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PubMed1987

Responses of Purkinje cells and mossy fibres in the flocculus of the monkey during sinusoidal movements of a visual pattern.

1. Discharges of Purkinje cells (P cells) and mossy fibres were recorded from the cerebellar flocculus of monkeys trained to fixate a stationary visual target. The units were tested with a sinusoidally moving random dot pattern (background) which was projected on an entire screen or on part of it. The receptive field organization of the units was tested by changing the area of stimulus presentation on the screen and by changing the direction of visual fixation. 2. When stimulated with sinusoidal movements of the background in the horizontal plane, ninety-two of 684 Purkinje cells (13.5%) responded to the retinal-slip velocity. Seventy-eight of the ninety-two visually responsive Purkinje cells (84.8%) also showed cyclic modulations in activity during horizontal smooth-pursuit eye movements (these were so-called horizontal gaze-velocity Purkinje cells). 3. In response to the sinusoidal retinal-slip velocity, the visual Purkinje cells showed six types of discharge patterns. Type 1 Purkinje cells (28/92 or 30.4%) were directionally selective: they showed a peak activity during background movement in one direction and a trough in the other. Both peaks and troughs were related to stimulus velocities. Their receptive fields were relatively large (greater than 45 deg) and included the fovea. 4. Type 2 (8/92 or 8.7%) and type 3 (5/92 or 5.4%) Purkinje cells showed sinusoidal responses similar to those of type 1 Purkinje cells, but the visual inputs were primarily excitatory in type 2 Purkinje cells and inhibitory in type 3 Purkinje cells. Only the peaks in type 2 and troughs in type 3 were related to stimulus velocities. 5. Type 4 (4/92 or 4.3%) and type 5 (11/92 or 12.0%) Purkinje cells showed responses to stimulus movements in both directions (bidirectional). When the moving background was projected with 10 deg of fixation, type 4 Purkinje cells were excited bidirectionally in relation to retinal-slip velocities. When the periphery of either hemiretina was stimulated, type 5 Purkinje cells were inhibited and the trough activity was stimulus-velocity dependent. Receptive fields were found in the ipsilateral hemiretinae in seven type 5 Purkinje cells and in the contralateral hemiretinae in the remaining four type 5 Purkinje cells. 6. Type 6 (36/92 or 39.1% Purkinje cells received an excitatory input from the central retina and an inhibitory input from the periphery. The peripheral receptive fields were either in the ipsilateral (69.4%) or contralateral (30.6%) hemiretinae of both eyes.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1996

Distribution of the inositol trisphosphate receptor in the catfish retina.

Inositol 1,4,5-trisphosphate (InsP3) mobilizes intracellular stored Ca2+ by binding to specific receptors that are similar to the ryanodine receptor of skeletal and cardiac muscle. We have immunolocalized the InsP3 receptor to the inner nuclear layer and limiting membranes of the catfish retina. Immunocytochemistry on dissociated retinal cells further localized the receptor in the horizontal, bipolar and Müller glial cells. Immunostaining of the rat retina localized the InsP3 receptor to the plexiform layers. These data show a different distribution of InsP3 receptor in the catfish retina compared to that of other vertebrates, that may be suggestive of a different functional role for this receptor in different species.

Animals↗

PubMedDate not supplied

Neurofibrillar bipolar cells in the capybara retina.

Using a reduced-silver neurofibrillar method, we stained a population of bipolar cells in the capybara retina. These cells are distributed throughout the retina following the same topography of other retinal cell classes as the A-type horizontal cells and ganglion cells. The level of axonal stratification, mosaic regularity, and dendritic coverage factor suggest that these neurofibrillar bipolar cells comprise a population of sublamina a cone bipolar cells.

Animals↗

PubMed2004

L and M cone contributions to the midget and parasol ganglion cell receptive fields of macaque monkey retina.

