Light on the moth-eye corneal nipple array of butterflies

The outer surface of the facet lenses in the compound eyes of moths consists of an array of excessive cuticular protuberances, termed corneal nipples. We have investigated the moth-eye corneal nipple array of the facet lenses of 19 diurnal butterfly species by scanning electron microscopy, transmission electron microscopy and atomic force microscope, as well as by optical modelling. The nipples appeared to be arranged in domains with almost crystalline, hexagonal packing. The nipple distances were found to vary only slightly, ranging from about 180 to 240 nm, but the nipple heights varied between 0 (papilionids) and 230 nm (a nymphalid), in good agreement with previous work. The nipples create an interface with a gradient refractive index between that of air and the facet lens material, because their distance is distinctly smaller than the wavelength of light. The gradient in the refractive index was deduced from effective medium theory. By dividing the height of the nipple layer into 100 thin slices, an optical multilayer model could be applied to calculate the reflectance of the facet lenses as a function of height, polarization and angle of incidence. The reflectance progressively diminished with increased nipple height. Nipples with a paraboloid shape and height 250 nm, touching each other at the base, virtually completely reduced the reflectance for normally incident light. The calculated dependence of the reflectance on polarization and angle of incidence agreed well with experimental data, underscoring the validity of the modelling. The corneal nipples presumably mainly function to reduce the eye glare of moths that are inactive during the day, so to make them less visible for predators. Moths are probably ancestral to the diurnal butterflies, suggesting that the reduced size of the nipples of most butterfly species indicates a vanishing trait. This effect is extreme in papilionids, which have virtually absent nipples, in line with their highly developed status. A similar evolutionary development can be noticed for the tapetum of the ommatidia of lepidopteran eyes. It is most elaborate in moth-eyes, but strongly reduced in most diurnal butterflies and absent in papilionids.     

Colour in the eyes of insects

Many insect species have darkly coloured eyes, but distinct colours or patterns are frequently featured. A number of exemplary cases of flies and butterflies are discussed to illustrate our present knowledge of the physical basis of eye colours, their functional background, and the implications for insect colour vision. The screening pigments in the pigment cells commonly determine the eye colour. The red screening pigments of fly eyes and the dorsal eye regions of dragonflies allow stray light to photochemically restore photoconverted visual pigments. A similar role is played by yellow pigment granules inside the photoreceptor cells which function as a light-controlling pupil. Most insect eyes contain black screening pigments which prevent stray light to produce background noise in the photoreceptors. The eyes of tabanid flies are marked by strong metallic colours, due to multilayers in the corneal facet lenses. The corneal multilayers in the gold-green eyes of the deer fly Chrysops relictus reduce the lens transmission in the orange-green, thus narrowing the sensitivity spectrum of photoreceptors having a green absorbing rhodopsin. The tapetum in the eyes of butterflies probably enhances the spectral sensitivity of proximal long-wavelength photoreceptors. Pigment granules lining the rhabdom fine-tune the sensitivity spectra.     

Post-larval development of compound eyes and stemmata of Chaoborus crystallinus (De Geer, 1776) (Diptera : Chaoboridae): Stage-specific reconstructions within individual organs of vision

During metamorphosis, the dioptric apparatus of the larval compound eye of Chaoborus crystallinus (Diptera : Nematocera) is radically reconstructed. The thin larval cornea of the ommatidia is replaced by strongly curved corneal lenses, and the eucone larval cone is replaced by an imaginal cone of the acone type. Curvature of the future lens is already apparent in very young pupae, in which the cornea consists only of a thin epicuticle with corneal nipples. Fibrillary cuticle is secreted by cone and primary pigment cells throughout pupal development. Lens formation is accompanied by movement of the nuclei of the accessory pigment cells. The larval cone disintegrates unexpectedly late in young, images. During late pupal development, 7 cone cell projections emerge. In contrast to the dioptric apparatus, the retinula cells and rhabdom remain almost unchanged during metamorphosis. The main refractive element of the larval ommatidium appears to be the cone, while that of the imaginal ommatidium is the corneal lens. In addition to the compound eyes, the pairs of stemmata are retained during the whole post-larval development. Pupal stemmata show no structural differences from the larval stemmata. The stemmata are still present in 2-day-old images (“retained stemmata”), but the primary stemma loses its dioptric apparatus and is proximally relocated to the basal region of the compound eye. The reconstructions in the visual system of Chaoborus, which occur during ontogeny, are probably connected with the change from aquatic living larvae to aerial adults, and appear to fulfill stage-specific needs of vision.     

