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.     

Temporal Co-Variation between Eye Lens Accommodation and Trapezius Muscle Activity during a Dynamic Near-Far Visual Task

Near work is associated with increased activity in the neck and shoulder muscles, but the underlying mechanism is still unknown. This study was designed to determine whether a dynamic change in focus, alternating between a nearby and a more distant visual target, produces a direct parallel change in trapezius muscle activity. Fourteen healthy controls and 12 patients with a history of visual and neck/shoulder symptoms performed a Near-Far visual task under three different viewing conditions; one neutral condition with no trial lenses, one condition with negative trial lenses to create increased accommodation, and one condition with positive trial lenses to create decreased accommodation. Eye lens accommodation and trapezius muscle activity were continuously recorded. The trapezius muscle activity was significantly higher during Near than during Far focusing periods for both groups within the neutral viewing condition, and there was a significant co-variation in time between accommodation and trapezius muscle activity within the neutral and positive viewing conditions for the control group. In conclusion, these results reveal a connection between Near focusing and increased muscle activity during dynamic changes in focus between a nearby and a far target. A direct link, from the accommodation/vergence system to the trapezius muscles cannot be ruled out, but the connection may also be explained by an increased need for eye-neck (head) stabilization when focusing on a nearby target as compared to a more distant target.     

Cell growth patterns and lens geometry: a quantitative study from three-dimensional reconstructions

The growth of the lens of the sea lamprey, Petromyzon marinus, was studied over the 5 years of larval development. Whole lenses (25) and Golgi-impregnated cells (393) were reconstructed with computer-assisted microscopy. Several cellular geometric parameters (length, width, curvature, surface, volume, shape) were correlated with the position of the cell's base on the lens capsular perimeter. Based on these correlations, the cells formed four groups that correspond to the central anterior, germinative, transitional and cortical fiber zones. A fifth zone, containing nuclear fiber cells, never stained. Lens growth is exponential during the 5 years. The anterior epithelium increases in size and in cell number by cell growth and division. The posterior mass increases in cell number by recruitment and increases in size by cell growth. A model is proposed to account for the size and shape of the lens based upon the coupling of anterior and posterior growth patterns. Four zonal boundaries are defined by changes in cell growth patterns. With growth, cells are subsumed into adjacent zones and zonal boundaries move away from the lens center. We find no support for the suggestion that cells migrate centrally.     

Focusing by shape change in the lens of the eye: a commentary on Young (1801) ‘On the mechanism of the eye’

In his Bakerian Lecture paper of 1801, Thomas Young provided the best account up to that time of the eye''s optical system, including refraction by the cornea and the surfaces of the lens. He built a device, an optometer, for determining the eye''s state of focus, making it possible to prescribe appropriate correction lenses. His main contribution, however, was to show that accommodation, the eye''s focusing mechanism, was not the result of changes to the curvature of the cornea, nor to the length of the eye, but was due entirely to changes in the shape of the lens, which he described with impressive accuracy. He was wrong, however, in believing that the reason the lens bulges when focusing on near objects was because it behaved as a contracting muscle. Half a century later, Helmholtz showed that the lens bulges not by its own contraction, but when it is relaxed as a result of contraction of newly discovered circular muscles in the ciliary body. This commentary was written to celebrate the 350th anniversary of the journal Philosophical Transactions of the Royal Society.     

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.     

Appearance of alpha-smooth muscle actin in human eye lens cells of anterior capsular cataract and in cultured bovine lens-forming cells

Using light and electron-microscopic immunolocalization techniques, and gel electrophoresis combined with immunoblotting, we have examined the expression of cytoskeletal proteins in normal human fetal, child and adult lenses, in human anterior capsular cataract and in bovine lens cells in vivo and in vitro. In this report, we focus our observations on the pattern of actin-isoform expression during normal and pathological situations in vivo and culture conditions. We have noted that cells of developing and mature human lenses as well as bovine lens cells in situ contain only beta- and gamma-actins. In contrast, alpha-smooth muscle (alpha-sm) actin, an isoform typical of smooth muscle differentiation, was demonstrated in bovine lens cells at different times of culture. Moreover, the multilayered cells observed in the subcapsular zone of human anterior capsular cataract were characterized by the presence of alpha-sm actin. Thus, extensive changes in actin-isoform expression take place in lens cells growing in culture and may also occur during cataractogenesis. The biological meaning of the appearance of a marker of myoid differentiation in the ectodermally derived lens-forming cells is discussed.     

How does plasma membrane surface area accommodate alterations in guard cell volume during stomatal closing?

