Distribution of small and medium-sized ganglion cells in the cat's retina

Morphological Characteristics of Retinal Ganglion Cells in the Retinas of Giant Pandas (Ailuropoda melanoleuca)

Vision plays a crucial role in the survival of animals, and the visual system has particularly selectively evolved in response to the visual environment, ecological niche, and species habitats in vertebrate species. To date, a horizontal streak of retinal ganglion cell (RGC) distribution pattern is observed across mammal species. Here, we report that the giant panda's vertically oriented visual streak, combined with current evidence of the animal's forward-placed eyes, ocular structure, and retinal neural topographic distribution patterns, presents the emergence of a well-adapted binocular visual system. Our results suggest that the giant panda may use a unique way to processing binocular visual information. Results of mathematical simulation are in favor of this hypothesis. The topographic distribution properties of RGCs reported here could be essential for understanding the visual adaptation and evolution of this living fossil.

Retinal Ganglion Cell Topography and Retinal Resolution in the Baikal Seal (Pusa sibirica)

The total number, size, topographic distribution, and cell density of ganglion cells were studied in retinal wholemounts of Baikal seals (Pusa sibirica). The ganglion cell size varied from 10 to 38 μm. A distinct cell group consisted of large ganglion cells of more than 30 μm in diameter. The topographic distribution of ganglion cells showed a definite area of high cell density similar to the area centralis of terrestrial carnivores. This area was located approximately 6-7 mm dorsotemporally of the geometric center of the wholemount. In this area, the peak cell densities in two wholemounts were 3,800 and 3,400 cells/mm2 (mean 3,600 cells/mm2). With a posterior nodal distance of 24 mm (underwater), this density corresponds to 631 cells/square degree. These values predict a retinal resolution of 2.4' in water and 3.0' in air. The topographic distribution of large cells featured the highest density in the same location as the total ganglion cell population.

Comparative study of visual pathways in owls (Aves: Strigiformes)

Although they are usually regarded as nocturnal, owls exhibit a wide range of activity patterns, from strictly nocturnal, to crepuscular or cathemeral, to diurnal. Several studies have shown that these differences in the activity pattern are reflected in differences in eye morphology and retinal organization. Despite the evidence that differences in activity pattern among owl species are reflected in the peripheral visual system, there has been no attempt to correlate these differences with changes in the visual regions in the brain. In this study, we compare the relative size of nuclei in the main visual pathways in nine species of owl that exhibit a wide range of activity patterns. We found marked differences in the relative size of all visual structures among the species studied, both in the tectofugal and the thalamofugal pathway, as well in other retinorecipient nuclei, including the nucleus lentiformis mesencephali, the nucleus of the basal optic root and the nucleus geniculatus lateralis, pars ventralis. We show that the barn owl (Tyto alba), a species widely used in the study of the integration of visual and auditory processing, has reduced visual pathways compared to strigid owls. Our results also suggest there could be a trade-off between the relative size of visual pathways and auditory pathways, similar to that reported in mammals. Finally, our results show that although there is no relationship between activity pattern and the relative size of either the tectofugal or the thalamofugal pathway, there is a positive correlation between the relative size of both visual pathways and the relative number of cells in the retinal ganglion layer.

Size and distribution of retinal ganglion cells in the St. Kitts green monkey (Cercopithecus aethiops sabeus)

The topographical distribution of density and the soma size of retinal ganglion cells (RGCs) were studied in the St. Kitts green monkey (Cercopithecus aethiops sabeus). The total number of RGCs, estimated from light microscopic analysis of wholemounted and of transversely sectioned retinae, ranged between 1,183,721 and 1,273,715 (mean 1,228,646). These estimates are comparable to the number of optic nerve fibres (1,220,000) estimated from semithin sections. The topographic distribution of RGCs shows a strong centroperipheral gradient. The soma size distribution of RGCs in Nissl-stained flatmounts falls within a range of between 5.7 microm and 22.9 microm and is comparable to other primate species. Somata of RGCs were found to be generally smaller within the fovea than in peripheral regions. Ganglion cells, as reported for other diurnal primates, are nonuniformly distributed with a slight nasotemporal elongation of isodensity contours, and they exhibit nasotemporal asymmetry in the frequency distribution of soma size. The topography of the RGC distribution of this semiarboreal, ground-dwelling monkey is similar to what has been found in other diurnal Old World species.

