The lens epithelium and radiation cataract. IV. Ultrastructural studies of interphase death in the meridional rows

[Features of accumulation of chemical elements in the volume of the lens in senile cataract]

PURPOSE

This study provides a detailed analysis of the bioinorganic chemical composition of lens substance in patients with senile cataract using classical and spatial statistics methods.

MATERIAL AND METHODS

The study included 30 isolated human lenses. The light scattering ability (LSA) of the lens substance was evaluated using an original method. Additionally, distribution of chemical elements in the lens substance was analyzed using a scanning electron microscope with energy dispersive spectrometer (SEM/EDS). Measurements by all methods were carried out in a single coordinate space, which made it possible to compare the spatial correlation of different parameters.

RESULTS

Small-angle light scattering of the lens substance has been quantitatively characterized for the first time. In contrast to the conventional norm, in senile cataract the accumulation fields of the majority of ion-forming elements (including Na, P, K, Cl) are distributed along the lines repeating the geometry of the lens capsule. At the same time, the light scattering ability of certain areas of the lens is significantly correlated with changes in the concentrations of Na, P, K, Ca in these areas. In particular, one ion-forming element can be distinguished - Na: spatial change of its concentration in senile cataract is strongly associated with a local change in LSA of the lens with opacities clustering of any degree. Thus, a change in the nature of the Na accumulation in the lens volume can be considered the main marker of senile cataract formation.

CONCLUSION

The distribution pattern of ion-forming elements indicates that the loss of barrier properties in the capsule plays a significant role in the development of senile cataract.

Biochemistry of Eye Lens in the Norm and in Cataractogenesis

The absence of cellular organelles in fiber cells and very high cytoplasmic protein concentration (up to 900 mg/ml) minimize light scattering in the lens and ensure its transparency. Low oxygen concentration, powerful defense systems (antioxidants, antioxidant enzymes, chaperone-like protein alpha-crystallin, etc.) maintain lens transparency. On the other hand, the ability of crystallins to accumulate age-associated post-translational modifications, which reduce the resistance of lens proteins to oxidative stress, is an important factor contributing to the cataract formation. Here, we suggest a mechanism of cataractogenesis common for the action of different cataractogenic factors, such as age, radiation, ultraviolet light, diabetes, etc. Exposure to these factors leads to the damage and death of lens epithelium, which allows oxygen to penetrate into the lens through the gaps in the epithelial layer and cause oxidative damage to crystallins, resulting in protein denaturation, aggregation, and formation of multilamellar bodies (the main cause of lens opacification). The review discusses various approaches to the inhibition of lens opacification (cataract development), in particular, a combined use of antioxidants and compounds enhancing the chaperone-like properties of alpha-crystallin. We also discuss the paradox of high efficiency of anti-cataract drugs in laboratory settings with the lack of their clinical effect, which might be due to the late use of the drugs at the stage, when the opacification has already formed. A probable solution to this situation will be development of new diagnostic methods that will allow to predict the emergence of cataract long before the manifestation of its clinical signs and to start early preventive treatment.

Localization of B-DNA and Z-DNA in terminally differentiating fiber cells in the adult lens

We examined histochemically and immunohistochemically the distribution of B- and Z-DNA in the epithelium and terminally differentiating dog lens fiber cells. On the basis of anti-DNA antibody reactivity, qualitative and quantitative data on B- and Z-DNA in cells were determined. Anti-B-DNA immunoreactivity gradually declined throughout nucleated fibers, with a precipitous decrease at approximately 90 microns. Anti-Z-DNA antibody binding decreased with a sudden loss of immunoreactivity at approximately 90 microns. The pattern of anti-B- and Z-DNA staining correlates with the loss of alpha-crystallin immunoreactivity, the major lens crystallin, and decreased eosin staining of proteins. Germinative zone cell nuclei showed the highest DNA probe binding values, followed by the superficial fibers, central zone, middle fibers, and deep fibers. The presence of single-stranded (ss)DNA in deeper fibers was detected by anti-ss-DNA antibodies. This is indicative of DNA degradation. These observations suggest that a dramatic reorganization of lens fiber cells' supramolecular order occurs at approximately 90 microns, the phase transition zone.

Calcimycin-induced lens epithelial cell apoptosis contributes to cataract formation

Previous studies have shown that calcimycin induces cataract in organ culture. To investigate the mechanism of this induction, the viability of lens epithelial cells in calcimycin (calcium ionophore, A23187)-treated rat lenses were examined. During incubation of lenses with 5 microM calcimycin, apoptotic epithelial cells were found after a 2-hr treatment as determined by terminal deoxynucleotidyl transferase (TdT) labeling. The percentage of apoptotic cells quickly rose as the incubation time increased. After a 12-hr incubation, more than 60% of the lens epithelial cells underwent apoptosis. Prolonged c-fos expression, previously shown to be an indicator of programmed cell death, was also observed during this treatment. DNA fragmentation assays further confirmed that the TdT labeled cells were indeed apoptotic. Under the same incubation conditions, the cultured lenses gradually lost transparency and became completely opaque in about 30 hr. Since the vertebrate lens contains only a single layer of epithelial cells, apoptotic death of these cells activated by calcimycin quickly destroys the lens epithelium, impairs homeostasis of the underlying fiber cells and initiates development of lens opacification.

