Retinal ganglion cells in two teleost species, Sebastiscus marmoratus and Navodon modestus

A lambda-shaped retractor lentis muscle in the yellowfin goby Acanthogobius flavimanus

We identified a morphologically uncommon piscine retractor lentis muscle in the yellowfin goby Acanthogobius flavimanus. This lentis muscle has a shape similar to the Greek small letter lambda (λ). The two legs of the muscle are attached to the retinal periphery at the ventral eyecup, while the tip is connected to the lens surface by a ligament. Scanning electron microscopy showed that the fibers of the lentis muscle run along the length of both the anterior and posterior legs. Immunolabeling with antiacetylated tubulin antibody and neuronal tracing with DiI of the whole lentis muscle revealed that the anterior leg is innervated by one or more nerves. The topographic distribution of ganglion cells in the retina was investigated to identify the visual axis. Three high cell density areas were observed in the dorso-temporal, ventro-nasals and ventro-temporal retina. These findings suggest that the λ-shaped lentis muscle may enable accommodatory movement of the lens toward the temporal as well as the nasal and/or ventral retina.

Visual system development of the spotted unicornfish, Naso brevirostris (Acanthuridae)

Ontogenetic changes of the visual system are often correlated to shifts in habitat and feeding behaviour of animals. Coral reef fishes begin their lives in the pelagic zone and then migrate to the reef. This habitat transition frequently involves a change in diet and light environment as well as major morphological modifications. The spotted unicornfish, Naso brevirostris, is known to shift diet from zooplankton to algae and back to mainly zooplankton when transitioning from larval to juvenile and then to adult stages. Concurrently, N. brevirostris also moves from an open pelagic to a coral-associated habitat before migrating up in the water column when reaching adulthood. Using retinal mapping techniques, we discovered that the distribution and density of ganglion and photoreceptor cells in N. brevirostris mostly changes during the transition from the larval to the juvenile stage, with only minor modifications thereafter. Similarly, visual gene (opsin) expression based on RNA sequencing, although qualitatively similar between stages (all fishes mainly expressed the same three cone opsins; SWS2B, RH2B, RH2A), also showed the biggest quantitative difference when transitioning from larvae to juveniles. The juvenile stage in particular seems mismatched with its reef-associated ecology, which may be due to this stage only lasting a fraction of the lifespan of these fishes. Hence, the visual ontogeny found in N. brevirostris is very different from the progressive changes found in other reef fishes calling for a thorough analysis of visual system development of the reef fish community.

Morphological characteristics of eyes and retinas of two sardines (Sardinops melanostictus and Etrumeus sadina, Clupeidae) and an anchovy (Engraulis japonicus, Engraulididae)

The morphology of the eyes and distribution of retinal ganglion cells in two sardine species (Sardinops melanostictus and Etrumeus sadina, Clupeidae) and the Japanese anchovy (Engraulis japonicus, Engraulididae) were investigated anatomically and histologically. The eyes of the sardines faced a slightly dorsolateral direction with the visual field extended obliquely upward. In contrast, the eyes in the anchovy were almost laterally directed. It was hypothesized that the sardines may have an advantage in receiving more downward irradiance compared with the anchovy. The lens muscle was larger in these three species than in many other teleosts, and its surface was entirely melanin-pigmented. Also, the lens muscle directly and tightly adhered to the backside surface of the iris. The relative area of the lens muscle to the area of the lens, a referential value of the relative power of visual accommodation were notably larger in the species studied than in other teleost values that have been previously reported. A higher M/L% value of these clupeid fishes could facilitate fast and wide ranging visual accommodation and was considered to be associated with maintaining and/or re-establishing school formations quickly. Analysis of topographical distributions of cells in the ganglion cell layer showed that cell density was highest in the ventrotemporal quadrant of the retina (temporal of the optic cleft) in all three species. Another potentially important role for the black-pigmented lens muscle may be to block the specialized retinal area from intense sunlight that scatters and irradiates upward or laterally in the surface waters that they inhabit. Thus, the sardine and anchovy may take advantage of efficient detection of visual signals in the frontal-upward direction and further improve visibility of the target in this direction.

A morphological classification of retinal ganglion cells in the Japanese catshark Scyliorhinus torazame

Retinal ganglion cells (GCs) in the Japanese catshark Scyliorhinus torazame were labeled retrogradely with biotinylated dextran amine (BDA3000). First the labeled cells were classified into 5 morphological types (types I-III: small GCs; types IV and V: large GCs) according to the size of the soma and the dendritic arborization pattern as seen in retinal wholemounts. Type I cells were stellate, with dendrites radiating in different directions. Type II cells had bipolar dendritic trees, with 2 primary dendrites extending in opposite directions. Type III cells had a single thick primary dendrite. Type IV cells were stellate, with dendrites covering a large area centered on the cell body. Type V cells were asymmetric, with most dendrites extending opposite to the axon as a large, fan-shaped dendritic field. Subsequently a wholemount was cross-sectioned, and we classified cells further into multiple subtypes according to the level of dendritic arborization within the inner plexiform layer. The present results suggest the existence of many types of GCs in elasmobranchs in addition to the 3 types of large GCs that have been characterized previously. Some of the newly described GC subtypes in the catshark retina appear to be similar to some of those reported in actinopterygians.

Retinal ganglion cell topography in juvenile Pacific bluefin tuna Thunnus orientalis (Temminck and Schlegel)

The retinal ganglion cell distribution, which is known to reflect fish feeding behavior, was investigated in juvenile Pacific bluefin tuna Thunnus orientalis. During the course of examination, regularly arrayed cells with a distinctive larger soma, which may be regarded as motion-sensitive cells, were found. The topographical distribution of ordinary-sized ganglion cells, which is usually utilized to estimate fish visual axis and/or visual field characteristics, showed that the highest-density area, termed the area centralis, was localized in the ventral-temporal retina. The retinal topography of ordinary-sized ganglion cells seems to reflect the bluefin tuna's foraging behavior; while cruising, cells in the area centralis may signal potential prey, such as small schooling pelagic fishes or squids, that are present in the upward-forward direction. Judging from morphological characteristics, the large ganglion cells localized in the small temporal retinal area seem to be equivalent to physiologically categorized off-center Y-cells of cat, which are stimulated by a transient dark spot in a bright visual field. It was inferred that presumed large off-center cells in the temporal retina detect movements of agile prey animals escaping from bluefin tuna as a silhouette against environmental light.