Distribution and coverage of beta cells in the cat retina

We have reexamined the retinal distribution and dendritic field dimensions of beta cells in the cat retina. Beta cells were labeled by retrograde transport from the A-layers of the lateral geniculate nucleus and distinguished from alpha cells on the basis of soma size. Dendritic fields of beta cells were visualized by intracellular staining in vitro. The fraction of cat ganglion cells that were beta cells varied with retinal location. Except near the area centralis, beta cells represented about half of all ganglion cells in the nasal hemiretina. They contributed as heavily as the other major ganglion cell classes to the nasal visual streak. In and near the area centralis and in the temporal retina, beta cells represented about two-thirds of all ganglion cells. The areas of beta cell dendritic fields were reciprocally related to beta cell density. For example, they were 3-fold smaller within the visual streak than at matched eccentricities outside it. For many cells, we could estimate both local beta cell density and dendritic field area. Coverage factor (dendritic field area x local density) remained constant at about 4 despite 100-fold variations in beta cell density, and was independent of eccentricity, nasotemporal location, or position relative to the visual streak. Analysis in terms of sampling theory suggests that the beta cell array is matched to X-cell spatial resolution so as to optimize acuity. The beta cell distribution and its systematic reflection in dendritic architecture predict acuity levels that apparently correlate well with actual visual performance across the cat's visual field.

Soma and axon diameter distributions and central projections of ferret retinal ganglion cells

Using a combination of retrograde horseradish peroxidase (HRP) labelling, silver staining, and electron microscopy, we have assessed the relationship between retinal ganglion cell soma size and axon diameter in the adult ferret (Mustela putorius furo). Retinal ganglion cells were labelled following injections of HRP into the lateral geniculate nucleus (LGN), superior colliculus (SC), or LGN+SC. The soma size distributions following LGN, SC, or LGN+SC injections were all unimodal showing considerable overlap between different cell classes. This was confirmed for alpha cells identified on the basis of dendritic filling or from neurofibrillar-stained retinae. Analysis of the soma size and axon diameters of a population of heavily labelled retinal ganglion cells showed a significant correlation between the two. However, the overall distribution of intraretinal axon diameter was bimodal with an extended tail. Analysis of the ganglion cell distributions in the adult ferret indicates that beta cells comprise about 50.5-55%, gamma 42.5-47%, and alpha 2.5% of the ganglion cell population. This implies that the proportion of gamma, beta, alpha cells in both cat and ferret retina is highly conserved despite differences in visual specialization in the two species.

On the distribution of gamma cells in the cat retina

Ganglion cells of the cat retina that are neither alpha nor beta cells are often lumped for convenience into a single anatomical group--the gamma cells (Boycott & Wässle, 1974; Stone, 1983; Wässle & Boycott, 1991). Defined in this way, gamma cells are the morphological counterpart to the physiological W-cell class, which includes all ganglion cells that are neither Y (alpha) nor X (beta) cells. We have estimated the retinal distribution of gamma cells by using retrograde transport to label ganglion cells innervating the superior colliculus and by assuming that these included virtually all gamma cells and no beta cells. We excluded labeled alpha cells on the basis of soma size. Our data suggest that gamma cells represent just under half of the ganglion cells in most of the nasal retina, but only about a third of those in the area centralis and temporal retina. Gamma cells do not appear to be more highly concentrated in the nasal visual streak than are other ganglion cells. In the temporal retina, gamma cells with crossed projections to the brain are apparently at least twice as common as those with uncrossed projections.

