Optical properties of a cephalopod eye (the short finned squid,Illex illecebrosus)

Summary The extent to which the cephalopod eye is optically similar to the teleost eye was determined by measuring refractive error, accommodative ability, spherical and chromatic aberrations, and refractive indices and radii of curvature of the ocular media. The squid eye is well corrected optically underwater although a tendency toward hyperopia exists. This may be due to the existence of chromatic aberration and the fact that an aquatic environment is somewhat limited to the blue end of the spectrum. Accommodation takes place by movement of the lens toward the retina in a manner similar to the teleost eye. However, the squid lens is not spherical but slightly flattened. The lens is overcorrected as far as spherical aberration is concerned. Thus peripheral light rays focus further from the lens than paraxial ones. The function of this unusual example of lens development is unknown.     

Theoretical study of the optimal shape of the front profile of the lens in the eye of the scallop,Pecten

The optimal shape of the front profile of the thick lens in the eye of the scallop,Pecten is theoretically, geometric optically investigated as a function of the refractive index of the lens and the retina, as well as of the geometrical parameters of the eye. The shape of the theoretical front surfaces is compared with that of the real, experimentally determined front face of the lens. The degree of correction of the lens for spherical aberration of the reflecting spherical mirror in thePecten eye is examined. The optimal shape of the front profile of the lens depends strongly on a set of parameters, such that a certain fine tuning is required among them to assure a full correction for spherical aberration. The extreme variability of the eye parameters and the shape of the front face of the lens in the scallop is inconsistent with this fine tuning requirement. The degree of correction of thePecten lens for spherical aberration might not be as good as it could be, a possible biooptical reason for which is discussed.     

Pharmacology of the retractor lentis muscle of the bluegill (Lepomis macrochirus)

We studied accommodative nerve-muscle transmission in a teleost fish, the bluegill (Lepomis macrochirus). The muscle contracted with electrical stimulation to move the spherical lens along the pupil plane in the eye ball so that the distance between the lens and retina changes. The dose-dependent effect on the lens muscle of acethylcholine and its related substances (nicotine, muscarine, curare and atropine), glutamate, gamma-aminobutyric acid, serotonin, glycine, dopamine and noradrenaline was observed with a binocular microscope. Its video image was processed by a computer to measure the shortening of the muscle. Among the transmitter candidates applied only acethylcholine caused a contraction and elicited the lens movement. Muscarine caused strong contraction of the muscle even in a low concentration (1 nM) while nicotine did not. The contraction was antagonized by atropine but not curare. Previous studies based on intraperitoneally injected reagents suggested a cholinergic innervation on the lens muscle. Our pharmacologic results verified by direct anatomical measurements on the isolated lens-muscle-nerve preparation unambiguously establish the dose-dependent muscarinic-cholinergic innervation to the lens muscle by parasympathetic postganglionic fibers.Abbreviations Ach Acetylcholine - GABA gamma-aminobutyric acid - TTX tetrodotoxin - RML Resting muscle length     

Optical quality of the ocular lens of the sea lamprey (<Emphasis Type="Italic">Petromyzon marinus</Emphasis>) during the mature and transformer periods of life

While larval sea lampreys exist as eyeless filter feeders for several years, they transform into free-swimming juveniles (transformers) that attach parasitically to prey fish as they develop sexual maturity. This study examines lamprey lens development and optics and, since the lens is often the only refractive component of an aquatic eye, the data also provide an indication of visual ability during transformer and adult periods of life. Seven adult sea lampreys (0.40–0.55 m) and eight transformers (0.15–0.18 m) were sacrificed, the eyes removed and lenses dissected, measured, and placed in an automated laser scanning instrument. Back vertex focal length (spherical aberration) was measured for 14 beam positions across each lens by using a digital camera to record the position of the refracted beam. Transformer lenses exhibit positive spherical aberration, with average focal lengths varying from about 2.40 mm near the lens center and 1.06 mm at the lens periphery. On the other hand, the lenses from adults are largely corrected for spherical aberration, with average focal lengths varying from 2.19 mm to 2.44 mm. This result indicates that the younger lenses do not have a gradient refractive index necessary to mitigate the aberration and that further study of this model may reveal the relation between lens embryology and the development of such a gradient.     