Vision in elasmobranchs and their relatives: 21st century advances

This review identifies a number of exciting new developments in the understanding of vision in cartilaginous fishes that have been made since the turn of the century. These include the results of studies on various aspects of the visual system including eye size, visual fields, eye design and the optical system, retinal topography and spatial resolving power, visual pigments, spectral sensitivity and the potential for colour vision. A number of these studies have covered a broad range of species, thereby providing valuable information on how the visual systems of these fishes are adapted to different environmental conditions. For example, oceanic and deep-sea sharks have the largest eyes amongst elasmobranchs and presumably rely more heavily on vision than coastal and benthic species, while interspecific variation in the ratio of rod and cone photoreceptors, the topographic distribution of the photoreceptors and retinal ganglion cells in the retina and the spatial resolving power of the eye all appear to be closely related to differences in habitat and lifestyle. Multiple, spectrally distinct cone photoreceptor visual pigments have been found in some batoid species, raising the possibility that at least some elasmobranchs are capable of seeing colour, and there is some evidence that multiple cone visual pigments may also be present in holocephalans. In contrast, sharks appear to have only one cone visual pigment. There is evidence that ontogenetic changes in the visual system, such as changes in the spectral transmission properties of the lens, lens shape, focal ratio, visual pigments and spatial resolving power, allow elasmobranchs to adapt to environmental changes imposed by habitat shifts and niche expansion. There are, however, many aspects of vision in these fishes that are not well understood, particularly in the holocephalans. Therefore, this review also serves to highlight and stimulate new research in areas that still require significant attention.

Parvalbumin-immunoreactive neurons in the inner nuclear layer of zebrafish retina

The purpose of this investigation is to characterize parvalbumin-immunoreactive (IR) neurons in the inner nuclear layer (INL) of zebrafish retina through immunocytochemistry, quantitative analysis, and confocal microscopy. In the INL, parvalbumin-IR neurons were located in the inner marginal portion of the INL. On the basis of dendritic stratification in the inner plexiform layer (IPL), at least two types of amacrine cells were IR for parvalbumin. The first one formed distinctive laminar tiers within s4 (PVs4) of the IPL, and the second within s5 (PVs5). The average number of PVs4 cells was 8263 cells per retina (n=3), and the mean density was 1671cells/mm(2). The average number of PVs5 cells was 1037 cells per retina (n=3), and the mean density was 210cells/mm(2). Quantitatively, 88.9% of anti-parvalbumin labeled neurons were PVs4 cells and 11.1% were PVs5 cells. Their density was highest in the midcentral region of the ventrotemporal retina and lowest in the periphery of the dorsonasal retina. The average regularity index of the PVs4 cell mosaic was 4.09, while the average regularity index of the PVs5 cell mosaic was 3.46. No parvalbumin-IR cells expressed calretinin or disabled-1, markers for AII amacrine cells, in several animals. These results indicate that parvalbumin-IR neurons in zebrafish are limited to specific subpopulations of amacrine cells and the expressional pattern of parvalbumin may not correspond to AII amacrine cells in several other animals. Their distribution suggests that parvalbumin-IR neurons are mainly involved in ON pathway information flow.

Retinotectal ganglion cells in the zebrafish, Danio rerio

The morphology of retinotectal ganglion cells was investigated by retrograde transport of dextran amines applied into the optic tectum in vitro. Based on criteria such as stratification pattern and size of the dendritic processes, as well as the shape and position of the soma within the dendritic field, three main groups of ganglion cell types with a total of nine different types were identified. The first group included monostratified cells, of which two types (Ma(2) and Mb(5)) may be ON- and OFF-variants, and the third (Mb(6)) had its dendritic field as a narrow band at the inner border of the inner plexiform layer. These three cells had the largest dendritic fields, with areas exceeding 40,000 microm(2). In two additional monostratified cells the dendrites were spread over the entire width of either sublamina a or sublamina b of the inner plexiform layer (Ma, Mb). They were of intermediate size with mean dendritic field areas between 10,000 and 20,000 microm(2). The second group contained two types of bistratified cells (Bb(4/5) and Bb(4,5/5,6)) with two distinct bands of dendritic stratifications in sublamina b. One of them had the smallest dendritic field (below 5,000,mm(2)) of all cell types in the sample. The diffuse cells of the third group had their dendrites across the entire width of the inner plexiform layer. The sample of retinotectal cells investigated in this study included types described previously (Mangrum et al. [2002] Vis Neurosci 19:767-779) but also new types not described previously.

Topographic analysis of the ganglion cell layer in the retina of the four-eyed fish Anableps anableps

Fish of the genus Anableps (Anablepidae, Cyprinodontiformes) have eyes that are adapted for simultaneous aerial and aquatic vision. In this study we investigate some of the corresponding retinal specializations of the adult Anableps anableps eye using retinal transverse sections and wholemounts. The linear dimensions of the retina were found to be asymmetric with a greater representation of the dorsal compared to the ventral visual field. The total number of neurons in the ganglion cell layer of the ventral hemiretina was on average 3.6 times greater than the values obtained in the dorsal hemiretina. Isodensity contour maps revealed a prominent horizontal visual streak in the ventral hemiretina with an average peak cell density of 18,286 cells/mm(2). A second less-well-developed horizontal visual streak was also observed in the dorsal hemiretina. A sub-population of large cells with soma areas between 74 and 188 microm(2) was identified and found to be distributed evenly across both hemiretinas. Together, these results show that the sampling gain of the ventral retina is significantly greater than the dorsal segment, that retinal specializations important for mediating acute vision are present in the parts of the visual field immediately above and below the surface of the water, and that visual functions related with the large ganglion cells require more even sampling across the visual field. The relevance of these retinal specializations to the feeding and other behavioral strategies adopted by Anableps is discussed.