Analysis of cone inputs to primate parvocellular ganglion cells suggests that red-green spectral opponency results when connections segregate input from long wavelength (L) or middle wavelength (M) sensitive cones to receptive field centers and surrounds. However, selective circuitry is not an obvious retinal feature. Rather, cone receptive field surrounds and H1 horizontal cells get mixed L and M cone input, likely indiscriminately sampled from the randomly arranged cones of the photoreceptor mosaic. Red-green spectral opponency is consistent with random connections in central retina where the mixed cone ganglion cell surround is opposed by a single cone input to the receptive field center, but not in peripheral retina where centers get multiple cone inputs. The selective and random connection hypotheses might be reconciled if cone type selective circuitry existed in inner retina. If so, the segregation of L and M cone inputs to receptive field centers and surrounds would increase from horizontal to ganglion cell, and opponency would remain strong in peripheral retina. We measured the relative strengths of L and M cone inputs to H1 horizontal cells and parasol and midget ganglion cells by recording intracellular physiological responses from morphologically identified neurons in an in vitro preparation of the macaque monkey retina. The relative strength of L and M cone inputs to H1 and ganglion cells at the same locations matched closely. Peripheral midget cells were nonopponent. These results suggest that peripheral H1 and ganglion cells inherit their L and M cone inputs from the photoreceptor mosaic unmodified by selective circuitry.

Animals↗

PubMed1988

Melatonin enhances horizontal cell sensitivity in salamander retina.

Intracellular electrophysiological recording techniques were utilized to investigate the possible function of retinal melatonin in the larval tiger salamander. Endogenous retinal melatonin was present and appeared to bind a membrane-enriched fraction of the salamander retina, as determined by radioimmunoassay and receptor binding studies. Melatonin added through the perfusion bath to flat-mounted retinas resulted in a horizontal cell (HC) hyperpolarization of 10-20 mV. Additionally, the amplitude of HC responses to short test flashes increased in the presence of melatonin. Voltage-intensity plots revealed that application of 500 microM of melatonin caused an increase of the HC light sensitivity and this effect was reversible. These results suggest that melatonin synthesized and released during the dark period of the diurnal cycle may alter the sensitivity of second-order neurons at a time of day when photopic input is at its lowest level.

Animals↗

PubMed1990

Neurotoxic effects of L-alpha-aminoadipic acid on the carp retina: a long term observation.

The hypothesis has been tested that the enantiomers of alpha-aminoadipic acid have different target effects; the L-isomer has both glio- and neurotoxic actions, while the DL-isomer has a gliospecific action in the CNS. Electrophysiological and morphological studies were carried out on the retina of the carp (Cyprinus carpio) for one to two months after intraocular injection with alpha-aminoadipic acids at various doses. Intracellular recording from horizontal cells and extracellular recording of spike discharges from ganglion cells in isolated retinal preparations were made from control and pretreated retinas at various intervals after intraocular injection with the enantiomers. In control retinas, application of 15 mM L-alpha-aminoadipic acid in the superfusate resulted in hyperpolarization of all horizontal cells and in a decrease in amplitude of their light responses (S-potentials). In the retinas pretreated with L-alpha-aminoadipic acid (8 mumol), low amplitude S-potentials were seen during an early phase 2-4 h after ocular injection, but the normal appearance of S-potentials was restored one day after injection. In control retinas, a brief period of iontophoretic application of L-alpha-aminoadipic acid resulted in a slight activation of the spontaneous spike firing of ganglion cells but a slight decrease in the rate of light-induced firing. In retinas pretreated with intraocular L-alpha-aminoadipic acid (4 mumol) 4 h prior to eye removal, however, light-induced spike discharges were abolished from nearly all spontaneously firing ganglion cells (greater than 90%). Their unresponsiveness to light stimuli lasted for more than two months after injection, and was accompanied by insensitivity to iontophoretically applied putative neurotransmitters.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Aminoadipic Acid↗

PubMed1994

Cloning, expression, and localization of a mouse retinal gamma-aminobutyric acid transporter.

