Asymmetry in the structure of average and large neurons of the ganglion layer of the frog retina

Morphometric analysis of the symmetry of middle and large ganglionic cells was performed on silver-impregnated retinal wholemounts of the frog. The nucleolus and the axis passing through the nucleolus in direction to optic disk were chosen as elements of symmetry characterizing the radial symmetry and bilateral one, respectively. It is demonstrated that the dendritic ramification angles of all cell types are smaller than 360 degrees and the angles of middle-type-GC are smaller than 180 degrees. In addition, their somata do not lie in the centre of the dendritic field, thus ganglionic cells have no radial symmetry. Directions of the axon and dendrites are opposite each other in the most of ganglionic cells, the terminals of dendrites being oriented from retinal centre to periphery in all quadrants of the retinal map. For estimation of bilateral symmetry the distance from the greatest remoted dendritic terminals to cell axis on the left and on the right from it was measured. Besides, the numbers of ramification knots and basal dendrites were counted. Most of ganglionic cells are asymmetrical in 2-3 mentioned structural parameters. Thus, the asymmetry in the structure of frog retinal neurons is rather norm than exception. Correlation between the asymmetry in ganglionic cell structure and functional asymmetry of their receptive fields is discussed.     

Amacrine cells in the ganglion cell layer of the cat retina

Following transection of the optic nerve, ganglion cells in the cat retina undergo retrograde degeneration. However, many small profiles (less than or equal to 10 micron) survive in the ganglion cell layer. Previously considered to be neuroglia, there is now substantial evidence that they are displaced amacrine cells. Their density increases from approximately 1,000 cells/mm2 in peripheral retina to 7,000 cells/mm2 in the central area. Their total number was found to be 850,000, which is five times the number of ganglion cells and also five times the number of astrocytes. Uptake of 3H-muscimol followed by autoradiography labelled 75% of the displaced amacrine cells; hence, the majority seem to be GABAergic. Immunocytochemistry with an antibody directed against choline-acetyl-transferase labelled approximately 10% of the displaced amacrines in the peripheral retina and 17% in the central area. Uptake of serotonin (5-HT) followed by immunocytochemistry was found in 25-30% of displaced amacrines. NADPH diaphorase histochemistry labelled approximately 5% of displaced amacrine cells. The sum of the various percentages make colocalization likely. Intracellular injection of Lucifer Yellow under microscopic control revealed that displaced amacrine cells constitute several morphological types.     

Neurogenesis and cell death in the ganglion cell layer of vertebrate retina

The correct formation of all central nervous system tissues depends on the proper balance of neurogenesis and developmental cell death. A model system for studying these programs is the ganglion cell layer (GCL) of the vertebrate retina because of its simple and well-described structure and amenability to experimental manipulations. The GCL contains approximately equal numbers of ganglion cells and displaced amacrine cells. Ganglion cells are the first or among the first cells born in the retina in all the studied vertebrates. Neurogenesis and cell death have been studied extensively in the GCL of various amniotes (rodents, chicks, and monkeys) and anamniotes (fish and frogs), and the two processes highlight developmental differences between the groups. In amniotes, neurogenesis occurs during a defined period prior to birth/hatch or the opening of the eyes, whereas in anamniotes, neurogenesis extends past hatching into adulthood-sometimes for years. Roughly half of GCL neurons die during development in amniotes, whereas developmental cell death does not occur in the GCL neurons of anamniotes. This review discusses the spatial and temporal patterns of neurogenesis, cell death, and possible explanation of cell death in the GCL. It also examines markers widely used to distinguish between ganglion cells and displaced amacrine cells, and methods employed to birth date neurons.     