Dimensional differences among the groups of retinal ganglion cells according to the retinal zones in chicks

The populations of retinal ganglion cell (RGC) groups (Groups I, II, III, IV) were similar each other between the central and intermediate zones, but the population in the peripheral zone were clearly different from those in the central and intermediate zones due to increase of Group III and IV cells and decrease of Group I cells. The dimensions of somal area and dendritic field of Group I cells increased very gradually toward the peripheral zone, but those of other three Groups grew steeply in the peripheral zone. The correlation index between somal area and dendritic field of RGCs showed high coefficient in the central (r=0.73) and intermediate (r=0.77) zones, but lowered clearly in the peripheral zone (r=0.64) due to increase of Group III cells, which showed nonlinear relation between somal area and dendritic field.

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.

Morphological features of chick retinal ganglion cells

On average, in chicks, the total number of retinal ganglion cells is 4.9 x 10(6) and the cell density is 10400 cells/mm2. Two high-density areas, namely the central area (CA) and the dorsal area (DA), are located in the central and dorsal retinas, respectively, in post-hatching day 8 (P8) chicks (19000 cells/mm2 in the CA; 12800 cells/mm2 in the DA). Thirty percent of total cells in the ganglion cell layer are resistant to axotomy of the optic nerve. The distribution of the axotomy resistant cells shows two high-density areas in the central and dorsal retinas, corresponding to the CA (5800 cells/mm2) and DA (3200 cells/mm2). The number of presumptive ganglion cells in P8 chicks is estimated to be 4 x 10(6) (8600 cells/mm2 on average) and the density is 13500 and 10200 cells/mm2 in the CA and DA, respectively, and 4300 cell/mm2 in the temporal periphery (TP). The somal area of presumptive ganglion cells is small in the CA and DA (mean (+/- SD) 35.7 +/- 9.1 and 40.0 +/- 11.3 microm2, respectively) and their size increases towards the periphery (63.4 +/- 29.7 microm2 in the TP), accompanied by a decrease in cell density. Chick ganglion cells are classified according to dendritic field, somal size and branching density of the dendrites as follows: group Ic, Is, IIc, IIs, Ills, IVc. The density of branching points of dendrites is approximately 10-fold higher in the complex type (c) than in the simple type (s) in each group. The chick inner plexiform layer is divided into eight sublayers according to the dendritic strata of retinal ganglion cells and 26 stratification patterns are discriminated.

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.

Morphologic analysis and classification of ganglion cells of the chick retina by intracellular injection of lucifer yellow and retrograde labeling with DiI

Retinal ganglion cells (RGCs) of chicks were labeled by using the techniques of intracellular filling with Lucifer Yellow and retrograde axonal labeling with carbocyanine dye (DiI). Labeled RGCs were morphologically analyzed and classified into four major groups: Group I cells (57.1%) with a small somal area (77.5 microm(2) on average) and narrow dendritic field (17,160 microm(2) on average), Group II cells (28%) with a middle-sized somal area (186 microm(2)) and middle-sized dendritic field (48,800 microm(2)), Group III cells (9.9%) with a middle-sized somal area (203 microm(2)) and wide dendritic field (114,000 microm(2)), and Group IV cells (5%) with a large somal area (399 microm(2)) and wide dendritic field (117,000 microm(2)). Of the four groups, Groups I and II were further subdivided into two types, simple and complex, on the basis of dendritic arborization: Groups Is, Ic, and Groups IIs, IIc. However, Group III and IV showed either a simple or complex type, Group IIIs and Group IVc, respectively. The density of branching points of dendrites was approximately 10 times higher in the complex types (18,350, 6,190, and 3,520 points/mm(2) in Group Ic, IIc, and IVc, respectively) than in the simple types (1,890, 640, and 480 points/mm(2) in Group Is, IIs, and IIIs). The branching density of Group I cells was extremely high in the central zone. The chick inner plexiform layer was divided into eight sublayers by dendritic strata of RGCs and 26 stratification patterns were discriminated. The central and peripheral retinal zones were characterized by branching density of dendrites and composition of RGC groups, respectively.

The eyes of suckermouth armoured catfish (Loricariidae, subfamily Hypostomus): pupil response, lenticular longitudinal spherical aberration and retinal topography

The dilated, round pupils of a species of suckermouth armoured catfish(Liposarcus pardalis) constrict slowly on illumination (over 35-40 min) to form crescent-shaped apertures. Ray tracing of He—Ne laser beams shows that the lenses of a related species (Pterygoplichthys etentaculus), which also has a crescent-shaped pupil, are well corrected for longitudinal spherical aberration, suggesting that the primary purpose of the irregular pupil in armoured catfish is not to correct such aberration. It is suggested that the iris operculum may serve to camouflage the pupil of these substrate-dwelling species. An examination of the catfish retina shows the photoreceptors to be exclusively single cones interspersed with elongate rods and demonstrates the presence of multiple optic nerve head papillae. Two areas of high ganglion cell density, each side of a vertically oriented falciform process, provide increased spatial resolving power along the axes examining the substrate in front of and behind the animal.

Retinal topography of ganglion cells and putative UV-sensitive cones in two Antarctic fishes: Pagothenia borchgrevinki and Trematomus bernacchii (Nototheniidae)

Accessory corner cones (ACC) have recently been suggested to be UV-sensitive photoreceptor cells. With a view toward explaining prey detection, we examined the topography of retinal ganglion cells and ACCs in two Antarctic nototheniids occupying different ecological niches: the cryopelagic Pagothenia borchgrevinki and the benthic Trematomus bernacchii. Isodensity maps of retinal ganglion cells showed that the main visual axis, coincident with the feeding vector, was in a forward direction in both species. Visual acuity was determined as 3.64 and 4.77 cycles/degree for the respective species. In P. borchgrevinki the highest density of ACCs was associated with the eye's main visual axis. This suggested that this species uses UV-vision during forward-swims and probably in encounters with prey. On the other hand, T. bernacchii possessed two horizontal band-shaped high-density areas of ACCs, which stretched from temporal to nasal and ventral to peripheral retinal regions. Therefore, this species appears to use UV-vision to watch prey across the entire circumference of the lateral area and in the water column above its head.