During stomatal movement, guard cells undergo considerable and repetitive variations in cell volume and consequently surface area over a period of minutes. Due to limited stretching capability of the plasma membrane, alterations in the surface area must accommodate the volume changes through membrane turnover. Using fluorescence imaging and electrophysiology techniques, extensive studies imply that endocytosis may be a critical mechanism for the plasma membrane turnover. In contrast to the conventional studies, using transmission electronic microscope in combination with laser confocal microscope so that the membrane turnover can be detected without a resolution limitation, our works, recently published in the Journal of Experimental Botany, has provided strong evidences that excretion and folding of plasma membrane are critical for the accommodation of the cell volume alterations in intact guard cells in Vicia faba L. These results have opened a new perspective on the mechanism for the membrane turnover during stomatal movement. In this addendum, we further discuss some key issues about the mechanisms for the accommodation of the cell volume alterations during stomatal movements.Key words: stomata, guard cell, plasma membrane, surface area, endocytosis, excretion, accommodationGuard cells control stomatal movement thereby regulating gas exchange in plants. During stomatal movement, guard cells undergo considerable and repetitive variations in cell volume and consequently surface area over a period of minutes. It was proposed that the alterations of the plasma membrane surface area could be up to 40%,¹ whereas the maximum possible stretching of membranes was limited to only about 2%.² Furthermore, due to the presence of a turgor pressure, it has been commonly thought that membrane infoldings should not occur in the guard cells.³ Therefore, it is reasonable to propose that alterations in the surface area must be accomplished by addition and removal of membrane material to and from the plasma membrane.⁴ While many studies imply that endocytosis most likely functions to accommodate the alterations in guard cell volume, many crucial questions about this mechanism deserve to be argued.     

Lens crystallin changes associated with amphibian metamorphosis: involvement of a beta-crystallin polypeptide

Lens crystallins isolated from the tadpole and frog lenses were compared with regard to the developmental changes of crystallin compositions. The major changes during the process of metamorphosis were (1) the total contents of alpha- and gamma-crystallins decrease from more than 70% to less than 60% and (2) one of the major beta-crystallin polypeptides increases from less than 1% to about 6% and (3) an amphibian-specific rho-crystallin also increases from about 6% to more than 10% of total soluble proteins of the lens. We have characterized the metamorphosis-dependent beta-crystallin polypeptide by peptide mapping and sequence determination of the protease-digested fragments. This polypeptide showed very high sequence homology to that of the major beta Bp-crystallin chain reported for the mammalian lenses. The changes of the relative abundance of various crystallins and the gradually-elevated levels of the expression of this beta Bp-like crystallin in the developing lens during metamorphosis may also have some bearing on the maintenance of lens stability in the adult frog lenses.     

Cellular changes in cultured lenses.

Observations were made on the frog lens epithelium after culture of the entire lens or of capsular explants. General deviations from normal lens structure as well as specific changes in two media were studied. DNA synthesis and mitosis were induced in the central epithelial cells. Disruption of the orderly, single, epithelial layer that is characteristic of normal lenses was accompanied by the appearance of multilayered plaques of epithelial cells and invasion of vacuolated regions of the lens fibers by epithelial cells. Cells that are fibroblast-like in appearance were observed in regions of the capsule depleted of cells and at the free edges of epithelial sheets in cell culture. Epithelial cells were surrounded by capsule-like material even situated in the lens interior. Nuclie derived from central epithelial cells of lenses cultured in L-15 medium and medium 199 had served as donors in previous nuclear transfer experiments in this laboratory. In our current observation of L-15-cultured lenses, cells were sparsely distributed on the capsule and nuclei were abnormally shaped; in 199-cultured lenses, cells were more densely distributed and nuclei resembled those of normal lenses. Medium 199 without serum could better maintain normal lens structure than L-15 medium without serum. In addition, the percentage of epithelial explants demonstrating cellular outgrowth was greater in medium 199. The differences in cellular behavior were shown not to be the result of different sugars, pH, or the presence of CO2. The nuclear transfer results may reflect the structural changes in the epithelium after lens culture in the two media.     

