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

The Anatomical RecordVolume 260, Issue 2 p. 124-131 ArticleFree Access Neuronal density in the human retinal ganglion cell layer from 16–77 years Alison Harman, Corresponding Author Alison Harman alison@psy.uwa.edu.au Department of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaDepartment of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaSearch for more papers by this authorBrett Abrahams, Brett Abrahams Department of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaSearch for more papers by this authorStephen Moore, Stephen Moore Department of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaSearch for more papers by this authorRobert Hoskins, Robert Hoskins Department of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaSearch for more papers by this author Alison Harman, Corresponding Author Alison Harman alison@psy.uwa.edu.au Department of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaDepartment of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaSearch for more papers by this authorBrett Abrahams, Brett Abrahams Department of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaSearch for more papers by this authorStephen Moore, Stephen Moore Department of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaSearch for more papers by this authorRobert Hoskins, Robert Hoskins Department of Psychology, University of Western Australia, Nedlands, WA 6907, AustraliaSearch for more papers by this author First published: 30 August 2000 https://doi.org/10.1002/1097-0185(20001001)260:2<124::AID-AR20>3.0.CO;2-DCitations: 81AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract 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. © 2000 Wiley-Liss, Inc. The vertebrate retina consists of three cellular layers, the innermost of which contains two types of neuron, displaced amacrine cells and retinal ganglion cells, the latter having axons which project to the brain. There have been a number of studies examining the mammalian retinal ganglion cell layer throughout life with the aim of discovering whether cells are lost as a result of aging. Two studies of sectioned rat retina have noted that retinal layers thin during aging (Weisse, 1995; Cano et al., 1986) implying that possibly neurons are lost. However, our recent study of the quokka wallaby retina showed that, in this species, neurons were not lost during a 14-year lifetime (Harman and Moore, 1999). An analysis of sample pieces of human wholemounted or sectioned tissue supports a cell density decrease throughout life (Gao and Hollyfield, 1992). It is possible that such a decrease in density is brought about by a reduction in cell number and/or by an increase in retinal area. Decreasing density of neurons in the ganglion cell layer can be accounted for, at least in some species such as the rat, the quokka wallaby and the horse, by a small but continuous increase in retinal area (Katz and Robison, 1986; Harman and Moore, 1999; Harman et al., 1999). The primate retina has a macula in which the packing density of neurons of the ganglion cell layer is very dense and, unlike those in other retinal regions in the mammalian retina, are arranged as several layers. For this reason it has been assumed that accurate counting of cells in the macula is not possible in stained wholemounts (Curcio and Allen, 1990; Curcio and Drucker, 1993). Therefore, human retinae have been examined in unstained, undehydrated wholemounts, with Nomarski optics (Curcio and Allen, 1990; Curcio and Drucker, 1993) or in sections (Gao and Hollyfield, 1992). The authors using wholemounts analysed ganglion cells as distinct from displaced amacrine cells as they were confident in distinguishing the two cell types. A decrease in ganglion cell density was observed to take place between young and old retinae. By contrast, in another wholemount study of the monkey retina, total numbers of ganglion cells throughout life were counted in stained wholemounts and no cell loss was seen (Kim et al., 1995). However, this finding may be the result of an inability to count all the densely packed, multilayered cells in the monkey macula. Another method for determining ganglion cell numbers is to count optic axons in the optic nerve. In some studies of human, axon numbers have been seen to decrease (Balaszi et al., 1984; Dolman et al., 1980; Johnson et al., 1987; Jonas et al., 1990; Mickelberg et al., 1989) and in others to remain unchanged (Repka and Quigley, 1989). Poor fixation of unperfused human nerves may well compromise an accurate count. A study of presumably