The eye of the hooded seal,Cystophora cristata,in air and water

Summary Multiple refractive state measurements were made on a male and female hooded seal (Cystophora cristata) when the eyes were exposed to air and to water. The measures, made by conventional retinoscopy and by photorefraction, show that the seals are moderately hyperopic (2–3 diopters) in water and moderately myopic (2–4 diopters) in air. No significant astigmatism was noted in either medium. The absence of refractive state variation over time suggests that an accommodative mechanism is insignificant or absent, although histological study indicates that the ciliary muscle is well developed.Photokeratoscopy, carried out on two animals with two keratoscopic instruments, show that the cornea is relatively flat (30 mm, or about one-half the diameter of the eye). Furthermore the cornea is only slightly astigmatic (less than 1 diopter). The refractive power of the external corneal surface (in air), calculated from a measurement of corneal refractive index of 1.378, amounts to only 10 or 11 diopters.As in the typical fish eye, hooded seal lenses are spherical or nearly spherical in shape (24–23 mm), and have short focal lengths (30–32 mm). Focal measures for rays at varying distances from the lens center indicate that spherical aberration is well corrected.There is no indication in this seal species, of a previously reported adaptation involving a highly astigmatic cornea which together with a slit pupil can minimize the optical effect of movement from water to air.     

Spherical aberration of the fish lens: interspecies variation and age

Summary Spherical aberration of the eyes of a spectrum of freshwater fishes was determined by photographing the refractive effects of excised crystalline lenses on multiple parallel split laser beams. In general, spherical aberration is minimized by the developmentally related variation in lens refractive index. However, spherical aberration is marked and non-monotonic in a non-visual species such as the bullhead. Furthermore, the size and variability of the aberration appears to be related to visual need as indicated by diet and feeding habits. For example, the lenses of predatory sight feeders such as the pike (Esox lucius) or rock bass (Ambloplites rupestris) are optically superior to that of an omnivorous feeder as the carp (Cyprinus carpio).The effect of age was tested by examining rock bass lenses from fish two to seven years of age. Lens quality, as indicated by the amount of change in posterior focal length for beams of varying eccentricity from the optic axis, is optimum in lenses from five year old fish. The significance of this variation in lens quality is uncertain and requires further study with greater attention to specimens of advanced age.     

Lens optical properties in the eyes of large marine predatory teleosts

The optical properties of the crystalline lenses were studied in a variety of large predatory teleosts (bony fishes) that forage in the open ocean, some of them at considerable depths. We found the first fish lenses that are free of measurable longitudinal spherical aberration, i.e., are perfectly monofocal, in contrast to the multifocal lenses that are typical for smaller fishes living close to the surface. In fact, none of the lenses investigated in this study were clearly multifocal. Most, but not all, of the lenses had long normalized focal lengths (focal length/lens radius) of up to 3.3 lens radii. A monofocal lens of long focal length, combined with spectrally suitably placed cone pigments, may be the optimal solution for vision of high spatial and spectral resolutions in a habitat where the available spectrum of light is limited.     

Reconstruction of the shape and optics of the lenses in the abathochroal‐eyed trilobite Neocobboldia chinlinica

Until now, the structure and optics of the calcite lenses in abathochroal trilobite eyes have not been investigated. So, the relationship of the abathochroal eye to other types of trilobite eyes has remained unclear. We have reconstructed the exact shape and optics of the lenses in the eodiscid trilobite Neocobboldia chinlinica to determine the mechanism of its abathochroal eye. The distal lens surface has a convex profile, while on the proximal lens surface there is a small central bulge, resulting in an undulating profile. Due to this bulge, the curvature and refractive power of the central region of the lens are greater than those of the peripheral zone. Consequently, the lens is bifocal. However, Neocobboldia could not take advantage of this bifocal property of its tiny lenses because of the diffraction of light and the infinite depth of field in object space. For the same reason, it is also sure that the undulating lower surface of the abathochroal lens did not evolve as a Huygensian profile, correcting for spherical aberration, as suggested earlier. This undulation is a result of the presence of the central bulge, the evolutionary significance of which remains enigmatic. On the basis of our results, we have outlined an evolutionary scenario for development of the optics of the lenses in trilobite eyes.     

