Analysis of human crystalline lens accommodation

The behavior of the human crystalline lens during accommodation is analytically studied. The lens is modeled as a closed axisymmetrical membrane shell of varying thickness enclosing an incompressible liquid. To simulate zonular tension associated with lenticular accommodation, an axisymmetrical radial force or displacement is imposed around the shell equator. Two second-order, simultaneous, nonlinear governing differential equations are derived. Numerical results, obtained from the investigation of human lens profiles of three independently published MRI images and a drawing of a microphotograph, demonstrate that when zonular traction within the physiological force range of the ciliary muscle is exerted, both central lens thickness and central optical power increase. Qualitatively, these increases are independent of lens shape. However, the magnitude of these changes is dependent on the initial profile of the lens and is enhanced by the "natural" variation in capsular thickness. Only when a pulling force significantly exceeds the force capacity of the ciliary muscle does the lens flatten and its central thickness and optical power decrease.     

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.     

Scanning electron microscopy of the zonular fibers in the rat eye

Summary The three-dimensional arrangement of the zonular fibers of Zinn and their ultrastructure was studied with the aid of scanning and transmission electron microscopy.Most of the thicker zonular fibers are arranged in straight bundles between the ciliary body and the lens, while the thinner fibers form a complex three-dimensional network interconnecting all the zonular fibers. These do originate from the limiting membrane covering the ciliary body. The zonular fibers are subdivided close to lens and form a complicated network on the surface of the lens capsule, i. e. the zonular lamella. The latter consists of a dense network of fibers and is from a structural point of view closely related to the zonular fibers and not to the lens capsule.The zonular fibers are continuous with those in the vitreous body close to the ciliary body but never in the lenticular two thirds of the zonular fibers or in the retrolental area.The ground substance is possible to demonstrate in freeze-dried specimens by scanning electron microscopy. It appeared granular or amorphous and coated the zonular fibers. It does not form membranes or fill all available space in contrast to its properties in the vitreous body. The many structural similarities between the zonular fibers and the vitreous body indicate perhaps a common origin.Supported by grants from Magnus Bergwalls Stiftelse and the Swedish Medical Research Council (B70-12 X -2543-02, B71-12 X -2543-03).     

Dynamic mechanism of visual accommodation in teleosts: structure of the lens muscle and its nerve control

The accommodatory system was examined in two teleosts (mackerel and bass). The fine structure and innervation of the lens muscle is presented to characterize the muscle organization. The neural pathway involved in the dynamic accommodation was examined by analysing the fibre spectrum of the ciliary nerve, and the nerve that controls the lens-muscle activity was studied by means of electrical stimulation. The lens muscle is composed of smooth-muscle cells, which contain numerous mitochondria. Many synaptic endings are also found on the muscle cells; these synaptic endings contain many agranular vesicles. From the results of the fibre analysis, it was found that the nerve that controls the lens muscle contains less than 100 myelinated nerve fibres in both fish: the electrical stimulation experiments demonstrate that the muscle is controlled by oculomotor (parasympathetic) nerve fibres. Ultrastructural features of the lens muscle and its nerve control resemble those of the mammalian ciliary muscle. The teleostean lens muscle is classified as a multi-unit smooth muscle.     

Quantitative investigation of calcimimetic R568 on beta cell adhesion and mechanics using AFM single-cell force spectroscopy

In this study we use a novel approach to quantitatively investigate mechanical and interfacial properties of clonal β-cells using AFM-Single Cell Force Spectroscopy (SCFS). MIN6 cells were incubated for 48 h with 0.5 mM Ca²⁺ ± the calcimimetic R568 (1 μM). AFM-SCFS adhesion and indentation experiments were performed by using modified tipless cantilevers. Hertz contact model was applied to analyse force–displacement (F–d) curves for determining elastic or Young’s modulus (E). Our results show CaSR-evoked increases in cell-to-cell adhesion parameters and E modulus of single cells, demonstrating that cytomechanics have profound effects on cell adhesion characterization.     

