Patterns of rod proliferation in deep-sea fish retinae

Retinal development in mandarinfish Siniperca chuatsi and morphological analysis of the photoreceptor layer

We describe the process of retinal development in mandarinfish Siniperca chuatsi from larvae to young fish. The developmental characteristics of the retinal structure and related cells were identified. Siniperca chuatsi were found to exhibit an altricial mode of retinal development that required considerable time to be completed after hatching. The retina was classed as a pure cone type during the early developmental stage. In the subsequent developmental stages, however, double cones gradually occupied the majority of the cone cells, while rod cells represented the majority of the photoreceptor cells. The outer segment (OS) of the rod cells were significantly longer compared with other morphological features, the OS of the two kinds of cone cells were significantly elongated and the diameters of the inner segment (IS) and OS of the double cone cells were significantly narrower in the later developmental stages. Combined with the scattered arrangement of cone cells at the different stages, the retina was found to have sacrificed a considerable part of visual acuity in the developmental process. The distribution of cone cells was observed to have gradually become regionalised during development. The findings of the present study also indicated that S. chuatsi have a high photosensitivity under dim light conditions as a result of specialised structures of the OS of photoreceptor cells and an increased number of rod cells. The loose arrangement of the cone mosaic presumably resulted in a poor imaging quality and, to some extent, the regionalisation of the cone-cell distribution compensated for the above shortcomings, which would enhance the ability of S. chuatsi to perceive targets in important directions for effective predation behaviour.

The Organization of the Inner Retina in a Pure-Rod Deep-Sea Fish

The pure rod retina of a deep-sea eel species was used as a model system for the study of the differentiation of horizontal, bipolar, amacrine and ganglion cells. We wanted to test the hypothesis that the functional organization of the inner retina is less complex than in species with duplex, rod- and cone-containing retinae. We used immunocytochemistry, backfilling ganglion cells with fluorescent dextranes and microinjection of Lucifer Yellow, to visualize the micromorphology of the various cell types in a confocal microscope. The pure rod retina contains a single type of horizontal cell. The inner plexiform layer is 10-15 microm thick and shows three main sublayers. Bipolar terminals are found in all sublayers, but the majority are found in the inner sublamina b (PKC-immunoreactive cells, that in fish with duplex retinae receive a mixed rod-cone input). The neurochemical diversity of amacrine cells in terms of immunoreactivity does not differ from other teleosts; this similarity includes the pattern of dendritic stratification and ramification as revealed by microinjection. Ten different types of ganglion cells are distinguished based on the sizes of their perikaryon and dendritic field, and the stratification pattern in the inner plexiform layer. This is similar to the situation in catfish with retinae containing a single type of cone in addition to a majority of rods. In this respect, the differences between pure rod retinae and duplex retinae containing a single cone type were less obvious than hypothesized. In the deep-sea eel, the density of dendritic ramification in amacrine and ganglion cells was strongly reduced. This may be functionally related to the fact that vision in the deep sea environment relies exclusively on bioluminescence and is represented by burst-like emissions of point sources. This requires a mode of retinal signal processing that is less complex than in duplex retinae and involves a lower density of dendritic branching and synapses.

Vision in the southern hemisphere lamprey Mordacia mordax: spatial distribution, spectral absorption characteristics, and optical sensitivity of a single class of retinal photoreceptor