Guanine-type retinal tapetum and ganglion cell topography in the retina of a carangid fish, Kaiwarinus equula

A guanine-type retinal tapetum was recorded in the eyes of a carangid fish Kaiwarinus equula (= Carangoides equula), spectrophotometric evidence of such being presented. The total amount of guanine in one eye was about 6.5 mg, the guanine density being ca. 1.3 mg cm(-2) over the retinal surface area. To examine the guanine distribution within the retina, the latter was divided into 21 regions. An area of high guanine density (more than 2.0 mg cm(-2)) was observed in the dorsal fundus of the retina, suggesting that the most sensitive vision was checked downward. Using whole-mount retinal preparations, the distribution of Nissl-stained cells within the retinal ganglion cell layer was examined. The greatest cell density area (area centralis) was observed only in the temporal retina. The visual acuity of the area centralis was 4.3 cycles deg(-1), suggesting that high resolution and binocular vision were directed frontally in this species. The eyes of a related carangid (Pseudocaranx dentex), lacking a tapetum, were also examined for comparison. The possible ecological advantage resulting from the tapetum is discussed in terms of visual threshold.

Temporal and mosaic distribution of large ganglion cells in the retina of a daggertooth aulopiform deep-sea fish (Anotopterus pharao)

The daggertooth Anotopterus pharao (Aulopiformes: Anotopteridae) is a large, piscivorous predator that lives within the epipelagic zone at night. In this species, the distribution of retinal ganglion cells has been examined. An isodensity contour map of ganglion cells shows that the cells concentrate in a slightly ventral region of the temporal retina. The region of high ganglion cell density contains 4.07 x 10(3) cells mm(-2), and the resulting visual acuity is 3.5 cycles deg(-1). Outside the area centralis, conspicuously large ganglion cells (LGCs) are observed in the temporal margin of the retina. The LGCs are regularly arrayed, and displaced into the inner plexiform layer. Thick dendrites extend into the outer part (sublamina a) of the inner plexiform layer. In the retinal whole mount, the total number of LGCs is 1590 (90.7 cm specimen), and the mean size of the LGCs is about four times larger than that of the ordinary ganglion cells. The morphological appearance of the LGCs was similar to the off-type alpha cells of the cat retina. The function of these distinctive LGCs is discussed in relation to specific head-up feeding behaviour.

The eyes of deep-sea fish. II. Functional morphology of the retina

Three different aspects of the morphological organisation of deep-sea fish retinae are reviewed: First, questions of general cell biological relevance are addressed with respect to the development and proliferation patterns of photoreceptors, and problems associated with the growth of multibank retinae, and with outer segment renewal are discussed in situations where there is no direct contact between the retinal pigment epithelium and the tips of rod outer segments. The second part deals with the neural portion of the deep-sea fish retina. Cell densities are greatly reduced, yet neurohistochemistry demonstrates that all major neurotransmitters and neuropeptides found in other vertebrate retinae are also present in deep-sea fish. Quantitatively, convergence rates in unspecialised parts of the retina are similar to those in nocturnal mammals. The differentiation of horizontal cells makes it unlikely that species with more than a single visual pigment are capable of colour vision. In the third part, the diversity of deep-sea fish retinae is highlighted. Based on the topography of ganglion cells, species are identified with areae or foveae located in various parts of the retina, giving them a greatly improved spatial resolving power in specific parts of their visual fields. The highest degree of specialisation is found in tubular eyes. This is demonstrated in a case study of the scopelarchid retina, where as many as seven regions with different degrees of differentiation can be distinguished, ranging from an area giganto cellularis, regions with grouped rods to retinal diverticulum.

Morphology of the brain and sense organs in the snailfish Paraliparis devriesi: neural convergence and sensory compensation on the Antarctic shelf

The Antarctic snailfish Paraliparis devriesi (Liparidae) is an epibenthic species, inhabiting depths of 500-650 m in McMurdo Sound. Liparids are the most speciose fish family in the Antarctic Region. We examine the gross morphology and histology of the sense organs and brain of P. devriesi and provide a phyletic perspective by comparing this morphology to that of four scorpaeniforms and of sympatric perciform notothenioids. The brain has numerous derived features, including well-developed olfactory lamellae with thick epithelia, large olfactory nerves and bulbs, and large telencephalic lobes. The retina contains only rods and exhibits a high convergence ratio (82:1). Optic nerves are small and nonpleated. The tectum is small. The corpus of the cerebellum is large, whereas the valvula is vestigial. The rhombencephalon and bulbospinal junction are extended and feature expanded vagal and spinal sensory lobes as well as hypertrophied dorsal horns and funiculi in the rostral spinal cord. The lower lobes of the pectoral fins have taste buds and expanded somatosensory innervation. Although the cephalic lateral line and anterior lateral line nerve are well developed, the trunk lateral line and posterior lateral line nerve are reduced. Near-field mechanoreception by trunk neuromasts may have been compromised by the watery, gelatinous subdermal extracellular matrix employed as a buoyancy mechanism. The expanded somatosensory input to the pectoral fin may compensate for the reduction in the trunk lateral line. The brains of P. devriesi and sympatric notothenioids share well-developed olfactory systems, an enlarged preoptic-hypophyseal axis, and subependymal expansions. Although the functional significance is unknown, the latter two features are correlated with habitation of the deep subzero waters of the Antarctic shelf.

