Melittin-induced changes in Na and Cl movements across the skin and cornea (in vitro) of the toad Bufo marinus

Melittin, from bee venom, increases short-circuit current (Isc) across the skin and cornea of toads. In skin this reflects a rise in the influx of Na and is inhibited by meclofenamic acid (inhibits prostaglandin synthetase). In corneas with melittin on the inside the rise in Isc is inhibited by bumetanide (inhibits Cl transport) and meclofenamic acid. Melittin on the tear side of the cornea causes a biphasic change in Isc, and a rise in all undirectional fluxes of Na and Cl. This effect was not changed by bumetanide or meclofenamic acid. Melittin apparently has two types of effects, one mediated by prostaglandins while the other is more direct.     

Quantification of collagen organization in the peripheral human cornea at micron-scale resolution

The collagen microstructure of the peripheral cornea is important in stabilizing corneal curvature and refractive status. However, the manner in which the predominantly orthogonal collagen fibrils of the central cornea integrate with the circumferential limbal collagen is unknown. We used microfocus wide-angle x-ray scattering to quantify the relative proportion and orientation of collagen fibrils over the human corneolimbal interface at intervals of 50 μm. Orthogonal fibrils changed direction 1–1.5 mm before the limbus to integrate with the circumferential limbal fibrils. Outside the central 6 mm, additional preferentially aligned collagen was found to reinforce the cornea and limbus. The manner of integration and degree of reinforcement varied significantly depending on the direction along which the limbus was approached. We also employed small-angle x-ray scattering to measure the average collagen fibril diameter from central cornea to limbus at 0.5 mm intervals. Fibril diameter was constant across the central 6 mm. More peripherally, fibril diameter increased, indicative of a merging of corneal and scleral collagen. The point of increase varied with direction, consistent with a scheme in which the oblique corneal periphery is reinforced by chords of scleral collagen. The results have implications for the cornea's biomechanical response to ocular surgeries involving peripheral incision.     

Familial lecithin:cholesterol acyltransferase deficiency. Biochemistry of the cornea

Opacification of the cornea from lipid accumulation is an early and characteristic feature of familial lecithin:cholesterol acyltransferase (LCAT) deficiency. Visual impairment in a female age 48 years led to keratoplasty and the first detailed analysis of cornea in this disorder. Multilaminar figures were present, and total lipid extracts were enriched with phospholipid and cholesterol; cholesteryl esters were reduced, and accounted for about 12% of the cholesterol. Linoleate C18:2 was the predominant residue in the cholesteryl ester fatty acid fraction, with a C18:1/18:2 ratio of 1:6.5. This ratio differs from that in normal cornea, and from that in plasma and in other tissue deposits in LCAT deficiency. Various disorders of the HDL/LCAT system in plasma can lead to corneal lipid accumulation and opacification. These disorders may share general defects of lipid clearance from the cornea, but this study of LCAT cornea indicates that the character of the accumulating lipid is significantly influenced by active local metabolism, irrespective of the defect in the HDL/LCAT system also present.     

A viscoelastic anisotropic hyperelastic constitutive model of the human cornea

A constitutive model based on the continuum mechanics theory has been developed which represents interlamellar cohesion, regional variation of collagen fibril density, 3D anisotropy and both age-related viscoelastic and hyperelastic stiffening behaviour of the human cornea. Experimental data gathered from a number of previous studies on 48 ex vivo human cornea (inflation and shear tests) enabled calibration of the constitutive model by numerical analysis. Wide-angle X-ray scattering and electron microscopy provided measured data which quantify microstructural arrangements associated with stiffness. The present study measures stiffness parallel to the lamellae of the cornea which approximately doubles with an increase in strain rate from 0.5 to 5%/min, while the underlying stromal matrix provides a stiffness 2–3 orders of magnitude lower than the lamellae. The model has been simultaneously calibrated to within 3% error across three age groups ranging from 50 to 95 years and three strain rates across the two loading scenarios. Age and strain-rate-dependent material coefficients allow numerical simulation under varying loading scenarios for an individual patient with material stiffness approximated by their age. This present study addresses a significant gap in numerical representation of the cornea and has great potential in daily clinical practice for the planning and optimisation of corrective procedures and in preclinical optimisation of diagnostic procedures.     