Retinal ganglion cell axon diameter spectrum of the cat: mean axon diameter varies according to retinal position

Axon diameters of retinal ganglion cells were measured from electron micrographs of the nerve fiber layer of the cat. Three adult retinae were examined which had mean axonal diameters of 1.18 +/- 0.86 (n = 5553), 1.12 +/- 0.79 (n = 7265), and 1.47 +/- 1.11 microns (n = 10,867). Cumulative histograms from several locations adjacent to the optic disc were unimodal (modal peaks: 0.6-0.8 microns). This unimodal distribution, however, did not reflect the regional differences in axonal diameters found throughout the retina. In many locations, especially those related to axons of the temporal retina, axon diameter distributions were clearly bimodal or even trimodal (modal peaks: 0.6-0.8, 1.4-2.1, and 3.3 microns). Measurements from one retina indicated that the mean diameters of axons arising from the area centralis and visual streak (0.94 +/- 0.63 and 0.98 +/- 0.68, respectively) were not significantly different from each other; however, when compared to other areas around the optic disc, the percentage of fibers with diameters between 1.5-2.0 microns was highest in the sample adjacent to the area centralis. Axons temporal to the optic disc were found to be on average larger than those nasal to the optic disc; similarly superior axons were larger than inferior axons. Axonal distributions at the retinal periphery were found to be significantly different from those at the optic disc (P < or = 0.05) and contained a higher percentage of medium-sized axons and fewer small axons. In each of the three retinae the proportions small, medium, and large axons were respectively gamma: 46; 47; 48, beta: 50; 49; 48, and alpha: 4; 4; 4; regional differences in the proportions of each axonal class are compared to previously published ganglion cell density maps. Differences between axonal bundles within each sample location were not significantly different; however, in one retina axons in the scleral half of the fiber layer were significantly larger (P < or = 0.01) than axons in the vitreal half of the nerve fiber layer adjacent to the optic disc. When compared to the axonal diameter distributions found within the optic nerve (Cottee et al., 1991) and optic tract (Reese et al., 1991), our data indicates that the diameter of retinal axons may increase by up to 30% along the length of the visual pathway.

Postnatal development of different classes of cat retinal ganglion cells

Previous investigators have documented the postnatal development of alpha and beta type ganglion cells in cat retinae (Ramoa et al. [1987] Science 237:522-525; Ramoa et al. [1988] J. Neurosci. 8:4239-4261; Dann et al. [1987] Neurosci. Lett. 80:21-26; Dann et al. [1988] J. Neurosci. 8(5):1485-1499). The development of the remaining cells (about 50%), which constitute a heterogeneous group and are referred to here collectively as gamma cells (Boycott and Wässle, '74), has not been studied in detail. The purpose of this study was to compare the postnatal development of alpha, beta, and gamma cells in kitten and adult retinae using horseradish peroxidase histochemistry and the fluorescent dye DiI. In the kitten, alpha, beta, and gamma cells are recognizable. We find, as have others, that kitten alpha and beta cell bodies and dendritic fields are significantly smaller than in the adult. However, kitten gamma cells are nearly adult sized. In fact, at birth the cell bodies of beta cells throughout the retina are significantly smaller than those of gamma cells. During the first 12 weeks of life, alpha and beta cell bodies increase in size from 90% to 680% depending upon eccentricity. Gamma cells hardly increase in size at all. Also, the normal adult center-to-peripheral cell size gradient for alpha and beta cells is not seen in the neonate. Gamma cells show no such gradient in the neonate or adult. Our results suggest that the morphological development of alpha and beta cells occurs later than that of gamma cells and may explain some of the differences in the effects of visual deprivation and surgical manipulation upon the parallel Y-, X-, and W-cell pathways.