well-fixed monkey optic nerve, however, also indicated that axon number did not change throughout life (Morrison et al., 1990). In summary, it is unclear whether mammalian, and in particular human, ganglion cells decrease in number during life. The aim of this study was to address this issue, by systematic sampling across the entire retinal surface of wholemounts (Stone, 1981). The method allows us to estimate total cell number and retinal area and thereby assess the causes of any observed decreases in cell density. Moreover, it is possible to determine accurately whether losses are evenly distributed across the retina or favour particular regions. We chose to count total neurons since we are concerned, from previous studies, that attempting to delineate amacrine from ganglion cells by morphological criteria is fairly arbitrary and there may be an age related bias in doing so (Harman and Moore, 1999). MATERIALS AND METHODS Tissue Human eyes were obtained from the Queensland Eyebank. The Eyebank had removed the corneas and immersion-fixed the remainder of the eye in 10% formalin soon after death (although the exact interval between death and fixation was unknown). The eyes used for ganglion cell counts were from donors aged between 16 and 77, as listed in Table 1. In most cases the eyes were the partners of those analysed in our previous paper on human RPE (Harman et al., 1997). Eyes from patients with a known history of eye disease were excluded from this study. Table 1. Retinal area and total neuron number in the retinal ganglion cell layer for each retina examined Tissue # Sex Age Retinal area mm2 Cell no. Cause of death 1341 M 16 967 1844502 haematoma 1686 M 24 1,069 2804188 gun shot wound 1190 M 26 1,030 2359668 carcinoma 1391 F 37 933 2767522 breast cancer 1288 M 41 1,021 2800317 myocardial ischeamia 1191 M 46 1,171 2655865 lung cancer 1592 F 51 987 2208513 cardiac arrest 1407 M 55 885 2119288 haematoma 1674 F 57 1,120 1911196 emphysema 1692 F 66 1,096 1667336 broncho pneumonia, breast cancer 1672 M 76 1,174 1738892 cardiac arrest, coronary artery disease 1453 M 77 1,078 1277396 cardiac arrythmia Fixation and Mounting The eyes were stored in 10% buffered formalin for 2–20 weeks before dissection. Retinae were dissected from eyes, radially cut, wholemounted onto 5% subbed microscope slides, stained with cresyl violet, and coverslipped. Analysis Cell identification. All neurons in the ganglion cell layer were counted, we did not attempt to distinguish between ganglion and displaced amacrine cells. Neurons could be distinguished from non-neuronal cells by their larger size, more irregular outlines, and clumped Nissl substance (Fig. 1). Non-neuronal cells comprised elongated blood vessel endothelial cells and smaller, more darkly staining glial cells with sparse cytoplasm (Fig. 1). Figure 1Open in figure viewerPowerPoint Photomicrograph of the retinal ganglion cell layer in the human. Neurons clearly identified as ganglion cells (large arrow) are large (l), medium (m), and small (s) and have recognisable cytoplasm. However, there are also cells (medium sized arrows) which may be either small ganglion cells, amacrine cells or possibly glial cells. Blood vessel endothelial cells (smallest arrow) have an elongated shape. Cresyl violet. Wholemount. Scale = 50 μm. Figure 2Open in figure viewerPowerPoint Macula disc (always 10 mm diameter) cut out of wholemount. The precise position was determined from the maps of neuronal cell density obtained from each wholemount. The relevant map for the wholemount illustrated is shown in the lower figure. Retinal area, cell counts, and neuronal density. Retinal area was determined by drawing around both fresh and dehydrated retinae, using a MD-2 digitiser (Minnesota Datametrics) and IBM-compatible personal computer. Estimates could then be made of retinal shrinkage resulting from dehydration and total cell counts adjusted accordingly. The digitiser was run with software developed in-house (Meyer, personal communication, 1995). We also included in the areal estimates all the retinae, aged 12–89 years, that we had used in a previous study of the RPE (Harman et al., 1997). The tissue was prepared in the same way as for this study except that the RPE was uppermost on the slide and details of the tissue have been previously published (Harman et al., 1997). For ganglion cell counts, initially the entire retina was examined as a wholemount. A 1% sample of cell numbers was taken as this value is considered appropriate (Stone, 1965). Cells were counted in regularly spaced samples across the entire retinal surface, using a light