On the contribution of the nucleus and cortex to human lens shape and size.

BACKGROUND: The shape of the human lens changes from almost spherical at birth to ellipsoid due to a decrease in sagittal thickness and an increase in equatorial diameter during the first two decades of life. Both dimensions increase thereafter. This study was undertaken to determine the reason for the change. METHODS: Published refractive index gradients, from 20 lenses aged from seven to 82 years, were used to calculate the protein contents of concentric shells of fibre cells in human lenses. The boundaries of nuclear cores containing from 2.5 to 45 mg, in 2.5 mg increments, were determined from the isoindicial shells. Cortex thickness was determined from the distance between the 30 mg nuclear boundary and the capsule. RESULTS: The sagittal thickness of every nuclear core decreased until age 40 years and remained constant thereafter. Over the same time frame, the equatorial diameter of the cores containing up to 30 mg of protein increased, while those of cores larger than 30 mg decreased. The volumes of the cores decreased and their shapes changed from near spherical to spheroidal. Equatorial and sagittal cortex thickness increased linearly with age at 0.0082 mm per year. The anterior sagittal cortex was 0.23 mm larger than the posterior and the equatorial cortex was 0.62 mm greater. CONCLUSIONS: Changes in lens shape observed during the first two decades of life are due to remodelling and compaction of the 30 mg nuclear core. Cortex growth is linear throughout life.

Understanding the α-crystallin cell membrane conjunction.

PURPOSE: It is well established that levels of soluble α-crystallin in the lens cytoplasm fall steadily with age, accompanied by a corresponding increase in the amount of membrane-bound α-crystallin. Less well understood, is the mechanism driving this age-dependent membrane association. The aim of this study was to investigate the role of the membrane and its associated proteins and peptides in the binding of α-crystallin. METHODS: Fiber cell membranes from human and bovine lenses were separated from soluble proteins by centrifugation. Membranes were stripped of associated proteins with successive aqueous, urea, and alkaline solutions. Protein constituents of the respective membrane isolates were examined by SDS-PAGE and western immunoblotting. Recombinant αA- and αB-crystallins were fluorescently-labeled with Alexa350® dye and incubated with the membrane isolates and the binding capacity of membrane for α-crystallin was determined. RESULTS: The binding capacity of human membranes was consistently higher than that of bovine membranes. Urea- and alkali-treated membranes from the nucleus had similar binding capacities for αA-crystallin, which were significantly higher than both cortical membrane extracts. αB-Crystallin also had a higher affinity for nuclear membrane. However, urea-treated nuclear membrane had three times the binding capacity for αB-crystallin as compared to the alkali-treated nuclear membrane. Modulation of the membrane-crystallin interaction was achieved by the inclusion of an NH2-terminal peptide of αB-crystallin in the assays, which significantly increased the binding. Remarkably, following extraction with alkali, full length αA- and αB-crystallins were found to remain associated with both bovine and human lens membranes. CONCLUSIONS: Fiber cell membrane isolated from the lens has an inherent capacity to bind α-crystallin. For αB-crystallin, this binding was found to be proportional to the level of extrinsic membrane proteins in cells isolated from the lens nucleus, indicating these proteins may play a role in the recruitment of αB-crystallin. No such relationship was evident for αA-crystallin in the nucleus, or for cortical membrane binding. Intrinsic lens peptides, which increase in abundance with age, may also function to modulate the interaction between soluble α-crystallin and the membrane. In addition, the tight association between α-crystallin and the lens membrane suggests that the protein may be an intrinsic component of the membrane structure.

Insights into the age-related decline in the amplitude of accommodation of the human lens using a non-linear finite-element model.

