The visual cells of the corsula mullet

The eye of Rhinomugil corsula has a duplex retina differentiated into dorsal and ventral halves, with the ventral retina 116·4 μm thicker than the dorsal retina. The rods of the ventral retina are relatively longer, with longer outer segments. The nuclei of the outer nuclear layer of the dorsal and ventral halves are in four and six to seven rows respectively. The rod outer segment bears a single incision. The mitochondria of cone and rod inner segments has a vitreal-scleral gradient. Single and double cones are present in both halves, with triple cones in the dorsal half only. The outer segments of double cones are equal and united. The single cones have two connecting cilia. The cone cells are arranged in a square mosaic with four double cones and five single cones to each unit in the dorsal half, and in a rhombic pattern in the ventral half.     

Duplex Retina in the Mesopelagic Deep-Sea Teleost Lestidiops affinis (Ege, 1930)

The eye of the deep-sea teleost Lestidiops affinis has been examined primarily by light microscopy and found to possess a duplex retina consisting of two main divisions, a pure-cone and a pure-rod region, with a narrow zone of transition, possessing both cones and rods, joining the two. The pure-cone region is located in the temporal (caudal) part of the retina subserving binocular vision in the rostral direction. It has an area temporalis retinae with particularly long and densely packed single cones arranged in a regular hexagonal mosaic. Joined (double or twin) cones have not been recognized with certainty in the pure-cone region. The pure-rod region, comprising the larger part of the retina, contains rods grouped in bundles separated by retinal pigment epithelium (RPE) processes with pigmented cores. The synaptic endings of the rods are arranged in separate clusters in the outer plexiform layer, there being apparently a separate rod pedicle cluster beneath (vitread to) each rod bundle. Structural comparisons with certain other deep-sea teleosts suggest the likely presence of a retinal tapetum in L. affinis, i.e. each single cone or rod bundle is situated in a reflecting pit formed by the RPE, with a discrete reflector apposed to the tip of each cone outer segment and the tips of the outer segments of each square-cut rod bundle.     

Changes in the Visual Cell Layer of the Duplex Retina During Growth of the Eye of a Deep-sea Teleost,Gempylus serpens Cuvier, 1829

Ontogenetic changes in the visual cell layer of the duplex retina during growth of the eye of the deep-sea teleost Gempylus serpens, the snake mackerel, are illustrated by comparing the retina of a small specimen with that of a previously studied adult fish. The small specimen has tightly packed cones spanning the whole width of the visual cell layer and small rods situated in its vitread part. Over most of the retina the cone population consists of single cones arranged in a very regular hexagonal mosaic. The temporalmost retina has a cone population consisting mainly of twin cones arranged in meridional rows. Growth of the eye is associated with an increase in the thickness of the visual cell layer and the density of rods and a total elimination of the densely packed single cones, the retina of the adult fish possessing only a temporally located population of double cones. The radical differences between the retina of the small and adult snake mackerel are probably associated with the different light regimes encountered by small and large specimens.     

Ontogeny of Double Cones in the Retina of Perch Fry (Perca fluviatilis,Teleostei)

The formation of double cones in the retina of fry of Perca fluviatilis has been investigated by light and electron microscopy. The retina of newly hatched fry is provided with single cones and rods, single cones being the predominant receptor type. Double cones are seen for the first time 22 days after hatching. Mitoses are observed in the periphery of the retina, but are also seen in more central parts of the retina containing differentiated receptors and a cone mosaic. The fate of the cells resulting from the centrally located mitoses is not known. No signs of longitudinal fission of differentiated single cones are seen. It is suggested that double cones in the retina of perch fry arise by fusion of single cones which associate closely and develop subsurface cisterns coextensive with the region of intimate contact in the ellipsoid. During the first few weeks after hatching, there is a gradual shift in arrangement of the cones. In the newly hatched fry, the single cones are arranged in rows. When double cones are first seen, square-pattern units appear, built up from four double cones and a single cone.     

Electron microscopic studies on the retinal photoreceptors of the spotted snakehead, Channa punctatus

The retinal photoreceptors of Channa punctatus include rods, single cones and the double cones. The double cones have the outer segments having lamellae which are distorted into tubules and vesicles, and the inner segments with hypertrophied mitochondria.     

