Somatic and dendritic mosaics formed by large ganglion cells in the retina of the common house gecko (Hemidactylus frenatus)

The structure and diversity of retinal ganglion cells in the masked greenling Hexagrammos octogrammus Pallas, 1814 (Pisces: Scorpaeniformes: Hexagrammidae)

The authors studied the structure and diversity of retinal ganglion cells (GC) in the masked greenling Hexagrammos octogrammus. In vivo labelling with horseradish peroxidase revealed GCs of various structures in retinal wholemounts. A total of 154 cells were camera lucida drawn, and their digital models were generated. Each cell was characterized by 17 structural and topological parameters. Using nine clustering algorithms, a variety of clusterings were obtained. The optimum clustering was found using silhouette analysis. It was based on a set of three variables associated with dendritic field size and dendrite stratification depth in the retina. A total of nine cell types were discovered. A number of non-parametric tests showed significant pair-wise between-cluster differences in at least four parameters with medium and large effect sizes. Three large-field types differed mainly in dendritic field size, total dendrite length, level of dendrite stratification in the retina and position of somata. Six medium- to small-field types differed mainly in the structural complexity of dendritic arbors and level of dendrite arborization. Cells similar and obviously homologous to types 1-4 were identified in many fish species, including teleosts. Potential homologues of type 5 cells were identified in fewer teleost species. Cells similar to types 6-9 in relative dendritic field size and dendrite arborization pattern were also described in several teleostean species. Nonetheless, their homology is more questionable as their stratification patterns do not match so well as they do in large types. Potential functional matches of the GC types were identified in a number of teleostean species. Type 1 and 2 cells probably match spontaneously active units with the large receptive field centre, so-called dimming and lightening detectors; type 4 may be a counterpart of changing contrast detectors with medium receptive field centre size preferring fast-moving stimuli. Type 3 (biplexiform) cells have no obvious functional matches. Probable functional matches of types 6, 8 and 9 belong to ON-centre elements with small receptive fields such as ON-type direction-selective cells, ON-type spot detectors or ON-type spontaneously active units. Type 5 and 7 cells may match ON-OFF type units, in particular, changing contrast detectors or orientation-selective units. Potential functional matches of GC types presently described are involved in a wide spectrum of visual reactions related to adaptation to gradual change in illumination, predator escape, prey detection and capture, habitat selection and social behaviour.

A Hierarchical View of Gecko Locomotion: Photic Environment, Physiological Optics, and Locomotor Performance

Terrestrial animals move in complex habitats that vary over space and time. The characteristics of these habitats are not only defined by the physical environment, but also by the photic environment, even though the latter has largely been overlooked. For example, numerous studies of have examined the role of habitat structure, such as incline, perch diameter, and compliance, on running performance. However, running performance likely depends heavily on light level. Geckos are an exceptional group for analyzing the role of the photic environment on locomotion as they exhibit several independent shifts to diurnality from a nocturnal ancestor, they are visually-guided predators, and they are extremely diverse. Our initial goal is to discuss the range of photic environments that can be encountered in terrestrial habitats, such as day versus night, canopy cover in a forest, fog, and clouds. We then review the physiological optics of gecko vision with some new information about retina structures, the role of vision in motor-driven behaviors, and what is known about gecko locomotion under different light conditions, before demonstrating the effect of light levels on gecko locomotor performance. Overall, we highlight the importance of integrating sensory and motor information and establish a conceptual framework as guide for future research. Several future directions, such as understanding the role of pupil dynamics, are dependent on an integrative framework. This general framework can be extended to any motor system that relies on sensory information, and can be used to explore the impact of performance features on diversification and evolution.

Ontogeny of cone photoreceptor mosaics in zebrafish

Cone photoreceptors in fish are typically arranged into a precise, reiterated pattern known as a "cone mosaic." Cone mosaic patterns can vary in different fish species and in response to changes in habitat, yet their function and the mechanisms of their development remain speculative. Zebrafish (Danio rerio) have four cone subtypes arranged into precise rows in the adult retina. Here we describe larval zebrafish cone patterns and investigate a previously unrecognized transition between larval and adult cone mosaic patterns. Cone positions were determined in transgenic zebrafish expressing green fluorescent protein (GFP) in their UV-sensitive cones, by the use of multiplex in situ hybridization labelling of various cone opsins. We developed a "mosaic metric" statistical tool to measure local cone order. We found that ratios of the various cone subtypes in larval and adult zebrafish were statistically different. The cone photoreceptors in larvae form a regular heterotypic mosaic array; i.e., the position of any one cone spectral subtype relative to the other cone subtypes is statistically different from random. However, the cone spectral subtypes in larval zebrafish are not arranged in continuous rows as in the adult. We used cell birth dating to show that the larval cone mosaic pattern remains as a distinct region within the adult retina and does not reorganize into the adult row pattern. In addition, the abundance of cone subtypes relative to other subtypes is different in this larval remnant compared with that of larvae or canonical adult zebrafish retina. These observations provide baseline data for understanding the development of cone mosaics via comparative analysis of larval and adult cone development in a model species.

