Traction on the retina induced by saccadic eye movements in the presence of posterior vitreous detachment

Posterior vitreous detachment is a fairly common condition in elderly people. Tractions exerted by the detached vitreous on the retina may result in retinal tears and detachments. We studied how these tractions can arise from saccadic eye movements. Numerical simulations have been performed on a two-dimensional model of the vitreous chamber within a rigid spherical sclera, subjected to prescribed finite-amplitude rotations about a vertical axis. The vitreous chamber was assumed to be split into two regions: one occupied by the detached vitreous, modeled as an elastic viscous solid, and the other occupied by the separated liquefied vitreous, modeled as a Newtonian fluid. At the interface between the two phases, we also considered the presence of the vitreous cortex, modeled as an elastic membrane. We tested several different configurations of the interface. In all cases, we found that eye rotations generate large tractions on the retina close to the attachment points of the membrane. Comparing them, we identified configurations of the vitreous detachment that exhibit higher tractions. We also investigated how the response to saccadic movements depends on some physical parameters, in particular on the rheological properties of the solid phase and the membrane. The numerical simulations show that the generated tractions may be of the same order of magnitude as the adhesive force between the retina and the pigment epithelium. Therefore, the model provides a sound physical justification for the hypothesis that saccadic movements, in the presence of posterior vitreous detachment, could be responsible for high tractions on the retina, which may trigger retinal tear formation.     

Evisceration of Mouse Vitreous and Retina for Proteomic Analyses

While the mouse retina has emerged as an important genetic model for inherited retinal disease, the mouse vitreous remains to be explored. The vitreous is a highly aqueous extracellular matrix overlying the retina where intraocular as well as extraocular proteins accumulate during disease.^1-3 Abnormal interactions between vitreous and retina underlie several diseases such as retinal detachment, proliferative diabetic retinopathy, uveitis, and proliferative vitreoretinopathy.^1,4 The relative mouse vitreous volume is significantly smaller than the human vitreous (Figure 1), since the mouse lens occupies nearly 75% of its eye.⁵ This has made biochemical studies of mouse vitreous challenging. In this video article, we present a technique to dissect and isolate the mouse vitreous from the retina, which will allow use of transgenic mouse models to more clearly define the role of this extracellular matrix in the development of vitreoretinal diseases.     

Whole Vitreous Humor Dissection for Vitreodynamic Analysis

The authors propose an effective technique to isolate whole, intact vitreous core and cortex from post mortem enucleated porcine eyes. While previous studies have shown the results of such dissections, the detailed steps have not been described, precluding researchers outside the field from replicating their methods. Other studies harvest vitreous either through aspiration, which does not maintain the vitreous structure anatomy, or through partial dissection, which only isolates the vitreous core. The proposed method isolates the whole vitreous body, with the vitreous core and cortex intact, while maintaining vitreous anatomy and structural integrity. In this method, a full thickness scleral flap in an enucleated porcine eye is first created and through this, the choroid tissue can be separated from the sclera. The scleral flap is then expanded and the choroid is completely separated from the sclera. Finally the choroid-retina tissue is peeled off the vitreous to leave an isolated intact vitreous body. The proposed vitreous dissection technique can be used to study physical properties of the vitreous humor. In particular, this method has significance for experimental studies involving drug delivery, vitreo-retinal oxygen transport, and intraocular convection.     

Structure for Hyaluronic Acid

THE mucopolysaccharide hyaluronic acid exists naturally as a hydrated gel. It is the constituent of synovial fluid which acts as a lubricant; it also occurs in the vitreous humour where its function is probably to maintain the precise shape of the eye. Chemically it is a regular, unbranched polymer with a repeating unit of the type (-N-G-)_n where N is N-acetylglucosamine and G is glucuronic acid. The glycosidic linkages N to G and G to N are 1e, 4e and le, 3e respectively (Fig. 1). Our arguments are based on the postulate that both glucopyranose rings are in the C1 chain conformation.     