AIM: To understand the effect of the geometric and material properties of the lens on the age-related decline in accommodative amplitude. METHODS: Using a non-linear finite-element model, a parametric assessment was carried out to determine the effect of stiffness of the cortex, nucleus, capsule and zonules, and that of thickness of the capsule and lens, on the change in central optical power (COP) associated with zonular traction. Convergence was required for all solutions. RESULTS: Increasing either capsular stiffness or capsular thickness was associated with an increase in the change in COP for any specific amount of zonular traction. Weakening the attachment between the capsule and its underlying cortex increased the magnitude of the change in COP. When the hardness of the total lens stroma, cortex or nucleus was increased, there was a reduction in the amount of change in COP associated with a fixed amount of zonular traction. CONCLUSIONS: Increasing lens hardness reduces accommodative amplitude; however, as hardness of the lens does not occur until after the fourth decade of life, the age-related decline in accommodative amplitude must be due to another mechanism. One explanation is a progressive decline in the magnitude of the maximum force exerted by the zonules with ageing.

Estimation of anterior nucleus of lens by Scheimpflug image before and after pupil dilatation.

PURPOSE: To compare the accuracy of lens transparency evaluations by Scheimpflug image in the anterior nucleus of the lens before and after pupil dilatation. METHODS: Scheimpflug lens images were recorded in 70 eyes of 38 subjects (age: 28-75 years) before and after pupil dilatation, and light scattering intensity measurements before and after dilatation were compared. RESULTS: There was a significant positive correlation between the light scattering intensity before and after dilatation at the anterior cortex, anterior nucleus, and central clear zone of the lens (r > 0.9, P <.0001). CONCLUSIONS: It is possible to estimate the transparency in the anterior nucleus of the lens from the Scheimpflug image without pupil dilatation. If nuclear type cataracts are regarded as a structural marker of aging in epidemiological studies, measuring the light scattering intensity in the anterior nucleus of the lens without dilatation seems to be a safe, useful, and quantitative method.

The nucleus of the human lens: demonstration of a highly characteristic protein pattern by two-dimensional electrophoresis and introduction of a new method of lens dissection.

A practical method for dissection of human lenses is described. The method utilizes the suture patterns as a guide to identify the developmental stage in which fiber cells were formed. Lenses were separated into cortex and adult, infantile, fetal and embryonic nuclear regions. Analysis of the proteins in each of these regions in adult lenses shows that the lens nucleus has a highly characteristic two-dimensional protein pattern distinct from that of the cortex. Each of the nuclear regions has essentially the same protein pattern. The data suggest that the conversion of cortical fibers to mature nuclear fibers involves well controlled processes.

Structure and regional water content of bovine, porcine, and human lenses examined with proton nuclear magnetic resonance imaging.

A proton nuclear magnetic resonance instrument with a 7-tesla field was used for nuclear magnetic resonance imaging (NMRI) to study bovine, porcine, and human lenses. The NMRI images show detailed changes in the water for normal and diseased tissues. The alterations in the nucleus and the cortex in relation to the health of the tissue are clearly illustrated.

Changes in light scatter and width measurements from the human lens cortex with age.

Light scatter and width measurements of the anterior cortical layers of the human lens were made in 50 eyes of 50 subjects using computerised linear scanning densitometry of Scheimpflug images. It was demonstrated that the amount of light scatter increased with age in all of the three major zones and that zone C3 showed the most marked increase. Most lens growth occurs in zone C2 with C3 showing little increase in width once it has become established. Zone C1 showed a tendency to decrease in width with age. In addition it was shown that the C3 zone, which is not present at birth and during early childhood, appears as a distinct layer during the second decade of life. Its scattering properties continue to increase throughout life, exceeding all other zones after approximately 30 years of age, in the absence of cataract. Possible explanations for the lens zone pattern are discussed.

[In vivo biomicroscopy of the human lens].

The human lens was observed in vivo by a specular microscope and specially designed cone contact lens. The lens fibers of central part of the anterior cortex were photographed and their widths measured in 16 healthy normal males in the third and fourth decades of life. The average width of the fibers was 12.35 +/- 1.17 microns and 12.36 +/- 0.80 microns. There was no statistically significant difference in average width between the two groups when the objectives were divided by their age in twenties and thirties.