Morphology and spatial arrangement of large retinal ganglion cells projecting to the optic tectum in the perciform fish Pholidapus dybowskii

Using retrograde HRP labeling from the optic nerve (ON) or optic tectum (OT), we have visualized large ganglion cells (LGCs) in wholemounted retinas of the teleost Pholidapus dybowskii and studied their morphology and spatial properties. In all, three LGC types were distinguished. In a previous paper, detailed data were provided on one type, biplexiform cells [Pushchin, I. I., & Kondrashev, S. L. (2003). Biplexiform ganglion cells in the retina of the perciform fish Pholidapus dybowskii revealed by HRP labeling from the optic nerve and optic tectum. Vision Research, 43, 1117-1133]. Here, we present data on the other two confirmed types, alpha(a) and alpha(ab) cells. The types differed in the level of dendrite stratification, dendrite arborization pattern, dendritic field size, and other features, and formed in the retina significantly non-random, spatially independent mosaics. Both types were labeled from the OT, indicating their participation in OT-mediated visual reactions. The comparison of spatial properties of alpha(a) and alpha(ab) mosaics labeled from the ON and OT suggests that the OT is the major or one of the major projection areas of both types. We also describe the morphology of cells resembling alpha(c) cells of other fishes, which were only labeled from the ON. The LGC types presently revealed were similar in their morphology to LGCs found in other teleosts supporting the hypothesis of LGC homology across the teleost lineage.

Evidence for a columnar organization of cones, Müller cells, and neurons in the retina of a cichlid fish

In the retina of many lower vertebrates, the arrangement of cells, in particular of cone photoreceptors, is highly regular. The data presented in this report show that in the retina of a cichlid fish (Astatotilapia burtoni) the regular arrangement is not restricted to cone photoreceptors and their synaptic terminals but can be found in elements of the inner retina as well. A variety of immunocytochemical and other markers was used in combination with confocal microscopy on whole-mount preparations and tangential sections. Nearest neighbor analysis was performed and density recovery profiles as auto- and cross-correlograms were generated. Cells displaying a regular arrangement of their synaptic processes in matching radial register to each other were identified for each major retinal neuronal cell type except ganglion cells (i.e. photoreceptors, horizontal cells, bipolar cells, and amacrine cells). The precise location of some of the corresponding cell bodies was not as regular but still non-random, however there was no spatial cross-correlation between cell bodies of different types. The radial processes of Müller glial cells displayed a distribution correlating to the arrangement of photoreceptors and neurons. Thus, for one Müller glial cell I found two PKC-positive cone bipolar cells, a spatially corresponding grid of parvalbumin-positive amacrine cell processes, one H1 horizontal cell, and two pairs of double cones. There was no evidence among ganglion cells matching this pattern, possibly due to the lack of suitable markers. Although many other cell types do not follow this matching regular mosaic arrangement, a basic columnar building block can be postulated for the retina at least in cichlid fish. This suggests a functional radial unit from photoreceptors to the inner plexiform layer.

The number, morphology, and distribution of retinal ganglion cells and optic axons in the Australian lungfishNeoceratodus forsteri(Krefft 1870)

Australian lungfishNeoceratodus forsterimay be the closest living relative to the first tetrapods and yet little is known about their retinal ganglion cells. This study reveals that lungfish possess a heterogeneous population of ganglion cells distributed in a horizontal streak across the retinal meridian, which is formed early in development and maintained through to adult stages. The number and complement of both ganglion cells and a population of putative amacrine cells within the ganglion cell layer are examined using retrograde labelling from the optic nerve and transmission electron-microscopic analysis of axons within the optic nerve. At least four types of retinal ganglion cells are present and lie predominantly within a thin ganglion cell layer, although two subpopulations are identified, one within the inner plexiform and the other within the inner nuclear layer. A subpopulation of retinal ganglion cells comprising up to 7% of the total population are significantly larger (>400 μm2) and are characterized as giant or alpha-like cells. Up to 44% of cells within the retinal ganglion cell layer represent a population of presumed amacrine cells. The optic nerve is heavily fasciculated and the proportion of myelinated axons increases with body length from 17% in subadults to 74% in adults. Spatial resolving power, based on ganglion cell spacing, is low (1.6–1.9 cycles deg−1,n= 2) and does not significantly increase with growth. This represents the first detailed study of retinal ganglion cells in sarcopterygian fish, and reveals that, despite variation amongst animal groups, trends in ganglion cell density distribution and characteristics of cell types were defined early in vertebrate evolution.