PURPOSE: To isolate a cDNA clone encoding a high-affinity gamma-aminobutyric acid (GABA) transporter from mouse retina, to examine its biochemical and pharmacologic properties, and to determine the sites of its mRNA expression in retinal cells. METHODS: A mouse retinal cDNA library was screened using a fragment of a rat brain GABA transporter (GAT-1) cDNA as a probe. One homologous clone, mouse retinal GAT-1, was chosen for further characterization. RNA transcribed from mouse retinal GAT-1 was microinjected into Xenopus oocytes, and pharmacologic properties of the expressed transporter were determined. Sites of mouse retinal GAT-1 mRNA expression were examined by in situ hybridization. RESULTS: The protein sequence deduced from the DNA sequence of mouse retinal GAT-1 cDNA was virtually identical to that of the rat and the mouse brain GAT-1. RNA transcribed from this clone induced a [3H]-GABA uptake activity in microinjected Xenopus oocytes that was both sodium and chloride dependent. The apparent Km and Vmax for the GABA uptake were 8.3 microM and 40.0 pmol/egg per hour, respectively. The mouse retinal GAT-1 induced GABA uptake was inhibited by L-diaminobutyric acid, guvacine, cis-4-hydroxynipecotic acid, nipecotic acid, and 4,5,6,7-tetrahydroisoxazolo [4,5c]-pyridin-3-ol with IC50 values of 320, 79, 71, 7.1, and 200 microM, respectively. However, beta-alanine was unable to inhibit the induced GABA uptake significantly (IC50 approximately 2,500 microM). In situ hybridization studies showed that mouse retinal GAT-1 mRNA was present in a subpopulation of amacrine, interplexiform, and displaced amacrine cells. Hybridization signal in the Müller cells was significantly lower, and GAT-1 transcripts were not detected in the bipolar, horizontal, or photoreceptor cells of mouse retina. CONCLUSIONS: The mouse retinal GAT-1 cDNA encodes a Na(+)-dependent, high-affinity GABA transporter that is mainly expressed in a subset of mouse retinal inter neurons.

Animals↗

PubMed2004

Characterization of receptors for glutamate and GABA in retinal neurons.

Glutamate and gamma-aminobutyric acid (GABA) are major excitatory and inhibitory neurotransmitters in the vertebrate retina, "a genuine neural center" (Ramón y Cajal, 1964, Recollections of My Life, C.E. Horne (Translater) MIT Press, Cambridge, MA). Photoreceptors, generating visual signals, and bipolar cells, mediating signal transfer from photoreceptors to ganglion cells, both release glutamate, which induces and/or changes the activity of the post-synaptic neurons (horizontal and bipolar cells for photoreceptors; amacrine and ganglion cells for bipolar cells). Horizontal and amacrine cells, which mediate lateral interaction in the outer and inner retina respectively, use GABA as a principal neurotransmitter. In recent years, glutamate receptors and GABA receptors in the retina have been extensively studied, using multi-disciplinary approaches. In this article some important advances in this field are reviewed, with special reference to retinal information processing. Photoreceptors possess metabotropic glutamate receptors and several subtypes of GABA receptors. Most horizontal cells express AMPA receptors, which may be predominantly assembled from flop slice variants. In addition, these cells also express GABAA and GABAC receptors. Signal transfer from photoreceptors to bipolar cells is rather complicated. Whereas AMPA/KA receptors mediate transmission for OFF type bipolar cells, several subtypes of glutamate receptors, both ionotropic and metabotropic, are involved in the generation of light responses of ON type bipolar cells. GABAA and GABAC receptors with distinct kinetics are differentially expressed on dendrites and axon terminals of both ON and OFF bipolar cells, mediating inhibition from horizontal cells and amacrine cells. Amacrine cells possess ionotropic glutamate receptors, whereas ganglion cells express both ionotropic and metabotropic glutamate receptors. GABAA receptors exist in amacrine and ganglion cells. Physiological data further suggest that GABAC receptors may be involved in the activity of these neurons. Moreover, responses of these retinal third order neurons are modulated by GABAB receptors, and in ganglion cells there exist several subtypes of GABAB receptors. A variety of glutamate receptor and GABA receptor subtypes found in the retina perform distinct functions, thus providing a wide range of neural integration and versatility of synaptic transmission. Perspectives in this research field are presented.

Animals↗

PubMed1993

Two types of neuronal precursor cells in the mammalian retina--a short review.

This short review summarizes current evidence that the mammalian retina is populated by two distinct groups of retinal neurons. One of them (ganglion cells, cone photoreceptor cells, horizontal cells, and a subpopulation of amacrine cells) is generated early in ontogenesis, and may be phylogenetically old. The other group (rod photoreceptor cells, bipolar cells, and another subpopulation of amacrine cells) is born late in ontogenesis, and seems to have been acquired later in phylogeny. It is suggested that the two groups of neurons are generated by two different types of precursor cells that may result from an asymmetrical final division of the undifferentiated retinal stem cells. Qualitative and quantitative features of precursor cell proliferation and differentiation are discussed.