Saccade movements effect on the intravitreal drug delivery in vitreous substitutes: a numerical study

In this study, the distributions of intravitreal injected drugs in post-vitrectomy human eyes, which are subjected to periodic saccade movements, are investigated. The computational model for the vitreous cavity of human eye is a sphere with one side truncated by the eye lens. A dynamic mesh technique was used to model the eye motion and the unsteady 3-D forms of continuity; Navier–Stokes and concentration transport of drug equations were solved numerically. The numerical model was validated earlier for the vitreous liquid flow field. The predicted drug concentration for idealized geometry was compared with the available analytic solution and excellent agreement was observed. The validated computer model was then used to simulate a real vitreous cavity filled with Balanced Salt Solution or aqueous humor as a vitreous substitute in order to obtain distribution of drugs in the post-vitrectomy eyes or liquefied vitreous. Additionally, effects of locations of drug injection, drug diffusion coefficients and saccade amplitude on the drug distribution and its uniformity were investigated. Although the earlier findings in the literature reported a day or a week as a needed time for drug uniform distribution in the vitreous substitutes, the present work depicts that saccade movements augment the transport of the drug in a way that the uniformity of the drug distribution can be achieved in a matter of minutes. Furthermore, in a vitreous cavity subjected to the saccade movements, the diffusion coefficient of drugs does not significantly affect their distribution after a few minutes. Even the injection location does not matter as uniform distribution is achieved after some time.     

Rheological properties of the vitreous and the role of hyaluronic acid

Coordinated rheological and biochemical measurements provide the linear and nonlinear mechanical properties of the vitreous and demonstrate the structural role of hyaluronic acid. "Cleated" tools are used to overcome wall slip and avoid tissue compression during measurements of the dynamic moduli of fresh porcine and bovine vitreous. Shear moduli decreased five-fold from initial to steady-state values in the first hour after dissection. Steady-state values (porcine: G'=2.8+/-0.9Pa, n=9; bovine: G'=7.0+/-2.0Pa, n=17) are significantly greater than previously reported. The decrease in modulus after removal from the eye correlates with a decrease in mass: even in the absence of external driving forces, porcine vitreous expels approximately 5% of its mass within 5min and continues to decay to a steady-state mass approximately 10% lower than its initial mass. The expelled fluid has a substantial hyaluronan concentration, but very low protein content. These results indicate that the vitreous network is under tension at its native volume and its high initial modulus results from this state of tension. We hypothesize that hyaluronan plays a role in sustaining the "internal tension" by Donnan swelling.     

Biochemical studies of the chicken vitreous in light-induced avian glaucoma.

Changes in several macromolecular species of the vitreous are traced during post-hatching development in normal and glaucomatous avian eyes. 1. Total gel vitreous, and its hydroxyproline content, change little during development. 2. Sol vitreous increases parallel eye growth, the concentrations of hexosamine and hexuronic acid remaining constant. 3. In light-induced avian glaucoma, the sol, but not the gel, is further increased, although the hyaluronic acid components are not diluted in the pathologically enlarged eye.     

Transport and steady-state concentration of plasma proteins in the vitreous humor of the chicken embryo: implications for the mechanism of eye growth during early development

The nature and origin of the proteins of the vitreous humor were examined in chickens during embryonic and early posthatching stages. The major proteins of the vitreous humor were similar in electrophoretic mobility to plasma proteins at all ages examined. Earlier studies from our laboratory and experiments described below showed that plasma proteins continuously entered and left the eye throughout its development. From these data it was concluded that the majority of vitreous-humor proteins were derived from the blood. The protein concentration of the vitreous humor was 13% of that of the plasma from embryonic Days 6 through 15 (E6 through E15). After E15, the relative protein concentration in the vitreous humor declined with respect to the plasma and reached 4% of the plasma protein concentration at hatching. Several possibilities were considered to account for how proteins can rapidly enter and leave the eye, yet maintain a steady-state concentration in the vitreous humor that is less than one-seventh of that in the blood.     