Quantitative Analysis of Cells in the Ganglion Cell Layer of the Chick Retina: Developmental Changes in Cell Density and Cell Size

Changes in cell density and size in the ganglion cell layer (GCL) of the retina were studied in chick embryos and post-hatching chicks. The total number of cells in the GCL increased from 3.64 million at embryonic day 8 (E8) to the maximal 7.85 million at E14. After E14, the number of cells decreased to 6.08 million at post-hatching day 1 (P1) and 4.87 million at P8. Cell density in the GCL decreased unevenly according to retinal regions; cell density in the presumptive central area (pCA) of P8-chicks decreased to approximately 45% of that in E8-embryos. Densities of the nasal peripheral retina (NP) and temporal peripheral retina (TP) of P8-chicks decreased to 23 and 18% of E8-embryos, respectively. Differentiation of the central (44,000 cells/mm(2) in pCA) - peripheral (28,000 cells/mm(2) in TP) gradient in cell density was formed by E8. The presumptive dorsal area (pDA) was shaped by E11, but became obscure with age. Although ganglion cell sizes were basically uniform at E8, differentiation occurred with the appearance of larger ganglion cells after E14. Mean size of retinal ganglion cells increased 2.8-fold in the pCA and 3.8-fold in the TP between E8 and P8, accompanying a similar scale of decreases in cell densities.

Changes in somal growth and dendritic patterns of the retinal ganglion cells in the chicks and chick embryos

Changes in the somal growth and dendritic patterns of retinal ganglion cells (RGCs) were studied in early chick embryos and post-hatching chicks by means of the retrograde axonal transport labeling with DiI. Branching patterns of the dendrites were relatively uniform on E8 (embryonic day 8) and became more complicated on E11. Variety of the branching pattern became plainly abundant after E14. On the other hand, somata of RGCs continued to grow until E14, corresponding the appearance of the central-peripheral gradient of the somal size. After E14, RGCs elaborated on the formation of the dendritic patterns as found in chick retina, and simultaneously the growth of somal sizes almost ceased.

Gecko vision--retinal organization, foveae and implications for binocular vision

Geckos comprise both nocturnal and diurnal genera, and between these categories there are several transitions. As their retinae have definitely to be classified as pure cone retinae, they provide an especially attractive model for comparison of organization and regional specializations adapted to very different photic environments. While the visual cells themselves show clear adaptations to nocturnal or diurnal lifestyles, the overall retinal organization is more related to that of diurnal vertebrates. Nocturnal geckos have lost any foveae of their diurnal ancestors, but they have retained a low convergence ratio and a high visual cell density. To enhance visual sensitivity, they exploit binocular - but not necessarily stereoscopic - vision. Diurnal species have retained binocular vision. Most diurnal species have developed new foveae, which are consequently located not in the central but in the temporal region of the retina.

Retinal mosaics: new insights into an old concept

It has been known since the middle of the 19th century that different neuronal types are distributed across the retinal surface in non-random arrays: indeed, these arrays, called 'mosaics', have long been considered to be a fundamental feature of retinal organization. However, until recently, little was known about how such mosaics are established during development. In the hope of stimulating further research, this article reviews the current status of three very different approaches to this intriguing general problem. The first postulates arrays of molecular markers, which are produced by specific cell types shortly after their final mitotic divisions and could be influential in the differentiation of other cell types. The second invokes a tangential dispersion of differentiating cells to generate spatial order, either while these cells are still migrating or soon after they reach their laminar destinations. The third involves the elimination of wrongly positioned cells through the process of naturally occurring cell death.

Species-dependent variation in the dendritic stratification of apparently homologous retinal ganglion cell mosaics in two neoteleost fishes

Large retinal ganglion cells of the marine neoteleost Bathymaster derjugini were labeled with horseradish peroxidase and studied in flatmounts. Four types formed regular, independent mosaics, of which three (biplexiform, alpha-a, alpha-c) resembled those in several other teleosts. The fourth (alpha-ab) appeared novel in one significant respect. Whereas we originally described similar cells in another neoteleost, Oreochromis spilurus, as monostratified in sublamina b of the inner plexiform layer, these were very clearly bistratified in a and b. Detailed re-analysis of our Oreochromis flatmounts showed that the difference is of one degree only: many Oreochromis cells do send fine dendrites into a. These observations strengthen the evidence that all four mosaics are homologous across a wide range of fishes, and clear away an obstacle to our earlier proposals that the alpha-a, alpha-ab and alpha-c mosaics of fishes, frogs, and perhaps other nonmammalian jawed vertebrates too, may all be homologous.