Photoreceptor cells

PurposeMelanolipofuscin (MLF) is a complex granule, exhibiting properties of both melanosomes and lipofuscin (LF) granules, which accumulates in retinal pigment epithelial (RPE) cells and may contribute to the etiology of age-related macular degeneration (AMD). MLF accumulation has been reported by Feeney-Burns to more closely reflect the onset of AMD than the accumulation of lipofuscin. In an effort to assess the possible contribution MLF may have to the onset of AMD, we analyzed the phototoxicity and protein composition of MLF and compared those results to that of LF.MethodsSpecifically, we observed the accumulation of MLF in human RPE from different decades of life, and assessed the phototoxicity of these granules. We also employed fluorescence spectroscopy, atomic force microscopy, transmission and scanning electron microscopy and proteomic analysis to examine the composition of MLF granules in an effort to ascertain their origin.ResultsOur results show that MLF granules are phototoxic and their accumulation more closely reflects the onset of AMD than does LF accumulation. Our compositional analysis of MLF has shown that while these granules contain some similarities to LF granules, MLF is substantially different. Of significant interest is the finding that MLF, in contrast to LF, does not contain photoreceptor-specific proteins, suggesting that MLF may not originate from the phagocytosis of photoreceptor outer segments. Instead the presence of RPE- and melanosome-specific proteins would suggest that MLF accumulates as a result of the melanosomal autophagocytosis of RPE cells.ConclusionsOur results provide significant insight into understanding the formation and toxicity of MLF and suggest a possible contribution to the etiology of retinal diseases.

We present a model of the mechanical and fluid forces associated with exudative retinal detachments where the retinal photoreceptor cells separate, typically from the underlying retinal pigment epithelium (RPE). By computing the total fluid volume flow arising from transretinal, vascular and RPE pump currents, we determine the conditions under which the subretinal fluid pressure exceeds the maximum yield stress holding

In the compound eyes of the fruitfly Drosophila, the dioptric system of each ommatidium is able to form virtual images of the receptor terminals (rhabdomere tips) throughout the whole depth of the eye. It is shown (§ 3) that 3 characteristic superposition phenomena occur for images formed by distinct ommatidia (Figs. 3b  and 5). The most remarkable superposition appears at the point where the optical axes of ail ommatidia converge (center of curvature of the eye). At this level, highly magnified virtual and erect images of corresponding rhabdomeres are superimposed, giving rise to a deep pseudopupil (Fig. 9). Since in the ommatidia of Drosophila the rhabdome shows a pattern of 7 distal endings (Fig. 8a), the resulting deep pseudopupil  consists of  7 light spots with a similar pattern (Figs. 8b, 7, 11). Conversely the deep pseudopupil of compound eyes which have fused rhabdomes consists of a single light spot (Fig. 19). Such pseudopupils can be best observed either with antidromic or with orthodromie illumination of the eye, according to the specific transmission or reflection properties of the rhabdomes.
The deep pseudopupil of Dipterans is not to be  confused with the corneal pseudopupil (Fig. 13a) and especially not with the reduced corneal pseudopupil observed with a reduced aperture of the microscope (Fig. 13b), in spite of the remarkable similarity of these phenomena  regarding  the  asymmetry  and the dimension of their pattern (comp. Figs. 7 and 13b). The reduced corneal pseudopupil consists of 7 facets whereas the deep pseudopupil consists of 7 virtual images of the receptor endings.
From the results of Kirschfeld (1967), the appearance of a reduced corneal pseudopupil like Fig. 13b on the eye  of Drosophila proves that 7 receptors located in 7 neighbouring ommatidia look in the same direction in space (Fig. 14). The existence of such an optical arrangement favors the view that the eye of Drosophila, like that of M usca, belongs to the "neural  superposition  type".
A comparative study between the deep pseudopupil and the reduced corneal pseudopupil leads to the following geometric relation, which is specific of the Drosophila eye and probably of ail compound eyes of the "neural superposition type":
D/R = e/f' 
where D is the diameter of a facet, e the  distance between the centers of two neighbouring rhabdomere endings,  R the radius of curvature of the eye, and f' the focal length (in air) of a corneal Jens.
Other types of pseudopupils, commonly appearing as dark spots in compound eyes, are explained on a basis similar to the deep pseudopupil of Drosophila ( § 5). In fact, the dioptric system of  an ommatidium  can give virtual images not only of its distal receptor endings but of the whole intensity distribution  (i.e. the whole "luminous structure") which is present in its internai focal plane. If this structure is simple, the deep pseudopupil,  resulting from superpositions of virtual images, is likewise simple (Figs. 16 and 17). If the "luminous structure" is complex, as for example in the eye of the butterfly Vanessa (Fig. 18a schematized in Fig. 18c), then the deep pseudopupil shows the same complexity (Fig. 18b and 18d).
In compound eyes which lack screening pigment between their  crystalline  cones,  one  can  see  seconda y  pupils  of  the lst and 2nd order as described by Exner. Again they may be explained by superpositions  of  virtual  images  in the  depth of the eye, according to Fig. 20. Moreover, the deep pseudopupil of the "optical superposition eye" may  be  due  to  the  fact that the more distal converging system of an ommatidium forms virtual images not of the rhabdome endings themselves but of real images of these endings (Fig. 21).
Although the phenomenon of the deep pseudopupil is not perceived by the animal, it is of interest for the experimenter who can use it: 1) to study the light receptors easily in the eye of live and intact animais, 2) to measure the physiological divergence angle between adjoining ommatidia,  3) to  study the movement of the visual axis and the retinomotor adaptation of the receptors, and 4) to stimulate simultaneously many corresponding receptors belonging to different ommatidia. The advantages  of  this  in vivo technique  are discussed  in  § 6.3.