Effect of intraocular silicone oil on ocular tissue

The aim of this study was to find out to which extent the instilled silicone oil affects the changes of lens, cornea, and the ocular pressure with regard to its time kept in the operated eye with a lens. The study was carried out on the sample of 40 patients, divided in three groups. The first group of patients with the silicone oil kept in the eye during 3 months, the second one during 3-6 months, and the third one during 6-9 months. Statistically important changes were observed in the progression of the lens opacification with the patients, progressively with the time of keeping the silicone oil in the eye. The followed cornea opacifications and increased ocular pressure were not statistically significantly linked with the length of keeping the silicone oil in the eye, so that it can be concluded that the silicone oil is the best substitute for the vitreous body but it should be removed from the eye, most frequently, in the period of 3-6 months from the date of its instillation, and after 3 weeks at the earliest.     

Experimental data on the penetration of gentamicin into the media of the eye]

Penetration of Soviet gentamicin into the humor of the anterior chamber and vitreous body of the eye with aseptic inflammation was studied after the antibiotic administration by various routed, i.e. instillations of 8 per cent antibiotic solution and 8 per cent antibiotic solution methylcellulose into the conjunctival sac, injections of 20 mg of gentamicin subconjunctivally and retrobulbarly, injections of gentamicin intramuscularly in doses of 0.6 mg/kg. The studies showed that gentamicin penetrated into the humor of the anterior chamber and vitreous body of the eye after all the administration routes mentioned above in concentrations sufficient for the antibiotic antimicrobial effect and persisted in the eye media for prolong periods of time (24--48 hours). The highest concentrations of the antibiotic in the tumor of the anterior chamber were achieved after its administration subconjunctivally or after instillation of its 8 per cent on methylcellulose, while in the vitreous body its highest concentrations were achieved after injections subconjunctively, retrobulbarly or intramuscularly. Instillations of gentamicin solution on methylcellulose provided higher and more persistant concentrations of the antibiotic in the humor as compared to instillations of its aqueous solutions. Retrobulbar injections of gentamicin had no advantages as compared to subconjunctival administration with respect to providing higher concentrations of the antibiotic in the eye media.     

Transient transport across an inhomogeneous blood-retina barrier

A mathematical model for transport across the blood-retina barrier and diffusion into the vitreous body of the human eye is formulated. The eye is modeled as a sphere, the surface of which represents the blood-retina barrier. The equations of the model are solved analytically, using an expansion in spherical harmonics and inversion of the Laplace transform in the time variable. The numerical properties of the solution are investigated and the applicability of the model to the analysis of data from three-dimensional vitreous fluorometry is discussed.     

A computational study of the flow through a vitreous cutter

Vitrectomy is an ophthalmic microsurgical procedure that removes part or all of the vitreous humor from the eye. The procedure uses a vitreous cutter consisting of a narrow shaft with a small orifice at the end through which the humor is aspirated by an applied suction. An internal guillotine oscillates back and forth across the orifice to alter the local shear response of the humor. In this work, a computational study of the flow in a vitreous cutter is conducted in order to gain better understanding of the vitreous behavior and provide guidelines for a new vitreous cutter design. The flow of a Newtonian surrogate of vitreous in a two-dimensional analog geometry is investigated using a finite difference-based immersed boundary method with an algebraically formulated fractional-step method. A series of numerical experiments is performed to evaluate the impact of cutting rate, aspiration pressure, and opening/closing transition on the vitreous cutter flow rate and transorifice pressure variation during vitrectomy. The mean flow rate is observed to increase approximately linearly with aspiration pressure and also increase nearly linearly with duty cycle. A study of time-varying flow rate, velocity field, and vorticity illuminates the flow behavior during each phase of the cutting cycle and shows that the opening/closing transition plays a key role in improving the vitreous cutter's efficacy and minimizing the potential damage to surrounding tissue. The numerical results show similar trend in flow rate as previous in vitro experiments using water and balanced saline solution and also demonstrate that high duty cycle and slow opening/closing phases lead to high flow rate and minor disturbance to the eye during vitrectomy, which are the design requirements of an ideal vitreous cutter.     