Patterns of cell death in the ganglion cell layer of the human fetal retina

The distribution of dying cells in the ganglion cell layer (GCL) of retinae from human fetuses has been analysed. Both whole-mounted and sectioned retinae have been studied. Results suggest that cells are lost from the GCL between weeks 14 and 30 of the gestation period, approximately. This period corresponds to the period during which axons are lost from the developing optic nerve. Cell loss is greatest between weeks 16 and 21 of the gestation period. The pattern of cell loss is nonuniform, and between weeks 16 and 24, the relative frequency of pyknotic cells (pyknotic cells:viable cells) in peripheral retina is considerably higher than in central retina. This pattern of cell loss predominates during the period in which a distinct centroperipheral gradient of cell densities emerges in the GCL of the human fetal retina (between 18 and 23 weeks gestation). It is suggested that the regional loss of ganglion cells may contribute to the formation of the cell density gradient.     

A regional specialization in the opossum's retina: Quantitative analysis of the ganglion cell layer

The distribution of ganglion cells in the opossum's retina was determined from flat-mounted preparations stained with cresyl-violet. The retinal area is 109 mm² (SD = 16 mm²). Maps of ganglion cell density were made from retinae of seven animals. in all maps iso-density lines were approximately concentric, showing a slight elongation towards the nasal region. Cell density varied from 400 cells/mm² at the extreme periphery to 2,900 cells/mm² in the region of highest count, the are centralis. The center of this region lies 1.85 mm (26.3°) temporal to the center of the optic nerve head. The average total number of ganglion cells is 77,384 (SD = 10,173). Based upon soma diameter histograms ganglion cells were classified into three groups, showing at area centralis peaks at 7 μm, 12 μm and 15 μm respectively. Cell soma diameter ranged from 6 μm to 21 μm, larger values being observed at the periphery.     

Intraretinal differentiation in the synaptic organization of the inner plexiform layer of the pigeon retina

The inner plexiform layer at ten retinal loci in pigeon was examined by electron microscopy. Photomontages of the entire depth of the inner plexiform layer at each locus were analyzed with respect to the number of amacrine and bipolar synapses, their respective ratios, synaptic densities, percent amacrine synapses in serial configuration, synaptic layering patterns, and the effect of staining procedures on these quantities. The results show that the pigeon retina is not homogeneous regarding the structural complexity of the inner plexiform layer, but may be divided into four general areas in decreasing order of complexity: red field, temporal yellow field, nasal yellow field, and the area centralis. Significant differences in the amacrine synapse to bipolar synapse ration and amacrine synaptic density were observed across the retina, while bi-polar synaptic density and the percent of serial synapses were rather constant. Amacrine synapses displayed a layering pattern which was consistent throughout the retina; while bipolar synapses showed two patterns. It was further observed that the density of amacrine and bipolar synapses bears little relationship to the density of amacrine and bipolar cells in the immediately overlying inner nuclear layer. This suggests that the various retinal loci may be characterized by different proportions of the morphological types of amacrine and bipolar cells present in the pigeon retina. Based on recent studies which have shown that a relationship exists between the complexity of ganglion cell receptive fields and the synaptic complexity of the inner plexiform layer, it is suggested that the ganglion cells of pigeon would show a physiological differentiation among retinal loci consistent with the observed differences in the anatomical complexity of the inner plexiform layer.     

The pigeon retina: Quantitative aspects of the optic nerve and ganglion cell layer

Studies of electron micrographs of pigeon optic nerve showed numerous unmyelinated fibers (29% of total) and more than twice as many total fibers as previously reported. The 2.4 million axons had a mean diameter below 1 μ, close to the limit of resolution of light microscopy. This probably accounts for the large discrepancy between light and electron microscopic counts. Studies of retinal whole mounts showed an irregular distribution of cells in the ganglion cell layer including a minor posterior fovea and an area of increased density in the posterior-superior quadrant. Sample counts produced an estimate of over 4.8 million cell bodies in this layer. Cytological investigation indicated that over 85% of these cells were neuronal. The considerable excess of neuron cell bodies over axons in thought to be explained by the presence of large numbers of amacrine cells, cells which do not project axons into the optic nerve.     