Aging↗

PubMed2001

Development of the primate retinal vasculature.

Human and macaque retinae have similar retinal vascular anatomy. The general features of the retinal vascular anatomy of these two primates have much in common with more widely studied animal models such as rat and cat. However, primates are unique amongst mammals in having a region in temporal retina specialized for high visual acuity, which includes the fovea centralis (or 'fovea'). Several features distinguish the fovea from other parts of the retina, including a very high local density of cone photoreceptors, a high density of inner retinal cells during development, and an absence of retinal blood vessels. The retinal vascular complex comprises a number of cell types, in addition to vascular endothelial cells, including pericytes, microglia, astrocytes-none of which is intrinsic to the retina. In addition, amacrine-like cells make bouton-like associations with retinal vessels and may be involved in the autoregulation of blood flow. During development endothelial cells 'invade' the retina, accompanied by a population of microglial cells; glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes are also seen associated with the developing vasculature, and are in advance of the vascular front by a few hundred microns. Recent findings indicate that astrocytes at the vascular front proliferate in response to factors released by endothelial cells, including leukemia inhibitory factor. Better understood is the role of GFAP-immunoreactive astrocytes just in advance of the developing vessels. These astrocytes are sensitive to hypoxia and in response release vascular endothelial growth factor (VEGF) which in turn promotes the migration, differentiation and proliferation of vascular endothelial cells. This hypoxia/VEGF-mediated process of migration, proliferation and differentiation appears common to the retinae of a variety of species, including human. However, in human and macaque retina, different mechanisms appear to govern the development of the retinal vessels growing along the horizontal meridian of the retina towards the central area, which contains the fovea. Despite the relatively advanced state of differentiation and maturation of cells in the central area compared with the periphery, the growth of retinal vessels into the central area has been described as 'retarded', and the incidence of cell proliferation associated with these vessels is lower than in peripheral vessels. Furthermore, neither retinal vessels nor their accompanying astrocytes grow into a circumscribed region which, at a later stage, develops into the foveal depression. These observations suggest that molecular markers define the foveal region and inhibit cell proliferation and vascular growth at the fovea and, perhaps, along the horizontal meridian. The findings also suggest that at the fovea, the retina is adapted morphologically to its blood supply, since in the vicinity of the fovea, the development of retinal vessels is retarded or inhibited. The limitations on vascularization of central retina has implications for its vulnerability to degenerative changes, as seen in age-related macular degeneration.

Animals↗

PubMed1997

A fundamental step-transition in retinal function at low light intensities.

There appears to be a fundamental step-transition in retinal function at low light intensities, close to the scotopic-mesopic transition. This step-transition is observed for elements of the retinal dark-light switch, which has been described in the chicken retina. Over the same range of light intensities, there is a step-transition in photoreceptor retinomotor movements and in the coupling of horizontal and All amacrine cells, which suggests a switch in retinal circuitry from rod-processing to cone-processing regimes. A similar step-transition in pineal function suggests that the retinal step-transition signals to the central circadian systems. Finally, this step-transition may also inhibit eye growth, and thus be responsible for the reported diurnal rhythm in eye growth. Disturbances to this step-transition may be the initial cause of disordered eye growth in the form-deprivation myopia paradigm.

Adaptation, Ocular↗

PubMed1981

Linear information processing in the retina: a study of horizontal cell responses.

A basic question about visual perception is whether the retina produces a faithful or a distorted neural representation of the visual image. It is now well known that in some retinal pathways there are significant nonlinear transductions which distort the neural image. The next natural question is, What are the locations of the nonlinear stages within the retinal network? We report here on an investigation of linearity and nonlinearity of responses of horizontal cells in the turtle retina as an assay of the degree of nonlinearity in the outer plexiform layer of the retina. The visual stimuli were sinusoidal gratings; these patterns were modulated by contrast reversal with a sinusoidal time course. The conclusion from our experiments is that the turtle's horizontal cell responses show evidence only of linear spatial summation even at moderately high contrasts on moderately high background levels. Our work thus indicates that there is no significant distortion of the visual image by the photoreceptors or by the neural summation of photoreceptor signals by horizontal cells under normal physiological conditions. These results are consistent with the view that the major nonlinearities of the retina are proximal to the outer plexiform layer.

