Large retinal ganglion cells that form independent, regular mosaics in the ranid frogs Rana esculenta and Rana pipiens

Retinal ganglion cell distribution and spatial resolution in the Asiatic toad Bufo gargarizans (Günther, 1859)

Vision plays a crucial role in the biology of anurans. The spatial arrangement of retinal ganglion cells (GCs) is closely related to visual behavior in vertebrates. There is scarce data on GC topography in anurans, in particular, in toads. I studied the number and distribution of GCs in the retina of the Asiatic toad Bufo gargarizans. GCs were unevenly distributed across the retina. Their spatial density was minimum in the dorsal periphery (3374 and 2486 cells/mm² in the smaller and larger toad, respectively). It increased towards the retinal equator, where a moderately pronounced visual streak was observed comprising several "patches" of a greater GC density. The streak had somewhat "vague" dorsal and ventral borders. The maximum GC density (8605 and 7282 cells/mm² in the smaller and larger toad, respectively) was found in the temporal retina, slightly dorsal to the equator. The respective zone was identified as an area centralis. The total GC number ranged from 266 × 10³ (smaller toad) to 309 × 10³ cells (larger toad). The spatial resolution as estimated from eye geometry and GC density in air was minimum in the dorsal periphery (0.90 and 0.79 cycles per degree in smaller and larger toads, respectively) and maximum in the area centralis (1.43 and 1.36 cycles per degree in smaller and larger toads, respectively). Both retinal specializations found in the Asiatic toad match its biology.

From random to regular: Variation in the patterning of retinal mosaics

The various types of retinal neurons are each positioned at their respective depths within the retina where they are believed to be assembled as orderly mosaics, in which like-type neurons minimize proximity to one another. Two common statistical analyses for assessing the spatial properties of retinal mosaics include the nearest neighbor analysis, from which an index of their "regularity" is commonly calculated, and the density recovery profile derived from autocorrelation analysis, revealing the presence of an exclusion zone indicative of anti-clustering. While each of the spatial statistics derived from these analyses, the regularity index and the effective radius, can be useful in characterizing such properties of orderly retinal mosaics, they are rarely sufficient for conveying the natural variation in the self-spacing behavior of different types of retinal neurons and the extent to which that behavior generates uniform intercellular spacing across the mosaic. We consider the strengths and limitations of these and other spatial statistical analyses for assessing the patterning in retinal mosaics, highlighting a number of misconceptions and their frequent misuse. Rather than being diagnostic criteria for determining simply whether a population is "regular," they should be treated as descriptive statistics that convey variation in the factors that influence neuronal positioning. We subsequently apply multiple spatial statistics to the analysis of eight different mosaics in the mouse retina, demonstrating conspicuous variability in the degree of patterning present, from essentially random to notably regular. This variability in patterning has both a developmental as well as a functional significance, reflecting the rules governing the positioning of different types of neurons as the architecture of the retina is assembled, and the distinct mechanisms by which they regulate dendritic growth to generate their characteristic coverage and connectivity.

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.

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.

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.

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.