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.     

Influence of absorption on polarization effects in light scattering from human red blood cells

Polarization effects in light scattering are sensitive indicators of cell structure and structural changes in time. In the spectral regions where the optical properties of the scatterers are relatively constant, the scattering pattern scales, it contracts or expands in a predictable manner as a function of the wavelength. In the spectral regions where the optical properties are strongly wavelength dependent (near absorption bands, etc.) the scattering curves do not scale, but change drastically in phase and amplitude as the wavelength is varied. Reported here is an empirical study of the magnitude of the influence of absorption on the polarization effects in light scattering. Scattering curves have been obtained for human red blood cells in the absorption band (blue light) and far from the absorption band (red light). The scattering at these wavelengths shows very strong nonscaling differences. These observations suggest the use of polarization effects in light scattering and their wavelength dependence for the studies of structural changes in cell nuclei. Nucleoproteins have strong absorption, optical rotatory dispersion and circular dichroism bands in the ultraviolet region of the spectrum, whereas there is little ψ-dependence in the visible range. There is also the possibility of binding specific chromophoric dyes to cell components, thus introducing absorption bands in the visible range, where scattering instrumentation and laser light sources are more readily available.     

The eye of the magellanic penguin (Spheniscus magellanicus): structure of the anterior segment

We undertook a light and scanning electron microscopic study of the eye in the Magellanic penguin (Spheniscus magellanicus). The anatomical peculiarities of the eyeball shape in Sphenisciformes have been previously described by others; here, we show that they are accompanied by several modifications in the organization of the anterior segment of the eye. The main change was found in the portion of opaque sclera extending from the cornea to the anterior border of the scleral ossicles, which was much broader than in other avian eyes. This scleral region was made of a very dense fibrous tissue and was as difficult to cut as the ossicles. The corneo-scleral boundary was also different from that of other birds, since the aqueous humor channel and the pectinate ligament were located 1.0-1.5 mm posterior to the cornea. The osseous ring was formed by 13 bones, including three pairs of over- and underplates. There was a single ciliary muscle, with meridionally oriented striated fibers. They were inserted on a circumference along the boundary between the fibrous sclera and the ossicles, far away from the wall of the aqueous humor channel. On their posterior end, the muscle fibers formed a tendinous structure attached to the inner surface of the sclera and to the outer surface of the ciliary body. Only short zonular fibrils were observed. These anatomical features are probably relevant for the adaptation of penguin eyes to vision on land and in the aquatic environment.     

A computational remodeling approach to predict the physiological architecture of the collagen fibril network in corneo-scleral shells

Organized collagen fibrils form complex networks that introduce strong anisotropic and highly nonlinear attributes into the constitutive response of human eye tissues. Physiological adaptation of the collagen network and the mechanical condition within biological tissues are complex and mutually dependent phenomena. In this contribution, a computational model is presented to investigate the interaction between the collagen fibril architecture and mechanical loading conditions in the corneo-scleral shell. The biomechanical properties of eye tissues are derived from the single crimped fibril at the micro-scale via the collagen network of distributed fibrils at the meso-scale to the incompressible and anisotropic soft tissue at the macro-scale. Biomechanically induced remodeling of the collagen network is captured on the meso-scale by allowing for a continuous re-orientation of preferred fibril orientations and a continuous adaptation of the fibril dispersion. The presented approach is applied to a numerical human eye model considering the cornea and sclera. The predicted fibril morphology correlates well with experimental observations from X-ray scattering data.     

Pore canals in the cornea of a functionally specialized area of the honey bee's compound eye

Summary The fine structure of the cornea in an anatomically and functionally specialized part of the honey bee's compound eye (dorsal rim area) was examined by light microscopy, transmission electron and scanning electron microscopy. Under incident illumination the cornea appears grey and cloudy, leaving only the centers of the corneal lenses clear. This is due to numerous pore canals that penetrate the cornea from the inside, ending a few m below the outer surface. They consist of (1) a small cylindrical cellular evagination of a pigment cell (proximal), and (2) a rugged-walled, pinetree-shaped extracellular part (distal). The functional significance of these pore canals is discussed. It is concluded that their light scattering properties cause the wide visual fields of the photoreceptor cells measured electrophysiologically in the dorsal rim area, and that this is related to the way this eye region detects polarization in skylight.     