Animals↗

PubMed1985

Spatial orientation of horizontal cell axon terminals in the carp retina.

In flatmounts of the carp (Cyprinus carpio) retina, 646 horizontal cells were singly marked by intracellular Lucifer yellow CH (LY) in the presence of dopamine or amphetamine, agents which were useful for restricting LY to single injected cells. Most axon terminals of cone-connected horizontal cells have a tendency to orient either radially or tangentially in the retinal field with respect to the optic disc as a center. Although the fluorescent cellular (dendritic field) area greatly varied depending upon the cell type (L-, RG- and YRB-type), the lengths of the axonal processes (axon plus terminal) were all comparable (400-600 microns). A few cells (4.1% of cells with visible axons) possessed a bifurcate axon with two axon terminals. Axons were not observed on rod-connected horizontal cells. The cellular area and the axonal length of L- and RG-type cells appeared to be smaller in the central than in the intermediate region of the retinal field.

Animals↗

PubMed2003

Contribution of connexin26 to electrical feedback inhibition in the turtle retina.

The first synaptic integration in the neuronal cascade of vision in vertebrates includes a feedback from horizontal cells to cones by a mechanism yet not fully understood. Recent observations in teleosts suggested an electrical feedback mechanism mediated by connexin26 (Cx26) hemichannels at the terminal dendrites of horizontal cells. By using reverse transcription-polymerase chain reaction and immunoblotting from retinal homogenate, we detected Cx26 mRNA transcripts in the turtle retina and demonstrated that they were translated into protein. Cx26 immunoreactivity was especially prominent in the outer plexiform layer. Subcellularly, immunoreactivity was located mainly between horizontal cell axon terminals and in horizontal cell dendrites forming the lateral elements at the ribbon synaptic complex of the cone pedicle. The label was confined to the horizontal cell membrane flanking the ribbon and was not found on the opposing photoreceptor membrane. No gap junctions at this location are known, so immunosignaling suggested the presence of hemichannels. Their relevance to the feedback mechanism was investigated by intracellular recordings from horizontal cells during application of the hemichannel blocker carbenoxolone. Carbenoxolone hyperpolarized the dark membrane potential by about 25 mV, decreased the amplitudes of responses to full-field light flashes, and suppressed the feedback-induced depolarizing inflexion in the response profile. These physiological findings are compatible with the involvement of hemichannels in the feedback between horizontal cells and cones and support the anatomical findings. Together, these data suggest the presence of an electrical feedback mechanism in the turtle retina, which therefore might be a common mechanism at the first visual synapse in vertebrates.

Animals↗

PubMed1984

The histochemical localization of cytochrome oxidase in the retina and lateral geniculate nucleus of the ferret, cat, and monkey, with particular reference to retinal mosaics and ON/OFF-center visual channels.

The histochemical localization of cytochrome oxidase within the normal retina and lateral geniculate nucleus (LGN) of cats, ferrets, and monkeys revealed that distinct layers, types of cells, and portions of neurons are more intensely stained than others. The dark staining of photoreceptor inner segments and cone pedicles and the light staining of photoreceptor outer segments, somata, and rod spherules demonstrates that different segments of the same cell may have disparate but distinct levels of oxidative enzyme activity. In tangential sections of retina, regular mosaic arrays were evident for each of several darkly reactive retinal components, such as cone inner segments, cone pedicles, and horizontal cells. In the cat and ferret, regular mosaic arrays were also formed by metabolically distinguishable populations of ganglion cells. Ia and IIa ganglion cells (OFF-; Nelson, R., E. V. Famiglietti, Jr., and H. Kolb (1978) J. Neurophysiol. 41: 472-483) were more darkly reactive than the other classes. The darker staining of sublamina a of the inner plexiform layer (OFF-; Famiglietti, E. V., Jr., and H. Kolb (1976) Science 194: 193-195) in the cat and ferret retina, as well as sublamina A' and A1' of the ferret LGN (OFF-; Stryker, M.P., and K.R. Zahs (1983a) J. Neurosci. 3: 1943-1951) suggest that, under typical rearing conditions, the OFF-channels may be metabolically more active than the ON-channels in these species. In Macaca and Saimiri, darker staining was observed in sublamina b of the inner plexiform layer (ON-; Famiglietti, E.V., Jr., and H. Kolb (1976) Science 194: 193-195) and laminae 1, 2, and 6 of the LGN, implying that, under similar rearing conditions, a different pattern is observed. The dark staining of many large retinal ganglion cells, as well as most of the larger LGN neurons (presumed Y/Y-like), in all species studied is evidence that the Y/Y-like pathway is also highly active.