Experimental data on methicillin tolerance and penetration into the eye media]

Tolerance of methicillin by the eye tissues was studied on its administration subconjunctively, into the front chamber and vitreous body of 20 rabbits. The studies showed that subconjunctival administration of the antibiotic was well tolerated in a dose of 50 mg, and its administration into the front chamber and vitreous body was well tolerated in doses of 1.0-2.5 mg. Penetration of methicillin into the fluids of the front chamber and vitreous body on its instillation into the conjunctive sac in a form of 2.5 per cent solution, its subconjunctival and retrobulbar injection in a dose of 50 mg and intramuscular administration in a dose of 40 mg/kg was studied. Animals with aseptic inflammation of the eyes due to burns of the cornea with 1 N hydrochloric acid were taken into the experiments. The method of agar diffusion with Staph. aureus 209P as the test-microbe was used. The studies showed that the highest methicillin concentrations in the eye media were observed an hour after the antibiotic subconjunctival administration. In the vitreous body they were 16 times lower than those in the front chamber fluid. The retrobulbar injections had no advantages over the subconjunctival administration for the antibiotic maximum concentrations in the vitreous body. The concentration of methicillin in the front chamber fluid on its local administration was many times higher than the minimum inhibitory concentration for staphylococci and may be considered as a therapeutic one.     

Review of recent research into cellulosic whiskers, their properties and their application in nanocomposite field

There are numerous examples where animals or plants synthesize extracellular high-performance skeletal biocomposites consisting of a matrix reinforced by fibrous biopolymers. Cellulose, the world's most abundant natural, renewable, biodegradable polymer, is a classical example of these reinforcing elements, which occur as whisker-like microfibrils that are biosynthesized and deposited in a continuous fashion. In many cases, this mode of biogenesis leads to crystalline microfibrils that are almost defect-free, with the consequence of axial physical properties approaching those of perfect crystals. This quite "primitive" polymer can be used to create high performance nanocomposites presenting outstanding properties. This reinforcing capability results from the intrinsic chemical nature of cellulose and from its hierarchical structure. Aqueous suspensions of cellulose crystallites can be prepared by acid hydrolysis of cellulose. The object of this treatment is to dissolve away regions of low lateral order so that the water-insoluble, highly crystalline residue may be converted into a stable suspension by subsequent vigorous mechanical shearing action. During the past decade, many works have been devoted to mimic biocomposites by blending cellulose whiskers from different sources with polymer matrixes.     

在大鼠玻璃体内移植猕猴神经前体细胞的方法探索

探索一种简单、可行的异种神经前体细胞眼内移植方法。采用机械性损伤方法造成大鼠视网膜局部受损,然后在损伤眼及对照眼玻璃体内移植绿色荧光蛋白(green fluorescence protein,GFP)标记的猕猴神经前体细胞,观察细胞能否存活。结果显示:经激光共聚焦显微镜检查发现移植细胞在损伤眼及对照眼内均可存活并整合至损伤眼视网膜。实验表明,玻璃体内异种移植GFP标记的猕猴神经前体细胞可以存活并整合,是一种可行的移植方法。     

Lack of collagen XVIII/endostatin results in eye abnormalities

Mice lacking collagen XVIII and its proteolytically derived product endostatin show delayed regression of blood vessels in the vitreous along the surface of the retina after birth and lack of or abnormal outgrowth of retinal vessels. This suggests that collagen XVIII/endostatin is critical for normal blood vessel formation in the eye. All basement membranes in wild-type eyes, except Descemet's membrane, showed immunogold labeling with antibodies against collagen XVIII. Labeling at sites where collagen fibrils in the vitreous are connected with the inner limiting membrane and separation of the vitreal matrix from the inner limiting membrane in mutant mice indicate that collagen XVIII is important for anchoring vitreal collagen fibrils to the inner limiting membrane. The findings provide an explanation for high myopia, vitreoretinal degeneration and retinal detachment seen in patients with Knobloch syndrome caused by loss-of-function mutations in collagen XVIII.     