Linear clusters of gold nanoparticles in quasinematic layers of DNA liquid-crystalline dispersion particles

The effects of small size (～2 nm) gold nanoparticles on the properties of particles of cholesteric liquid-crystalline dispersions formed by double-stranded DNA molecules were analyzed. It has been shown that gold nanoparticles induce two different processes. First, they facilitate reorganization of the spatial cholesteric structure of dispersion particles to nematic one. This process is accompanied by the fast decrease in the amplitude of abnormal band in the CD spectrum. Second, they can form ensembles consisting of gold nanoparticles. This process is accompanied by the development and displacement of surface plasmon resonance band in the visible region of the absorption spectrum. The appearance of this band is analyzed by considering two different models of the formation of ensembles consisting of gold nanoparticles. By small-angle X-ray scattering we performed structural analysis of phases formed by DNA cholesteric liquid-crystalline dispersion particles treated with gold nanoparticles. As a result of this study it was possible to prove the formation of linear clusters of gold nanoparticles in the “free space” between the adjacent DNA molecules fixed in the quasinematic layers of liquid-crystalline particles. It has been hypothesized that the formation of linear clusters of gold nanoparticles is most likely related to DNA molecules, ordered in the spatial structure of quasinematic layers, and the toxicity of these nanoparticles in biological systems hypothesized.     

Action Spectra of Photosynthetic Activities of Chlorella ellipsoidea

• 1) Suspensions of Chlorella show an even stronger light scattering than suspensions of chloroplasts of spinach. The bands of absorption are thus broadened and, at higher concentrations, moved to lower wave-lengths. The intensity of the photosynthesis closely follows the curves of light scattering, a fact partly explaining the high efficiency of green light. Calculated per unit thermoelectrically measured incident energy the action spectrum shows bands at 660–670 nm and c. 500 nm and a comparatively high level of the whole region 500–560 nm.
• 2) Flash experiments show the existence of a steady state carotene/xanthophyll that is moved to reduction (c/x > 1) in blue and green light and to oxidation (c/x < 1) in red light. All experiments point to the existence of two light reactions, the first one involving excitation of carotenoids, with ferredoxin-TPN as acceptor, the second one involving excitation of chlorophyll, with the cytochrome system of the chloroplasts acting as donors of electrons and thus completing an energy converting circulation between pigments and enzyme systems.
• 3) The operation of combined light reactions appears also from experiments with simultaneous or succedaneous illumination with monochromatic light of different wave-lengths. Some effects may be explained from separate excitations of carotenoids and chlorophylls, others may depend on still unknown photic reactions.
• 4) The action spectrum in ultrared shows a positive band at c. 900 nm but no or only very small effects in the region 950–1400 nm. Ultrared radiation has on the other hand an enhancing effect on the light excitation in the visible spectrum. A combination of infrared and visible radiation shows a roughly linear relation between incident energy and photosynthetic effect.
• 5) All experiments were performed in the region of linear relation between intensity of incident light and O₂-production. Induced effects of combined monochromatic regions show a very rapid initial change in the steady states that in one or two minutes simmers down to a balanced state of continued photosynthesis. No change was observed in the total quantity of the pigments.

     

Material properties of the cornea and sclera: a modelling approach to test experimental analysis

Material properties of cornea and sclera are important for maintaining the shape of the eye and the requisite surface curvatures for optics. They also need to withstand the forces of external and internal musculature and fluctuations in intraocular pressure (IOP). These properties are difficult to measure and variable results have been reported. A previously published experimental procedure, from which the material properties of the eyeball coats were obtained, has been modelled in this paper using Finite Element Analysis, in order to test the accuracy of the experiment. Material parameters were calculated from the model and the resulting relationships between stress and strain for the cornea and sclera compared to their experimentally obtained counterparts. The comparison between model and experiment was close for the sclera but more varied for the cornea. The pressure vessel model can be applied for measuring the material properties of the sclera but is less accurate for the cornea.     