Biofeedback of accommodation to reduce functional myopia

Functional myopia may be defined as the refractive condition of the eye due to spasm of the ciliary muscle. As a result of the ciliary muscle spasm, the crystalline lens becomes more convex, creating a myopic refractive condition. The normal increase and decrease in the refractive power of the crystalline lens is known as accommodation and is controlled by the autonomic nervous system innervation to the ciliary muscle. Previous studies have reported that voluntary control of accommodation is possible by biofeedback training (Cornsweet & Crane, 1973; Randle, 1970). The present research investigated the application of biofeedback control of accommodation to reduce functional myopia. A double-reversal, multiple-baseline design was used to conduct the experiment. The results revealed that the three adult male subjects achieved the preset criterion, a 1/2-diopter reduction from initial baseline to a subsequent baseline. Further analysis of the data revealed even greater changes between initial baseline and feedback periods. Although generalization to a nonexperimental environment was not trained, each subject showed a reduction in myopia and an increase in visual acuity. The results of the experiment clearly demonstrated that functional myopia is subject to voluntary control.The research presented in the text was submitted in partial fulfillment of the requirements for the degree of doctor of philosophy in the Ferkauf Graduate School, Yeshiva University. The author wishes to express appreciation to his dissertation committee, professors Carl Auerbach, Abraham Givner, and Allan C. Goldstein. Also, appreciation is given to Ralph Dippner and John Orzuchowski for their assistance.     

In vivo measurement of age-related stiffening in the crystalline lens by Brillouin optical microscopy

The biophysical and biomechanical properties of the crystalline lens (e.g., viscoelasticity) have long been implicated in accommodation and vision problems, such as presbyopia and cataracts. However, it has been difficult to measure such parameters noninvasively. Here, we used in vivo Brillouin optical microscopy to characterize material acoustic properties at GHz frequency and measure the longitudinal elastic moduli of lenses. We obtained three-dimensional elasticity maps of the lenses in live mice, which showed biomechanical heterogeneity in the cortex and nucleus of the lens with high spatial resolution. An in vivo longitudinal study of mice over a period of 2 months revealed a marked age-related stiffening of the lens nucleus. We found remarkably good correlation (log-log linear) between the Brillouin elastic modulus and the Young's modulus measured by conventional mechanical techniques at low frequencies (~1 Hz). Our results suggest that Brillouin microscopy is potentially useful for basic and animal research and clinical ophthalmology.     

Hyaluronan and chondroitin sulfate proteoglycans are colocalized to the ciliary zonule of the rat eye: a histochemical and immunocytochemical study

 In previous studies, chondroitin sulfate proteoglycans have been localized to the periphery of the zonular fibers and the individual zonular fibrils (or microfibrils) after Cuprolinic blue staining in conjunction with chondroitinase digestions and immunogold labelling with 2-B-6 antibody. In the present study, we wished to determine if these proteoglycans are linked to hyaluronan to form a large multimolecular aggregate. To accomplish this, we localized the hyaluronan using a biotinylated hyaluronan-binding protein fragment of chondroitin sulfate proteoglycan, containing also the link protein, purified from bovine nasal cartilage. The results showed that the ciliary zonule of the rat eye was reactive with the biotinylated hyaluronan-binding probe as demonstrated by streptavidin-peroxidase-diaminobenzidine staining and streptavidin-gold labelling. Hyaluronan-gold labelling showed that the gold particles were mostly localized on the periphery of the zonular fibers, which was similar to the localization pattern of the zonule associated-proteoglycans. This hyaluronan-binding probe also strongly labelled the sites of zonule insertion over the basement membrane of the inner ciliary epithelium at the pars plana and the lens capsule at the equatorial region, which suggests its probable role in the attachment of ciliary zonule to the basement membranes. To demonstrate whether these two molecules are linked to one another, ultrastructural colocalization of both hyaluronan and chondroitin sulfate proteoglycans was performed on the same sections by double-gold labelling, and combined Cuprolinic blue staining and hyaluronan-gold labelling. Gold particles of 15 and 10 nm in sizes labelling both hyaluronan and chondroitin 4-sulfate, were colocalized to the surface of the zonular fibers. The combined Cuprolinic blue staining and hyaluronan-gold labelling showed that the gold particles were localized towards the ends of the Cuprolinic blue-stained rodlets, which strongly suggests that these chondroitin sulfate proteoglycans are linked to the hyaluronan chain to form a large aggregate surrounding the periphery of the zonular fibers. These ciliary zonule-associated proteoglycan-hyaluronan aggregates may play a role in organizing the individual zonular fibrils (microfibrils) into bundles of zonular fibers. Accepted: 5 November 1996     