Studying nanostructured nipple arrays of moth eye facets helps to design better thin film solar cells

Nipples on the surface of moth eye facets exhibit almost perfect broadband anti-reflection properties. We have studied the facet surface micro-protuberances, known as corneal nipples, of the chestnut leafminer moth Cameraria ohridella by atomic force microscopy, and simulated the optics of the nipple arrays by three-dimensional electromagnetic simulation. The influence of the dimensions and shapes of the nipples on the optics was studied. In particular, the shape of the nipples has a major influence on the anti-reflection properties. Furthermore, we transferred the structure of the almost perfect broadband anti-reflection coatings to amorphous silicon thin film solar cells. The coating that imitates the moth-eye array allows for an increase of the short circuit current and conversion efficiency of more than 40%.     

Effects of dietary deficiency of selective amino acids on the function of the cornea and lens in rats

Summary.  Effects of dietary deficiencies of tryptophan and methionin on the transparency of cornea and lens were investigated in young rats (Brown-Norway, BN; Sprague-Dawley, SD) over 3 months. Transparency of the cornea and lens were evaluated in weekly intervals using a photo-slitlamp microscope. After sacrifice and lens fresh weight determination the lenses were prepared for histopathology. Methionin deficiency had no effect on the parameters investigated. Tryptophan deficiency caused severe loss of body weight in both strains, with additional loss of hair in SD rats. These developed corneal neovascularisations and cataracts. BN rats showed an enhanced zone of discontinuity in the lens. Diet intermission arrested the pathological processes in the eye which restarted when feeding the diet again. This observation is supported by lens fresh weight data. DNA staining evidenced that tryptophan deficiency arrested lens fiber maturation in both strains but stimulated corneal neovascularisation only in SD rats. Received June 29, 2001 Accepted August 6, 2001 Published online July 31, 2002     

Dipeptidyl peptidase IV (DPPIV) activity in the tear fluid as an indicator of the severity of corneal injury: a histochemical and biochemical study

Comparative histochemical and biochemical studies on the catalytically active protease Dipeptidyl peptidase IV (DPPIV), have been performed in the rabbit cornea and the tear fluid using a sensitive fluorogenic substrate, Gly-Pro-7-amino-4-Trifluoromethyl Coumarine (AFC). In both normal and experimentally injured corneas, DPPIV activity was detected histochemically and in the tear fluid biochemically. In contrast to the normal cornea where DPPIV activity was absent and in the tear fluid where it was low, during continuous wearing of contact lenses or repeated irradiation of the cornea with UVB rays, slight DPPIV activity appeared first in the superficial layers of the corneal epithelium, while later increased activity was present in the whole epithelium. This paralleled elevated DPPIV activity in the tear fluid. Moreover, during continuous contact lens wear, the increased DPPIV activity in the tear fluid was, in many cases, coincidental with the presence of capillaries in the limbal part of the corneal stroma. After severe alkali burns when corneal ulcers appeared, collagen fragments were active for DPPIV, which was associated with high DPPIV activity in the tear fluid. In conclusion, Gly-Pro-AFC was found to be useful for comparative histochemical and biochemical studies on DPPIV activity in the experimentally injured rabbit eye. Using the method of the tear film collection by a short touch of substrate punches to the respective site of the cornea or conjunctiva we can show that in experimental injuries (wearing of contact lenses, irradiation of the cornea with UVB rays), the damaged corneal cells were the main source for DPPIV activity in the tear fluid. It is suggested that the activity of DPPIV measured in the tear fluid might serve as an indicator of early corneal disorders, e.g. corneal vascularization related to contact lens wear.     

Structural models of the supramolecular organization of AQP0 and connexons in junctional microdomains

Membrane proteins perform many essential cellular functions. Over the last years, substantial advances have been made in our understanding of the structure and function of isolated membrane proteins. However, like soluble proteins, many membrane proteins assemble into supramolecular complexes that perform specific functions in specialized membrane domains. Since supramolecular complexes of membrane proteins are difficult to study by conventional approaches, little is known about their composition, organization and assembly. The high signal-to-noise ratio of the images that can be obtained with an atomic force microscope (AFM) makes this instrument a powerful tool to image membrane protein complexes within native membranes. Recently, we have reported high-resolution topographs of junctional microdomains in native eye lens membranes containing two-dimensional (2D) arrays of aquaporin-0 (AQP0) surrounded by connexons. While both proteins are involved in cell adhesion, AQP0 is a specific water channel whereas connexons form cell–cell communication channels with broad substrate specificity. Here, we have performed a detailed analysis of the supramolecular organization of AQP0 tetramers and connexon hexamers in junctional microdomains in the native lens membrane. We present first structural models of these junctional microdomains, which we generated by docking atomic models of AQP0 and connexons into the AFM topographs. The AQP0 2D arrays in the native membrane show the same molecular packing of tetramers seen in highly ordered double-layered 2D crystals obtained through reconstitution of purified AQP0. In contrast, the connexons that surround the AQP0 arrays are only loosely packed. Based on our AFM observations, we propose a mechanism that may explain the supramolecular organization of AQP0 and connexons in junctional domains in native lens membranes.     