Fine structure of the retina of black bass, Micropterus salmoides (Centrarchidae, Teleostei).

The structure of light- and dark-adapted retina of the black bass, Micropterus salmoides has been studied by light and electron microscopy. This retina lacks blood vessels at all levels. The optic fiber layer is divided into fascicles by the processes of Müller cells and the ganglion cell layer is represented by a single row of voluminous cells. The inner nuclear layer consists of two layers of horizontal cells and bipolar, amacrine and interplexiform cells. In the outer plexiform layer we observed the synaptic terminals of photoreceptor cells, rod spherules and cone pedicles and terminal processes of bipolar and horizontal cells. The spherules have a single synaptic ribbon and the pedicles possess multiple synaptic ribbons. Morphologically, we have identified three types of photoreceptors: rods, single cones and equal double cones which undergo retinomotor movements in response to changes in light conditions. The cones are arranged in a square mosaic whereas the rods are dispersed between the cones.     

Photoreceptor fine structure in the southern fiddler ray (Trygonorhina fasciata).

The fine structure of the retinal photoreceptors has been studied by light and electron microscopy in the southern fiddler ray or guitarfish (Trygonorhina fasciata). The duplex retina of this species contains only rods and single cones in a ratio of about 40:1. No multiple receptors (double cones), no repeating pattern or mosaic of photoreceptors and no retinomotor movements of these photoreceptors were noted. The rods are cylindrical cells with inner and outer segments of the same diameter. Cones are shorter, stouter cells with a conical outer segment and a wider inner segment. Rod outer segment discs display several irregular incisures to give a scalloped outline to the discs while cone outer segment discs have only a single incisure. In all photoreceptors a non-motile cilium joins the inner and outer segments. The inner segment is the synthetic centre of photoreceptors and in this compartment is located an accumulation of mitochondria (the ellipsoid), profiles of both rough and smooth endoplasmic reticulum, prominent Golgi zones and frequent autophagic vacuoles. The nuclei of rods and cones have much the same chromatin pattern but cone nuclei are invariably located against or particularly through the external limiting membrane (ELM). Numerous Landolt's clubs which are ciliated dendrites of bipolar cells as well as Müller cell processes project through the ELM, which is composed of a series of zonulae adherentes between these cells and the photoreceptors. The synaptic region of both rods (spherules) and cones (pedicles) display both invaginated (ribbon) synapses and superficial (conventional) synapses with cones showing more sites than the rods.     

Development of a rod photoreceptor mosaic revealed in transgenic zebrafish

The number and distribution of neurons within the vertebrate retina are tightly regulated. This is particularly apparent in the highly ordered, crystalline-like arrangement of the cone photoreceptors in the teleost. In this report, using a transgenic line of zebrafish, a novel and developmentally regulated mosaic pattern of the rod photoreceptors is described. The spatial and temporal expression of EGFP, under the control of the Xenopus rhodopsin gene promoter, was nearly identical to the endogenous rhodopsin. EGFP was first detected in the ventral nasal retinal in an area of precocious neurogenesis referred to as the "ventral patch". Subsequent expression of EGFP was observed in isolated cells sporadically distributed across the dorsal and central retina. However, confocal microscopy and spatial analysis of larval eyes or retinal explants from adults revealed a precise arrangement to the rod photoreceptors. The rod terminals were arranged in regularly spaced rows with clearly identifiable telodendria linking neighboring cells. The rod inner segments projected through the cone mosaic in a predictable pattern. In the adult, the rod mosaic originated near the retinal margin where clusters of rods differentiated around the immature short single cone. In the embryo, the sporadic differentiation of the rods led to the gradual formation of the mosaic pattern. With the growing interest in neuronal stem cells, revisiting this model of neurogenesis provides an avenue to uncover mechanisms underlying the precise integration of new neuronal elements into a preexisting neural network.     