Morphological and genetic characteristics of the open-eyelid mutant spontaneously occurring in NC-strain mice

A heritable open-eyelid mutant which arose spontaneously in the NC strain of mouse in 1982 has been maintained as the NC-eob line through brother-sister matings. When the line exceeded the 12th generation, its reproductive capabilities were initially compared with those of the NC strain. Then eye abnormalities were investigated morphologically and genetically. Although most of the reproductive parameters in the NC-eob line were comparable to those in the NC strain, a significant decrease was observed in the mean number of pups delivered, and the viability of pups at 7 days of age was reduced to 55%. The eyelids of pups were widely open at birth. The exposed cornea dried within a few hours and a bloody exudate appeared around the margin of the eyelids. At 1 day of age, the eye opening was smaller in size and covered with a scab. Histological examination demonstrated apparent inflammatory changes in the cornea and conjunctiva. At 21 days of age, the eyes revealed opacity, a rough surface, and microphthalmia. Histological findings in the cornea, such as keratosis and vascularization, coincided with the eye opacity. The weight and size of the eyeballs were almost the same as those in normal NC pups. However, a significant decrease in the length of the palpebral rima was evident and this was considered to be the cause of the microphthalmia. The results of genetic analysis indicated that these eye abnormalities were caused by an autosomal recessive gene, eob. These results suggested that the eob gene might be a new mutation manifesting semi-lethality as a pleiotropic effect.     

Age in angle-closure glaucoma and the normal - a study in profile photography

The horizontal corneal diameters of normal subjects of differing age and of those with angle-closure glaucoma were measured by surface and profile photography in the absence of the influence of axial length. The correlationship found between age and corneal diameter of surface photography for normal subjects was not found in the case of measurement by the technique which was independent of opacification of the peripheral zones of the cornea i.e., profile photography. No relationship was found between age and corneal diameter by either technique for the angle-closure glaucoma subjects.     

Reproducibility and age-related changes of ocular parametric measurements in rabbits

ABSTRACT: BACKGROUND: The rabbit is a common animal model for ophthalmic research, especially corneal research. Ocular structures grow rapidly during the early stages of life. It is unclear when the rabbit cornea becomes mature and stabilized. We investigated the changes of keratometry, refractive state and central corneal thickness (CCT) with age. In addition, we studied the intra- and inter-observer reproducibility of anterior chamber depth (ACD) and anterior chamber width (ACW) measurements in rabbits using anterior segment-optical coherence tomography (AS-OCT). RESULTS: The growth of New Zealand White rabbits (n = 16) were monitored from age 1 to 12 months old. Corneal keratometric and refractive values were obtained using an autorefractor/keratometer, and CCT was measured using an AS-OCT. Keratometry and CCT changed rapidly from 1 to 7 months and appeared to be stabilizing after 8 months. The reduction of corneal curvature was approximately 1.36 diopter (D)/month from age 1 to 7 months, but the change decelerated to 0.30 D/month from age 8 to 12 months. An increase of 10 mum/month in CCT was observed from age 1 to 7 months, but the gain was reduced to less than 1 mum/month from age 8 to 12 months. There was a hyperopic shift over the span of 12 months, albeit the increase in spherical equivalent was slow and gradual. Rabbits of random age were then selected for 2 repeated ACD and ACW measurements by 2 independent and masked observers. Bland-Altman plots revealed a good agreement of ACD and ACW measurements inter- and intra-observer and the ranges of 95% limit of agreement were acceptable from a clinical perspective. CONCLUSIONS: Corneal keratometry, spherical equivalent refraction and CCT changed significantly during the first few months of life of rabbits. Young rabbits have been used in a large number of eye research studies. In certain settings, the ocular parametric changes are an important aspect to note as they may alter the findings made in a rabbit experimental model. In this study, we have also demonstrated for the first time a good between observer reproducibility of measurements of ocular parameters in an animal model by using an AS-OCT.     

The importance of the Maillard reaction in ophtalmology

Non enzymatic glycosylation( glycation) of proteins, described by L. C. Maillard in 1912, results in the formation of advanced glycation end products (AGE-s). These exhibit a number of harmful reactions, increasing with age and involved in several age-associated pathologies. In ocular pathology, their role was demonstrated at several levels of age-associated eye-diseases, such as the rigidification of cornea, in the separation of vitreous fibers from the hyaluronan jelly, which might result in retinal detachment. AGE-s are involved also in retinal microvascular alterations in diabetics as well as in age-related macular degeneration. We compared the cytotoxic effect of several AGE-s on human skin fibroblasts and corneal keratocytes. Keratocytes were shown to be much more resistant to the cytotoxic effect of several AGE-products than fibroblasts. This higher resistance of keratocytes to the free radical mediated cytotoxic effect of AGE-s might be the result of the constant exposure of cornea to UV-light possibly mediating the appearance of more efficient protective mechanisms during evolution.     