Ocular dimensions and cone photoreceptor topography in adult Nile Tilapia <Emphasis Type="Italic">Oreochromis niloticus</Emphasis>

Information on the anatomy of the eye and the topography of cone photoreceptor cells in the retina is presented for the Nile Tilapia (Oreochromis niloticus). In adults, the shape and proportions of the ocular components of the prominent eye conform to the general form of fish eyes, as determined using cryo-sectioned eyes. The lens is approximately spherical and there is little variation in the distance from the centre of the lens to the border between the choroid and retina at a range of angles about the optical axis. The average ratio of the distance from the centre of the lens to the retina: lens radius (Matthiessen’s ratio) is 2.44:1. In retinal wholemounts, single and double (twin) cone photoreceptors, forming a square mosaic, are present. Peak photoreceptor densities for both morphological cone types are found in the temporal retina. Using peak cone densities and estimates of focal length from cryo-sectioned eyes, visual acuity is calculated to be 5.44 cycles per deg. The lack of apparent specific ocular or retinal specializations and the relatively low visual acuity reflect the lifestyle of the Nile Tilapia and may allow it to adapt to changes in visual environment in its highly variable natural habitat as well as contributing to the ‘ecological flexibility’ of this species.     

Structure of the eye of Haideotriton wallacei, a North American troglobitic salamander

The eye of Haideotriton wallacei is more reduced histologically than those of othe troglobitic salamanders. The tiny eye is imbedded in a mass of adipose tissue. No extrinsic eye muscles are present. A rudimentary lens is present in about half of the eyes examined. In two instances the lens is surrounded by a small chamber; most eyes lack a chamber. The retina and iris are relatively undifferentiated. The relatively massive retina lacks rods and cones, an outer plexiform layer and subdivided nuclear layers. A tiny optic nerve runs to the brain.     

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. J. Morphol. 276:415–424, 2015. © 2014 Wiley Periodicals, Inc.     

The lens controls cell survival in the retina: Evidence from the blind cavefish Astyanax

The lens influences retinal growth and differentiation during vertebrate eye development but the mechanisms are not understood. The role of the lens in retinal growth and development was studied in the teleost Astyanax mexicanus, which has eyed surface-dwelling (surface fish) and blind cave-dwelling (cavefish) forms. A lens and laminated retina initially develop in cavefish embryos, but the lens dies by apoptosis. The cavefish retina is subsequently disorganized, apoptotic cells appear, the photoreceptor layer degenerates, and retinal growth is arrested. We show here by PCNA, BrdU, and TUNEL labeling that cell proliferation continues in the adult cavefish retina but the newly born cells are removed by apoptosis. Surface fish to cavefish lens transplantation, which restores retinal growth and rod cell differentiation, abolished apoptosis in the retina but not in the RPE. Surface fish lens deletion did not cause apoptosis in the surface fish retina or affect RPE differentiation. Neither lens transplantation in cavefish nor lens deletion in surface fish affected retinal cell proliferation. We conclude that the lens acts in concert with another optic component, possibly the RPE, to promote retinal cell survival. Accordingly, deficiency in both optic structures may lead to eye degeneration in cavefish.     

Regional differences in a nematoceran retina (Insecta,Diptera)

Summary The compound eye of Psychoda cinerea comprises two types of ommatidia, arranged so as to divide the retina into distinct dorsal and ventral regions. The P-type ommatidium, in the ventral part of the eye, differs fundamentally from the other dipteran ommatidia so far described, and is regarded as a primitive ommatidium. The acone dioptric apparatus is the same in both types, with a spherical lens and four Semper cells, the processes of which expand below the rhabdom to form a ring of pigment sacs. Only the distal region of the rhabdom is surrounded by a continuous ring of screening pigment, formed by 2 primary and 12–16 secondary pigment cells. The highly pigmented retinula cells penetrate the basement membrane proximally at about the level of their nuclei; in this region they are separated from the hemolymph by glial elements. The rhabdomeres R1–6 are fused to form a tube. The two types of ommatidia are defined by the arrangement of the retinula cells R7/8: in the T type the central rhabdomeres are one below the other, in the usual tandem position, whereas in the P type only R8 is central, with R7 in the peripheral ring. In the proximal region of the retina, retinula cells with parallel microvilli in neighboring ommatidia are joined in rows by lateral processes from the R8 cells. All the rhabdomeres are short and not twisted, which suggests that the retinula cells are highly sensitive to direction of polarization. The eye can adapt by a number of retinomotor processes. These findings, together with observations of behavior, imply that the psychodids have well-developed visual abilities.     