On the eye of the Goldeye Hiodon alosoides (Teleostei: Hiodontidae)

In the eye of the Goldeye the photoreceptors are arranged in bundles and the pigment epithelium contains a massive reflector or tapetum lucidum. Photoreceptor bundles are arranged in parallel rows, the bundles alternating in position from row to row. Each bundle contains about 60 photoreceptors, of which 30 or so are cones. Rod outer segments lie in the scleral half of the outer retinal region of the light-adapted eye. Processes of the pigment epithelium cells extend vitread almost to the external limiting membrane; they envelop the bundles of rods and cones, and a ring of four processes surrounds each bundle. A process contains two kinds of reflecting crystals (composed of uric acid). A large part of the epithelium cell is packed with small disc-shaped crystals (crystallites) enclosed in thin membranes; the tip of the process, in the region of the photoreceptor bundle, contains orderly arrays of small rod-shaped crystals (rodlets). It is suggested that the crystallites form a diffuse reflector backscattering light into the rods; and that the rodlets reflect light regularly from their surfaces into the photoreceptor bundles. In the light-adapted state, rods are enveloped by pigment and crystallites. The organization is compared with that of other fishes that have photoreceptors in bundles (grouped retinae) and tapeta lucida.     

Peculiarities of the eye morphology and the spectral sensitivity of the retinal photoreceptors of the Pacific saury <Emphasis Type="Italic">Cololabis saira</Emphasis>

This study examines some peculiarities of the eye organization and spectral properties of retinal photoreceptors of the Pacific saury Cololabis saira. The saury has relatively large eyes with a developed accomodation apparatus and an area of enhanced visual acuity (the fovea) in the retina. A specialized pigmented septum is observed in the vitreal cavity, which is supposed to function as a light-shading screen. The retina contains numerous rods and single and double cones arranged in a square mosaic pattern. Microspectrophotometric measurements indicated that their max occurs at 502 (rods), 380 (single cones), and 478/565 (double cones) nm. Such properties can provide color vision in a broad spectral range, including UV light. The peripheral visual apparatus of the Pacific saury is typical of active diurnal predatory fish that inhabit shallow and upper pelagic water layers.     

Regional variations in the outer retina of atherinomorpha (Beloniformes,Atheriniformes, Cyprinodontiformes: Teleostei): photoreceptors,cone patterns,and cone densities

The outer retinae of adults of 13 atherinomorph species, representing nine different families, were examined by both light and electron microscopy. The retinae were investigated with respect to photoreceptor types, cone densities, and cone patterns. All data were composed to eye maps. This procedure allows an interspecific comparison of the regional differences within the outer retina among these shallow-water fish. Furthermore, for a more detailed pattern analysis nitro-blue tetrazolium chloride- (NBT)-stainings in the retina of Melanotaenia maccullochi are presented. Apart from rods, eight morphologically different cone types could be identified: short, intermediate, and long single cones, double cones (equal and unequal), triple cones (triangular and linear), and in Ameca splendens one quadruple cone. Dimensions and occurrence of photoreceptors vary among the respective species and within the retinal regions. In the light-adapted state, the cones are arranged in highly ordered mosaics. Five different cone tessellation types were found: row patterns, twisted row patterns, square patterns, pentagonal patterns, and, exclusively in Belone belone, a hexagonal pattern. In Melanotaenia maccullochi the different spectral photoreceptor classes correspond well with the distribution of morphological photoreceptor classes within the mosaic. Double cone density maxima together with a highly ordered cone arrangement usually occur in the nasal and/or ventral to ventrotemporal retina. In most of the species that were examined these high-density regions are presumed to process visual stimuli from the assumed main directions of vision, which mainly depend on feeding behavior and predator pressure. Our findings are discussed with respect to the variable behavioral and visual ecology and phylogeny of the respective species.     

A cytological study on the development of the different types of visual cells in the chicken (Gallus domesticus)

The formation of visual cells and their intracellular organelles was studied in the embryonic chicken (Gallus domesticus) between stage 36 and hatching. Cilia formation was observed at stage 30 and by stage 42, outer segment formation from the cilia was evident. The inner segments appeared as buddings at stage 36. By stage 37, the buddings of double cones were observed clearly and such buddings elongated by stage 42. Both the single cones and rods appeared as buddings by stage 38 and elongation of the buddings was seen by stage 42. Oil droplets initially appeared by stage 39 in accessory cones and were observed in other cones by stage 42. Glycogen bodies were demonstrated firstly in rods and accessory cones at stage 43 and their development was completed by stage 45. In essence, all the essential elements of the visual cells were fully developed by hatching.     

