Alison M. Harman

Neuronal density in the human retinal ganglion cell layer from 16-77 years

Literature assessing whether or not neurons (retinal ganglion cells and displaced amacrine cells) are lost from the retinal ganglion cell layer in mammals with age is still controversial, some studies finding a decrease in cell density and others not. To date there have been no studies estimating the total number of neurons in the retinal ganglion cell layer of humans throughout life. Recent studies have concentrated on the macular region and examined cell densities, which are reported to decrease during aging. In a study of the human retinal pigment epithelium (RPE), we showed that, while RPE cell number does not change, cell density increases significantly in central temporal retina (macular region) as the retina ages. We speculated that the increase in density represents a “drawing together” of the retinal sheet to maintain high cell densities, in this region of the neural retina, in the face of presumed cell loss from the ganglion cell layer due to aging. Here, therefore, we have sampled the entire ganglion cell layer of the human retina and estimated total neuron numbers in 12 retinae aged from 16 to 77 years. Human retinae, fixed in formalin, were obtained from the Queensland Eye Bank and whole‐mounted, ganglion cell layer uppermost. The total number of neurons was lower in the older than younger retinae and neuronal density was lower in most retinal regions in older retinae. Retinal area increased with age and neuronal density fell throughout the retina with a mean reduction of 0.53% per year. However, the percentage reduction in density was much lower for the macular region, with a value of 0.29% per year. It is possible that this lesser reduction in cell density in the macula is a result of the drawing together of the retinal sheet in this region as we speculated from RPE data. Anat Rec 260:124–131, 2000.

A Strong Correlation Exists between the Distribution of Retinal Ganglion Cells and Nose Length in the Dog

The domestic dog, Canis lupus familiaris, is a subspecies of the gray wolf, Canis lupus, with almost identical mitochondrial DNA. The dog is the most diverse species on earth, with skull length varying between 7 and 28 cm whereas the wolf skull is around 30 cm long. However, eye size in dogs does not appear to vary as much. For example, small dogs such as the chihuahua appear to have very large eyes in proportion to the skull. Our aim was to examine eye size and retinal cell numbers and distribution to determine whether the dog eye exhibits as much variation as the skull. We found a correlation between eye radius and skull dimensions. However, the most surprising finding was that the distribution of ganglion cells in the eye varied tremendously from a horizontally aligned visual streak of fairly even density across the retina (as seen in the wolf) to a strong area centralis with virtually no streak (for example, as observed in a pug from the current series). This variation in ganglion cell density within a single species is quite unique. Intriguingly, the ratio of peak ganglion cell density in the area centralis to visual streak was highly negatively correlated with skull length (r = –0.795, n = 22) and positively correlated with cephalic index (r = 0.687, n = 22). The orientation of eyelid aperture was also correlated with cephalic index (r = 0.648, n = 22). Therefore, the genetic manipulation of selective breeding, which has produced an abnormal shortening of the skull and eyelids with less lateral apertures, has also produced a considerably more pronounced area centralis in the dog.

Anatomical comparison of the macaque and marsupial visual cortex: common features that may reflect retention of essential cortical elements.

This study identifies fundamental anatomical features of primary visual cortex, area V1 of macaque monkey cerebral cortex, i.e., features that are present in area V1 of phylogenetically distant mammals of quite different lifestyle and features that are common to other regions of cortex. We compared anatomical constituents of macaque V1 with V1 of members of the two principal marsupial lines, the dunnart and the quokka, that diverged from the eutherian mammalian line over 135 million years ago. Features of V1 common to both macaque and marsupials were then compared with anatomical features we have previously described for macaque prefrontal cortex. Despite large differences in overall area and thickness of V1 cortex between these animals, the absolute size of pyramidal neurons is remarkably similar, as are their specific dendritic branch patterns and patterns of distribution of intrinsic axons. Pyramidal neuron patchy connections exist in the supragranular V1 in both the marsupial quokka and macaque as well as in macaque prefrontal cortex. Several specific types of aspinous interneurons are common to area V1 in both marsupial and macaque and are also present in macaque prefrontal cortex. Spiny stellate cells are a common feature of the thalamic‐recipient, mid‐depth lamina 4 of V1 in all three species. Because these similarities exist despite the very different lifestyles and evolutionary histories of the animals compared, this finding argues for a highly conserved framework of cellular detail in macaque primary visual cortex rather than convergent evolution of these features. J. Comp. Neurol. 400:449–468, 1998.