Neuropeptide Y immunoreactivity identifies a group of gamma-type retinal ganglion cells in the cat

Ganglion cells within the cat retina have been traditionally grouped by morphological criteria into three major classes: alpha, beta, and gamma. The gamma-type cells have been least well characterized, but the available evidence indicates that this class comprises a relatively heterogeneous population of neurons. In the present study we demonstrate that an antibody for neuropeptide Y (NPY) recognizes a subpopulation of about 2,000 gamma-type ganglion cells. The NPY-immunoreactive (IR) neurons project to the superior colliculus and to the C layers of the lateral geniculate nucleus as demonstrated by retrograde labeling with fluorescent tracers (fluorogold or rhodamine latex microspheres). Virtually all of these cells disappear following lesions of the optic nerve. The NPY-IR ganglion cells were identified as gamma cells on the basis of soma size and dendritic branching patterns. The somas of these neurons are small (8-22 microns in diameter), and each cell is characterized by sparsely branching dendritic processes, usually extending into the middle third of the inner plexiform layer, the physiologically defined ON sublamina. These neurons are distributed across the entire retina, with the highest density at the area centralis. Within local regions of the retina, however, there was no indication that the NPY-IR gamma cells are arrayed in a regular mosaic pattern. These results provide the first evidence that the gamma class of ganglion cells of the cat retina can be subdivided on the basis of immunocytochemical properties.

Retinal afferents to the tectum opticum and the nucleus opticus principalis thalami in the pigeon

The retinal afferents of the tectum opticum and the n. opticus principalis thalami (OPT) were studied with fluorescent tracers in pigeons. Injections into the tectum opticum revealed topographically related areas of high density labelling in the contralateral retina. In these areas up to 15,000 cells/mm2 were labelled. After tectal injections the soma sizes of labelled retinal ganglion cells in the area centralis ranged from 5 to 23 microns with a mean of 7.5 microns. Afferents from the ipsilateral retina could not be demonstrated. Injections into the OPT labelled neurons throughout the retina without a clear topographical relation to the locus of injection. The density never exceeded 150 cells per mm2. The soma size range was 8 to 35 microns with a mean of 14.6 microns. Independently of the injection area within the OPT, the red field in the dorsotemporal retina was always extremely sparsely labelled. The number of labelled ganglion cells in this area never exceeded 25 neurons/mm2. After OPT injections the average density of labelling per unit area was six times higher in the yellow than in the red field. The results confirm previous reports of a massive and topographically organized retinal projection onto the optic tectum. The projection onto the OPT was clearly smaller and with the retrograde tracing techniques in use, an orderly topography has not been demonstrated. The paucity of red field projections onto the OPT suggests that the role of the thalamofugal pathway in binocular integration is very limited.

Anatomical correlations between soma size, axon diameter, and intraretinal length for the alpha ganglion cells of the cat retina

Retinal ganglion cells within the same region of the retina may have different lengths of axon before reaching the optic disc depending on the route they take with respect to the temporal raphe. We have investigated whether there is a correlation between soma and intraretinal axon diameter and how these parameters relate to intraretinal axon length on both sides of the cat temporal raphe. Retinas were wholemounted and alpha-cell somata and fibers stained with a modified neurofibrillar method. Moving peripherally from the area centralis along the raphe there was a progressively increasing difference between the intraretinal axon lengths for nearly adjacent cells across the raphe, which reached a maximum of 4-5 mm at the retinal periphery. Cells on the nasal aspect of the raphe had shorter axons than did adjacent cells on the temporal aspect of the raphe. Comparison of soma diameter samples across the raphe showed there was no clear trend between soma diameter and intraretinal length. Replotting the raphe and sample areas on a cell density map indicated that differences in soma diameter could be attributed to ganglion-cell density differences between the sampled areas. Examination of the stained cells revealed that within the initial length of the axon there was a region showing a reduction of axon diameter (diameter less than 1 micron), which varied in length from cell to cell. The axon was, therefore, divided into three segments: the portion of axon prior to thinning (A), the thin segment itself (B), and the part of the axon after the thin segment (C). The diameter of each segment (A,B,C) and the lengths of the first and second segments (A,B) were significantly correlated with soma diameter (P less than 0.001). From measurements of the axon diameter of segment C, it was concluded that alpha-cell axons continue to increase in diameter along their path towards the optic disc. The present report indicates that alpha-cell soma size, when going from the area centralis to the periphery along the raphe, reaches a plateau and then declines within more peripheral retinal locations in spite of increasing intraretinal axon length. Thus, there is no positive correlation between soma or axon diameter and intraretinal axon length. The anatomical findings are discussed in relation to previous reports of retinal development and complementary conduction times within intraretinal and extraretinal visual pathways.