Geometric optical optimization of the corneal lens of Notonecta glauca

The optimal shape of the corneal lens of the water bug backswimmer (Notonecta glauca) and the optimal shape and position of the thin transition layer between the distal and proximal units of its cornea are theoretically determined. Using a geometric optical method, first the shape of a geometric interface between the lens units is determined, which eliminates the longitudinal spherical aberration. This interface is investigated for differently formed thick lenses when the medium in contact with the entrance surface of the lens is water or air. The optimal transition layer for the amphibious backswimmer is that, the boundaries of which are the theoretical interfaces for water and air, and the refractive index varies continuously in it. The optimal shape of the corneal lens is determined, with the disadvantageous lenses, with respect to the possible minimal spherical aberration and amount of reflected light from the transition layer, being rejected. The optimal position of the transition layer in the cornea can be obtained from the minimization of the amount of diffracted light on the marginal connection of the layers. The optimal corneal lens for backswimmer has ellipsoid boundary surfaces; the optimal transition layer in it is thin bell-shaped, at the marginal connection of which there is no dimple, the maximum of the layer is on the margin of the cornea. The shape of the theoretically optimal corneal lens, the shape and position of the theoretically optimal transition layer agree well with those of Notonecta glauca. The question posed, the geometric optical method used and the results presented are of general importance, and not only with respect to vision in the bug Notonecta, but also in the fossil trilobites, or in the wave guide theories which have been employed in similar modelling problems, in design of system of lenses without spherical aberration, for example.     

Tropomodulin 1 constrains fiber cell geometry during elongation and maturation in the lens cortex

Lens fiber cells exhibit a high degree of hexagonal packing geometry, determined partly by tropomodulin 1 (Tmod1), which stabilizes the spectrin-actin network on lens fiber cell membranes. To ascertain whether Tmod1 is required during epithelial cell differentiation to fiber cells or during fiber cell elongation and maturation, the authors quantified the extent of fiber cell disorder in the Tmod1-null lens and determined locations of disorder by confocal microscopy and computational image analysis. First, nearest neighbor analysis of fiber cell geometry in Tmod1-null lenses showed that disorder is confined to focal patches. Second, differentiating epithelial cells at the equator aligned into ordered meridional rows in Tmod1-null lenses, with disordered patches first observed in elongating fiber cells. Third, as fiber cells were displaced inward in Tmod1-null lenses, total disordered area increased due to increased sizes (but not numbers) of individual disordered patches. The authors conclude that Tmod1 is required first to coordinate fiber cell shapes and interactions during tip migration and elongation and second to stabilize ordered fiber cell geometry during maturation in the lens cortex. An unstable spectrin-actin network without Tmod1 may result in imbalanced forces along membranes, leading to fiber cell rearrangements during elongation, followed by propagation of disorder as fiber cells mature.     

Evidence for gelsolin as a corneal crystallin in zebrafish

We have shown that gelsolin is one of the most prevalent water-soluble proteins in the transparent cornea of zebrafish. There are also significant amounts of actin. In contrast to actin, gelsolin is barely detectable in other eye tissues (iris, lens, and remaining eye) of the zebrafish. Gelsolin cDNA hybridized intensely in Northern blots to RNA from the cornea but not from the lens, brain, or headless body. The deduced zebrafish gelsolin is approximately 60% identical to mammalian cytosolic gelsolin and has the characteristic six segmental repeats as well as the binding sites for actin, calcium, and phosphatidylinositides. In situ hybridization tests showed that gelsolin mRNA is concentrated in the zebrafish corneal epithelium. The zebrafish corneal epithelium stains very weakly with rhodamine-phalloidin, indicating little F-actin in the cytoplasm. In contrast, the mouse corneal epithelium contains relatively little gelsolin and stains intensely with rhodamine-phalloidin, as does the zebrafish extraocular muscle. We propose, by analogy with the diverse crystallins of the eye lens and with the putative enzyme-crystallins (aldehyde dehydrogenase class 3 and other enzymes) of the mammalian cornea, that gelsolin and actin-gelsolin complexes act as water-soluble crystallins in the zebrafish cornea and contribute to its optical properties.     