microscope attached to the same computer-linked equipment. Cells were counted, at 1,000× magnification, in 100 × 100 μm areas, at 1 mm intervals. Total cell numbers were obtained by proportionality. Sectioning of the macular area. Since neurons in the ganglion cell layer around the fovea are several layers thick in primates, counting them in stained wholemounts is thought to be inaccurate (Curcio and Allen, 1990) and could give a false impression that neuron numbers are retained. Without taking special care, foveal neurons may be undersampled since they are present in several layers. If so, when cells are lost from the macula, underlying ones, which could not be seen in young retinae, would be counted in older retinae giving the false impression that cell density remains constant. Therefore, on completion of the analysis of the wholemount, the macular region was removed and sectioned for future analysis. In this way, we ensured accurate assessment of cell numbers in this multilayered region. The region was marked out and removed using a standard trephine, 10 mm in diameter. The circular punches were then removed from the slide, embedded in wax, and sectioned at 10 μm. These sections were counted using the disector method to eliminate double-counting of cells split between sections (West, 1993). Briefly, all cells of neuronal appearance were counted at 1 mm intervals along each section, each counted region being 100 μm long. Cells overlapping the left edge of the square graticule were excluded while those overlapping the right edge were included. The disector method further requires that cells that are cut through on the top surface of the section are also excluded and, by focussing through the section, all other cells are counted. The total numbers of cells in the circular punches were then calculated by proportionality allowing for shrinkage due to wax processing. Total numbers of neurons per retina were then adjusted accordingly after allowing for shrinkage due to wax processing. The procedure allowed the production of an accurate map of neuron densities in the retinal ganglion cell layer across the entire retinal surface. Retinal regional analysis. Retinal maps were geometrically divided into quadrants (dorsal, ventral, nasal, and temporal) and six concentric rings, using the posterior pole of the eye as the centre (Fig. 3). This division allowed statistical comparison of neuronal cell densities in different regions, a method used by us previously (Fleming et al., 1996a,Fleming et al., b, Harman et al., 1997). In this way each region of the retina could be distinguished by a quadrant and eccentricity designation, with ring 1 being centremost and ring 6 indicating the periphery (Fig. 3). The foveal region fell within the temporal region of rings 2 to 4, with the optic nerve lying in the nasal region of ring 1. For statistical purposes the three inner rings (1–3) were combined and considered “central retina.” The average cell density was calculated for each of the 16 remaining regions. Figure 3Open in figure viewerPowerPoint Diagram illustrating how the retina was divided into six concentric rings and four quadrants. The inner three were then grouped together (rings 1–3). Neuronal soma diameter. Average neuron size per retina was determined for all retinae by drawing around all neurons in 15 regularly spaced locations of equal area for each quadrant, resulting in around 250 neurons sampled for each quadrant, combining results and obtaining means for each retina. RESULTS Retinae The ganglion cell layer in retinal wholemounts (n = 12) from donors aged from 16 to 77 years were analysed and retinal areas and total neuron number in the retinal ganglion cell layer are shown for each (Table 1). Retinal areal shrinkage was a mean of 3.1% (SD 0.8%) and was not more pronounced in younger or older retinae. Retinal Area Retinal area is increasing throughout life, the correlation between area and age is significant (r = 0.52, P = 0.00014; Fig. 4). When retinae were divided into three groups, 12–30 years, 35–57 years, and 61–89 years, an analysis of variance reveals a significant difference between the three groups (F = 6.9, P = 0.002). Figure 4Open in figure viewerPowerPoint Graph showing retinal area before dehydration (top) and also retinal area grouped by age (bottom), error bars are standard error of the mean. The retinal areas shown in the top graph include the retinae analysed for this study and also retinae used for previous studies of the retinal pigment epithelium (Harman et al., 1997). Retinal Maps Six representative retinal maps for eyes aged between 24 