Hydroxyproline concentrations in ocular tissues of Arabian camel (Camelus dromedarius Linn.)

Hydroxyproline (Hyp) concentrations (total, free, peptide-bound and protein-bound) in camel eye tissues were determined. Total Hyp concentration was highest in iris, followed by ciliary body, sclera, cornea, lens and retina; the difference between total Hyp concentration of iris and sclera (P < 0.05) and cornea, lens and retina (P < 0.001) was statistically significant. Cornea had the highest concentration of free Hyp, followed by ciliary body, retina, iris, sclera and lens (P < 0.001). Peptide-bound Hyp concentration was highest in iris, followed by lens, cornea, ciliary body, retina and sclera (P < 0.001). Iris also had the highest concentration of protein-bound Hyp, followed by ciliary body, sclera, cornea, retina and lens; the difference in the protein-bound Hyp concentration between iris and sclera (P < 0.05) and cornea, retina and lens (P < 0.001) was statistically significant. Iris was also found to have the highest concentration of collagen, followed by ciliary body, sclera, cornea, lens and retina; the difference between the collagen concentration of iris and sclera (P < 0.05) and cornea, lens and retina (P < 0.001) was statistically significant. These variations may result from differences in the collagen structure and/or composition in these tissues.     

The transparent lens and cornea in the mouse and zebra fish eye

The lens and cornea combine to form a single optical element in which transparency and refraction are the fundamental biophysical characteristics required for a functional visual system. Although lens and cornea have different cellular and extracellular specializations that contribute to transparency and refraction, their development is closely related. In the embryonic mouse, the developing cornea and lens separate early. In contrast, zebra fish lens and cornea remain connected during early development and the optical properties of the cornea and lens observed by slit lamp and quasielastic laser light scattering spectroscopy (QLS) are more similar in the zebra fish eye than in the mouse eye. Optical similarities between cornea and lens of zebra fish may be the result of similarities in the cellular development of the cornea and lens.     

Light transmission in the human cornea as a function of position across the ocular surface: theoretical and experimental aspects

This article investigates the theoretical basis for differences in visible light transmission through the human cornea as a function of distance from the center. Experimentally, transmission decreases approximately linearly up to 3 mm from the central axis, then quadratically beyond this. It is known that collagen fibril number density and collagen fibril radii change from the central region to the corneal periphery. We modeled, using the direct-summation-of-scattered-fields method, the effects these ultrastructural changes would be expected to have on light transmission, accounting for the increase in corneal thickness from center to edge. Fibril positions for the modeling were obtained from electron micrographs of human cornea. Theoretically, transmission remains fairly constant across the central cornea; then, as the fibril diameter increases, the predicted scattering increases. Interfibrillar spacing changes alter the refractive index ratio between matrix and fibril; this was modeled in our theoretical deductions. Fibril number density had a minimal effect on light propagation. Our theoretical deductions were in broad agreement with our experimental data. It is concluded that the reduced transparency in the peripheral stroma is primarily caused by changes in fibril radius and an increase in refractive index ratio between the fibril and the interfibrillar substance.     

X-ray scattering used to map the preferred collagen orientation in the human cornea and limbus

Many properties of connective tissues are governed by the organization of the constituent collagen. For example, the organization of collagen in the cornea and the limbus, where the cornea and sclera meet, is an important determinant of corneal curvature and hence of the eye's focusing power. We have used synchrotron X-ray scattering to map the orientation of the collagen fibrils throughout the human cornea, limbus, and adjacent sclera. We demonstrate a preferred orientation of collagen in the vertical and horizontal directions that is maintained to within about 1 mm from the limbus, where a circular or tangential disposition of fibrils occurs. The data are also used to map the relative distribution of both the total and the preferentially aligned collagen in different parts of the tissue, revealing considerable anisotropy. The detailed structural information provided is an important step toward understanding the shape and the mechanical properties of the tissue.     