Dissection of Human Vitreous Body Elements for Proteomic Analysis

The vitreous is an optically clear, collagenous extracellular matrix that fills the inside of the eye and overlies the retina. ^1,2 Abnormal interactions between vitreous substructures and the retina underlie several vitreoretinal diseases, including retinal tear and detachment, macular pucker, macular hole, age-related macular degeneration, vitreomacular traction, proliferative vitreoretinopathy, proliferative diabetic retinopathy, and inherited vitreoretinopathies. ^1,2 The molecular composition of the vitreous substructures is not known. Since the vitreous body is transparent with limited surgical access, it has been difficult to study its substructures at the molecular level. We developed a method to separate and preserve these tissues for proteomic and biochemical analysis. The dissection technique in this experimental video shows how to isolate vitreous base, anterior hyaloid, vitreous core, and vitreous cortex from postmortem human eyes. One-dimensional SDS-PAGE analyses of each vitreous component showed that our dissection technique resulted in four unique protein profiles corresponding to each substructure of the human vitreous body. Identification of differentially compartmentalized proteins will reveal candidate molecules underlying various vitreoretinal diseases.     

Mathematical model of flow in the vitreous humor induced by saccadic eye rotations: effect of geometry

Saccadic eye rotations induce a flow in the vitreous humor of the eye. Any such flow is likely to have a significant influence on the dispersion of drugs injected into the vitreous chamber. The shape of this chamber deviates from a perfect sphere by up to 10–20% of the radius, which is predominantly due to an indentation caused by the lens. In this paper we investigate theoretically the effect of the domain shape upon the flow field generated by saccades by considering an idealized model. The posterior chamber geometry is assumed to be a sphere with a small indentation, undergoing prescribed small-amplitude sinusoidal torsional oscillations, and, as an initial step towards understanding the problem, we treat the vitreous humor as a Newtonian fluid filling the chamber. The latter assumption applies best in the case of a liquefied vitreous or a tamponade fluid introduced in the vitreous chamber after vitrectomy. We find the flow field in terms of vector spherical harmonics, focusing on the deviation from the flow that would be obtained in a perfect sphere. The flow induced by the departure of the domain geometry from the spherical shape has an oscillating component at leading order and a smaller-amplitude steady streaming flow. The oscillating component includes a circulation cell formed every half-period, which migrates from the indentation towards the center of the domain where it disappears. The steady component has two counter-rotating circulations in the anterior part of the domain. These findings are in good qualitative agreement with the experimental results of Stocchino et al. (Phys Med Biol 52:2021–2034, 2007). Our results predict a significant reduction in the expected time for drug dispersal across the eye compared with the situation in which there is no fluid flow present.     

The nature and origin of the glycosaminoglycans of the embryonic chick vitreous body

Glycosaminoglycans of the embryonic chicken vitreous were characterized and then were used as markers to establish which tissues synthesize the vitreous humor during development. The glycosaminoglycans are predominantly chondroitin sulfates by several criteria. They are resistant to streptomyces hyaluronidase, an enzyme which degrades only hyaluronate, and are digested by testicular hyaluronidase and chondroitinase AC, enzymes which degrade hyaluronate plus chondroitin 4- and 6-sulfates. On electrophoresis on cellulose acetate in 0.15 M phosphate buffer, pH 6.7, the vitreous glycosaminoglycans migrate slightly slower than authentic chondroitin sulfate, but, in 0.1 N HCl, they migrate very close to chondroitin sulfate standards. Finally, the disaccharides produced by digestion of these radioactively labeled glycosaminoglycans with chondroitinases AC and ABC were identified as Δdi-4S and Δdi-6S, as expected for chondroitin 4- and 6-sulfate. By using incorporation of radioactive precursors into chondroitin sulfates in vitro, we than determined which tissues synthesize the vitreous humor in the developing eye. Late in development, on Day 12–13, the isolated vitreous is very active in chondroitin sulfate synthesis, while the neural retina, the lens, and the pecten are less active and produce a high proportion of enzyme-resistant GAG. The eye tissues isolated from embryos labeled in ovo retain similar amounts and types of glycosaminoglycans, indicating that cells within the vitreous synthesize the vitreous humor glycosaminoglycans at this time. Earlier in development, from Days 6 to 8, the isolated vitreous incorporates very low levels of radioactivity into GAG, but the neural retina incorporates high levels of radioactivity into chondroitin sulfate. When the embryos are labeled in ovo and the same tissues are isolated following incorporation, the vitreous retains more radioactive chondroitin sulfate than does the neural retina. Thus, the vitreous humour glycosaminoglycan is initially synthesized by the neural retina and is secreted into the vitreous space.