Blindsight in Subjects with Homonymous Visual Field Defects

Brain damage in the visual system can lead to apparently blind visual areas. However, more elaborate testing indicates that some visual ability may still exist for specific stimuli in the otherwise blind regions. This phenomenon is called blindsight if subjects report no conscious awareness of visual stimuli but when forced to guess, nevertheless perform better than chance. It has mainly been suggested that secondary visual pathways are responsible for this phenomenon. However, no published study has clearly shown the neural mechanism responsible for blindsight. Furthermore, experimental artifacts may have been responsible for the appearance of the phenomenon in some subjects. In the present study, the visual fields of nine subjects were mapped and residual visual performance was examined in many areas using three different experimental procedures. Artifacts such as stray light or eye movements were well controlled. In addition, confidence ratings were required after each trial in the forced-choice tests. The results show that only one subject with a lesion in the optic radiation had blindsight in two discrete areas of the affected visual field. Spared optic radiation fibers of the main (primary) geniculo-striate visual pathway were most likely to account for this finding.

Retinal Structure and Visual Acuity in a Polyprotodont Marsupial, the Fat-Tailed Dunnart (Sminthopsis crassicaudata)

The visual system of the fat-tailed dunnart (Sminthopsis crassicaudata), a small polyprotodont marsupial, has been examined both anatomically and behaviourally. The ganglion cell layer was examined in cresyl-violet stained wholemounts and found to contain a mean of 81,400 ganglion cells (SD ± 3,360); the identification of ganglion cells was supported by a correspondence to optic axon counts. Ganglion cells were distributed as a mid-temporally situated area centralis, embedded in a pronounced visual streak. Localised implants of horseradish peroxidase into retinal wholemounts revealed both A-type and B-type horizontal cells. Sections of the outer retina showed it to be rod-dominated, with a rod-to-cone ratio of 40:1 at the area centralis; cones were found to contain oil droplets but double cones were not a prominent feature. The retinal pigment epithelium consisted of squamous cells. Visual acuity, estimated from counts of peak ganglion cell density (8,300/mm2, SD ± 1,180) and measurements of posterior nodal distance (2.9 mm), was found to be 2.30 cycles per degree. The value was close to that of 2.36 cycles per degree estimated by behavioural tests using a Mitchell jumping stand; values were similar at low, intermediate and high light levels. Our findings are discussed in relation to the lifestyle of the dunnart.

Variability in the Location of the Retinal Ganglion Cell Area Centralis Is Correlated with Ontogenetic Changes in Feeding Behavior in the Black Bream, Acanthopagrus butcheri (Sparidae, Teleostei)

The development of neural cell topography in the retinal ganglion cell layer was examined in a teleost, the black bream (Acanthopagrus butcheri). From Nissl-stained wholemounts, it was established that fish between 10 and 15 mm standard body length (SL) possess high cell densities throughout the dorso-temporal retinal quadrant, with peak cell densities located in temporal regions of the retina. However, in fish between 15 and 80 mm SL, a wide variation in the position of the peak cell density is revealed with the locations of the areae centrales (AC) ranging from exclusively temporal to periphero-dorsal retina. Fish larger than 80 mm SL always possess an AC located in the dorsal region of the dorso-temporal retinal quadrant. The topography of ganglion cells within the ganglion cell layer was determined by comparing the numbers of ganglion cells retrogradely-labeled from the optic nerve with the total population of Nissl-stained neurons (ganglion plus displaced amacrine cells) in a range of different-sized individuals. Ganglion cell topography was the same as that recorded for all Nissl-stained neurons. The feeding behavior of juveniles from metamorphosis to 80 mm SL was observed, where fish were given the choice of feeding on live food in mid-water (until 15 mm SL) or obtaining pellets from the surface or the bottom. A range of feeding patterns was recorded, with the smallest fish taking food from mid-water but individuals between 15 and 80 mm SL taking food either from the surface or the bottom or both. A correlation between the preferred mode of feeding and the position of the AC was found, such that those individuals feeding in mid-water or at the surface possess a temporal or intermediate (dorso- temporal) AC, whereas those predominantly feeding from the bottom possess a dorsal AC.