Corneal Thickness Profile and Associations in Chinese Children Aged 7 to 15 Years Old

Corneal thickness (CT) maps of the central (2-mm diameter), para-central (2 to 5-mm diameter), peripheral (5 to 6-mm diameter), and minimum (5-mm diameter) cornea were measured in normal Chinese school children aged 7 to 15 years old using Fourier-domain optical coherence tomography. Multiple regression analyses were performed to explore the effect of associated factors [age, gender, refraction, axial length and corneal curvature radius (CCR)] on CT and the relationship between central corneal thickness (CCT) and intraocular pressure (IOP). A total of 1228 eyes from 614 children were analyzed. The average CCT was 532.96 ± 28.33 μm for right eyes and 532.70 ±28.45 μm for left eyes. With a 10 μm increase in CCT, the IOP was elevated by 0.37 mm Hg, as measured by noncontact tonometry. The CT increased gradually from the center to the periphery. The superior and superior nasal regions had the thickest CTs, while the thinnest points were primarily located in the inferior temporal cornea. The CCT was associated with CCR (p = 0.008) but not with gender (p = 0.075), age (p = 0.286), axial length (p = 0.405), or refraction (p = 0.985). In the para-central region and the peripheral cornea, increased CT was associated with younger age, male gender, and a flatter cornea.     

Morphogenesis of the eye of Siberian sturgeon

The most relevant changes in Acipenser baeri eye organization were detected between hatching and 5 days post hatch. At this age, the eye had an anterior chamber, lens, iris, choroid gland, scleral cartilage, cornea and a vitreous chamber lined by the retina (with two photoreceptors: rods and single cones).     

Cholesterol biosynthesis by the cornea. Comparison of rates of sterol synthesis with accumulation during early development

The origin of the cholesterol needed by the cornea for growth and cell turnover was addressed by comparing absolute rates of sterol synthesis with rates of sterol accumulation during early development of the rabbit. Linearity of incorporation of 3H2O and [14C]mevalonate into digitonin-precipitable sterols with time of incubation in vitro and a lack of accumulation of 14C in intermediates of sterol biosynthesis indicated that tritiated water can validly be used to measure rates of sterol synthesis by the cornea. The rate of sterol synthesis per unit weight of rabbit cornea was constant between 14 and 60 days of age at an average 1.03 nmol of 3H of 3H2O incorporated/mg dry cornea per 8 h. Essentially all of the synthesized cholesterol and most of the cholesterol mass was present in corneal epithelium. The cumulative sterol synthesized over the 46-day period studied exceeded the observed rate of cholesterol accumulation by sixfold. Cholesterol synthesized in excess of the growth requirement was likely used to support turnover of the epithelium which was estimated at 9 days. Removal of cholesterol from the cornea by excretion into tear fluid and clearance by high density lipoproteins are also considered.     

Changes of superoxide dismutase, catalase and glutathione peroxidase in the corneal epithelium after UVB rays. Histochemical and biochemical study

In this study, the effects of UVA and UVB rays on antioxidant enzymes (superoxide dismutase, glutathione peroxidase, catalase) were examined in the corneal epithelium. The corneas of albino rabbits were irradiated with a UV lamp generating UVA (365 nm wavelength) or UVB rays (312 nm wavelength), 1 x daily for 5 min, from a distance of 0.03 m, over 4 days (shorter procedure) or 8 days (longer procedure). In contrast to UVA rays, which did not evoke significant disturbances, UVB rays changed the activities of antioxidant enzymes. The longer repeated irradiation with UVB rays was performed, the deeper the observed decrease in antioxidant enzymes. The shorter procedure evoked a more profound decrease of glutathione peroxidase and catalase (the enzymes cleaving hydrogen peroxide) than of superoxide dismutase, an enzyme scavenging superoxide radical and producing hydrogen peroxide during the dismutation reaction of a superoxide free radical. This may contribute to an insufficient hydrogen peroxide cleavage at the corneal surface and danger to the cornea from oxidative damage. After the longer procedure (UVB rays), the activities of all antioxidant enzymes were very low or completely absent. In conclusion, repeated irradiation of the cornea with UVB rays evokes a deficiency in antioxidant enzymes in the corneal epithelium, which very probably contributes to the damage of the cornea (and possibly also deeper parts of the eye) from UVB rays and the reactive oxygen products generated by them.     