Biofluorescence has recently been found to be widespread in marine fishes, including sharks. Catsharks, such as the Swell Shark (Cephaloscyllium ventriosum) from the eastern Pacific and the Chain Catshark (Scyliorhinus retifer) from the western Atlantic, are known to exhibit bright green fluorescence. We examined the spectral sensitivity and visual characteristics of these reclusive sharks, while also considering the fluorescent properties of their skin. Spectral absorbance of the photoreceptor cells in these sharks revealed the presence of a single visual pigment in each species. Cephaloscyllium ventriosum exhibited a maximum absorbance of 484 ± 3 nm and an absorbance range at half maximum (λ1/2max) of 440-540 nm, whereas for S. retifer maximum absorbance was 488 ± 3 nm with the same absorbance range. Using the photoreceptor properties derived here, a "shark eye" camera was designed and developed that yielded contrast information on areas where fluorescence is anatomicall...

The morphology and molecular mechanisms of animal photoreceptor cells and eyes reveal a complex pattern of duplications and co-option of genetic modules, leading to a number of different light-sensitive systems that share many components, in which clear-cut homologies are rare. On the basis of molecular and morphological findings, I discuss the functional requirements for vision and how these have constrained the evolution of eyes. The fact that natural selection on eyes acts through the consequences of visually guided behaviour leads to a concept of task-punctuated evolution, where sensory systems evolve by a sequential acquisition of sensory tasks. I identify four key innovations that, one after the other, paved the way for the evolution of efficient eyes. These innovations are (i) efficient photopigments, (ii) directionality through screening pigment, (iii) photoreceptor membrane folding, and (iv) focusing optics. A corresponding evolutionary sequence is suggested, starting at no...

For determination of possible neurotransmitters synthesized by photoreceptor cells, turtle retinas were dissociated into single cells with proteolytic enzymes. These cells were partially separated by velocity sedimentation to yield a fraction rich in photoreceptors. Individual photoreceptor cells were then sucked into a micropipette and incubated with labeled precursors of known or suspected neurotransmitters. After incubation, the radioactive products were analyzed by high-voltage electrophoresis. Of all the chemicals tested, turtle photoreceptor cells synthesized only acetylcholine, suggesting that these cells may be cholinergic.

Teleost retinal cones contract in the light and elongate in the dark. In the green sunfish, Lepomis cyanellus, the necklike myoid region of the cone contracts from as much as 120 ,um (midnight dark-adapted) to 6 ,um in the fully light-adapted state. When ...

Elevation of erythropoietin (Epo) concentrations by hypoxic preconditioning or application of recombinant human Epo (huEpo) protects the mouse retina against light-induced degeneration by inhibiting photoreceptor cell apoptosis. Because photoreceptor apoptosis is also the ...

Rapid concentration jumps of Ins(1,4,5)P3 or ATP were made inside Limulus ventral photoreceptors by flash photolysis of the parent caged compounds. In intact ventral photoreceptors, the photolysis flash evokes a maximum amplitude light-activated current; therefore, a procedure was developed for uncoupling phototransduction by blocking two of the initial reactions in the cascade, rhodopsin excitation and G protein activation. Rhodopsin was inactivated by exposure to hydroxylamine and bright light. This procedure abolished the early receptor potential and reduced the quantum efficiency by 325 +/- 90-fold (mean +/- SD). G protein activation was blocked by injection of guanosine-5'-O-(2-thiodiphosphate) (GDP beta S). GDP beta S injection reduced the quantum efficiency by 1,881 +/- 1,153-fold (mean +/- SD). Together hydroxylamine exposure and GDP beta S injection reduced the quantum efficiency by 870,000 +/- 650,000-fold (mean +/- SD). After the combined treatment, photoreceptors pro...

