key role in regulating retinal development.

Subcortical afferent and efferent connections of the superior colliculus in the rat and comparisons between albino and pigmented strains.

SummarySubcortical connections of the superior colliculus were investigated in albino and pigmented rats using retrograde and anterograde tracing with horseradish peroxidase (HRP), following unilateral injection of HRP into the superior colliculus. Afferents project bilaterally from the parabigeminal nuclei, the nucleus of the optic tract, the posterior pretectal region, the dorsal part of the lateral posterior-pulvinar complex and the ventral nucleus of the lateral lemniscus; and ipsilaterally from the substantia nigra pars reticulata, the pars lateralis of the ventral lateral geniculate nucleus, the intergeniculate leaflet, the zona incerta, the olivary pretectal nucleus, the nucleus of the posterior commissure, the lateral thalamus, Forels field H2, and the ventromedial hypothalamus. Collicular efferents terminate ipsilaterally in the anterior, posterior and olivary pretectal nuclei, the nuclei of the optic tract and posterior commissure, the ventrolateral part of the dorsal lateral geniculate nucleus, the pars lateralis of the ventral lateral geniculate nucleus, the intergeniculate leaflet, and the zona incerta; and bilaterally in the parabigeminal nuclei and lateral posterior-pulvinar complex (chiefly its dorsal part). The general topographical patterns of some of the afferent and efferent projections were also determined: the caudal and rostral parts of the parabigeminal nucleus project to the caudal and rostral regions, respectively, of the superior colliculus; caudal superior colliculus projects to the most lateral, and lateral superior colliculus to the most caudal part of the terminal field in the dorsal lateral geniculate nucleus; caudolateral superior colliculus projects to the caudal ventrolateral part of the ventral lateral geniculate nucleus, while rostromedial parts of the colliculus project more rostrally and dorsomedially. Following comparable injections in pigmented and albino animals, fewer retrogradely labelled cells were found in subcortical structures in the albino than in the pigmented rats. The difference was most marked in nuclei contralateral to the injected colliculus. Thus, the effects of albinism on the nervous system may be more widespread than previously thought.

Bifurcating retinal ganglion cell axons in the rat, demonstrated by retrograde double labelling

SummaryBy injecting two different tracers, one in each lateral geniculate nucleus, it was shown that some retinal ganglion cells project to both hemispheres in the rat. The double-labelled ganglion cells were confined to the temporal retina corresponding to the binocular visual field.

Retinal mitosis is regulated by dopa, a melanin precursor that may influence the time at which cells exit the cell cycle: analysis of patterns of cell production in pigmented and albino retinae.

A melanin‐associated agent seems to play a role in regulating retinal development. When absent, diverse deficits occur. There is evidence that this agent regulates patterns of mitosis. This study examines retinal development in pigmented and albino rats to identify the regulating agent and its mode of action. Throughout neurogenesis, many more mitotic profiles are found in albinos than pigmented animals. At the peak of retinal neurogenesis, approximately 50% more mitotic profiles are found in albinos than in matched pigmented animals, resulting in abnormal retinal thickening. Concurrently, increasing numbers of pyknotic nuclei are identified, such that later in development retinal thickness normalises. However, the crude centre‐to‐periphery pattern of cell production is preserved. Abnormal cell proliferation is found in a range of albino rat strains, but it is not present in their brains, confirming that the abnormality is ocular and melanin related. Dopa is a critical element in initial stages of melanin synthesis and is present in abnormally low levels in developing albino retinae. Furthermore, it is an antimitotic agent. Addition of dopa to albino eyes in vitro normalises patterns of cell production. These results are consistent with the hypothesis that dopa is a major regulator of retinal cell production and that it influences the capacity of cells to exit the cell cycle. J. Comp. Neurol. 405:394–405, 1999.

Melanopsin retinal ganglion cells and the maintenance of circadian and pupillary responses to light in aged rodless/coneless (rd/rd cl) mice

Melanopsin‐expressing ganglion cells have been proposed as the photoreceptors mediating non‐rod, non‐cone ocular responses to light. Here we use the aged (approximately 2 years) rodless and coneless (rd/rd cl) mouse to assess the impact of progressive inner retinal cell loss on melanopsin expression, circadian entrainment and pupillary constriction. Aged rd/rd cl mice show substantial transneuronal retinal degeneration leaving only the ganglion cell layer and little of the inner nuclear layer. Despite this loss, quantitative reverse transcriptase‐polymerase chain reaction showed normal levels of melanopsin expression, and immunocytochemistry demonstrated both the presence and normal cellular appearance of these cells. Furthermore, the optic nerves of the two genotypes (rd/rd cl and +/+) were not obviously different in animals older than 2 years. However, this massive level of retinal degeneration left both pupillary and circadian responses to light intact, even in rd/rd cl mice older than 2 years. Our data provide the first positive correlation between the persistence of melanopsin‐expressing cells and the maintenance of both circadian and pupillary responses to light in the absence of rods and cones. These findings, together with recent studies on melanopsin knockout mice, are consistent with the hypothesis that melanopsin‐expressing ganglion cells are photosensitive and mediate a range of irradiance‐detection tasks.

