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.

Effect of chronic smoking on lens nucleus as assessed by Pentacam HR lens densitometry in young adults.

OBJECTIVE: To evaluate the effects of chronic tobacco smoking on lens nucleus by Pentacam HR lens densitometry (LD) in young adults. DESIGN: Prospective cross-sectional case series. METHODS: Thirty subjects (23 M, 7 F) who were chronic cigarette smokers (≥10 cigarettes/day for at least 2 years) (group 1) and another 30 subjects (23 M, 7 F) who did not smoke (group 2), were included in this study. The patients were matched for age and sex between the groups. The exclusion criteria were any history of ocular surgery, any systemic disorders and any ocular diseases except for mild refractive disorders. Lens densitometry measurements were done with the Pentacam HR (Oculus, Wetzlar, Germany). The Schirmer test and pachymetry measurements were also performed. RESULTS: Mean age of the patients for both groups was 28.90 ± 8.20 years (range: 18-40 years). Mean lens densitometry (LD) measurements of Group 1 (chronic cigarette smoking group) were higher than those of Group 2 (control group) in all LD techniques; however only mean "peak" LD measurements showed a statistically significant difference between these two groups (Group 1: 8.67 ± 0.61, Group 2: 8.44 ± 0.70, p = 0.04). The mean Schirmer test value was 12.43 ± 5.60 mm in Group 1 and 13.00 ± 4.26 mm in Group 2 (p = 0.55). The mean central corneal thickness (CCT) value was 564.23 ± 34.61 µm in Group 1 and 550.47 ± 32.94 µm in Group 2 (p = 0.03). DISCUSSION: The Pentacam HR LD seems to be an important option for the evaluation of lens nucleus in young adults, because it gives objective and quantitative data. CONCLUSION: Although chronic smoking increases lens nucleus density in young adults, the effect is not statistically significant when compared with the control group.

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.

The shape of the human lens nucleus with accommodation.

Knowledge about geometric properties such as shape and volume and Poisson's ratio of the nucleus can be used in the mechanical and optical modeling of the accommodation process. Therefore, Scheimpflug imaging was used to determine the shape of the human lens nucleus during accommodation in five subjects. To describe the shape of the nucleus, we fitted a parametric model of the cross-sectional geometry to the gradient of the Scheimpflug images using the Hough transform. The geometric model made it possible to estimate the anterior and the posterior central radius, central thickness, equatorial diameter, and cross-sectional area of the nucleus. Assuming that the nucleus is rotationally symmetric, the volume of the nucleus can be estimated by integrating around the circumference. For all five subjects, the results show that during accommodation the nucleus became more convex and that the central thickness increased whereas the equatorial diameter decreased. This decrease in equatorial diameter of the nucleus with accommodation is in accordance with the Helmholtz accommodation theory. Finally, the volume of the nucleus (on average 35 mm(3)) showed no significant change during accommodation in any of the subjects, presumably due to the fact that the human nucleus consists of incompressible material with a Poisson's ratio that is near .5.

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.

[Anatomical evaluation of hydrodissected human lens nucleus].

We measured the diameter and central thickness of the central nucleus, isolated by repeated hydrodissection, in 100 eyes undergoing extracapsular cataract extraction. The average diameter and central thickness were 6.43 +/- 0.86 mm and 2.93 +/- 0.36 mm, respectively. The average ratio between the two measurements was 2.12, which is similar to that of an extracted crystalline lens. Statistical analysis revealed a positive correlation between central thickness and age (p < 0.05). There was a weak positive correlation between central thickness and hardness of the nucleus. Since specimens were obtained from clinical cases in this study, further evaluation should be made using cadaver eyes.

Size of the lens nucleus separated by hydrodissection.

We measured the extracted central nucleus (diameter and thickness) of 88 eyes (average patient age, 72 years) undergoing extracapsular cataract extraction (ECCE). The average diameter was 6.51 +/- 0.75 mm; the average thickness, 2.96 +/- 0.33 mm. Since the average lens diameter is approximately 10 mm, a corneoscleral incision of 7 to 8 mm in chord length should be sufficient for its removal in most ECCE cases. Accumulation of these data and further analysis correlating them with age and degree and type of cataract in larger series will be useful in determining the smallest possible incision size required for individual patients.

An enriched set of features of nuclear cataract identified by multidimensional scaling.

Development of an improved system for visual classification of cataracts requires a three-step procedure: first, to identify the full range of visible features of cataracts; second, to develop and test scales for the visual assessment of each feature; and third, to establish the epidemiological or clinical validity of each scale for cataract classification. This paper focuses on the first step, applying a powerful psychometric technique for identifying the visible features of nuclear cataracts. New visual features of nuclear cataract were identified using the psychometric procedure of multidimensional scaling (MDS). Each of 5 observers independently examined pairings of slitlamp photographs of 24 cases of pure nuclear cataract, making two different ratings of dissimilarity of each of the 276 possible pairs. The two dissimilarity ratings were, first, of nuclear color and, second, of nuclear structure. MDS analysis of the dissimilarity ratings of nuclear color revealed two major visual features underlying the judgments: one a combination of hue and saturation, and the other brightness. Analysis of the ratings of nuclear structure identified a total of nine features: one distinguishing between immature and mature cataracts, four describing features of the immature cataracts (aspect ratio, background haze, clarity of the embryonal nucleus, and clarity of the outer nuclear shell), and four describing features of the mature cataracts (opalescence, aspect ratio, color of the nucleus, and symmetry). We conclude that there are many more systematic distinctions to be made in the appearance of nuclear cataracts than are now recognized in clinical practice.

Morphological study of age-related changes in mouse lens.

Age-related changes in the lenses of ddy strain mice, ranging from 4 to 24 months old, were studied histologically. The early change of aging lens was noted as a slight extension of fiber cells into the posterior capsule in the area behind the nuclear bow around 6 months of age. Those cell extensions formed villous projections reaching up to one half of the total capsule by 18 months of age. Another notable change was a decreasing cell population of the lens epithelium and the appearance of their nuclear inclusions. The latter contained rough endoplasmic reticulum, free ribosomes and filamentous matrix at the early stage and a mass of filamentous material at the later stage. This change was initiated by 12 months of age, and became profound with the increase of age. Moreover, the bow nuclei became markedly displaced posteriorly after 14 months of age. At about 18 months of age, when the pathological changes in both epithelial and bow regions became severe, the superficial cells in the posterior cortex showed swelling and atrophy, which soon led to the lens opacity due to accumulation of cell debris and fluid. These histopathological changes associated with aging appear to correspond to the incipient form of posterior subcapsular cataract which is detectable by clinical examinations.