Coloration mediates the relationship between an organism and its environment in important ways, including social signaling, antipredator defenses, parasitic exploitation, thermoregulation, and protection from ultraviolet light, microbes, and abrasion. Methodological breakthroughs are accelerating knowledge of the processes underlying both the production of animal coloration and its perception, experiments are advancing understanding of mechanism and function, and measurements of color collected noninvasively and at a global scale are opening windows to evolutionary dynamics more generally. Here we provide a roadmap of these advances and identify hitherto unrecognized challenges for this multi- and interdisciplinary field.

DHA, released from membrane phospholipids upon oxidative damage, activates retinoid X receptors (RXR), thus triggering multiple mechanisms, including upregulation or activation of pro-survival proteins, antioxidant defense mechanisms and the ERK/MAPK pathway to promote photoreceptor survival.

Although the presence of feathers in the nest is widespread among birds, it has not been previously suggested that feathers can be used as sexual signals. Females of the spotless starling ( Sturnus unicolor ) regularly carry feathers to their nest, mostly during laying and incubation. We show that the arrangement of these feathers was non-random with respect to the side (obverse or reverse) placed upwards (which can be viewed from the nest entrance). Feathers of the wood pigeon ( Columba palumbus ) and the spotless starling, which exhibit higher ultraviolet and visible reflectance on their reverse side, were predominantly placed with this side upwards. On the contrary, feathers of the jay ( Garrulus glandarius ) were predominantly found exhibiting the obverse side, which possesses higher reflectance in this species. Feathers of the azure-winged magpie ( Cyanopica cyana ), with similar reflectance values on either side, were placed indiscriminately in obverse and reverse positions. T...

The uptake of acidic amino acids by goldfish photoreceptors was investigated by light microscope autoradiography. Isolated retinas were incubated in media containing micromolar amounts of L-[3H]aspartate, L-[3H]glutamate, and D-[3H]aspartate. We have four major observations. (i) Rods accumulate L-[3H]glutamate with a high-affinity transport system; they exhibit a glutamate-to-aspartate selectivity ratio of 30:1. When incubated in 1-10 microM L-[3H]glutamate, rods label more densely than cones. A unit area of rod membrane transports glutamate 30 times better than a unit area of cone membrane. (ii) Red-sensitive and green-sensitive cones show accumulation of L-[3H]aspartate, D-[3H]aspartate, and L-[3H]glutamate, apparently with high affinity, but with little selectivity. Because rods have poor aspartate uptake, red-sensitive and green-sensitive cones may be preferentially labeled with L-[3H]aspartate or D-[3H]aspartate, (iii) Blue-sensitive cones show no uptake of L-[3H]aspartate, D-[...

Visual transduction begins with the detection of light within the photoreceptor cell layer of the retina. Within this layer, specialized cells, termed rods and cones, contain the proteins responsible for light capture and its transduction to nerve impulses. The phototransductive proteins reside within an outer segment region that is connected to an inner segment by a thin stalk rich in cytoskeletal elements. A unique property of the outer segments is the presence of an elaborate intracellular membrane system that holds the phototransduction proteins and provides the requisite lipid environment. The maintenance of normal physiological function requires that these postmitotic cells retain the unique structure of the outer segment regions--stacks of membrane saccules in the case of rods and a continuous infolding of membrane in the case of cones. Both photoreceptor rod and cone cells achieve this through a series of coordinated steps. As new membranous material is synthesized, transpor...

Many animals use the spectral distribution of light to guide behaviour, but whether they have colour vision has been debated for over a century. Our strong subjective experience of colour and the fact that human vision is the paradigm for colour science inevitably raises the question of how we compare with other species. This article outlines four grades of ‘colour vision’ that can be related to the behavioural uses of spectral information, and perhaps to the underlying mechanisms. In the first, even without an (image-forming) eye, simple organisms can compare photoreceptor signals to locate a desired light environment. At the next grade, chromatic mechanisms along with spatial vision guide innate preferences for objects such as food or mates; this is sometimes described as wavelength-specific behaviour. Here, we compare the capabilities of di- and trichromatic vision, and ask why some animals have more than three spectral types of receptors. Behaviours guided by innate preferences ...