Development of the human retina: patterns of cell distribution and redistribution in the ganglion cell layer

Neurogenesis in the ventricular layer and the development of cell topography in the ganglion cell layer have been studied in whole-mounts of human fetal retinae. At the end of the embryonic period mitotic figures were seen over the entire outer surface of the retina. By about 14 weeks gestation mitosis had ceased in central retina and differentiation of photoreceptor nuclei was evident within a well-defined area which constituted about 2% of total retina area. This area was approximately centered on the site of the putative fovea, identified by the exclusive development of cone nuclei at that location. The area of retina in which mitosis had ceased increased as gestation progressed. By mid-gestation mitosis in the ventricular layer occupied about 77% of the outer surface of the retina and by about 30 weeks gestation mitosis in the ventricular layer had ceased. Cell density distributions in the ganglion cell layer were nonuniform at all stages studied (14-40 weeks). Densities were highest at about 17 weeks gestation, and by mid-gestation the adult pattern of cell topography was present with maps showing elevated cell densities in posterior retina and along the horizontal meridian. Cell densities generally declined throughout the remainder of the gestation period, except in the posterior retina, where densities in the perifoveal ganglion cell layer remained high during the second half of gestation. There is a rapid decline in cell density in the foveal ganglion cell layer toward the end of gestation, and it is suggested that the persistence of high densities in the perifoveal region may be related to migration of cells away from the developing fovea. The total population of cells in the ganglion cell layer was highest (2.2-2.5 million cells) between about weeks 18 and 30 of gestation. After this the cell population declined rapidly to 1.5-1.7 million cells. It is suggested that naturally occurring neuronal death is largely responsible for this decline.     

Variations in cell density in the ganglion cell layer of the retina as a function of ocular pigmentation

Ocular melanin regulates retinal development, including cell density gradients in the central retina, a region essential for normal visual acuity. In albinos this region is underdeveloped and peak cell numbers are reduced. It is not known whether there is a dosage relationship between pigmentation and the degree of this underdevelopment, as studies of the retinal effects of albinism have commonly used rodents. These have poorly developed central regions even in the wild type. Rabbits, however, have a unique, highly specialized, visual streak in the central retina where cell density gradients are very steep and these are reduced in albinos. Here, cell densities in the ganglion cell layer of separate groups of rabbits, with different levels of ocular pigmentation and known mutations of the tyrosinase gene coding sequence, were examined. These revealed reductions in peak cell densities and/or in the regions over which high cell densities were maintained in all hypopigmented phenotypes. There was no dosage relationship between levels of pigmentation and deficits in the ganglion cell layer as animals with relatively small reductions in retinal pigment had deficits comparable to those found in albinos. The greatest variability between pigmentation phenotypes was between the two completely unpigmented strains. Consequently, although pigment may regulate the development of the central retina, this study failed to show that it does so in a dose-dependent manner.     

Topographical development of the ganglion cell fiber layer in the chick retina. A whole mount study

Three hundred and forty whole mounts of retinas from domestic fowl aged two days of incubation to several months post-hatching were examined using a modified pyridine-silver technique. The first visible ganglion cell axons appeared near the posterior retinal pole between two and three days of incubation. These early axons grew immediately toward the optic stalk and became grouped in fascicles even before reaching it. The fascicles increased in number, diameter and length, became interconnected and became more parallel and orderly with age. The mean fascicle diameter and concentration increased with nearness to the optic fissure. Growth cones were seen only in the most vitread portion of the fiber layer. Two main groups of fascicles, one superficial and one deep, surrounded the head of the fissure by five days of incubation, each group entering the fissure at a different angle. At this time an arcuate pattern of fibers arose near the posterior pole of the retina. Fiber bundles were relatively thin and sparse in the center of this pattern. The distance between the fissure and the center of the pattern increased with age. The arcuate pattern virtually disappeared within several weeks, possibly indicating an abortive formation of a macula and fovea. The fibers at the periphery of the older retinas ran parallel to the iris, encircling but not entering it. These circumferential fibers, which first developed on the temporal side of the retina and later nasally, diverged from a point on the dorsal periphery of the retina. The relative position of this divergence point swung about 90° around the retinal periphery between five and one-half and eight days of incubation. Probable centrifugal fibers were first seen at 17 days of incubation.     