The fine structure of the pigment epithelium and photoreceptor cells of the newt,Triturus viridescens dorsalis (Rafinesque)

The morphology of the retinal pigment epithelium and photoreceptor cells has been studied in the common newt Triturus viridescens dorsalis by light, conventional transmission and scanning electron microscopy. The pigment epithelium is formed by a single layer of low rectangular cells, separated by a multilayered membrane (Bruch's membrane) from the vessels of the choriocapillaris. The scleral border of the pigment epithelium is highly infolded and each epithelial cell contains smooth endoplasmic reticulum, myeloid bodies, mitochondria, lysosomes, phagosomes and an oval nucleus. Inner, pigment laden, epithelial processes surround the photoreceptor outer and inner segments. The three retinal photoreceptor types, rods, single cones and double cones, differ in both external and internal appearance. The newt, rod, outer segments appear denser than the cones in both light and electron micrographs, due to a greater number of rod lamellae per unit distance of outer segment and to the presence of electron dense intralamellar bands. The rod outer segments possess deep incisures in the lamellae while the cone lamellae lack incisures. Both rod and cone outer segments are supported by a peripheral array of dendritic processes containing longitudinal filaments which originate in the inner segment. The inner segment mitochondria, forming the rod ellipsoid, arelong and narrow while those in the cone are spherical to oval in shape. The inner segments of all three receptor cell types also contain a glycogen-filled paraboloid and a myoid region, just outside the nucleus, rich in both rough and smooth endoplasmic reticulum. The elongate, cylindrical nuclei differ in density. The rod nuclei are denser than those of the cones, contain clumped chromatin and usually extend further vitreally. Similarly, the cytoplasm of the rod synaptic terminal is denser than its cone counterpart and contains synaptic vesicles almost twice as large as those of the cones. Photoreceptor synapses in rods and cones are established by both superficial and invaginated contacts with bipolar or horizontal cells.     

The spectral sensitivity of the retinal photoreceptors of the Asiatic smelt <Emphasis Type="Italic">Osmerus dentex</Emphasis> (Steindachner et Kner, 1870)

The spectral sensitivity and complement of the retinal photoreceptors of the Asiatic smelt from the Sea of Japan were studied by microspectrophotometry and light microscopy. Apart from rods, one type of single cones and one type of unequal double cones were found in major parts of the retina. The dominant type of the cone pattern (mosaic) is a row pattern consisting of various linear arrangements of separate single and double cones. The absorbance maxima of rods and a majority of singe cones and double cones equaled 516, 425 and 514/565 nm, respectively. It has been established that all of the pigments are based on retinal. The findings are compared with data on the osmerid retina from the literature and discussed with respect to the adaptations to light conditions, peculiarities of behavior, and seasonal migrations of smelts.     

Ultrastructure of the retina of two subspecies of Coregonus lavaretus (Teleostei) from Lake Constance (Germany)

In the two studied subspecies of Coregonus lavaretus , the pollan ( C. l. wartmanni ) (which lives deep in the pelagial) and the gangfish ( C. l. macrophthalmus ) (which lives near the slope, closer to the bottom), duplex retinae containing rod and cone photoreceptors are found. Four morphologically different cone types were observed: unequal double cones, short single cones, long single cones and triple cones. The cones are arranged in a square pattern (four double cones around a central short single cone) in the ventral and ventrotemporal and in a row pattern in the nasal and dorsal areas of the retina. Moreover, intermediate patterns can be observed in several regions indicating that double cone twisting occurs, i.e. double cones twist about their longitudinal axis. The highest cone densities are found in the ventrotemporal area. Conversely, the rod photoreceptor density is the highest in the dorsal retina. While the basic morphology of the retina is the same in both subspecies, the distribution of long single and triple cones differs between the studied animals. While these cone types are very rare in the pollan, they are common in the gangfish, though not exhibiting a regular pattern. The findings are discussed with regard to the photic habitat conditions, the systematic position of coregonids and variation of retinal morphology in the two subspecies.     

