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A. Kinoshita, Z. Lu, T. Kitaoka, T. Amemiya; The Ultrastructure and Elelement Canges in the Tear Film With Aging . Invest. Ophthalmol. Vis. Sci. 2003;44(13):2534.
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Purpose: To study the ultrastructure and element changes in the tear film with aging. Materials and method: We used 7 normal 5-month-old rats and 7 normal 15-month-old rats. Immediately after eyeballs were enucleated under pentobarbital anesthesia, they were frozen with an isopentan-propane mixture cooled by liquid nitrogen and then put into liquid nitrogen. For freeze substitution, the corneas were cut into small peaces in liquid nitrogen and transferred to 2% osmium tetroxide in absolute acetone in a screw-capped vial surrounded by a bath of acetone and dry ice. The vial was transferred to a -20°c freezer for 2 h, then to a -4°c refrigerator for 2 h and finally to room temperature. The specimens were washed with absolute acetone and then embedded in epoxy resin. Ultrathin sections were examined with a transmission electron microscope. And the specimens were coated with gold in a vacuum evapolater after being dried in a critical point-drying apparatus and then observed with a scanning electron microscope. After we examined the ultrastructure of the tear film, elements in tear film were analyzed with EDAX DX-4. Results: The tear film consists of a thin film-like structure at the most superficial layer and net-shaped structure at the bottom. The thickness of tear film was 282.1 ±7.7 nm (n=15) in 5-months-old rats, and 665 ±34.0 nm (n=15) in15-months-old rats (p<0.0001). The electron density of the net-shaped structure was higher in 15-month-old rats than in 5-months-old rats. Microvilli in the corneal surface were longer in 5-month-old rats than in15-month-old rats. Calcium and iron were detected in the tear film with EDAX in 5-month- old and 15-month- old rats. Discussion: The results suggest that a tear film becomes thin with aging and microvilli getting shorter. These conditions may hardly keep healthy tear film with aging. Higher electron density of the net-shaped structure might be related to calcium and iron concentration, deposition of calcium and iron with aging. Conclusion: The structure and elements of the tear film may be changeable with aging.
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