Imaging depth extension of optical coherence tomography in rabbit eyes using optical clearing agents

Optical coherence tomography has become an indispensable diagnostic tool in ophthalmology for imaging the retina and the anterior segment of the eye. However, the imaging depth of optical coherence tomography is limited by light attenuation in tissues due to optical scattering and absorption. In this study of rabbit eye both ex vivo and in vivo, optical coherence tomography imaging depth of the anterior and posterior segments of the eye was extended by using optical clearing agents to reduce multiple scattering. The sclera, the iris, and the ciliary body were clearly visualized by direct application of glycerol at an incision on the conjunctiva, and the posterior boundary of sclera and even the deeper tissues were detected by submerging the posterior segment of eye in glycerol solution ex vivo or by retro-bulbar injection of glycerol in vivo. The ex vivo rabbit eyes recovered to their original state in 60 s after saline-wash treatment, and normal optical coherence tomography images of the posterior segment of the sample eyes proved the self-recovery of in vivo performance. Signal intensities of optical coherence tomography images obtained before and after glycerol treatment were compared to analysis of the effect of optical clearing. To the best of our knowledge, this is the first study for imaging depth extension of optical coherence tomography in both the anterior and posterior segments of eye by using optical clearing agents.     

Localization of the ultraviolet-sensor Opn5m and its effect on myopia-related gene expression in the late-embryonic chick eye

Recent studies show that exposure to ultraviolet (UV) light suppresses ocular elongation, which causes myopia development. However, the specific mechanisms of this process have not been elucidated. A UV-sensor, Opsin 5 (Opn5) mRNA was shown to be present in extraretinal tissues. To test the possibility that UV-signals mediated by Opn5 would have a direct effect on the outer connective tissues of the eye, we first examined the expression patterns of a mammalian type Opn5 (Opn5m) in the late-embryonic chicken eye. Quantitative PCR showed Opn5m mRNA expression in the cornea and sclera. The anti-Opn5m antibody stained a small subset of cells in the corneal stroma and fibrous sclera. We next assessed the effect of UV-A (375 nm) irradiation on the chicken fibroblast cell line DF-1 overexpressing chicken Opn5m. UV-A irradiation for 30 min significantly increased the expression of Early growth response 1 (Egr1), known as an immediate early responsive gene, and of Matrix metalloproteinase 2 (Mmp2) in the presence of retinal chromophore 11-cis-retinal. In contrast, expression of Transforming growth factor beta 2 and Tissue inhibitor of metalloproteinase 2 was not significantly altered. These results indicate that UV-A absorption by Opn5m can upregulate the expression levels of Egr1 and Mmp2 in non-neuronal, fibroblasts. Taken together with the presence of Opn5m in the cornea and sclera, it is suggested that UV-A signaling mediated by Opn5 in the extraretinal ocular tissues could influence directly the outer connective tissues of the chicken late-embryonic eye.     

虻复眼感杆光导的模式与适应状态的关系

利用逆向照明方法,不经角膜中和,通过光学显佩镜快速观察和记录了暗适应,短时间明适应和长时间明适应状态下,虻(Atylotus miser Szilady)复眼感杆光导的模式。同时与角膜中和的实验结果作了比较。本文还讨论了憎黄虻复眼角膜透镜的作用以及该复眼对视觉信憎黄息抽样方式与适应状态的关系。     

南五台蝎蛉成虫复眼的超微结构

采用扫描电镜和组织切片法,观察南五台蝎蛉Panorpa nanwutaina Chou成虫复眼的超微结构。南五台蝎蛉复眼近半椭球形,包括1500～1600个小眼。小眼表面光滑,由角膜、晶体、2个初级和12个次级色素细胞、视杆、以及基膜组成。角膜为多层片状纤维结构;晶体含有4个晶锥细胞;视杆由若干个视网膜细胞组成。晶体、视杆周围、和色素细胞内含有大量的色素颗粒,基膜两侧也有色素颗粒分布。南五台蝎蛉的复眼属于并列像眼。与普通蝎蛉P.communis L.小眼的次级色素细胞数目不同。讨论了南五台蝎蛉角膜的功能以及感觉毛和次级色素细胞在分类中的作用。     