Animals↗

PubMedDate not supplied

Synaptic feedback, depolarization, and color opponency in cone photoreceptors.

For some 20 years, synaptic feedback from horizontal cells to cones has often been invoked, more or less convincingly, in discussions of retinal action and vision. However, feedback in cones has proved to be rather complex and difficult to study experimentally. The mechanisms and consequences of feedback are therefore still only partly understood. This review attempts to assess the knowns and unknowns. The limitations of the evidence for feedback are reviewed to support the position that unequivocal evidence still largely rests on intracellular recording from cones. Of the three distinct types of depolarization observed in cones, the graded depolarization is taken as the fundamental manifestation of feedback. The evidence for the hypothesis that GABA is the neurotransmitter for feedback appears reasonably strong but several complications will have to be resolved to make the hypothesis more secure. There is evidence that feedback contributes to aspects of light adaptation and spatiotemporal processing of visual information. The contributions seem modest in magnitude. The role of feedback in shaping the color-opponent responses of retinal neurons is evaluated with particular emphasis on pharmacological studies, spatial and temporal aspects of the response of chromatic horizontal cells, and the enigmatic nature of depolarizations in blue- and green-sensitive cones. On this and other evidence, it is suggested that feedback may impress some detectable wavelength dependency in some cones but the dominant mechanisms for color opponency probably reside beyond the photoreceptors.

Adaptation, Ocular↗

PubMed1996

Glia cells of the monkey retina--II. Müller cells.

In this paper, for the first time a quantitative description of the morphology and distribution of Müller cells in the macaque monkey retina using immunohistochemistry and high resolution confocal laser scanning microscopy is given. By their morphological features Müller cells are ideally adapted to their neuronal environment in the various retinal layers, with a dense network of horizontal processes, especially in the inner plexiform layer, and close contacts to neuronal somata especially in the outer nuclear layer and ganglion cell layer. Morphology varies with retinal eccentricity. The thickness of the inner trunk increases significantly with increasing retinal eccentricity. According to the overall thickness of the retina, Müller cells in central retina are longer than in peripheral regions. In the parafoveal region, the outer trunks of Müller cells in the outer plexiform layer are immensely elongated. These Müller fibres can reach lengths of several hundred micrometers as they travel through the outer plexiform layer from the foveal centre towards the foveal border where they enter the inner nuclear layer. Müller cell density varies between 6000 cells/mm2 in far peripheral and peak densities of > 30,000 cells/mm2 in the parafoveal retina. There is a close spatial relationship between Müller cells and blood vessels in the monkey retina, suggesting a role of Müller cells in the formation of the blood-retinal barrier, in the uptake of nutrients and the disposal of metabolites.

Animals↗

PubMed2001

Photoreceptor-horizontal cell reaggregation in monolayer cultures of neonatal rabbit retina.

It has been widely reported that trypsin-dissociated embryonic or neonatal retinal neurons have the ability to reaggregate into histotypic structures when grown in suspension. We now demonstrate that suspension culture is not required and that cell-specific aggregates can also be generated in papain-dissociated cell cultures of neonatal rabbit retina grown on laminin-coated glass coverslips. This culture preparation permitted the direct observation of a rapid and specific reaggregation of retinal cell types into two discrete types of clusters characterized by different morphologies as well as distinct labeling patterns of cell-specific antibodies and lectins. The first cluster type was composed of cells of the outer retina (photoreceptors in specific arrangements with horizontal cells); the second was composed of cells of the inner retina (amacrines and perhaps ganglion cells). The two cluster types were not intermingled, but rather they were segregated from each other, creating a spatial separation of inner from outer retinal cells across the coverslip. Video microscopy revealed that the separation was achieved rapidly (<1 hour) and that neither cell migration nor cell division was required for construction and segregation of cell-specific reaggregates. This novel observation suggests that repulsive as well as attractive factors exist that result in an initial sorting of cells from the outer and inner retina during early development.

Animals↗