Large retinal ganglion cells in the channel catfish (Ictalurus punctatus): three types with distinct dendritic stratification patterns form similar but independent mosaics

Retinal ganglion cells in the channel catfish (Ictalurus punctatus) were retrogradely labelled, and those with the largest somata and thickest primary dendrites were categorized by their levels of dendritic stratification. Three types were found, each forming a mosaic making up approximately 1% of the ganglion cell population. Using a system based on established sublaminar terminology, we call these the alpha-a (alpha a), alpha-b (alpha b), and alpha-c (alpha c) ganglion cell mosaics. Cells of the alpha a mosaic had large, sparsely branched trees in sublamina a at 10-30% of the depth of the inner plexiform layer (IPL), sclerad to those of all other large ganglion cells. Some alpha a somata were displaced into the IPL or inner nuclear layer (INL) but belonged to the same mosaic as their orthotopic counterparts. Cells of the alpha b mosaic had dendrites that branched a little more and arborized in sublamina b at 50-60% of the IPL depth. Many also sent fine branches into sublamina a, and some were fully bistratified in a and b. The alpha c cells arborized in the most vitread sublamina, sublamina c, at 80-95% of the IPL depth. The soma areas of the three types in the largest retina studied ranged between 139 microns 2 and 670 microns 2 with significant differences in the order alpha a > alpha c > or = alpha b. Analyses based on nearest-neighbour distance (NND) and on spatial auto- and cross-correlograms showed that each mosaic was statistically regular and independent of the others. Mosaic spacings were similar for each type, giving mean NNDs of 242-279 microns in the largest retina and 153-159 microns in a smaller one. Correspondences between these mosaics, previously defined large ganglion cell types in catfish, and other mosaic-forming large ganglion cells in fish and frogs are discussed along with their implications for neuronal classification, function, development, and evolution.

The diencephalon of the Pacific herring, Clupea harengus: retinofugal projections to the diencephalon and optic tectum

The pattern of retinofugal projections to nuclei in the diencephalon and to the optic tectum was analyzed with horseradish peroxidase and autoradiographic methods in Clupea harengus, a clupeomorph teleost, for comparison with osteoglossomorph, elopomorph, and euteleost teleosts and with non-teleost actinopterygians. Most retinal fibers decussate in the optic chiasm and project to nuclei in the preoptic area, ventral and dorsal thalamus, posterior tuberculum, synencephalon, and pretectum, as well as to the accessory optic nuclei and optic tectum. Some ipsilateral projections do not decussate in the optic chiasm, while others decussate and recross via the supraoptic (minor) and posterior commissures. The pattern of projections is similar to that seen in other actinopterygian fishes with several exceptions. The terminal field usually present lateral to nucleus anterior in the dorsal thalamus is extremely reduced despite the relatively large size of the nucleus. A dense terminal field lies within the cell plate of nucleus corticalis in the pretectum rather than dorsal to it. The tectal hemisphere is composed of two distinct lobules, and the dorsal optic tract projects to the more rostromedial lobule while the ventral optic tract projects to the more caudolateral lobule. The lack of a significant projection to nucleus anterior and the lobular morphology of the optic tectum appear to be apomorphic for Clupea. Other features of the pattern of retinal projections are also analyzed in actinopterygian fishes including Clupea, and several hypotheses are advanced as to which traits are plesiomorphic for actinopterygians and/or for teleosts.

Retinal morphology of cyprinid fishes: a quantitative histological study of ontogenetic changes and interspecific variation

Morphological patterns of the retina, cone size and density, rod density, rod-cone ratio, ganglion cell density, convergence of receptor cells, resolving power (RP) and regionalization were examined throughout life history in roach and in adults of asp, bream, common carp, roach and sabre carp. The retina of hatchlings is packed with small cones. During larval and juvenile growth the retina stretches, cones increase in diameter and rods are present in increasing numbers. Photopic and scotopic sensitivity as well as resolving power increase. Comparison of adults shows distinct interspecific differences in retinal parameters, which can be related to life style.

A correlative study of the physiology and morphology of the retinotectal pathway of the perch

The anatomy and physiology of the retinotectal pathway of the perch was investigated using physiological and histological techniques. Massed responses of the optic nerve to single shocks exhibited five distinct peaks. Single-unit responses to shocks indicate two groups of fast fibers correlating well with peaks I and II of the massed response. The flash-evoked response in nerve and tectum has three major phases (PSPI-III), with a marked low-threshold fast component. Patterns of flash-evoked response from single fibers vary, but the responses of fast transient fibers coincide with the timing of PSPI, and longer latency groups with PSPII-III. Units reflexly activated by efferents were also seen, and 12% of units were photically inexcitable. Surprisingly, few fibers responded well to a scanned spot light, unlike tectal cells, and receptive fields were often large (greater than 70 deg). ON/OFF responses, evoked either by whole field or local illumination, were much commoner than pure ON or OFF responses. Effects of electrical stimulation or cautery of the tectum on the flash-evoked response of fiber bundles, via the efferents were marginal, but repetitive stimulation or section of the optic nerve produced clear-cut deficits in the slow components of the flash-evoked response of the nerve. Stimulation of the eighth nerve produced a complex long-latency, large-amplitude response in the optic nerve. The fiber spectrum of the optic nerve taken from electron micrographs revealed the presence of a relatively small group (less than 1%) of thick fibers with diameters between 3 microns and 10 microns that could be correlated with fast responses recorded from the optic nerve, and the remainder with axon diameters down to 0.2 microns providing the slow responses. The distribution of cell-body diameters from sectioned and wholemount material indicated a marked distinction between small and large ganglion cells. The total number of fibers in the nerve was estimated 868,840.