Viewing Ageing Eyes: Diverse Sites of Amyloid Beta Accumulation in the Ageing Mouse Retina and the Up-Regulation of Macrophages

Background Amyloid beta (Aβ) accumulates in the ageing central nervous system and is associated with a number of age-related diseases, including age-related macular degeneration (AMD) in the eye. AMD is characterised by accumulation of extracellular deposits called drusen in which Aβ is a key constituent. Aβ activates the complement cascade and its deposition is associated with activated macrophages. So far, little is known about the quantitative measurements of Aβ accumulation and definitions of its relative sites of ocular deposition in the normal ageing mouse. Methodology/Principal Findings We have traced Aβ accumulation quantitatively in the ageing mouse retina using immunohistochemistry and Western blot analysis. We reveal that it is not only deposited at Bruchs membrane and along blood vessels, but unexpectedly, it also coats photoreceptor outer segments. While Aβ is present at all sites of deposition from 3 months of age, it increases markedly from 6 months onward. Progressive accumulation of deposits on outer segments was confirmed with scanning electron microscopy, revealing age-related changes in their morphology. Such progress of accumulation of Aβ on photoreceptor outer segments with age was also confirmed in human retinae using immunohistochemistry. We also chart the macrophage response to increases in Aβ showing up-regulation in their numbers using both confocal laser imaging of the eye in vivo followed by in vitro immunostaining. With age macrophages become bloated with cellular debris including Aβ, however, their increasing numbers fail to stop Aβ accumulation. Conclusions Increasing Aβ deposition in blood vessels and Bruchs membrane will impact upon retinal perfusion and clearance of cellular waste products from the outer retina, a region of very high metabolic activity. This accumulation of Aβ may contribute to the 30% reduction of photoreceptors found throughout life and the shortening of those that remain. The coating of Aβ on outer segments may also have an impact upon visual function with age.

Retinal nerve fibre layer polarimetry: histological and clinical comparison

AIMS To compare histological thickness of the retinal nerve fibre layer in the primate with retardation measurements obtained in vivo using the Mark II Nerve Fiber Analyzer (NFA, Laser Diagnostic Technologies, San Diego, USA). METHODS Scanning laser polarimetry was performed on both eyes of a healthy anaesthetised adult primate (Macaca mulatta). The retinal nerve fibre layer thickness was measured in the eye with the best polarimetry image. A nerve fibre layer thickness map was scaled and aligned to a retardation map to permit correlation of retardation and thickness measurements. RESULTS Retinal nerve fibre layer thickness measurements could be satisfactorily aligned with corresponding retardation values at 216 locations. The overall correlation coefficient for nerve fibre layer thickness and retardation wasr = 0.70 (n = 216, p <0.001). Regional comparison showed the best correlation (r = 0.76, n = 45, p <0.001) occurred inferior to the optic disc. Less positive but still highly significant correlations were seen superiorly and temporally (r = 0.52, n = 26, p = 0.007 andr = 0.49, n = 86, p = <0.001 respectively), with the lowest correlation occurring at the nasal aspect of the disc (r  = 0.06, n  = 67, p = 0.64). CONCLUSIONS In the primate eye, retinal nerve fibre layer thickness shows a positive correlation with retardation measurements obtained with the nerve fibre analyser. However, since the correlation coefficient varied around the optic disc, further evaluation of the device is advised before its routine clinical use.

Retinal cell addition and rod production depend on early stages of ocular melanin synthesis.

Retinal mitosis is regulated by dopa, a melanin precursor present in the developing retinal pigment epithelium. Its absence results in retinal deficits including a failure of ≈30% of the rod population to develop. Here, 3H‐thymidine labelling is used to analyse patterns of cell addition spanning the main period of retinal development in rat litters containing both pigmented and albino phenotypes. Many more thymidine‐labelled cells are found in each cellular layer at maturity in albinos than in their pigmented littermates. Normal spatial patterns of photoreceptor addition are seen in albinos during cone production and for most of the subsequent period of rod addition. However, abnormal spatial patterns of cell addition occur across the retinal when rod production peaks. A delay in the centre to periphery gradient of cell addition is apparent in both nuclear layers. These data are related to deficits in the mature architecture of the albino retina. The results are consistent with there being significant cell cycle and/or exit point irregularities in hypopigmented retinae. It is probable that reduced dopa levels in albinos result in the cell cycle rate not slowing appropriately with development, which may lead to cells missing their exit points. This produces abnormal patterns of cell addition at key stages and delays in the gradient of retinal maturation along with a large cell loss at critical stages of rod production. J. Comp. Neurol. 420:437–444, 2000.