Volume change of the ocular lens during accommodation

Abstract

tion, mammalian lenses change shape from a rounder configuration (near focusing) to a flatter one (distance focusing). Thus the lens must have the capacity to change its volume, capsular surface area, or both. Because lens topology is similar to a torus, we developed an approach that allows volume determination from the lens cross-sectional area (CSA). The CSA was obtained from photographs taken perpendicularly to the lenticular anterior-posterior (A-P) axis and computed with software. We calculated the volume of isolated bovine lenses in conditions simulating accommodation by forcing shape changes with a custom-built stretching device in which the ciliary body-zonulae-lens complex (CB-Z-L) was placed. Two measurements were taken (CSA and center of mass) to calculate volume. Mechanically stretching the CB-Z-L increased the equatorial length and decreased the A-P length, CSA, and lens volume. The control parameters were restored when the lenses were stretched and relaxed in an aqueous physiological solution, but not when submerged in oil, a condition with which fluid leaves the lens and does not reenter. This suggests that changes in lens CSA previously observed in humans could have resulted from fluid movement out of the lens. Thus accommodation may involve changes not only in capsular surface but also in volume. Furthermore, we calculated theoretical volume changes during accommodation in models of human lenses using published structural parameters. In conclusion, we suggest that impediments to fluid flow between the aquaporin-rich lens fibers and the lens surface could contribute to the aging-related loss of accommodative power.lens volume calculation; intralenticular fluid movement; presbyopia; mammalian lens THE EYE IS ABLE TO FORM a clear image on the retina from objects situated within a wide range of distances due to a process called accommodation. In higher vertebrates, including humans, accommodation results from changes in crystalline lens shape and surface radii of curvature (13).When the ciliary muscle is relaxed and flattened against the sclera, the zonulae adjoining the ciliary body and the lens capsule are under tension and thus pulling eccentrically on the lens equator. This action causes the lens to adopt a relatively flattened shape with a larger equatorial diameter and a shorter A-P length, which allows focusing on virtual infinity (zero accommodation). When an individual focuses on a near object, the ciliary muscle contracts (shortening its distance to the lens equator), the zonulae relax, and the lens as a whole adopts a relatively rounded shape, which is its normal tendency. Physically, these changes in lens shape inevitably must involve changes in either capsular surface area or lens volume, or both.Classic theories of lenticular accommodation suggest that the volume of the intraocular crystalline lens remains constant during the accommodation process (16,17,30). No empirical studies have demonstrated that the volume actually stays constant. Because of the physical principle mentioned abov...

Fluid transport phenomena in ocular epithelia

“…With a more advanced technology, we were able to determine that the bovine lens regains its volume in less than 200 milliseconds after the release of the stretching tension (unpublished data). Although we demonstrated a change in volume of the lens, it turned out from our analysis that a change in the surface area of the lens during accommodation is also required (Gerometta et al, 2007). We are presently in the processes of developing an approach to quantify the extent to which the lens surface area might also change.…”

“…We recently described in detail our approach to calculate volume from lens CSA (Gerometta et al, 2007). In this work we characterized 1) an in vitro model: the isolated iris-ciliary bodyzonulae-lens complex (CB-Z-L) dissected from bovine, and 2) a theoretical model: the graphical construction of human lenses from structural parameters acquired from the literature.…”

“…Hence, we developed a method to compute the volume of the lens from lateral photographs (Gerometta et al, 2007). These photographs enabled the CSA to be computed with software.…”

“…We don't know the force developed inside the lens when stretched, but it should be sufficient to move fluid at the rate required. For technical reasons related to the design of our "stretching apparatus" (rotating a wheel), it took several seconds to stretch the lens, but we felt no particular resistance (Gerometta et al, 2007). With a more advanced technology, we were able to determine that the bovine lens regains its volume in less than 200 milliseconds after the release of the stretching tension (unpublished data).…”

Dynamic control of slow water transport by aquaporin 0: Implications for hydration and junction stability in the eye lens

“…The magnitude of this force is comparable with cell-adhesion forces sustained by, for example, integrins (48). However, this estimate does not take into account water release and reabsorption by crystallin proteins or by the lens itself (49,50), which would lower the bulk modulus and in turn the force on the cavity. Second, in our simulation of the octameric junction, water molecules pass between the intermembrane cavity and the extracellular environment through ''portals'' in the extracellular domains of AQP0, as suggested by Harries et al (17).…”