Residual Vision in a Subject with Damaged Visual Cortex

It is well known that a lesion in the optic radiation or striate cortex leads to blind visual regions in the retinotopically corresponding portion of the visual field. However, various studies show that some subjects still perceive certain stimuli even when presented in the blind visual field. Such subjects either perceive stimuli abnormally or only certain aspects of them (residual vision) or, in some cases, deny perception altogether even though visual performance can be shown to be above chance (blindsight). Research on monkeys has suggested a variety of parallel extrastriate visual pathways that could bypass the striate cortex and mediate residual vision or blindsight. In the present study, we investigated a subject with perimetrically blind visual areas caused by bilateral brain damage. Black and white stimuli were presented at many locations in the intact and affected areas of the visual field. The subjects task was to state, using confidence levels, whether the target stimulus was black or white. The results revealed an area in the blind visual field in which the subject perceived a light flash when the experimental black stimulus was presented. We hypothesize that a spared region in the visual cortex most likely accounts for these findings.

Neurogenesis in the Hippocampus of an Adult Marsupial

In the adult eutherian brain, stem cells in the dentate gyrus continually divide throughout adult life and into old age producing new granule cells. However, it was not known whether this is also the case for marsupials. Previously, in fact, it was thought that marsupials did not have continued neurogenesis in the mature brain. Here we examined neurogenesis in the adult brain of a small marsupial, the fat-tailed dunnart, using 3H-thymidine to label newly generated cells. We showed that neurogenesis takes place in the adult dentate gyrus along its inner margin, as seen in eutherian mammals. Control animals had similar numbers of labeled cells 24 h and 1 month after 3H-thymidine injection. An enriched environment resulted in similar numbers of cells being generated as controls. However, there was a significant decline in the number of labeled cells one month later. Stress and old age resulted in significantly lower numbers of new cells being generated. In immunohistochemically treated control brains, 3H-thymidine-labeled cells at the early stage were sometimes GFAP positive, were not calbindin positive at either stage examined and at the later stage were PSA-NCAM positive. We hypothesize that, as seen in eutherian mammals, the new cells progressed from being GFAP positive at stem cell stage to PSA-NCAM positive during outgrowth of mossy fibers 1 month later, to calbindin positive when mature. It is possible that maturity of these cells was not reached by 1 month as marsupials have a slower metabolic rate and this species also undergoes daily periods of torpor.

The Retinal Ganglion Cell Layer and Visual Acuity of the Camel

We examined the retinal ganglion cell layer of the dromedary camel, Camelus dromedarius. We have estimated that there are 8 million neurons in the ganglion cell layer of this large retina (mean area of 2,300 mm–2). However, only approximately 1 million are considered to be ganglion cells. The ganglion cells are arranged as two areas of high cell density, one in the temporal and one in the nasal retina. Densities of ganglion cells between these two high density regions is much lower, often less than 100 per mm–2. In between these two high density regions, on the nasal side of the optic nerve head, is a unique and dense vertical streak of mostly non-ganglion cells; the function of this specialization is unknown. On the basis of ganglion cell density we estimate that the peak acuity in the dromedary camel is about 10 and 9.5 cycles per degree in the temporal and nasal high density regions respectively and falls to 2–3 cycles per degree in the central retina. Behavioral acuity was estimated for one bactrian camel and was found to be approximately 10 cyc deg–1. The camel has a retina with a mean thickness of 104 µm, less than the 143 µm thickness that has previously been thought to be necessary for a retinal vasculature. Nevertheless, there is an extensive vitreal vasculature that does not appear to spare any retinal region.

Experimental eye enlargement in mature animals changes the retinal pigment epithelium

Form deprivation has been shown to result in myopia in a number of species such that the eye enlarges if one eye is permanently closed at the time of eye opening. In the quokka wallaby, the eye grows slowly throughout life. After form deprivation, the eye enlarges by 1-1.5 years of age to the size of that in a 4-6-year-old animal and the number of multinucleated retinal pigment epithelial (RPE) cells in the enlarged retina remains much lower than would be expected in eyes of comparable size. Here we have repeated the experiment but examined animals at 4 years of age. The sutured eye grew significantly larger than did its partner. Numbers of RPE cells were comparable between sutured and partner eyes but were lower than in normal animals of similar age. Reductions in RPE cell density were greater in nasal than in dorsal or ventral retina and were not seen in temporal retina. The distribution of multinucleated cells was quite different in the sutured and open eyes. As in normal eyes, partner eyes had most multinucleated cells in ventral retina, while in the sutured eyes such cells were located mainly in the far periphery. In conclusion, the RPE is significantly changed by the eye enlargement process. However, it is not known whether this change results from an active part played by the RPE in the retinal expansion process or whether the changes are simply a result of a passive increase in area of the RPE.