Immunohistochemical subpopulations of retinopetal neurons in the nucleus olfactoretinalis in a teleost, the whitespotted greenling (Hexagrammos stelleri)

Two subpopulations of retinopetal neurons in the nucleus olfactoretinalis (NOR) were revealed by means of general histological, tract tracing, and immunohistochemical methods in a marine teleost, the whitespotted greenling (Hexagrammos stelleri). Two types of cells in the NOR (M cell, L cell) were morphologically distinguishable from one another. The cell bodies of M cells were medium-size and fusiform, while those of L cells were large and ellipsoidal or irregular. M cells were located at the ventralmost region of the junction between the olfactory bulb and telencephalon, while L cells were scattered more rostrally along the ventromedial surface of the olfactory bulb. Following applications of HRP or Fluoro-Gold to the optic nerve, almost all M cells and about half of the L cells were labeled in the contralateral NOR. L cells showed strong immunoreactivities to gonadotropin releasing hormone (GnRH) and Phe-Met-Arg-Phe-NH2 (FMRFamide), whereas M cells showed only weak GnRH-immunoreactivity and no FMRFamide-immunoreactivity. Axons of L cells were detected in the optic nerve by means of GnRH- and FMRFamide-immunohistochemistry. The axons were concentrated in the myelinated edge (the oldest area) of the "ribbon" optic nerve. GnRH- and FMRFamide-immunoreactive fibers constituted a plexus at the junction between the inner nuclear layer and inner plexiform layer. The existence of two structurally separate classes of NOR cells suggests two functionally separate channels from the NOR to the retina.

Centrifugal pathways to the retina: influence of the optic tectum

Two types of centrifugal pathways to the retina have been found in the vertebrates, according to the location of the cell bodies and presence or absence of connections with the optic tectum. One type is represented by the isthmo-optic nucleus (ION) of birds and, therefore, termed "ION-type" retinopetal system. The other type is termed "non-ION-type" retinopetal system. The ION-type retinopetal systems have been found in the cyclostomes, teleosts, reptiles, and birds. This review describes the anatomy and physiology of the ION-type retinopetal systems, mainly of birds and teleosts. On the basis of anatomical and physiological evidence cited in this review, the ION-type retinopetal systems can be regarded as the tectofugal pathways to the retina. The function of the ION-type retinopetal systems is discussed in detail, with special emphasis on their relation to the role of the tectum in mediating visuomotor behavior.

Topography and morphology of retinal ganglion cells in the coral trout Plectropoma leopardus (Serranidae): a retrograde cobaltous-lysine study

The retinal topography of the adult coral trout Plectropoma leopardus (Serranidae, Perciformes) is examined in Nissl-stained material and confirmed by means of retrograde labelling with cobalt-lysine from the optic nerve. Concentric isodensity contours surround a temporoventral area centralis of over 1.5 x 10(4) cells per mm2 which lies ventral of the elongated optic nerve head. Peripheral densities of ganglion cells fall to less than 0.25 x 10(4) cells per mm2. A total of 1.17 x 10(5) ganglion cells is found over the entire retina. Silver intensifications of cobalt-lysine-stained cells allow a detailed analysis of ganglion cell morphology. Soma and dendritic field size increase with eccentricity, and within the confines of the area centralis only the smallest cell size is found. Dendritic arbors of peripheral ganglion cells are all oriented parallel to the retinal margin with an axon lying close to the vitreous humor. Nine classes of cells are characterized on the criteria of soma size, dendritic field size, primary and secondary branching patterns, soma position, and dendritic termination areas. Possible morphological homologies with ganglion cells of other teleosts are also discussed.

Quantitative electrophysiological studies of regenerating visuotopic maps in goldfish--II. Delayed recovery of sensitivity to small light flashes

The regenerating retinotectal projection of goldfish was mapped with punctate flashes of light produced by red light-emitting diodes. The characteristics of multiunit receptive fields were studied in fish kept at 25 degrees C at different times after unilateral optic nerve crush. From about 20 days, when the first visually-evoked responses to black-on-white stimuli appeared, until about 40 days, no consistent responses to light-emitting diodes could be obtained, although high-contrast, long-duration light-emitting diode stimuli elicited weak off-responding. At around 40 days, responses to light-emitting diodes reappeared as the amplitude of evoked multiunit activity increased sharply. At their emergence, light-emitting diode-sensitive multiunit receptive fields were irregular and only slightly enlarged, but quickly regained normal shape and size. Conformity to a linear and uniform visuotopography recovered more slowly and, in some individuals, incompletely. The results suggest that "on" and "off" optic fiber systems, probably with small terminal arbors, are functionally expressed at a later time in regeneration than the predominantly "off" system manifested earlier. The different time courses of recovery in these systems explain several aspects of the recovery of visual behavior during optic nerve regeneration.

Topographic analysis of the retinal ganglion cell layer and optic nerve in the sandlance Limnichthyes fasciatus (Creeiidae, Perciformes)

The sandlance or tommy fish Limnichthyes fasciatus (Creeiidae, Perciformes) is a tiny species that lives beneath the sand with only its eyes protruding and is found throughout the Indopacific region. The retina of the sandlance possesses a deep convexiclivate fovea in the central fundus of its minute eye (1.04 mm in diameter). A Nissl-stained retinal whole mount in which the pigment epithelium had been removed by osmotic shock was used to examine the retinal topography of the ganglion cell layer. There was a foveal density of between 13.0 x 10(4) cells per mm2 (S.D. +/- 1.8 x 10(4) cells per mm2), counted in the retinal whole mount, and 15.0 x 10(4) cells per mm2, counted in transverse sections, which diminished to a peripheral density of 4.5 x 10(4) cells per mm2 (S.D. +/- 0.8 x 10(4) cells per mm2). The total population of axons within the optic nerve was assessed by electron microscopy. Optic axon densities ranged from 2 x 10(6) axons per mm2 in the caudal apex to over 16 x 10(6) axons per mm2 within a specialized region of unmyelinated axons in the rostral apex. The topography of the proportion of unmyelinated axon population (26%) follows closely that of the total population of optic nerve axons. There was a total of 104,452 axons within the optic nerve compared with 102,918 cells within the retinal ganglion cell layer. A close relationship is revealed between ganglion cell soma areas and axon areas where the organization in the optic nerve and retina may reflect some functional retinotopicity.

