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O indicates an oil droplet in the accessory cone. The double cones of the sturgeon where C stands for the slender chief cone and A stands for accessory cone. Note 1 The oil droplet O in these cones and 2 arrow points to glycogen particles inside the accessory cones. Further appreciate that the nuclei N of accessory cones are usually above the external limiting membrane at the base of the microvilli M. The accessory A and chief C cones of the double cone complex. It was widely accepted that birds, reptiles all the way to the primitive fish have inner segments with oil droplets.

In the study with different fishes, we had recorded the presence of oil droplets in the cones of a phylogenetically old fish, the sturgeon, though most of these droplets were colourless in this species Figure 5 , unlike those in the chicken Figure It is not entirely clear whether these droplets inside the inner segments exist as a light filter, as storage of energy or have other purposes.

The fact that these oil droplets could have many colours led to a hypothesis that they might be related to spectral sensitivity [ 1 ]. The yellowish oil droplets in the retina of the fish had been related with reduction of chromatic aberration, reduction of glare and dazzle, and the improvement of contrast vision [ 2 ].

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Most of the birds, if not all, have oil droplets in their inner segments of the retina. These include two active predators, which are the eagles and the owls. These birds display sharp acuity of vision. In addition, oil droplets are also seen in birds that are not good at flying such as the chicken and the duck. Our group performed an experiment years ago to observe retinal changes in the birds after occlusion of one eye. After a month, many of the oil droplets were gone.

It was, however, not certain whether there was a disintegration of the oil droplets alone or a death of the cones with droplets or both [ 3 ]. It was also doubtful whether the oil droplets were used as an energy source in the eyes of the birds as birds had a pectin oculi in their eyes, which were highly vascularised and could be an energy source [ 4 ]. Another peculiar structure inside the inner segments is the presence of glycogen bodies in the accessory cones.

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These glycogen bodies are present as significant structure in the myoid portion of the inner segments. Developmental studies on the chicken retina indicated an initial accumulation of rough endoplasmic reticulum as parallel columns along the long axis in the myoid region of the inner segment of the accessory cones. As the retina developed, the Golgi apparatus became active in this region and by around 6—7 days before hatching, glycogen began to form in between the rough endoplasmic reticulum.

Collections of glycogen increased before hatching and these glycogen bodies with the rough endoplasmic reticulum formed round structures termed as paraboloid [ 3 ].

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Paraboloids Figure 12 persisted in the adult chicken retina and those of other birds. The function of the paraboloid is not clarified at this stage. One of the possibilities is this represents high energy storage and perhaps equipped for the high energy demand of the avians, particularly for those active predative species. One support for this was from the previous work of our laboratory in which we occluded the eyes of the chicken for 1 month, the paraboloid began to lose their glycogen indicated by the less Periodic Acid Schiff-positive staining.

In addition, most species in the lower vertebrate scale do have an equal and equivalent structure in their cones.

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This is exemplified by the sturgeon, which has a collection of glycogen in their accessory cones although this collection was often semilunar in shape Figure 7. Furthermore, the appearance of glycogen was often documented in degenerating retinae of the fish [ 5 ] , which obviously was not for providing energy in this already degenerating structure.

Paraboloid P in the accessory cones of the chicken retina. They contain glycogen and thus are PAS positive. In the albino rat, it was generally agreed that the great majority of the retinal visual cells were rods, and cones were extremely rare. In the deep sea species, as exemplified by the sharks, cones were present in the retina but in very small quantities Figure 13 , while in the amphibians, double cones persisted Figure However, one must be aware that the sharks do not use vision to catch prey. In the fresh water fishes, such as the sturgeon, twin cones are demonstrated in the retina Figure In these animals, the ratio of twin cone to single cone was about 1 to 10 and the double cone to single cone ratio was about 3 to 4, with the rest as rods.

For detailed comparison between animal groups, please refer to our comparative atlas published in ref. Since the active predators, which dwell in shallow waters such as the crocodiles, possess a fair amount of twin cones in the retina, their functional role in the deep sea does not appear to be supported.

In these predators, the fact that the number of double cones and single cones were close to each other suggested that both were important in vision e. Electrically, the double cones and twin cones possess diverse properties.

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Injection of fluorescent dye into one component of the twin cone reported a lack of dye coupling into the other cell of the twin cone but electrical coupling was evident while double cones displayed dye coupling between the two components [ 7 ]. Electrical coupling was observed in the gap junction of many cones in the mammalian retina [ 8 ] and in between red and green cones in the primates [ 9 ].

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In our morphological studies, it was clear that components of double cones adhered tightly in the proximal region above the outer limiting membrane but not so tightly in the more distal areas Figure Furthermore, both components of twin cones adhered tightly all the way Figure Retina of the Siamese fighting fish showing pairs of double cones arrow. A possible type of double cone in the monkey retina with a slender chief cone C and a slightly round accessory cone A. The cones in the retina are usually present in specific pattern. It is already well known that cones are concentrated more in the central retina than the periphery.

This statement is not flawless. Belonidae, Teleostei during light and dark adaptation — photoreceptors, cone patterns and densities Frank Reckel , Roland R. Melzer , Ulrich Smola. Cronin , Daniel Osorio.

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Photoreceptor types and their relation to the spectral and polarization sensitivities of clupeid fishes I. Noveles Flamarique , Craig W. The pupil response of a teleost fish, Porichthys notatus: description and comparison to other species R Hackler Douglas , Robert D. Millions of books are added to our site everyday and when we find one that matches your search, we'll send you an e-mail.


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