Volume change of the ocular lens during accommodation

During accommodation, mammalian lenses change shape from a rounder configuration (near focusing) to a flatter one (distance focusing). Thus the lens must have the capacity to change its volume, capsular surface area, or both. Because lens topology is similar to a torus, we developed an approach that allows volume determination from the lens cross-sectional area (CSA). The CSA was obtained from photographs taken perpendicularly to the lenticular anterior-posterior (A-P) axis and computed with software. We calculated the volume of isolated bovine lenses in conditions simulating accommodation by forcing shape changes with a custom-built stretching device in which the ciliary body-zonulae-lens complex (CB-Z-L) was placed. Two measurements were taken (CSA and center of mass) to calculate volume. Mechanically stretching the CB-Z-L increased the equatorial length and decreased the A-P length, CSA, and lens volume. The control parameters were restored when the lenses were stretched and relaxed in an aqueous physiological solution, but not when submerged in oil, a condition with which fluid leaves the lens and does not reenter. This suggests that changes in lens CSA previously observed in humans could have resulted from fluid movement out of the lens. Thus accommodation may involve changes not only in capsular surface but also in volume. Furthermore, we calculated theoretical volume changes during accommodation in models of human lenses using published structural parameters. In conclusion, we suggest that impediments to fluid flow between the aquaporin-rich lens fibers and the lens surface could contribute to the aging-related loss of accommodative power. lens volume calculation; intralenticular fluid movement; presbyopia; mammalian lens     

Multiaxial mechanical behavior of the porcine anterior lens capsule

The biomechanics of the lens capsule of the eye is important both in physiologic processes such as accommodation and clinical treatments such as cataract surgery. Although the lens capsule experiences multiaxial stresses in vivo, there have been no measurements of its multiaxial properties or possible regional heterogeneities. Rather all prior mechanical data have come from 1-D pressure–volume or uniaxial force-length tests. Here, we report a new experimental approach to study in situ the regional, multiaxial mechanical behavior of the lens capsule. Moreover, we report multiaxial data suggesting that the porcine anterior lens capsule exhibits a typical nonlinear pseudoelastic behavior over finite strains, that the in situ state is pre-stressed multiaxially, and that the meridional and circumferential directions are principal directions of strain, which is nearly equibiaxial at the pole but less so towards the equator. Such data are fundamental to much needed constitutive formulations.     

Cerebral cortical and brainstem areas related to the central control of lens accommodation in cat and monkey.

1. Studies on the central nervous system related to lens accommodation in cat and monkey were reviewed. 2. During the last decade, a considerable amount of neurophysiological data on the peripheral innervation of the ciliary muscle, properties of parasympathetic oculomotor neurons and mesencephalic reticular neurons have accumulated. 3. Neurophysiological and anatomical evidence supporting the contribution of the cerebellum to lens accommodation has been obtained. 4. Recently, cerebral cortical neurons in the parieto-occipital cortex with activities related to lens accommodation were found in cat and monkey.     

Comparative and functional studies on the suspensory apparatus of the lens (Mammalia)

Summary The suspensory apparatus of the lens has been studied in two species of placental mammals (dog and horse) and in two species of marsupials (phalanger and wallaby). Particular attention has been paid to the suspensory apparatus of the dog and the phalanger. It has been found that the suspensory apparatus of the lens shows great morphological variability, although some features generally are more common in one species than in the others. For instance, in the dog there is a tendency to form fibrous bundles, an uneven distribution of fibres around the lens, and a fan-like arrangement of these fibres at their attachment to the lens. In the phalanger the fibres are evenly distributed around most of the circumference of the lens and the bundles, if formed, are slender. An area devoid of fibres, described as a diastema,is also present.The morphology of ciliary folds, and of the fibrous bundles and the various types of interconnections between them, have been described and illustrated, and the comparative morphology of the suspensory apparatus of the lens discussed. The reduction of the accommodative mechanism from that situated on both horizontal and vertical planes to that on a horizontal plane only has been discussed. The latter type is found in mammals. The mechanics of the suspensory apparatus of the lens has also been considered and attention given to the rotation of the ciliary folds and the twisting of the fibrous bundles, as observed in the dry specimens. A possible relationship of this finding to the adaptation of the lens has also been discussed. The suggestion has been put forward that the suspensory apparatus of the lens is not only suspensory but also regulatory in function, the regulation depending on its ability to disperse stresses between the ciliary muscle and the lens. The formation of bundles, the existence of interconnecting fibres and interconnecting nets, and the spreading of the fibres of a bundle at its attachment to the lens, are the main factors contributing to the regulation of stresses.The morphology of the suspensory apparatus of the dog and the phalanger was compared with that of man.     