Retinal ganglion cell topography in teleosts: a comparison between Nissl-stained material and retrograde labelling from the optic nerve

The retinal topography of cells within the ganglion cell layer of three teleost species is examined in Nissl-stained material in which all neuronal elements containing Nissl substance in the cytoplasm are counted. A topographic comparison is made with retrogradely labelled ganglion cells to differentiate the proportion of nonganglion cells not possessing an axon joining the optic nerve. In the three species studied 92%, 80%, and 66% were found to be the maximum proportion of true ganglion cells in the area centralis, horizontal streak, and periphery, respectively. The proportion of nonganglion cells in the total population of cells counted was 24%. The major contribution to this discrepancy is from peripheral nonspecialized regions of the retina. There is little difference in both topography and peak densities of retinal ganglion cells between the two techniques. The soma areas of both populations are analysed, with the homogeneous nonganglion cell population possessing cells between 5 and 15 micron2 and the heterogeneous ganglion cell soma between 5 and 68 micron2, increasing in size with eccentricity.

Asymmetric distribution of retinal ganglion cells in goldfish

The distribution of retinal ganglion cells (RGCs) in goldfish was determined by removing an eye and applying cobaltous-lysine to the optic nerve for 24 hr. This procedure allowed the cobalt label to be in continuous contact with the cut ends of the optic axons and thereby backfilled many RGCs. RGC density was determined across three different sizes of retinae by using fish with different eye sizes. Confirming earlier work, we found that RGC density diminished as retinal area increased. However, irrespective of the retinal size, the density of RGCs was elevated along the temporal boundary between the dorsal and the ventral retina. A conservative estimate indicated that the RGC density in the temporal retina was at least 1.8-2.5 times higher than the mean RGC density of the entire retina. Thus, the goldfish retina does not appear to have a homogeneous distribution of RGCs as was previously considered. Small and large retinae differed with respect to the percentage of cells in the RGC layer that was RGCs. In small retinae, even when the noncobalt-filled cells (glia and displaced amacrine cells) were added to the cobalt-filled RGCs, the density of all cell types was elevated in the temporal retina relative to the remainder of the retina. Furthermore, in small retinae, the percentage of cells in the RGC layer that was RGCs (75%) was constant across the radial and circumferential aspects of the retina. In marked contrast, in medium-large retinae, a homogeneous distribution of cells across the entire retina resulted when the noncobalt-filled cells were added to the cobalt-filled cells. However, the percentage of cells that was cobalt-filled RGCs was significantly greater in the temporal retina (50%) than in the remainder of the retina (35%). In large retinae, as in small retinae, the percentage of cells that was RGCs did not vary as a function of distance from the optic disc. These data suggest that, in the course of retinal maturation, cell density in the temporal retina is elevated initially and then declines subsequently to the level of the surrounding retina. Over time, more displaced to the level of the surrounding retina. Over time, more displaced amacrine cells may be added to the tissue surrounding the temporal retina. Alternatively, more RGCs outside the temporal retina may become displaced amacrine cells. Such events could account for the growth-associated, disproportionate decrease in the percentage of cells that is RGCs in the tissue surrounding the temporal retina.(ABSTRACT TRUNCATED AT 400 WORDS)

Visual system of the channel catfish (Ictalurus punctatus): II. The morphology associated with the multiple optic papillae and retinal ganglion cell distribution

The retinal organization associated with the multiple optic papillae of the catfish Ictalurus punctatus was examined. In each retina from ten to 17 papillae form an oval ring (which is wider dorsoventrally than mediolaterally). The dorsalmost papilla in this ring lies at the center of the retina. In addition, up to seven small papillae lie within the ring. Bundles of fibers leave the neural retina via the papillae. These bundles remain separate as they pass through the distal portions of the neural retina and then merge before passing through the choroid. Bundles running through dorsal papillae receive fibers from a roughly wedge-shaped retinal area; bundles running through ventral papillae receive fibers from a small area of central retina and a disproportionately large area of peripheral retina. A band of high ganglion cell density was observed extending between the nasal and temporal poles of the retina. No correlation was found between the retinal areas contributing fibers to the bundles of axons emerging from individual papillae and the areas of high cell density. Furthermore, no correlation was found between the average area of the retinal ganglion cells and the ganglion cell density. From HRP preparations and axon counts we estimate that each retina of 95-mm catfish contains about 50,000 ganglion cells.

Distribution and morphology of retinal ganglion cells in the Japanese quail

A ganglion cell density map was produced from the Nissl-stained retinal whole mount of the Japanese quail. Ganglion cell density diminished nearly concentrically from the central area toward the retinal periphery. The mean soma area of ganglion cells in isodensity zones increased as the cell density decreased. The histograms of soma areas in each zone indicated that a population of small-sized ganglion cells persists into the peripheral retina. The total number of ganglion cells was estimated at about 2.0 million. Electron microscopic examination of the optic nerve revealed thin unmyelinated axons to comprise 69% of the total fiber count (about 2.0 million). Since there was no discrepancy between both the total numbers of neurons in the ganglion cell layer and optic nerve fibers, it is inferred that displaced amacrine cells are few, if any. The spectrum in optic nerve fiber diameter showed a unimodal skewed distribution quite similar to the histogram of soma areas of ganglion cells in the whole retina. This suggests a close correlation between soma areas and axon diameters. Retinal ganglion cells filled from the optic nerve with horseradish peroxidase were classified into 7 types according to such morphological characteristics as size, shape and location of the soma, as well as dendritic arborization pattern. Taking into account areal ranges of somata of each cell type, it can be assumed that most of the ganglion cells in the whole retinal ganglion cell layer are composed of type I, II and III cells, and that the population of uniformly small-sized ganglion cells corresponds to type I cells and is an origin of unmyelinated axons in the optic nerve.