Corneal microprojections in coleoid cephalopods

The cornea is the first optical element in the path of light entering the eye, playing a role in image formation and protection. Corneas of vertebrate simple camera-type eyes possess microprojections on the outer surface in the form of microridges, microvilli, and microplicae. Corneas of invertebrates, which have simple or compound eyes, or both, may be featureless or may possess microprojections in the form of nipples. It was previously unknown whether cephalopods (invertebrates with camera-type eyes like vertebrates) possess corneal microprojections and, if so, of what form. Using scanning electron microscopy, we examined corneas of a range of cephalopods and discovered nipple-like microprojections in all species. In some species, nipples were like those described on arthropod compound eyes, with a regular hexagonal arrangement and sizes ranging from 75 to 103?nm in diameter. In others, nipples were nodule shaped and irregularly distributed. Although terrestrial invertebrate nipples create an antireflective surface that may play a role in camouflage, no such optical function can be assigned to cephalopod nipples due to refractive index similarities of corneas and water. Their function may be to increase surface-area-to-volume ratio of corneal epithelial cells to increase nutrient, gas, and metabolite exchange, and/or stabilize the corneal mucous layer, as proposed for corneal microprojections of vertebrates.     

Homozygous mutations in PXDN cause congenital cataract, corneal opacity, and developmental glaucoma

Anterior segment dysgenesis describes a group of heterogeneous developmental disorders that affect the anterior chamber of the eye and are associated with an increased risk of glaucoma. Here, we report homozygous mutations in peroxidasin (PXDN) in two consanguineous Pakistani families with congenital cataract-microcornea with mild to moderate corneal opacity and in a consanguineous Cambodian family with developmental glaucoma and severe corneal opacification. These results highlight the diverse ocular phenotypes caused by PXDN mutations, which are likely due to differences in genetic background and environmental factors. Peroxidasin is an extracellular matrix-associated protein with peroxidase catalytic activity, and we confirmed localization of the protein to the cornea and lens epithelial layers. Our findings imply that peroxidasin is essential for normal development of the anterior chamber of the eye, where it may have a structural role in supporting cornea and lens architecture as well as an enzymatic role as an antioxidant enzyme in protecting the lens, trabecular meshwork, and cornea against oxidative damage.     

Ectopic expression of BEAF32A in the Drosophila eye imaginal disc inhibits differentiation of photoreceptor cells and induces apoptosis

Transgenic flies were established in which ectopic expression of boundary element-associated factor (BEAF) 32A was targeted to the Drosophila eye imaginal disc. The eyes of the adult fly displayed a severe rough eye phenotype. When these eyes were sectioned, most ommatidia were found to be fused and irregularly shaped rhabdomeres were observed. In the developing eye imaginal disc, expression of BEAF32A inhibited differentiation of photoreceptor cells. Expression of BEAF32A also induced extensive apoptosis of eye imaginal disc cells and, consistent with this, co-expression of baculovirus P35 in the eye imaginal disc suppressed the BEAF32A-induced rough eye phenotype. To investigate the effects of BEAF32A on regulation of chromatin structure, genetic crosses of the BEAF32A-overexpressing flies with loss-of-function mutants for genes encoding other boundary element-binding factors or regulators of chromatin structure were conducted. Interestingly, half-dose reduction of the su(Hw) gene strongly enhanced the rough eye phenotype induced by BEAF32A. Furthermore, genetic crosses of the transgenic flies with loss-of-function mutants for genes interacting with Polycomb revealed specific links between BEAF32A and genes such as Distalless and kohtalo, suggesting a relation to the chromatin insulator function of BEAF. In addition, genetic crosses of transgenic flies expressing BEAF32A with a collection of Drosophila deficiency stocks allowed us to identify several genomic regions, deletions of which caused enhancement or suppression of the BEAF32A-induced rough eye phenotype. The transgenic flies established in this study should be useful to identify targets of BEAF32A and its positive or negative regulators in Drosophila.     

Contact Lens Related Corneal Infections

This article describes microbial keratitis, infection of the cornea by micro-organisms. Contact lens wear is a predisposing factor for the development of microbial keratitis. Micro-organisms probably adhere to the contact lens, transfer from the contact lens to a damaged or compromised corneal epithelial surface, penetrate into the deeper layers of the cornea and produce corneal damage. Host responses to the invading micro-organisms, while designed to protect the eye, can often exacerbate the situation and the resulting microbial keratitis can lead to permanent blindness. The microbial, biochemical and immunological aspects of MK will be described in detail.     