The action of ciliary muscle contraction on accommodation of the lens explored with a 3D model

Biomechanics and Modeling in Mechanobiology - The eye’s accommodative mechanism changes optical power for near vision. In accommodation, ciliary muscle excursion relieves lens tension,...     

Fibrillin microfibrils are stiff reinforcing fibres in compliant tissues

Fibrillin-rich microfibrils have endowed tissues with elasticity throughout multicellular evolution. We have used molecular combing techniques to determine Young's modulus for individual microfibrils and X-ray diffraction of zonular filaments of the eye to establish the linearity of microfibril periodic extension. Microfibril periodicity is not altered at physiological zonular tissue extensions and Young's modulus is between 78 MPa and 96 MPa, which is two orders of magnitude stiffer than elastin. We conclude that elasticity in microfibril-containing tissues arises primarily from reversible alterations in supra-microfibrillar arrangements rather than from intrinsic elastic properties of individual microfibrils which, instead, act as reinforcing fibres in fibrous composite tissues.     

Probing elasticity and adhesion of live cells by atomic force microscopy indentation

Atomic force microscopy (AFM) indentation has become an important technique for quantifying the mechanical properties of live cells at nanoscale. However, determination of cell elasticity modulus from the force–displacement curves measured in the AFM indentations is not a trivial task. The present work shows that these force–displacement curves are affected by indenter-cell adhesion force, while the use of an appropriate indentation model may provide information on the cell elasticity and the work of adhesion of the cell membrane to the surface of the AFM probes. A recently proposed indentation model (Sirghi, Rossi in Appl Phys Lett 89:243118, 2006), which accounts for the effect of the adhesion force in nanoscale indentation, is applied to the AFM indentation experiments performed on live cells with pyramidal indenters. The model considers that the indentation force equilibrates the elastic force of the cell cytoskeleton and the adhesion force of the cell membrane. It is assumed that the indenter-cell contact area and the adhesion force decrease continuously during the unloading part of the indentation (peeling model). Force–displacement curves measured in indentation experiments performed with silicon nitride AFM probes with pyramidal tips on live cells (mouse fibroblast Balb/c3T3 clone A31-1-1) in physiological medium at 37°C agree well with the theoretical prediction and are used to determine the cell elasticity modulus and indenter-cell work of adhesion. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Proteoglycans associated with the ciliary zonule of the rat eye: a histochemical and immunocytochemical study

The structural organization of integral and associated components of the ciliary zonule is still not fully understood. The present study is to localize and characterize the proteoglycans associated with the ciliary zonule of the rat eye by Cuprolinic blue (CB) staining and immunocytochemistry. After CB staining, the proteoglycans appeared as electron dense elongated rodlets and were localized with the zonular fibers. They were seen lying on the periphery of the zonular fibers or along the length of the individual fibrils. Most of the CB rodlets had a size of 60–170 nm long (average 130 nm) and 25 nm wide. Smaller CB rodlets measuring 25–60 nm long (average 45 nm) and 12 nm wide were sometimes found associated with the individual zonular fibrils. The CB rodlets were removed after chondroitinase ABC or chondroitinase AC treatment, but were resistant to heparitinase, nitrous acid, keratanase orStreptomyces hyaluronidase digestions. The ciliary zonule was also immunostained with three monoclonal antibodies: 2-B-6 specific for chondroitin 4-sulfate, 3-B-3 for chondroitin 6-sulfate and 1-B-5 for unsulfated chondroitin, using indirect immunoperoxidase or immuno-colloidal gold methods. The zonular fibers were immunoperoxidase stained and immunogold labeled by 2-B-6, but were not reactive to 3-B-3 and 1-B-5. The results demonstrate that chondroitin sulfate proteoglycan is associated with the ciliary zonule of the rat eye.     

Characterizing the normal proteome of human ciliary body

Background

The ciliary body is the circumferential muscular tissue located just behind the iris in the anterior chamber of the eye. It plays a pivotal role in the production of aqueous humor, maintenance of the lens zonules and accommodation by changing the shape of the crystalline lens. The ciliary body is the major target of drugs against glaucoma as its inhibition leads to a drop in intraocular pressure. A molecular study of the ciliary body could provide a better understanding about the pathophysiological processes that occur in glaucoma. Thus far, no large-scale proteomic investigation has been reported for the human ciliary body.