Use of a triaxial magnetometer for respiratory measurements

We describe a triaxial magnetometer (Tri-mag) system, which consists of a transmitter, four sensors, a processing unit, and a personal computer (PC). The Tri-mag processing unit outputs the position of each sensor relative to the transmitter in three orthogonal coordinates, and this information is communicated to the PC. First, we demonstrated that within a defined octant of a sphere in which the center is the transmitter, we can measure radial distances with an accuracy of +/- 1 mm over a range extending from 10 to 70 cm from the transmitter. Second, we recorded the three-dimensional movement of sensors on the anterior and posterior surfaces of the chest wall during maximum voluntary ventilation in four normal men; all sensors were placed in the midsagittal plane of the body. Anterior sensors were located on the sternum at the level of the third intercostal space and at 2 cm above the umbilicus, whereas posterior sensors were located on the posterior spine at the same vertical levels as the anterior sensors. In all subjects the following was found. 1) Both anterior sensors moved anterior and cephalad during inspiration. The anterior thoracic sensor showed greater vertical than anteroposterior (A-P) movement, whereas the anterior abdominal sensor showed greater A-P than vertical movement. 2) Inspiration was associated with spinal extension, whereas expiration was associated with spinal flexion. Third, we used Tri-mag information to 1) measure tidal volume (VT) over a range extending from 500 ml to inspiratory capacity and 2) measure the change in end-expiratory lung volume (EELV) over a range extending from FRC to FRC plus a minimum of 1.5 liters. Our results indicate that greater than 96% of the changes in VT and greater than 82% of the changes in EELV can be accounted for by changes in A-P, vertical, and lateral dimensions of the chest wall.     

Short-term culturing of teleost crystalline lenses combined with high-resolution optical measurements

Culturing whole lenses is a frequently used method for studying regulatory events on the lens in controlled environments. The evaluation methods used often fall under two categories, molecular or optical. The main benefit from optical measurements is that they directly detect changes in the lens’ main function, i.e. refracting light. However, these measurements often have rather low resolution or yield results open for subjective interpretation. Here we present a short-term crystalline lens culturing technique combined with a high-resolution optical measuring method. There are two main advantages of using teleost lenses compared to mammalian lenses. Teleost tissue generally has a higher tolerance than mammalian tissue with regard to temperature and nutrient fluctuations. Teleost lenses are structurally more robust and can be excised from the eye without disturbing form or function. The technique is developed for short-term culturing (3 h), however, the lenses appear viable for at least 24 h and longer culturing may be possible. The technique is resistant to small variations in osmolarity and yields quantitative datasets for further analyses and statistical treatment.     

Cellular changes in cultured lenses

Summary Observations were made on the frog lens epithelium after culture of the entire lens or of capsular explants. General deviations from normal lens structure as well as specific changes in two media were studied. DNA synthesis and mitosis were induced in the central epithelial cells. Disruption of the orderly, single, epithelial layer that is characteristic of normal lenses was accompanied by the appearance of multilayered plaques of epithelial cells and invasion of vacuolated regions of the lens fibers by epithelial cells. Cells that are fibroblast-like in appearance were observed in regions of the capsule depleted of cells and at the free edges of epithelial sheets in cell culture. Epithelial cells were surrounded by capsule-like material even when situated in the lens interior. Nuclei derived from central epithelial cells of lenses cultured in L-15 medium and medium 199 had served as donors in previous nuclear transfer experiments in this laboratory. In our current observation of L-15-cultured lenses, cells were sparsely distributed on the capsule and nuclei were abnormally shaped; in 199-cultured lenses, cells were more densely distributed and nuclei resembled those of normal lenses. Medium 199 without serum could better maintain normal lens structure than L-15 medium without serum. In addition, the percentage of epithelial explants demonstrating cellular outgrowth was greater in medium 199. The differences in cellular behavior were shown not to be the result of different sugars, pH, or the presence of CO₂. The nuclear transfer results may reflect the structural changes in the epithelium after lens culture in the two media. This work was supported by grants 2RO1 EY 00555-06 and 5SO1 RR 05510-10 from the National Institutes of Health.     

Connexins are mechanosensitive

Connexins form gap junction channels that provide a hydrophilic path between cell interiors. Some connexins, particularly the lens connexins, Cx46 and Cx50 and their orthologs, can form functional hemichannels in nonjunctional membranes. These hemichannels are a nonselective conduit to the extracellular medium and may jeopardize cell survival. The physiological function of hemichannels has remained elusive, but it has been postulated that hemichannels are involved in ATP-release caused by mechanical stimulation. Here we show with single-channel and whole cell electrophysiological studies that Cx46 hemichannels are mechanosensitive, like other families of ion channels and membrane-bound enzymes. The hemichannel response to mechanical stress is bipolar. At negative potentials stress opens the channel, and at positive potentials stress closes it. Physiologically, Cx46 hemichannels may assist accommodation of the ocular lens by providing a transient path for volume flow as the lens changes shape. mechanical stress; lens     

Ultrastructure of traumatic cataractogenesis in the frog: a comparison with mouse and human lens.

