Biomicroscopy and scanning electron microscopy of early opacities in the aging human lens.

A large sample of lenses obtained from donor eyes spended for cornea transplantation were biomicroscopically screened and a representative subsample of lenses (about 30) exhibiting early opacities were studied at the scanning electron microscopic (SEM) level. Two types of early cortical opacities (Lens Opacities Classification System cortical grade 1a, 1b) could be distinguished biomicroscopically: (1) radial shades, and (2) circular shades. The prevalence of both types is age related, with an earlier occurrence of radial shades than of circular shades. In addition, radial shades proved to occur as single entities in isolation in lenses from people younger than the age of 40, whereas circular shades are only occasionally found in isolation in older lenses. SEM analysis substantiated the existence of the two early forms of opacity. Radial shades were found to consist of restricted parts of a small group of fibers in the deep cortex. These affected parts had membranes with a fine globular aspect and in cross-section proved to be filled with medium to large globular elements. Neighboring fibers and the nonaffected parts of the fibers involved in the radial shades had a normal SEM ultrastructure. Circular shades proved to originate as fractures of a large cohort of fibers in the deep equatorial cortex perpendicular to the course of the fibers. The fractured faces were slightly swollen, but the membranes on both sides of the fracture had a normal ultrastructure. In cross-sections of more advanced circular shades it proved that the broken fibers are filled with globular elements and are opaque. This opacification proved to extend only in one direction: either anteriorly or posteriorly. The results strongly suggest that two distinct early cataractogenic cellular processes are operating in the aging human lens. The possible induction of circular shades by radial shades, as can tentatively be concluded on account of the difference in appearance during aging of circular and radial shades, needs further verification by SEM analysis of more severely affected lenses. The observations corroborate the suggestion by Bron and Brown of the existence of mechanisms within the lens that separate damaged from undamaged fibers (segregation) or separate affected parts from unaffected parts of individual fibers (sealing).