Pseudomonas aeruginosa proteases and corneal virulence

Pseudomonas aeruginosa is a common cause of corneal infections, particularly among users of soft contact lenses. Previous studies with chemically induced mutants deficient in alkaline protease (AP) or elastase (LasB) suggested that these proteases contributed to the rapid liquifactive stromal necrosis characteristic of P. aeruginosa corneal infections. Because these mutants might harbor other chromosomal changes that could affect virulence, the role of these proteases in the pathogenesis of corneal disease (as well as a second elastase, LasA protease) was reexamined by constructing isogenic mutants deficient only in these enzymes. Allelic exchange was used to construct mutants of P. aeruginosa PAO1-V deficient in AP (PAO1-V AP[ - ]), LasB and LasA protease (PDO801 LasB[ - ]), or all three proteases (PDO801 TM). These mutants were then evaluated for virulence using mouse scratch and rabbit intrastromal injection models of corneal disease. Loss of AP significantly increased disease scores in the rabbit (P < 0.030) but not the mouse (P > 0.060) model of infection. Loss of both elastases had no effect on ocular virulence in either animal model of corneal disease (P > 0.100). The loss of all three proteases significantly decreased disease scores in the rabbit (P < 0.035), but not in the mouse (P > 0.110). Taken together, these data suggest that AP, LasB, and LasA protease are not essential for initiating or maintaining a corneal infection. Furthermore, AP appears to be an important mediator of pathology depending on the location of the organism within the cornea and whether or not concomitant elastolytic activity is present.     

Centering mechanism for soft contact lenses

A soft contact lens, because of its elasticity, can re-center itself over the cornea after it has been displaced by a blink. In this paper, a potential energy method is used to simulate the centering mechanism. Specifications for commercially available soft contact lenses and realistic eye shapes are used for the examples. With this technique we can compare the centering attributes of different lens and eye geometries.     

GROWTH OF LENS AND OCULAR ENVIRONMENT: ROLE OF NEURAL RETINA IN THE GROWTH OF MOUSE LENS AS REVEALED BY AN IMPLANTATION EXPERIMENT

The lens of 6-day-old normal mouse was implanted into the lentectomized eye of adult mouse to examine the effect of retina upon the growth of the implanted lens in vivo. The implanted lens grew normally and its transparency was kept for more than 5 months after implantation. The connection between the implanted lens and the ciliary part of the recipient iris was well established with the regeneration of zonular fibers from the recipient. In young lenses implanted reversely into adult eyes, the epithelial cells facing the retina elongated and a new epithelium was formed on the corneal side of the lens within 5 days. Young lenses implanted either in normal or reverse orientation into eyes from which the retina was previously removed did not grow. The cells of the original lens epithelium of these lenses were randomly accumulated beneath the posterior lens capsule, while the anterior portion of the implanted lenses became an epithelial structure without cell elongation. These results suggest that the growth of the implanted lens may be dependent on the retina of the adult eye.     

The role of corneal crystallins in the cellular defense mechanisms against oxidative stress

The refracton hypothesis describes the lens and cornea together as a functional unit that provides the proper ocular transparent and refractive properties for the basis of normal vision. Similarities between the lens and corneal crystallins also suggest that both elements of the refracton may also contribute to the antioxidant defenses of the entire eye. The cornea is the primary physical barrier against environmental assault to the eye and functions as a dominant filter of UV radiation. It is routinely exposed to reactive oxygen species (ROS)-generating UV light and molecular O(2) making it a target vulnerable to UV-induced damage. The cornea is equipped with several defensive mechanisms to counteract the deleterious effects of UV-induced oxidative damage. These comprise both non-enzymatic elements that include proteins and low molecular weight compounds (ferritin, glutathione, NAD(P)H, ascorbate and alpha-tocopherol) as well as various enzymes (catalase, glucose-6-phosphate dehydrogenase, glutathione peroxidase, glutathione reductase, and superoxide dismutase). Several proteins accumulate in the cornea at unusually high concentrations and have been classified as corneal crystallins based on the analogy of these proteins with the abundant taxon-specific lens crystallins. In addition to performing a structural role related to ocular transparency, corneal crystallins may also contribute to the corneal antioxidant systems through a variety of mechanisms including the direct scavenging of free radicals, the production of NAD(P)H, the metabolism and/or detoxification of toxic compounds (i.e. reactive aldehydes), and the direct absorption of UV radiation. In this review, we extend the discussion of the antioxidant defenses of the cornea to include these highly expressed corneal crystallins and address their specific capacities to minimize oxidative damage.     

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.