Cytoarchitecture and fiber connections of the superficial pretectum in a teleost, Navodon modestus

Fiber connections of the so-called nucleus geniculatus lateralis (or the nucleus pretectalis superficialis pars parvocellularis) in a teleost, Navodon modestus, were examined by means of the horseradish peroxidase (HRP) tracing method. The nucleus receives fibers from the contralateral retina, ipsilateral optic tectum and nucleus isthmi, and projects bilaterally to the nucleus intermedius of Brickner and ipsilaterally to the optic tectum and raphe nuclei. The fiber connections suggest that the nucleus relays mainly visual information to the inferior lobe (hypothalamus) but not to the telencephalon. The nucleus is not a homologous structure to the lateral geniculate nucleus in other vertebrate classes.

The visual system of the channel catfish (Ictalurus punctatus). I. Retinal ganglion cell morphology

Horseradish peroxidase was applied to lesions in the optic nerve of catfish (Ictalurus punctatus). The retinae were processed to reveal HRP-labelled ganglion cells. The histochemical techniques employed allowed fine details of the dendritic arbor to be resolved. Flat-mounted retinae were examined and the following characteristics were noted in individual ganglion cells: Soma area, shape, and depth; number and diameter of major dendrites; shape, area, and depth(s) within the inner plexiform layer (ipl) of the dendritic arbor; origin of the axon (from the soma or a dendrite). On the basis of these characteristics, eleven classes of ganglion cells were delineated: four classes of giant cells (G1-G4) and seven classes of smaller cells (S1-S7). G1 cells had dendrites arborizing in the most distal sublamina of the ipl. G1 cells in the dorsal retina had nasotemporally elongated dendritic arbors. G2 cells had dendrites in the proximal portion of the ipl. G3 cells were almost completely confined to a band running between the nasal and temporal retinal poles, through the center of the retina. In this location, the cells had dorsoventrally elongated dendritic arbors, which were bistratified in the ipl. G4 cells were displaced into the inner nuclear layer. S1 and S4 cells had axons arising from their somata, and dendrites arborizing in the distal and the proximal ipl, respectively. S2 cells were typified by their unstratified dendritic arbors. Similarly, S3 cells were characterised by their bistratified arbors. S5 cells arborized in the most proximal ipl sublamina. S6 cells were small ganglion cells with their somata lying in the inner nuclear layer. S7 cells tended to have complex dendritic arbors, and their axons arose from dendrites.

A quantitative study of the relative contribution of different retinal sectors to the innervation of various thalamic and pretectal nuclei in goldfish

The contribution of retinal ganglion cells situated in different retinal quadrants to the innervation of eight nontectal, retinorecipient targets was examined in goldfish. In some fish, cobaltous-lysine was used to selectively fill severed intraretinal ganglion cell axons and the number of filled axons within each nucleus was determined. In other fish, either the dorsal or ventral or nasal or temporal retina was ablated and the remaining axons from the intact retina were filled with cobalt. The density of the cobalt-filled axons within the retinorecipient targets was quantified with a microdensitometer. All of the eight targets received different degrees of innervation when the contributions from dorsal and ventral retina were compared. The suprachiasmatic nucleus received axons from ventral, but not from dorsal, retinal ganglion cells (RGCs), while the nucleus opticus dorsolateralis, nucleus opticus commissurae posterior, and nucleus opticus pretectalis dorsalis received more axons from ventral than from dorsal RGCs. The tuberal region, nucleus corticalis, and the accessory optic nucleus received axons from dorsal, but not from ventral, RGCs. The nucleus opticus pretectalis ventralis received more axons from dorsal then from ventral RGCs. Only one target, nucleus corticalis, appeared to receive more axons from nasal than from temporal RGCs. In general, those nuclei that were closest to the dorsal optic tract were innervated exclusively or predominantly by ventral RGC axons, whereas those nuclei that were closest to the ventral optic tract were innervated exclusively or predominantly by dorsal RGC axons. These data indicate that in this particular vertebrate, the dorsal and ventral retinal regions are not homogeneous with respect to their projections to nontectal nuclei. The possible role that the nontectal nuclei play in determining the course of optic axons is discussed.

The organization of retinal projections to the diencephalon and pretectum in the cichlid fish, Haplochromis burtoni

The organization of retinofugal projections was studied in a cichlid fish by labelling small groups of retinal ganglion cell axons with either horseradish peroxidase or cobaltous lysine. Two major findings resulted from these experiments. First, optic tract axons show a greater degree of pathway diversity than was previously appreciated, and this pathway diversity is related to the target nuclei of groups of axons. The most striking example is the formation of the medial optic tract. Fibers that will become the medial optic tract move abruptly away from their neighbors, at about the level of the optic chiasm, and coalesce at the dorsomedial edge of the marginal optic tract. The medial optic tract projects to the thalamus, the dorsal pretectum, and the deep layer of the optic tectum. The axial optic tract is a group of fibers which segregates from the most medial portion of the marginal optic tract, at about the level of the optic chiasm. The axial tract stays medial to the marginal optic tract for a few hundred microns and then curves laterally to rejoin the marginal optic tract. At least some axial trat axons terminate in the suprachiasmatic nucleus. Within the marginal optic tract, retinal ganglion cell axons from a given retinal quadrant are always segregated into at least two groups. The smaller group projects to the superficial pretectal nucleus. The larger group projects to the superficial layer of the optic tectum. Second, each nontectal retinal termination site receives a unique pattern of retinal input. Within the pretectum the parvocellular superficial pretectal nucleus receives a highly retinotopically organized input from all retinal regions; the basal optic nucleus receives a roughly retinotopically organized input from all retinal regions; the dorsal pretectum receives an input from all retinal regions; and the central pretectal nucleus receives input only from the ventral hemiretina. Within the diencephalon the thalamus receives an input from all retinal regions, but this input is not retinotopically organized; the suprachiasmatic nucleus receives input from the region of central retina that lies just dorsal to the optic nerve head, via the axial optic tract. The accessory optic nucleus receives input from the dorsal hemiretina.