The dorso-laterally located eyes of the southern hemisphere lamprey Mordacia mordax (Agnatha) contain a single morphological type of retinal photoreceptor, which possesses ultrastructural characteristics of both rods and cones. This photoreceptor has a large refractile ellipsosome in the inner segment and a long cylindrical outer segment surrounded by a retinal pigment epithelium that contains two types of tapetal reflectors. The photoreceptors form a hexagonal array and attain their peak density (33,200 receptors/mm2) in the ventro-temporal retina. Using the size and spacing of the photoreceptors and direct measures of aperture size and eye dimensions, the peak spatial resolving power and optical sensitivity are estimated to be 1.7 cycles deg-1 (minimum separable angle of 34'7'') and 0.64 microm2 steradian (white light) and 1.38 microm2 steradian (preferred wavelength or lambdamax), respectively. Microspectrophotometry reveals that the visual pigment located within the outer segment is a rhodopsin with a wavelength of maximum absorbance (lambdamax) at 514 nm. The ellipsosome has very low absorptance (<0.05) across the measured spectrum (350-750 nm) and probably does not act as a spectral filter. In contrast to all other lampreys studied, the optimized receptor packing, the large width of the ellipsosome-bearing inner segment, together with the presence of a retinal tapetum in the photophobic Mordacia, all represent adaptations for low light vision and optimizing photon capture.

Timing and location of rhodopsin expression in newly born rod photoreceptors in the adult teleost retina

Labeling of newly divided retinal cells with bromodeoxyuridine (BrdU) and a rhodopsin mRNA probe revealed that rhodopsin is first expressed by new rod photoreceptors 2 days after cell birth in an adult cichlid fish. Most new cells that expressed rhodopsin had nuclei located in the vitreal half of the outer nuclear layer (ONL), lending further support to the hypothesis that movement from scleral to vitreal ONL is associated with rod differentiation.

Fine structure and development of the retina of the grenadier anchovy Coilia nasus (Engraulididae, Clupeiformes)

A study of the morphogenesis of the grenadier anchovy retina was undertaken using light and electron microscopy. Five developmental stages from prelarvae 3 days after fertilization to adult fish were studied. In addition to the general morphology of the eye and retina, special emphasis was given to the development of the photoreceptors and pigment epithelium (PE). The earliest retinae showing structural features indicative of a functioning eye are pure cone retinae composed of rows of alternating long and short cones forming a transient, tesselated pattern. At this stage there is a conventional PE containing melanin. In older stages cone rows are separated by the newly formed rods and by PE wedges filled with diffusely reflecting guanine crystallites. The findings are compared with the retinae of other engraulidids and with the development of teleost retinae in general. Moreover, the observed structural changes are discussed with respect to the photic habitat conditions of these anadromous fish that move between coastal waters, estuary, and river.

Immunochemical localization of calretinin in the retina of the turbot (Psetta maxima) during development

The expression of the calcium-binding protein calretinin was analysed by immunohistochemistry techniques in the retina of turbot (Psetta maxima) from embryonic to juvenile stages. Calretinin immunoreactivity was first detected in retinae from newly hatched larvae, in which the anlage of the inner plexiform layer and a subset of amacrine and ganglion cells displayed a faint immunolabelling. First appearance of photoreceptors during larval life coincided with an increase in the intensity of the labelling. During subsequent larval development, the expression of calretinin affected distinctive retinal components. The inner plexiform layer, optic fiber layer, and a population of amacrine and ganglion cells were invariably labelled. Occasional bipolar cells were labelled at the end of the larval period. By metamorphosis, calretinin is sequentially expressed in horizontal cells, and bipolar immunoreactive cells become numerous. The pattern of calretinin immunoreactivity of the inner plexiform layer changes from the larval to juvenile period. In all cases, calretinin immunoreactivity exhibited variations between the peripheral retina, which contains the most recently differentiated retinal components, and the remainder of the differentiated retina. Our results suggest that the progressive expression of calretinin in the turbot retina appears associated with some degree of neuronal differentiation. Once the definitive pattern of calretinin immunoreactivity is established in the turbot retina, both similarities and differences with the calretinin location in the retina of other vertebrates can be demonstrated.