Taurine,amino acid transmitters,and related molecules in the retina of the Australian lungfish Neoceratodus forsteri: a light-microscopic immunocytochemical and electron-microscopic study

The morphology of the retina of the Australian lungfish Neoceratodus forsteri was investigated by means of light- and electron microscopy, whilst immunocytochemical studies were performed to determine the cellular distributions of the major amino acid neurotransmitters and other amino acids. The distributions of glycine and GABA were similar to those previously described for teleost, amphibian and mammalian retinae. Labelling was abundant in amacrine cells, whilst GABA was also present in one layer of horizontal cells and some bipolar cells. Taurine was present in both rods and cones, but, unlike the mammalian or avian retina, was absent from other cellular structures, including glial elements. Unexpectedly, the photoreceptor terminals lacked an apparent content of the excitatory amino acid transmitter glutamate. The glutamate that was present in the rods and cones occupied a crescentic arc corresponding to the location of glycogen-rich paraboloids. Asparagine was also present in rods, albeit in the modified mitochondria that formed the elipsoids of the rod inner segments. Arginine, the precursor for formation of nitric oxide, was present in glial cells, and in the paraboloids of both rods and cones.     

The foveal photoreceptor mosaic in the pipefish, Corythoichthyes paxtoni (Syngnathidae, Teleostei)

The foveal and non-foveal retinal regions of the pipefish, Corythoichthyes paxtoni (Syngnathidae, Teleostei) are examined at the level of the light and electron microscopes. The pipefish possesses a deep, pit (convexiclivate) fovea which, although lacking the displacement of the inner retinal layers as described in other vertebrate foveae, is characterised by the exclusion of rods, a marked increase in the density of photoreceptors and a regular square mosaic of four double cones surrounding a central single cone. In the perifoveal and peripheral retinal regions, the photoreceptor mosaic is disrupted by the insertion of large numbers of rods, which reduce spatial resolving power but may uniformly increase sensitivity for off-axis rays. In addition to a temporal fovea subtending the frontal binocular field, there is also a central area centralis subtending the monocular visual field. Based on morphological comparisons with other foveate teleosts, four foveal types are characterised and foveal function discussed with respect to the theoretical advantage of a regular square mosaic.     

Monkey photoreceptor calycal processes and interphotoreceptor matrix as observed by scanning electron microscopy.

Photoreceptors and the interphotoreceptor matrix of monkey retina were observed by scanning electron microscopy. Cone photoreceptors are easily distinguished from rod photoreceptors by their wide conical inner segments. The calycal processes of 50 rods and 50 cones were counted and measured. The calycal processes of cones were distinct, short, and uniform in diameter (0.1 micron). They were arranged equidistantly and, in most cases, were not continuous with longitudinal inner segment ridges, as previously suggested. In contrast to cones, rod calycal processes were fewer in number, were about one-fourth the number of the cones, were of variable length (0.7 micron to 3.0 micron), and tapered to a fine point at their distal ends. The interphotoreceptor matrix appeared spongelike, made up of anastomosing plates and strands filling the interphotoreceptor space. Other than an increased amount of matrix around cones, no structural difference between rod- and cone-associated interphotoreceptor matrix was observed.     

Posttranslational modifications of tubulin in teleost photoreceptor cytoskeletons

1. Posttranslational modifications of tubulin by acetylation and detyrosination have been correlated previously with microtubule stability in numerous cell types. 2. In this study, posttranslational modifications of tubulin and their regional distribution within teleost photoreceptor cones and rods are demonstrated immunohistochemically using antibodies specific for acetylated, detyrosinated, or tyrosinated tubulin. 3. Immunolocalization was carried out on isolated whole cones and mechanically detached rod and cone inner/outer segments. 4. Acetylated tubulin within rods and cones is found only in microtubules of the ciliary axoneme of the outer segment. Detyrosinated tubulin is also enriched in axonemes of both rod and cone outer segments. 5. Distributions of tyrosinated and detyrosinated cytoplasmic microtubules differ within cones and rods. In cones, detyrosinated and tyrosinated tubulins are both abundant throughout the cell body. In rods, the ellipsoid and myoid contain much more tyrosinated tubulin than detyrosinated tubulin. Comparisons between whole cones and cone fragments suggest that detyrosinated microtubules are more stable than tyrosinated microtubules in teleost photoreceptors. 6. Our findings provide further evidence that microtubules of teleost cones differ from rod microtubules in their stabilities and rapidity of turnover within the photoreceptor inner segment.