Relationships between eye factors and lens-forming transformations in the cornea and pericorneal epidermis of larval Xenopus laevis

Larval Xenopus laevis at stage 56 (Nieuwkoop and Faber, '56) were subjected to various types of lentectomy: (1) simple lentectomy, from the pupillary space after incision of outer and inner cornea; (2) lentectomy from the dorsal region of the eye; (3) lentectomy from the dorsal region of the eye and simultaneous incision of the outer cornea; (4) lentectomy from the dorsal region of the eye and simultaneous incision of the outer and inner cornea. The results obtained show that the outer cornea underwent lens-forming transformations only when the inner cornea had been incised, thus permitting outer cornea (Experiments I-IV). No lens regeneration occurred when the inner cornea was left intact (Experiments II, III). It was concluded that the factor(s) allowing the lens-forming transformations of the outer cornea is not an aspecific nutritional factor(s) but a more specific factor(s) that cannot reach the outer cornea when the inner cornea is intact. Therefore, the absence of the lens and sufficient nutrient available to the outer cornea are not enough to allow lens regeneration from the outer cornea. When lens removal was carried out through the dorsal part of the eye (Experiments III-IV) the lens regenerated from the pericorneal epidermis of this region in a large number of cases.     

Relationship of turbidity to the stages of platelet aggregation

The process of platelet aggregation as detected by turbidity changes in the platelet aggregometer was studied relative to light scattering by large particles. For latex beads a plot of light scattering intensity/unit mass versus particle size gave increased light scattering intensity for small particle sizes but decreased scattering at large particle size. This behavior is predicted by Rayleigh-Gans theory. These results were related to the platelet aggregometer, an optical instrument used to measure the association of small particles (monomeric platelets) to large particles (platelet aggregates). Formalin-fixed platelets do not show changes in light transmission due to energy-requiring processes, such as shape change, so that turbidity changes in the presence of aggregating agents could be attributed to a change in platelet aggregation state. Small platelet aggregates showed increased turbidity compared to a similar mass of monomeric platelets. In fact, very large platelet aggregates that were visible to the unaided eye were needed to produce a decrease in light scattering intensity. Thus, turbidity can either increase or decrease with platelet aggregation depending on the size of the aggregates. Studies of platelet aggregation that show no initial increase in turbidity must be characterized by dominance of large platelet aggregates and monomeric platelets.     

Material properties and effect of preconditioning of human sclera,optic nerve,and optic nerve sheath

The optic nerve (ON) is a recently recognized tractional load on the eye during larger horizontal eye rotations. In order to understand the mechanical behavior of the eye during adduction, it is necessary to characterize material properties of the sclera, ON, and in particular its sheath. We performed tensile loading of specimens taken from fresh postmortem human eyes to characterize the range of variation in their biomechanical properties and determine the effect of preconditioning. We fitted reduced polynomial hyperelastic models to represent the nonlinear tensile behavior of the anterior, equatorial, posterior, and peripapillary sclera, as well as the ON and its sheath. For comparison, we analyzed tangent moduli in low and high strain regions to represent stiffness. Scleral stiffness generally decreased from anterior to posterior ocular regions. The ON had the lowest tangent modulus, but was surrounded by a much stiffer sheath. The low-strain hyperelastic behaviors of adjacent anatomical regions of the ON, ON sheath, and posterior sclera were similar as appropriate to avoid discontinuities at their boundaries. Regional stiffnesses within individual eyes were moderately correlated, implying that mechanical properties in one region of an eye do not reliably reflect properties of another region of that eye, and that potentially pathological combinations could occur in an eye if regional properties are discrepant. Preconditioning modestly stiffened ocular tissues, except peripapillary sclera that softened. The nonlinear mechanical behavior of posterior ocular tissues permits their stresses to match closely at low strains, although progressively increasing strain causes particularly great stress in the peripapillary region.