years and 77 years are shown in Figure 5. There is reasonable variability between retinae both in the macular region and in peripheral regions. Cell densities are lower in the older eyes. Figure 5Open in figure viewerPowerPoint Retinal wholemount maps of six retinae aged from 23–77 years old. Densities are cells per (100 × 100) μm2. Number and Density of Neurons in the Retinal Ganglion Cell Layer Neuron numbers in the retinal ganglion cell layer decreased with age (Fig. 6). There was considerable variation in neuron number between individual eyes however, correlations between age and number were significant for the entire retina, (r = -0.693, P = 0.013) but not for the macular region, (r = −0.33, NS). The average density of cells per retina (Fig. 7) decreased significantly with age (r = 0.725, P = 0.008) although values did not fall significantly in the macula (r = 0.387, NS). Differential changes in neuron density were more apparent when the retina was divided into regions (Table 2). Density decreased in most regions. Dorsal and nasal regions showed the strongest correlations between cell density and age. There was no significant correlation between density and age in temporal regions (Table 2). Figure 6Open in figure viewerPowerPoint Graphs showing total neuron number per retina and total number of neurons per macular punch. Figure 7Open in figure viewerPowerPoint Graphs showing average neuron density per retina and average neuron density per macular punch. Table 2. Correlation for each region Region Correlation Significance V1–3 −0.637 0.026** Indicates this correlation is significant at the 0.05 level, ** at the 0.01 level. V4 −0.613 0.034** Indicates this correlation is significant at the 0.05 level, ** at the 0.01 level. V5 −0.535 0.073 V6 −0.525 0.136 N1–3 −0.332 0.292 N4 −0.825 0.001** N5 −0.828 0.001** N6 −0.697 0.012** Indicates this correlation is significant at the 0.05 level, ** at the 0.01 level. D1–3 −0.686 0.014** Indicates this correlation is significant at the 0.05 level, ** at the 0.01 level. D4 −0.775 0.003** D5 −0.663 0.019** Indicates this correlation is significant at the 0.05 level, ** at the 0.01 level. D6 −0.653 0.021** Indicates this correlation is significant at the 0.05 level, ** at the 0.01 level. T1–3 −0.439 0.154 T4 −0.493 0.103 T5 −0.464 0.129 T6 −0.263 0.410 * Indicates this correlation is significant at the 0.05 level, ** at the 0.01 level. Soma Diameters Mean soma diameter of neurons in the ganglion cell layer did not change throughout life (Fig. 8). Values ranged from 4 to 24 μm throughout life with 95% being below 13 μm. There was no correlation between the percentage of neurons over 10 μm and age (Fig. 8). Since mean soma size did not change, we assume that a preferential loss of large or small neurons was not taking place. Figure 8Open in figure viewerPowerPoint Graph showing mean soma diameter (top) and percentage of neurons larger than 10 μm (bottom) per retina. DISCUSSION In this paper we have investigated changes in retinal area along with number, density, and size of neurons in the human retinal ganglion cell layer from the second decade onwards. Our data support the hypothesis that neurons are lost due to aging as we have seen lower numbers for oldest retinae. Previous research has demonstrated that densities of neurons in the ganglion cell layer decrease but these studies were restricted to relatively small regions of the retina, either a circular region 12 mm in diameter and centred on the fovea (Curcio and Drucker, 1993) or 1–2 mm square tissue fragments from the foveal and equatorial regions (Gao and Hollyfield, 1992). Few studies have examined the ganglion cell layer of the entire retina at various stages during adult life in a mammal; the two that have done so analysed the retina of the monkey (Kim et al., 1996) and quokka wallaby (Harman and Moore, 1999). Neither found a decrease in neuron number. However, there is always the added problem when analysing human tissue that lifestyle, medical problems, cause of death and so on are variables whose effects on tissue are largely unknown. By contrast, animal tissue is generally obtained from animal colonies whose members have been carefully maintained throughout their lives in controlled conditions and sacrificed before accident or death through natural causes intervenes. We decided to analyse the macular region in sections since cells here are multilayered. When we reanalysed the macula using the sectioned retinal punches, we discovered that we were indeed undersampling in some retinae by around 10% but that nevertheless, even from the sectioned retina