Retinal ganglion-cell densities and soma sizes are unaffected by long-term monocular deprivation in the cat

Three cats were raised with monocular deprivation for 5.2-7.2 years, and ganglion-cell densities and soma sizes were measured in their flat-mounted retinae. The retinae were Nissl-stained so that ganglion cells could be measured whether or not they maintained normal central projections. Measurements were made in the area centralis, peripheral binocular segment, and monocular segment of the retinae. There were no significant differences between the deprived and non-deprived retinae in the densities or soma-sizes of alpha cells or other (non-alpha) ganglion cells at any of these retinal locations. These results support the view that the most distal effects of monocular deprivation occur at the retino-geniculate contact, and they suggest that even after long-term monocular deprivation, effects in the lateral geniculate nucleus do not produce secondary, retrograde changes in the retina.

Retinal wholemounts: a simple method for precise mapping

A simple and inexpensive method to analyse retinal wholemounts is described. It allows the definition of regularly spaced sites to map regional specializations of the retina under light microscopy. After attaching a piece of translucent millimetre graph paper behind the slide holding the preparation, the two grid patterns of the graph paper and of the eye-piece grid are used in combination for the definition of sites for analysis and the return to specific areas as small as 50-100 microns2. The present method is simpler and offers several advantages when compared to others based on X-Y movements of the microscope stage or on photographic montage.

Cell death in the developing retinal ganglion cell layer of the wallaby Setonix brachyurus

The distribution and number of dying cells in the developing retinal ganglion cell layer of the wallaby Setonix brachyurus were assessed by using cresyl violet stained tissue. The density of dying cells has been expressed per 100 live cells for the entire retinal surface, data being presented as a grid of 500 micron squares. For statistical analysis, retinae were divided into 8 regions; dorsal, ventral, nasal, and temporal quadrants, each further divided into center and periphery. This method allowed comparison of the extent of cell death at different retinal locations as the high density area centralis of live cells developed temporal to the optic disk from 60 days onward. Between 30 and 70 days, dying cells were seen across the entire retina; beyond 100 days very few were seen. Initially, there was a significantly higher incidence of dying cells in the central retina compared to the periphery, whereas from 50 days this situation was reversed. Analysis of the central retina before and during area centralis formation consistently indicated a significantly lower number of dying cells per 100 live cells in temporal compared to other retinal quadrants. This differential pattern suggests that cell death lowers live cell densities less in the emerging area centralis than elsewhere, and therefore must play a part in establishing live cell density gradients. However, we cannot exclude the possibility that other factors are also instrumental. Indeed, factors such as areal growth (Beazley et al., in press) presumably operate at later stages since live cell density gradients continue to be accentuated even after cell death is complete. Numbers of dying cells peaked by 50 days, reaching approximately 1% of the live cell population. At this stage, counts were also maximal for live cells with values up to 30% above the adult range.