Results

In this study, we have carried out an in-depth LC-MS/MS-based proteomic analysis of normal human ciliary body and have identified 2,815 proteins. We identified a number of proteins that were previously not described in the ciliary body including importin 5 (IPO5), atlastin-2 (ATL2), B-cell receptor associated protein 29 (BCAP29), basigin (BSG), calpain-1 (CAPN1), copine 6 (CPNE6), fibulin 1 (FBLN1) and galectin 1 (LGALS1). We compared the plasma proteome with the ciliary body proteome and found that the large majority of proteins in the ciliary body were also detectable in the plasma while 896 proteins were unique to the ciliary body. We also classified proteins using pathway enrichment analysis and found most of proteins associated with ubiquitin pathway, EIF2 signaling, glycolysis and gluconeogenesis.

Conclusions

More than 95% of the identified proteins have not been previously described in the ciliary body proteome. This is the largest catalogue of proteins reported thus far in the ciliary body that should provide new insights into our understanding of the factors involved in maintaining the secretion of aqueous humor. The identification of these proteins will aid in understanding various eye diseases of the anterior segment such as glaucoma and presbyopia.     

Surface change of the mammalian lens during accommodation

Classical theories suggest that the surface area of the crystalline lens changes during accommodation while the lens volume remains constant. Our recent work challenged this view by showing that the lens volume decreases as the lens flattens during unaccommodation. In this paper we investigate 1) the magnitude of changes in the surface of the in vitro isolated cow lens during simulated accommodation, as well as that of human lens models, determined from lateral photographs and the application of the first theorem of Pappus; and 2) the velocity of the equatorial diameter recovery of prestretched cow and rabbit lenses by using a custom-built software-controlled stretching apparatus synchronized to a digital camera. Our results showed that the in vitro cow lens surface changed in an unexpected manner during accommodation depending on how much tension was applied to flatten the lens. In this case, the anterior surface initially collapsed with a reduction in surface followed by an increase in surface, when the stretching was applied. In the human lens model, the surface increased when the lens unaccommodated. The lens volume always decreases as the lens flattens. An explanation for the unexpected surface change is presented and discussed. Furthermore, we determined that the changes in lens volume, as reflected by the speed of the equatorial diameter recovery in in vitro cow and rabbit lenses during simulated accommodation, occurred within a physiologically relevant time frame (200 ms), implying a rapid movement of fluid to and from the lens during accommodation.     

Identification of the teleost Edinger-Westphal nucleus by retrograde horseradish peroxidase labeling and by electrophysiological criteria

Summary A homolog of the Edinger-Westphal nucleus of other vertebrates is described in two species of serranid basses of the genusParalabrax, a group possessing a wide range of ocular accommodation but lacking a pupillary reflex to light. The nucleus was found by retrograde labeling from the ciliary ganglion and lies dorsolateral to the ipsilateral oculomotor nucleus. The nucleus consists of 60 to 100 neurons with an average soma diameter of about 20 m in animals weighing 70 to 150 g. Electrophysiological experiments support the identification. Microstimulation of the nucleus evokes contraction of the ipsilateral lens retractor muscle and slight constriction of the caudal ipsilateral iris. Multi- and single-unit recordings in the nucleus reveal spontaneous firing (about 30 spikes/s in single units), the rate of which decreases during visually-evoked lens retractor relaxations (accommodation to near stimuli). Recordings of muscle fiber activity in the lens retractor show essentially the same behavior, which suggests that the ciliary ganglion and neuromuscular junctions simply relay impulses with little if any synaptic integration. The existence of a discrete Edinger-Westphal nucleus devoted largely to accommodation makesParalabrax a good model system for the further tracing of central accommodation control pathways.Abbreviations CNS central nervous system - EW Edinger-Westphal nucleus - HRP horseradish peroxidase - WGA wheat germ agglutinin     

Mechanisms of accommodation in the bird eye

Summary Keratoscopic study of corneal curvature before and after accommodation in two common bird species failed to provide evidence of a corneal accommodative mechanism. Accommodative changes in refractive state measured retinoscopically are presumably brought about by the effect of ciliary muscle contraction on lens curvature. However, retinoscopic and freeze-sectioning study of accommodation in diving ducks supports the long suspected existence of an iris accommodative mechanism capable of producing dramatic changes in lens curvature. This mechanism is believed to be a means of compensating for the refractive loss of the cornea in water.This research was supported in part by a grant from the Canadian National Sportsmen's Fund. The assistance of Dr. W.F. Long and Mr. R. Ortleib (Niska Wildlife Foundation) is gratefully acknowledged.