Normal and needle-punctured lenses of Rana pipiens were examined with the electron microscope in order to characterize the sequence of ultrastructural changes that follow the injury over a 5-month period. Results were compared with those obtained previously in experimentally injured mouse and accidentally injured human lenses. The normal adult frog lens was found to have a morphology similar to that of mammalian lenses. As in the human, frog lens epithelial cells contained scattered microfilaments and were connected by desmosomes and gap junctions. They differed from mouse cells, which had been shown to lack desmosomes and to have microfilaments organized into dense bundles. These differences are postulated to be related to the degree of accommodative deformation of the lens displayed by these species. After injury, cellular debris and fibrin, accumulated in the wound, were phagocytized by extrinsic cells derived from the blood and ocular tissues. Leucocytes, pigmented cells and fibroblasts remained in the wound for eight weeks, along with epithelial cells which proliferated and migrated from the wound margins.Epithelial cells showed an increase in those organelles associated with protein synthesis and transport, and in microfilaments. In cataractous lenses, epithelial cells showed changes in matrix, and lens fibers became organized into smaller, denser compressed units. At five months, considerable healing had taken place, but localized opacities persisted in many frog lenses.     

Transport processes of fluid exchange in the crystalline lens of the rabbit eye

Processes of fluid exchange in the crystalline lens of the rabbit eye were investigated. The direction of movement of fluid in the crystalline lens was investigated from the movement of fluorescein by the method of "stopped diffusion". It has been found that the mechanism of fluid transport in the crystalline lens is active and is carried out by means of the Na-kappa-ATPase transport system. The energy necessary for the active transport of fluid inside the crystalline lens is in the range (1.5-6) x 10(-2) J. Owing to the active fluid transport, the pressure inside the crystalline lens constantly increases by 6 mm Hg. In rabbit's life-time, the movement of fluid in the crystalline lens occurs in the direction from the anterior to the posterior surface followed by the exit to vitreous humor.     

Accommodation of the human lens capsule using a finite element model based on nonlinear regionally anisotropic biomembranes

Accommodation of the eyes, the mechanism that allows humans to focus their vision on near objects, naturally diminishes with age via presbyopia. People who have undergone cataract surgery, using current surgical methods and artificial lens implants, are also left without the ability to accommodate. The process of accommodation is generally well known; however the specific mechanical details have not been adequately explained due to difficulties and consequences of performing in vivo studies. Most studies have modeled the mechanics of accommodation under assumptions of a linearly elastic, isotropic, homogenous lens and lens capsule. Recent experimental and numerical studies showed that the lens capsule exhibits nonlinear elasticity and regional anisotropy. In this paper we present a numerical model of human accommodation using a membrane theory based finite element approach, incorporating recent findings on capsular properties. This study seeks to provide a novel perspective of the mechanics of accommodation. Such findings may prove significant in seeking biomedical solutions to restoring loss of visual power.     

Bacterial adhesion measurements on soft contact lenses using a Modified Vortex Device and a Modified Robbins Device

S. marcescens 8100 andP. aeruginosa 15442 were used to study bacterial adhesion to hydrogel contact lenses which had not been worn. Bacterial removal from unworn lens materials was assessed with a calibrated vortex device modified with a digital rpm readout and fitted with a test tube attachment (MVD). The MVD, which relies on a whirlpool-like force to remove the bacteria, showed that bacteria adhered to the same degree to etafilcon A, vifilcon A and polymacon lenses under standardized conditions. Tracking the isoenzyme patterns of these bacterial species over time showed instability ofS. marcescens upon repeated passage. This instability was not evident withP. aeruginosa. Bacterial adhesion ofP. aeruginosa 15442, to human worn and unworn etafilcon A materials was determined with a Modified Robbins Device. The MRD was closed off at both ends stopping medium and bacterial movement after 1 h of fluid flow over the lens surface. The results show that immediately following this 1-h period more bacteria adhere to unworn contact lenses than to worn lenses. However, bacterial counts were equivalent on worn and unworn lenses following 5 h of static incubation.     

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.     

Sphingolipids in human lens membranes: an update on their composition and possible biological implications

The unique nature of the most abundant phospholipids in human lens membranes remained overlooked until the 1990s when it was possible to discern dihydrosphingomyelins (DHSMs) from the more common sphingomyelins (SMs). Unlike in other mammalian membranes, DHSMs comprise nearly half of the phospholipids in adult human lenses. Compared to SMs with a trans double bond between carbons 4 and 5 of the sphingoid backbone, the absence of this unsaturation site in DHSMs allows the participation of the OH group on C3 in intermolecular H-bonds and leads to stronger interlipid interactions with both neighboring DHSMs and cholesterol. Phospholipid compositional changes with age and lens region observed in mammals with various life spans and lens growth rates, suggest that the highest levels of DHSMs along with the lowest amounts of phosphatidylcholines and SMs are found in lenses with the lowest growth rate, namely human lenses. The participation of phospholipid metabolites in the control of mitosis and elongation of lens cells is plausible and deserves investigation.     

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.