Apoptosis induced by high- and low-LET radiations

Cell death after irradiation occurs by apoptosis in certain cell populations in tissues. The phenomenon also occurs after high linear energy transfer (LET) irradiation, and the relative biological effectiveness (RBE) is 3 to 4 (with respect to low-LET radiation and apoptosis in intestinal crypts) for neutrons with energies of 14 MeV and up to 600 MeV. It is thought that p53 plays a role in the phenomenon, as radiation-induced apoptosis is not observed in p53-null animals.

Accelerated heavy ions and the lens. IX. Late effects of LET and dose on cellular parameters in the murine lens

Lenses of mice irradiated with 250 MeV protons, 670 MeV/amu 20Ne, 600 MeV/amu 56Fe, 350 MeV/amu 56Fe, 600 MeV/amu 93Nb or 593 MeV/amu 139La ions were evaluated by analysing cytopathological indicators which have been implicated in the cataractogenic process. The LETs ranged from 0.39 to 953 keV/microns and the fluences from 1.31 x 10(3)/mm2 to 5.12 x 10(7)/mm2. The lenses were assessed 64 weeks post-irradiation in order to observe the late effects of LET and dose on the target cell population of the lens' epithelium. Our studies showed that growth-dependent pathological changes occurred at the cellular level as a function of dose and LET. For a given particle dose, as the LET rose, the number of abnormal mitotic figures, micronuclei frequency, and the disorganization of meridional rows increased to a maximum and then reached a plateau or decreased. For particles of the same LET, the severity of meridional rows disorganization and micronuclei frequency increased with increasing dose. The numbers of cells surviving at late times post-irradiation were comparable with those of controls. In addition, the cellular density was similarly unaffected. These observations are consistent with the current theory of the mechanism of radiation cataractogenesis which posits that genomic damage to the epithelial cells surviving the exposure is responsible for opacification.

Selenite-induced epithelial damage and cortical cataract

The purposes of these experiments were 1) to measure microscopic changes in the epithelium associated with selenite cataract, and 2) to describe the formation and subsequent clearing of selenite cortical cataract. Fourteen-day old suckling rat pups received a single subcutaneous injection of an overdose of sodium selenite at 2.25 mg Se/kg b.w. Development of cortical cataract was observed by biomicroscopy, and changes in epithelium were studied by light microscopy of flat-mounted lens epithelia. Selenite administration caused cortical cataract 15-30 days after injection in addition to previously characterized nuclear cataract. The cortical cataract progressed through equatorial vacuolization, opacity, and finally clearing of the cataract. Mitosis was suppressed and karyorrhexis was observed in the germinative zone of the epithelium 5 hours after selenite injection. Pathological disorganization of the epithelium followed. Changes included vacuolization, loss of meridional rows, and defective fiber formation. Restoration of epithelial morphology was associated with clearing of cortical opacity. Epithelial damage at 5 hours was the earliest change yet recorded for selenite cataract, and these data are consistent with our working hypothesis that the initial site of attack of selenium in both cortical and nuclear cataract is the lens epithelium.

Further observations on the effect of galactose on the development of X-ray-induced cataract in mice

The effect of a 30% galactose diet on the progression of X-ray-induced cataract in mice was evaluated by following morphological changes as seen by light and transmission electron microscopy in different regions of the lens. Lens opacities as observed with the slit-lamp biomicroscope developed at a slower rate in galactose-fed animals than in those on a normal diet. The protective effect of galactose on X-ray cataract was seen whether galactose feeding was initiated either 1 week before or after exposure to X-ray. At 4 months after X-ray approximately 50% of galactose-fed animals had mature cataracts, compared to 100% in the control group. Similarly, at two weeks after exposure to X-ray, before any lens opacities were observed, morphological changes were more severe in the control group; cells in the meridional row were more disorganized in the control than in the galactose-fed groups. However, the progression of mature cataracts in the two galactose-fed groups were not significantly different. Since free radicals produced by X-ray are thought to be short-lived, the protective effect of galactose feeding after X-ray was unexpected, raising the possibility that some of the active species may be long-lasting. The nature of such radicals, if any, is unknown and remains to be investigated.

Lens epithelium and radiation cataract. V. Observations on acid phosphatase and meridional row nuclear fragmentation

The influence of X-radiation on acid phosphatase activity in differentiating meridional row cells of rat lens epithelia was examined by ultrastructural cytochemistry. X-ray doses of 10-12 Gy (1000-1200 rads) produced clearly observable nuclear and cytoplasmic damage at 13-19 hr postirradiation. In those cells, acid phosphatase reaction product was associated with much of the electron dense material of fragmented nuclei and various parts of the cytoplasm. In addition, many irradiated cells without observable damage were positive for reaction product. The presence of acid phosphatase activity in these otherwise normal-appearing cells is suggestive of more subtle radiation damage than that observed in the more severely damaged cells.

Cytoarchitectural changes in lens epithelium of galactose-fed rats

Whole mounts of lens epithelia from rats fed on a galactose-rich diet for one, two, three or seven days were examined for morphological alterations. The meridional rows of epithelia from lenses of animals fed on galactose for seven days were found to be grossly disorganized or abnormally elongated. There was some indication that the cells were edematous and that the epithelium had become multilayered. These abnormalities apparently extended into the transitional zone. Evidence of the cytoarchitectural changes was present as early as one day after institution of the 30% galactose diet and progressed steadily over the period of observation. The findings provide support for the hypothesis that loss of meridional row integrity is a phenomenon common to development of most types of cortical cataracts.