Displaced retinal ganglion cells in the wallaby Setonix brachyurus

Displaced ganglion cells have been examined in wholemounted and sectioned retinae following bilateral injection of horseradish peroxidase into optic tracts of the wallaby, Setonix brachyurus, "quokka". Such cells, which lie in the vitread part of the inner nuclear layer, are located mainly in superior retina as a streak-like band dorsal to the area centralis and visual streak of orthotopic ganglion cells. Only between 1 and 2% of the total ganglion cell population were displaced, but an analysis of cell morphology and soma diameter suggested that displaced ganglion cells represented several cell types.

Specific labelling of ganglion cells in the cat retina by a monoclonal antibody

The introduction of monoclonal antibody technology has made it possible to produce specific markers for various retinal cell types. We report here on a monoclonal antibody, AB5, which specifically labels the retinal ganglion cells of the cat. The labelled cells could be categorized as either alpha, beta or gamma subtype based on morphological criteria. This antibody will be useful for studies of the morphology, localization and synaptic connectivity of ganglion cells in the cat retina.

Retinal ganglion cell loss produced by intraocular kainic acid in cats: variation with somal size and eccentricity

Eight eyes of adult cats were injected with different doses of kainic acid (KA) and examined following survival times of either 5 or 12 days. At a survival time of 12 days, a dose of 76 nmol produced an 18% loss of ganglion cells in the center of the area centralis (AC), 70% loss at a location 2 mm from the AC, and 95% loss at a location 6 mm from the AC. Larger doses (240, 760 and 2400 nmol) produced losses comparable to that observed for 76 nmol. For example, 2400 nmol produced a 35% loss in the AC, 81% at 2 mm, and 88% at 6 mm. At a survival time of 5 days, doses of 240 and 760 nmol produced a loss of ganglion cells comparable to that seen at 12 days. In one eye, a large dose of KA (7600 nmol) produced total loss of ganglion cells at a survival time of 5 days. By comparing loss of cells in restricted somal diameter ranges at different retinal eccentricities, it was possible to distinguish two significant correlations that were largely independent of survival time and dose: (1) at 2 mm, loss of cells with somal diameters larger than 21 micron significantly exceeded loss of cells with smaller somata. In particular, alpha cells were totally eliminated in 6 of the 8 KA-treated eyes. (2) The mean loss of ganglion cells with somal diameters less than 21 micron was significantly greater at 2 mm and 6 mm than in the AC. Together, these results show that loss of ganglion cells produced by KA varies somal size and retinal eccentricity.

Distribution and soma size of ganglion cells in the retina of the eastern chipmunk (Tamias sibiricus asiaticus)

Topographic distribution and soma size of ganglion cells were studied in Nissl-stained, whole-mounted retinas of the eastern chipmunk. High density areas in the central retina were elongated horizontally, making up the visual streak. The total count of ganglion cells was estimated as 410,000. Throughout the retina soma size of ganglion cells showed a uimodal distribution, although a distinct population of large cells was found in the dorso-temporal periphery.

Structure/function relationships of retinal ganglion cells in the cat

Intracellular recording and horseradish peroxidase (HRP) iontophoresis was used to define structure/function relationships for single retinal ganglion cells in the intact cat eye. Fifteen physiologically characterized cells were labeled as follows. Five W-cells had gamma morphology, 6 X-cells had beta morphology, and 1 Y-cell had alpha morphology, and these relationships support earlier conclusions. However, one cell could not be physiologically classified despite beta morphology, one X-cell was not a beta cell, and one Y-cell was not an alpha cell. Whether these unusual structure/function relationships represent an artifact of methodology or complications to be added to prevailing notions requires further study.

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.

Representation of the visual streak in visuotopic maps of the cat's superior colliculus: influence of the mapping variable

A series of visuotopic maps has been prepared from recordings of electrical potentials related to W-cell afferent activity in the cat's superior colliculus. These maps clearly exhibit an expected exaggeration of the representation of the upper visual field, due to tilt of the retina's visual streak in the 'position of paralysis'. This asymmetry disappears when the visual field's coordinate system is rotated by an angle equal to the tilt of the axis of the nasal streak. Previously published maps, based on recordings from postsynaptic collicular units, have failed to reflect this tilt of the nasal visual streak, perhaps in part because the centers of unit receptive-fields are biased estimators of the retinal origin of axons terminating near a collicular recording site.