counts, the correlations between neuronal number and density in the macula and age were still non-significant. This finding is in contrast to that for the entire retina, for which mean neuron density was significantly correlated with age. Previously we have suggested, from our examination of the retinal pigment epithelium, that the retina slowly increases in area during adult life and that centro-temporal retina, containing the macular region, undergoes the least areal expansion (Fleming et al., 1996a; Harman et al., 1997). Our present data support these suggestions, there is strong evidence for an increase in retinal area with a concurrent decrease in neuron density, but that neuronal density decreases minimally in the macula. We would suggest that this minimal decrease in density in the macula results, in part, from a minimal retinal areal expansion in this region (Fleming et al., 1996a; Harman et al., 1997; Harman and Moore, 1999). We have also shown that, even though it is possible that neurons may be lost throughout life from the ganglion cell layer, the loss is not of a particular cell size. This result is in contrast to the observation made by Sadun and Bassi (1990) that in Alzheimer's disease, axons of the largest retinal ganglion cells, M cells, were preferentially lost from the optic nerve. It has previously been thought that cells of the magnocellular pathway would be preferentially lost due to aging (Schefrin et al., in press). We have not seen any evidence for this hypothesis. It is generally believed that neuronal loss and aging in general is not linked to absolute time but rather to a “rate of living” which is reflected in metabolic rate (Economos, 1982). Therefore, the rate of neuron loss will be in direct proportion to metabolic rate. However, this hypothesis does not explain why the quokka wallaby and rhesus monkey (average life expectancies 14 and 35 years, respectively) do not show signs of cell loss in the ganglion cell layer whereas humans do. Retinal ganglion cells in the quokka accumulate large quantities of lipofuscin in the last few years of life from around 10 years of age (Harman and Moore, 1999). This finding indicates that, even though neurons of this species undergo an aging process considerably more rapid than that in humans, it does not lead to cell loss. In general however, it is likely that the loss of neurons from the central nervous system due to aging is considerably less than has generally been assumed (for review see Morrison and Hof, 1997). For example, the visual system of the monkey, from ganglion cell layer to primary visual cortex, shows no neuronal loss due to aging (Kim et al., 1996, 1997; Peters et al., 1997; Ahmad and Spear, 1993; Peters and Sethares, 1993; Vincent et al., 1989). The main changes due to aging in the monkey brain appear to be a degeneration of myelin and various glial cell types (Peters et al., 1989, 1991, 1994). Acknowledgements We thank the Queensland Eye Bank for providing tissue in excellent condition without which this study would have been impossible. We also thank Dr. Trish Fleming for preparation of some of the tissue and Professor Lyn Beazley for very helpful comments on the manuscript. The study was funded by a Program Grant from The National Health and Medical Research Council of Australia. LITERATURE CITED Ahmad A, Spear PD. 1993. Effects of aging on the size, density and number of rhesus monkey lateral geniculate neurons. J Comp Neurol 334: 631– 643. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Balaszi AG, Rootman J, Drance SM, Schulzer M, Douglas GR. 1984. The effect of age on the nerve fiber population of the human optic nerve. Am J Opthalmol 97: 760– 766. CrossrefPubMedWeb of Science®Google Scholar Cano J, Machado A, Reinoso-Suarez F. 1986. Morphological changes in the retina of ageing rats. Arch Gerontol Geriatr 5: 41– 50. CrossrefCASPubMedWeb of Science®Google Scholar Curcio CA, Allen KA. 1990. Topography of ganglion cells in human retina. J Comp Neurol 300: 5– 25. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Curcio CA, Drucker DN. 1993. Retinal ganglion cells in Alzheimer's disease and aging. Ann Neurol 33: 248– 257. Wiley Online LibraryPubMedWeb of Science®Google Scholar Dolman CL, McCormick AQ, Drance SM. 1980. Aging of the optic nerve. Arch Ophthalmol 98: 2053– 2058. CrossrefCASPubMedWeb of Science®Google Scholar Economos AC. 1982. Mammalian design and rate of living. Exp Gerontol 17: 145– 152. CrossrefCASPubMedWeb of Science®Google Scholar Fleming PA, Harman AM, Beazley LD. 1996a. Development and aging of the RPE in a marsupial, the quokka. Exp Eye Res 62: 457– 470. CrossrefCASPubMedWeb of Science®Google Scholar Fleming PA, Harman AM, Beazley LD. 1996b. Retinal pigment epithelium topography in the mature quokka, Setonix brachyurus. Exp Eye Res 62: 85– 93. CrossrefCASPubMedWeb of Science®Google Scholar Gao H, Hollyfield JG. 1992. Aging of the human retina. Invest Ophthalmol Vis Sci 33: 1– 17. CASPubMedWeb of Science®Google Scholar Harman AM, Fleming PA, Moore S, Hoskins R. 1997. Development and aging of cell topography in the human retinal pigment epithelium. Invest Ophthalmol Vis Sci 38: 2016– 2026. CASPubMedWeb of Science®Google Scholar Harman AM, Moore S. 1999. Number of neurons in the retinal ganglion cell layer of the quokka wallaby do not change throughout life. Anat Rec 256: 78– 83. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Harman AM, Moore S, Hoskins R, Keller P. 1999. Horse vision and an explanation for the visual behaviour originally explained by the ramp retina. Equine Vet J 31: 384– 390. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Johnson BM, Miao M, Sadun AA. 1987. Age-related decline of human optic nerve axon populations. Age 10: 5– 9. CrossrefWeb of Science®Google Scholar Jonas JB, Muller-Bergh JA, Schlotzer-Schrehardt UM, Naumann GOH. 1990. Human optic nerve fibre count and optic disc size. Invest Ophthalmol Vis Sci 33: 2012– 2018. Web of Science®Google Scholar Katz ML, Robison WG Jr. 1986. Evidence of cell loss from the rat retina during senescence. Exp Eye Res 42: 293– 304. CrossrefCASPubMedWeb of Science®Google Scholar Kim CB, Tom BW, Spear PD. 1996. Effects of aging on the densities, numbers and sizes of retinal ganglion cells in rhesus monkey. Neurobiol Aging 17: 431– 438. CrossrefCASPubMedWeb of Science®Google Scholar Kim CB, Pier LP, Spear PD. 1997. Effects of aging on numbers and sizes of neurons in histochemically defined subregions of monkey striate cortex. Anat Rec 247: 119– 128. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Mickelberg FS, Drance SM, Schulzer M, Yidegiligne HM, Weis MM. 1989. The normal human optic nerve. Ophthalmology 96: 1325– 1328. CrossrefPubMedWeb of Science®Google Scholar Morrison JC, Cork LC, Dunkelberger GR, Brown A, Quigley HA. 1990. Aging changes of the rhesus monkey optic nerve. Invest Ophthalmol Vis Sci 31: 1623– 1627. CASPubMedWeb of Science®Google Scholar Morrison JH, Hof PR. 1997. Life and death of neurons in the aging brain. Science 278: 412– 419. CrossrefCASPubMedWeb of Science®Google Scholar Peters A, Sethares C. 1993. Aging and the Meynert cells in rhesus monkey primary visual cortex. Anat Rec 236: 721– 729. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Peters A, Josephson K, Vincent SL. 1991. Effects of aging on the neuroglial cells and pericytes within area 17 of the rhesus monkey cerebral cortex. Anat Rec 229: 384– 398. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Peters A, Leahu D, Moss MB, McNally KJ. 1994. The effects of aging on Area 46 of the frontal cortex of the rhesus monkey. Cerebral Cortex 4: 621– 635. CrossrefCASPubMedWeb of Science®Google Scholar Peters A, Rosene DL, Moss MB, Kemper TL, Abraham CR, Tigges J, Albert MS. 1996. Neurobiological bases of age-related cognitive decline in the rhesus monkey. J Neuropathol Exp Neurol 55: 861– 874. CrossrefPubMedWeb of Science®Google Scholar Peters A, Nigro NJ, McNAlly KJ. 1997. A further evaluation of the effect of age on the striate cortex of the rhesus monkey. Neurobiol Aging 18: 29– 36. CrossrefCASPubMedWeb of Science®Google Scholar Repka MX, Quigley HA. 1989. The effect of age on normal human optic nerve fiber number and diameter. Ophthalmology 96: 26– 32. CrossrefPubMedWeb of Science®Google Scholar Sadun AA, Bassi C. 1990. Optic nerve damage in Alzheimer's disease. Ophthalmology 97: 9– 17. CrossrefCASPubMedWeb of Science®Google Scholar Schefrin BE, Tregear SJ, Harvey LO, Werner JS. In press. Senescent changes in scotopic contrast sensitivity. Vis Res. PubMedGoogle Scholar Stone J. 1981. The retinal wholemount handbook. Sydney, Australia: Maitland Publications, University of Sydney. Google Scholar Vincent SL, Peters A, Tigges J. 1989. Effects of aging on the neurons within area 17 of rhesus monkey cerebral cortex. Anat Rec 223: 329– 341. Wiley Online LibraryCASPubMedWeb of Science®Google Scholar Weisse I. 1995. Changes in the aging rat retina. Ophthal Res 27: 154– 163. CrossrefPubMedWeb of Science®Google Scholar West MJ. 1993. New stereological methods for counting neurons. Neurobiol Aging 14: 275– 285. CrossrefCASPubMedWeb of Science®Google Scholar Citing Literature Volume260, Issue21 October 2000Pages 124-131 FiguresReferencesRelatedInformation