Ultraviolet light transmission through the human corneal stroma is reduced in the periphery

This article investigates in vitro light transmission through the human cornea in the ultraviolet (UV) portion of the electromagnetic spectrum as a function of position across the cornea from center to periphery. Spectrophotometry was used to measure UV transmission in the wavelength range 310–400 nm, from the central cornea to its periphery. UV transmission decreases away from the center, and this is attributed to scattering and absorbance. Corneal endothelial cells, which line the back of the cornea and are more numerous in the periphery, therefore receive a lower dose of UV than do those in the central cornea. This is consistent with the recent observation that endothelial cells in the corneal periphery exhibit less nuclear oxidative DNA damage than those in the central cornea.     

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.     

A microstructurally-based finite element model of the incised human cornea.

A mechanical model of the human cornea is proposed and employed in a finite element formulation for simulating the effects of surgical procedures, such as radial keratotomy, on the cornea. The model assumes that the structural behavior of the cornea is governed by the properties of the stroma. Arguments based on the microstructural organization and properties of the stroma lead to the conclusion that the human cornea exhibits flexural and shear rigidities which are negligible compared to its membrane rigidity. Accordingly, it is proposed that to a first approximation, the structural behavior of the cornea is that of a thick membrane shell. The tensile forces in the cornea are resisted by very fine collagen fibrils embedded in the ground substance of the stromal lamellae. When the collagen fibrils are cut, as in radial keratotomy, it is argued that they become relaxed since there is negligible transfer of load between adjacent fibrils due to the low shear modulus of the ground substance. The forces in the cornea are then resisted only by the remaining uncut fibrils. The cutting of fibrils induces an anisotropy and inhomogeneity in the membrane rigidity. By assuming a uniform angular distribution of stromal lamellae through the corneal thickness, geometric arguments lead to a quantitative representation for the anisotropy and inhomogeneity. All material behavior is assumed to be in the linear elastic regime and with no time-dependency. The resulting constitutive model for the incised cornea has been employed in a geometrically non-linear finite element membrane shell formulation for small strains with moderate rotations. A number of numerical examples are presented to illustrate the effectiveness of the proposed constitutive model and finite element formulation. The dependence of the outcome of radial keratotomy, measured in terms of the immediate postoperative shift in corneal power, on a number of important factors is investigated. These factors include the value of the elastic moduli of the stromal lamellae (dependent on the patient's age), the incision depth, the optic zone size, the number of incisions and their positions, and the intraocular pressure. Results have also been compared with expected surgical corrections predicted by three expert surgeons and show an excellent correspondence.     

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.     

Angiogenic effect of colchicine

The effect of colchicine on the microcirculatory vessels of eye perilimbal area was revealed in 2 groups of experimental rabbits. Colchicine was found to promote the growth of capillaries both during intravenous injections of colchicine solution and during local colchicine application as an ointment. The proper substance of cornea was also changed under the influence of colchicine. Changes in the ultrastructure of the capillary wall and endothelial cells forming the capillaries have been noted.     

Characteristics of the structure of the cornea of the reptilian eye]

Submicroscopic structural peculiarities in the cornea are described in some reptiles (grass-snake, lizard, turtle). Certain stages in histogenesis of the grass-snake cornea are presented. Poor differentiation of epithelium and its resemblance to endothelium is demonstrated in the grass-snake cornea. In cytoplasm of cornea cells of lizard and turtle a high content of mitochondria is noted. Great majority of mitochondria compactly arranged in the middle part of cytoplasm provide the appearance of zones. There is a middle zone of mitochondrial arrangement, there are also two zones in cytoplasm (internal-perinuclear and peripheral) without mitochondria but with supporting structures.     

Role of a regulatory protein from the bovine cornea in activation of cell sources of corneal regeneration in vitro

A highly purified regulatory protein isolated from the bovine cornea (RPC) was tested for the effect on the rat and newt corneas in vitro under different culture conditions. In the newt cornea, RPC stimulated limbus epithelial cells in roller cultures and cells in the basal layer of corneal epithelium in both roller and stationary cultures. In roller cultures of the rat cornea, RPC had no effect on weakly differentiated limbus cells but stimulated progenitor cells of the basal cornea layer to differentiation into definitive epithelial cells. In stationary cultures, RPC did not activate the involvement of cells in regeneration of the rat corneal tissue.