Effects of prenatal ionizing irradiation on the development of the ganglion cell layer of the mouse retina

Prenatal exposure to ionizing irradiation has been shown to be an effective method to eliminate selectively certain neuronal population. This investigation studied the effects on the ganglion cell layer of the retinae of adult mice exposed to a gamma source (total dose=3 Gy) at 16 days gestation. There was a significant reduction in the total number of neurons (displaced amacrine+ganglion cells) in the ganglion cell layer (33%) that was mainly caused by a pronounced loss (59%) of displaced amacrine cells. The diameters of the surviving retinal ganglion cells were consistently larger than those of the controls. Prenatal irradiation is the first experimental approach that partially eliminates displaced amacrine cells. It is suggested that the morphogenesis of retinal ganglion cells may be affected by displaced amacrine cells.     

GABAergic ganglion cells in the rabbit retina

The ganglion cells are the output neurons of the retina. There is, however, relatively little known about the neurotransmitters used by these cells. In the present study, ganglion cells identified with a ganglion cell-specific monoclonal antibody (AB5) are shown in separate double-label experiments to be gamma-aminobutyric acid (GABA)-like immunoreactive and to possess a high-affinity uptake mechanism for [3H]GABA accumulation. The localization of these markers of GABA activity to AB5-labelled ganglion cells provides the first definitive evidence for the presence of a classical transmitter in retinal ganglion cells and suggests that GABA may perform a role as a neurotransmitter in these cells.     

Alpha ganglion cells in the rabbit retina

In the rabbit retina a distinctive morphological class of large ganglion cells was demonstrated by a combination of intracellular staining with Lucifer Yellow and the quantification of reduced silver-stained preparations. The class is called alpha because of the qualitative and quantitative resemblance to the alpha cells of the cat's retina. Rabbit alpha cells change their size with location on the retina. In the high ganglion cell density region of the visual streak, their somata are about 15 micron in diameter, and their dendritic fields have diameters as small as 180-220 micron. The largest alpha cells in the inferior periphery have soma diameters of 30 micron and dendritic field diameters of 960 micron. There is a considerable scatter of sizes at any retinal location. Alpha cell density changes from about 55/mm2 in the streak to about 3/mm2 in far periphery, and the cells make up 1-1.4% of the ganglion cell population. Dendritic trees stratify in either an inner or an outer sublamina of the inner plexiform layer, suggesting an on/off dichotomy in the response to light. Each of the inner and outer branching subtypes is distributed in a regular mosaic, and the dendritic trees cover the retina completely and economically. The possibility is discussed that the alpha cells are the brisk transient/Y cells of physiology.     

GABA-immunoreactivity in ganglion cells of the rat retina

Ganglion cells in the rat retina were labeled with the fluorescent dye, Diamidino-yellow, by retrograde transport from the superior colliculus and subsequently reacted for GABA-like immunoreactivity with a rhodamine-conjugated antiserum. Examination of sectioned retinas by fluorescence microscopy showed double labeling in approximately 6% of the ganglion cells. The presence of GABA in these neurons suggests that they may be involved in providing direct inhibitory input to the rat tectum.     