The larval eye of the aeolid nudibranch Trinchesia aurantia (Alder and Hancock)

Summary The larval eye of the aeolid nudibranch Trinchesia aurantia has been investigated at three different stages; in all, the eyes remain closely attached to, and in cellular contact with, the central ganglia. The larval eye is a simplified version of the adult eye in that, the eye and the constituent cells, nuclei, lens, microvilli and pigment granules are all smaller, and the interdigitation between the retinal cells is not developed. The absence of the small cells of the cornea and of the spherical vesicles in the cytoplasm of the sensory cells, is further evidence of the incomplete formation of the eye. The possible origin of the eye of Trinchesia is discussed and compared with that of other gastropods.I am very grateful for the help and guidance of my supervisor Dr. D. A. Dorsett throughout the preparation of this paper. I was sponsored by a grant from the N.E.R.C.     

Regressive evolution: ontogeny and genetics of cavefish eye rudimentation

Two hypotheses exist to explain ontogenetic eye reduction in Astyanax cave fish: first, after lens induction by the primordial eye cup, the lens plays the role of a central regulator of eye and retina regression or, second, the retina itself is an independent unit of eye development. A comparative study of five blind cave fish populations and their surface sister form was performed to investigate the differences of ontogenetic eye regression between the cave populations during different stages of development. The study revealed that, in addition to the initial formation of smaller primordia, eye regression is also caused during later ontogeny by different relative growth and specific histological characteristics. Whereas the cave fish lens never properly differentiates, the regressive process of the retina is transitorily interrupted by ongoing differentiation. In the newly-discovered Molino cave population, even visual cells with well-organized outer segments develop, which are secondarily reduced at a later ontogenetic stage. This result shows that the retina and lens are independent developmental units within the eye ball. Presumably, the genetic systems responsible for both show independent inheritance, which is also corroborated by hybrids of F ₂-crosses between the cave and surface fish, in which lens and retina development do not correlate. During ontogeny, the eye size differs between the cave populations. In Pachón cave fish, the relatively large eye size correlates with an ancient introgression from a surface population, which may have delayed eye regression.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 92 , 287–296.     

Determination of optical parameters of the porcine eye and development of a simulated model

The eye is a very sophisticated system of optical elements for the preeminent sense of vision. In recent years, the number of laser surgery to correct the optical aberration such as myopia or astigmatism has significantly increased. Consequently, improving the knowledge related to the interactions of light with the eye is very important in order to enhance the efficiency of the surgery. For this reason, a complete optical characterization of the porcine eye is presented in this study. Kubelka‐Munk and Inverse Adding‐Doubling methods were applied to spectroscopy measurement to determine the absorption and scattering coefficients. Furthermore, the refractive index has been measured by ellipsometry. The different parameters were obtained for the cornea, lens, vitreous humor, sclera, iris, choroids and eyelid in the visible and infrared region. Thereafter, the results are implemented in a COMSOL Multiphysics® software to create an eye model. This model gives a better understanding of the propagation of light in the eye by adding optical parts such as the iris, the sclera or the ciliary bodies. Two simulations show the propagation of light from the cornea to the retina but also from the sclera to the retina. This last possibility provides a better understanding of light propagation during eye laser surgery such as, for example, transscleral cyclophotocoagulation. Figure: Eye simulation models allow the development of new laser treatments in a simple and safe way for patients. To this purpose, the creation of an eye simulated model based on optical parameters obtained from experimental data is presented in this study. This model will facilitate the understanding of the light propagation inside the porcine eye.     

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

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

Visual accommodation in vertebrates: mechanisms, physiological response and stimuli

The mechanism and stimulation of the accommodative reflex in vertebrate eyes are reviewed. Except for lampreys, accommodation is brought about by intraocular muscles that mediate either a displacement or deformation of the lens, a change of the corneal radius of curvature or a combination of these mechanisms. Elasmobranchs have little accommodation and are emmetropic in water rather than hyperopic as commonly stated. Accommodation in teleosts and amphibians is well understood and achieved by lens displacement. The accommodative mechanism of amniotes is of considerable diversity and reflects different lifestyles rather than phylogenetical relationships. In all amniotes, the ciliary muscle never has a direct impact on the lens. It relaxes the tension applied to the lens by zonular fibers and/or ligaments. In birds and reptiles the ciliary muscle is usually split into two parts, of which the anterior portion changes the corneal radius of curvature. The deformation of the lens is generally achieved either by its own elasticity (humans, probably other mammals and sauropsids) or by the force of circular muscle fibers in the iris (reptiles, birds, aquatic mammals). In the second part of the paper, some of the current hypotheses about the accommodative stimulus are reviewed together with physiological response characteristics.