Retinal W-cell projections to the medial interlaminar nucleus in the cat: implications for ganglion cell classification

The perikaryal sizes and retinal distribution of ganglion cells labeled after small iontophoretic injections of horseradish peroxidase (HRP) into the medial interlaminar nucleus (MIN) were studied. Injections were also made into the LGNv and the C-laminae of the dorsal lateral geniculate nucleus (LGNd) for comparison. The results are consistent with suggestions that the MIN contains three approximately vertically oriented laminae which, from medial to lateral, receive their input from, respectively, contralateral nasal, ipsilateral temporal, and contralateral temporal retina. Each MIN lamina receives afferents from two distinct groups of retinal ganglion cells (1) cells with large somas (over 25 micron), coarse primary dendrites, large dendritic trees (500-900 micron in diameter), and coarse axons; (2) cells with medium-sized somas (14-20 micron), medium-caliber primary dendrites, large dendritic trees (350-700 micron), and fine axons. The large cells are clearly Y-cells or alpha cells, and they provide approximately 50% of the retinal input to all layers of the MIN. The medium-sized cells, which provide the remaining 50% of the retinal output in the MIN, are, we argue, W-cells, since they do not differ in soma size, dendritic morphology, axon caliber, or receptive field properties from medium-sized W-cells which project to other thalamic or midbrain structures. These results suggest two phylogenetic trends within the W-cell group: (1) the differentiation of thalamic and midbrain components; and (2) the further differentiation of ipsilateral and contralateral projections within the midbrain component. This latter division corresponds to the distinction between W1 and W2 cells described previously (Rowe and Stone, '77, '80).

The distribution of ganglion cells in the retina of the North American opossum (Didelphis virginiana)

The distribution of ganglion cells in the retina of the opossum was determined from whole-mounted retinae stained with cresyl violet. Isodensity lines were approximately circular with a peak density of 2,000 to 2,700 cells/mm2 in superior temporal retina (area centralis). The total number of retinal ganglion cells was estimated to be 72,000 to 135,000 (mean 101,026) in retinae ranging from 125 to 187 mm2 in total area. Three groups of ganglion cells were distinguished on the basis of soma size and retinal topography. Large cells (24 to 32 micrometer diameter) were fairly evenly distributed across the retina. Medium cells (12 to 23 micrometer diameter) were more numerous in the superior temporal quadrant than in other regions of the retina. Small cells (7 to 11 micrometer diameter) were prominent in all retinal regions, but particularly in nasal and inferior retina. An analysis of topographical differences in soma size distribution suggests that the medium size cells can be further subdivided into small-medium and large-medium groups.

The distribution and sizes of ganglion cells in the retinas of five Australian marsupials

Maps of ganglion-cell distribution have been constructed from whole-mounted retinas of five Australian marsupial species. The pademelon wallaby (Thylogale billiardieri), the scrub wallaby or tammar (Macropus eugenii), and the carnivorous Tasmanian devil (Sarcophilus harissi) have both a visual streak and an area centralis. The retina of the brown bandicoot (Isoodon obesulus) also shows both these features but they are less prominent than in the former three species, whereas the burrow-dwelling, hairy-nosed wombat (Lasiorhinus latifrons) possesses a well-developed visual streak but seems to lack an area centralis. A study of ganglion-cell sizes comparing nasal and temporal retina, the visual streak, and/or the area centralis was undertaken in each species. Results show that as in the cat, small ganglion cells tend to concentrate in the visual streak. However, the temporal-nasal differences in cell sizes described in the cat (Stone et al., '80) could be detected only in those marsupials in which an area centralis was clearly recognizable.