Isolation of ganglion cells from the retina

A microdissection technique was developed to isolate cellular layers of the goldfish retina. Three regions of the neural retina, the photoreceptor-interneuron layer (PL), the vascular layer (VL), and the ganglion cell layer (GCL) were recovered. A cleavage plane between the PL and GCL occurred at the level of the inner plexiform region as shown by light microscopy. Scanning electron microscopy found the predominant cells of the PL to include rods and cones while the VL contained a capillary network that arose from the retinal artery. The GCL consisted of pear-shaped ganglion cells about 5 μm in size with single, unbranched axons with diameters of about 0.5 μm. This preparation was virtually free of other cell types or debris. Transmission electron microscopy demonstrated that axons of the GCL were unmyelinated in contrast to those fibers distal to the optic nerve head. On the basis of morphology, microdissection could isolate a single population of retinal neurons free of glia.Biochemical indicators were used to determine the degree of separation of the GCL from the PL and VL. Markers of the cholinergic system, acetylcholinesterase and α-bungarotoxin binding, predominate only within the PL. These findings are consistent with known histochemical features of the retina and suggest that the GCL is not contaminated by the neighboring photoreceptor-interneuron region. Ganglion cell axons distal to the optic nerve head are myelinated and contain high levels of the marker enzyme 2′,3′-cyclic nucleotide-3′-phosphodiesterase. In contrast, this enzymatic activity is not detected within the unmyelinated axons of the glia free GCL preparation.To examine retinal events during axonal regeneration, transection of ganglion cell axons was carried out at the level of the optic tract. Axotomy led to stimulated amino acid incorporation detected within the GCL and not other regions of the retina. This specific response of the GCL was observed both in vivo and in vitro indicating that isolated ganglion cells remain intact and biologically active. Microdissection provides, therefore, a population of retinal ganglion cells that may be used to analyze the biochemical events of neuronal regeneration.     

Displaced ganglion cells in the retina of the rat

The distribution, laterality of projection, and perikaryal sizes of displaced ganglion cells (DGCs) were examined in whole-mounted retinae after massive unilateral injections of horseradish peroxidase along the optic tract in pigmented rats. The DGCs were found predominantly in the lower temporal periphery of the retina. Nearly all DGCs labeled had contralaterally projecting axons. The sizes of the labeled DGCs spanned the range of ordinary ganglion cells, but few middle-sized DGCs were labeled. The results support the hypothesis that displaced ganglion cells are late-developing neurons that do not complete their migration toward the ganglion cell layer during retinal histogenesis.     

Topography of ganglion cells in human retina

We quantified the spatial distribution of presumed ganglion cells and displaced amacrine cells in unstained whole mounts of six young normal human retinas whose photoreceptor distributions had previously been characterized. Cells with large somata compared to their nuclei were considered ganglion cells; cells with small somata relative to their nuclei were considered displaced amacrine cells. Within the central area, ganglion cell densities reach 32,000-38,000 cells/mm2 in a horizontally oriented elliptical ring 0.4-2.0 mm from the foveal center. In peripheral retina, densities in nasal retina exceed those at corresponding eccentricities in temporal retina by more than 300%; superior exceeds inferior by 60%. Displaced amacrine cells represented 3% of the total cells in central retina and nearly 80% in the far periphery. A twofold range in the total number of ganglion cells (0.7 to 1.5 million) was largely explained by a similar range in ganglion cell density in different eyes. Cone and ganglion cell number were not correlated, and the overall cone:ganglion cell ratio ranged from 2.9 to 7.5 in different eyes. Peripheral cones and ganglion cells have different topographies, thus suggesting meridianal differences in convergence onto individual ganglion cells. Low convergence of foveal cones onto individual ganglion cells is an important mechanism for preserving high resolution at later stages of neural processing. Our improved estimates for the density of central ganglion cells allowed us to ask whether there are enough ganglion cells for each cone at the foveal center to have a direct line to the brain. Our calculations indicate that 1) there are so many ganglion cells relative to cones that a ratio of only one ganglion cell per foveal cone would require fibers of Henle radiating toward rather than away from the foveal center; and 2) like the macaque, the human retina may have enough ganglion cells to transmit the information afforded by closely spaced foveal cones to both ON- and OFF-channels. Comparison of ganglion cell topography with the visual field representation in V1 reveals similarities consistent with the idea that cortical magnification is proportional to ganglion cell density throughout the visual field.