The eyes of deep-sea fish. I: Lens pigmentation, tapeta and visual pigments

Deep-sea fish, defined as those living below 200 m, inhabit a most unusual photic environment, being exposed to two sources of visible radiation; very dim downwelling sunlight and bioluminescence, both of which are, in most cases, maximal at wavelengths around 450-500 nm. This paper summarises the reflective properties of the ocular tapeta often found in these animals, the pigmentation of their lenses and the absorption characteristics of their visual pigments. Deep-sea tapeta usually appear blue to the human observer, reflecting mainly shortwave radiation. However, reflection in other parts of the spectrum is not uncommon and uneven tapetal distribution across the retina is widespread. Perhaps surprisingly, given the fact that they live in a photon limited environment, the lenses of some deep-sea teleosts are bright yellow, absorbing much of the shortwave part of the spectrum. Such lenses contain a variety of biochemically distinct pigments which most likely serve to enhance the visibility of bioluminescent signals. Of the 195 different visual pigments characterised by either detergent extract or microspectrophotometry in the retinae of deep-sea fishes, ca. 87% have peak absorbances within the range 468-494 nm. Modelling shows that this is most likely an adaptation for the detection of bioluminescence. Around 13% of deep-sea fish have retinae containing more than one visual pigment. Of these, we highlight three genera of stomiid dragonfishes, which uniquely produce far red bioluminescence from suborbital photophores. Using a combination of longwave-shifted visual pigments and in one species (Malacosteus niger) a chlorophyll-related photosensitizer, these fish have evolved extreme red sensitivity enabling them to see their own bioluminescence and giving them a private spectral waveband invisible to other inhabitants of the deep-ocean.

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.

Density ratio of dopaminergic versus serotonergic cells correlates with cone-to-rod ratio in teleost retinas

Dopaminergic and serotonergic cells were visualised immunohistochemically with antibodies against tyrosine hydroxylase and serotonin in retinal wholemounts of eight teleosts from different habitats and with different rod-to-cone ratios. The cell densities were calculated, and the density ratio of dopaminergic cells versus serotonergic cells was compared among these fish species. The density ratio was high (1.9-2.7) in three out of the four species of cichlid fish studied with cone densities roughly equalling rod densities, medium in roach (0.8) where rods dominate cone numbers, and low in deep-sea fish (0.2-0.4) with pure rod retinas. These observations confirm earlier findings on the species-specificity of the ratio of dopaminergic versus serotonergic cells in the retina and further demonstrate a close correlation between the rod-to-cone ratio, and the density ratio of dopaminergic versus serotonergic cells in the inner retina. The possible significance for the processing of photopic and scotopic information in the inner retina is discussed.

Rod outer segment renewal in the retinae of deep-sea fish

Outer segment renewal involves the synthesis of disc material in the photoreceptor inner segments, the shedding of the tips of the photoreceptor outer segments, and their phagocytosis by the retinal pigment epithelial cells. It has been suggested that in the retinae of deep-sea fish no renewal of outer segments may take place. In order to assess outer segment renewal in deep-sea fish retinae we counted (i) periciliary vesicles in rod inner segments as a parameter for disc-synthesis activity and (ii) phagosomes in retinal pigment epithelial cells as a parameter of shedding and phagocytosis in 12 species of deep-sea fish with multibank or single bank retinae. We also measured the lengths of rod outer segments in order to evaluate the balance between synthesis and phagocytotic activity. In four of these species (Synaphobranchus kaupi, Nematonurus armatus, Coryphaenoides guentheri and Halosauropsis macrochir) we further recorded size-related changes of these parameters and their relation to the position of a given rod within the banks in the retina. The number of periciliary vesicles was highest in inner segments of the most vitread bank and in the periphery of the retina. Phagosomes were most abundant in retinal pigment epithelial cells of the central retina. Long rod outer segments were most frequently recorded in the peripheral retina indicating that in this region new synthesis may outbalance shedding. Vitread rod outer segments were only slightly longer than sclerad ones. Larger animals had shorter rod outer segments than small ones. We present evidence that rod outer segment renewal takes place in the retina of all deep-sea fish. Vitread rods may be more active in this respect than sclerad ones.