Surface change of the mammalian lens during accommodation

Abstract

Classical theories suggest that the surface area of the crystalline lens changes during accommodation while the lens volume remains constant. Our recent work challenged this view by showing that the lens volume decreases as the lens flattens during unaccommodation. In this paper we investigate 1) the magnitude of changes in the surface of the in vitro isolated cow lens during simulated accommodation, as well as that of human lens models, determined from lateral photographs and the application of the first theorem of Pappus; and 2) the velocity of the equatorial diameter recovery of prestretched cow and rabbit lenses by using a custom-built software-controlled stretching apparatus synchronized to a digital camera. Our results showed that the in vitro cow lens surface changed in an unexpected manner during accommodation depending on how much tension was applied to flatten the lens. In this case, the anterior surface initially collapsed with a reduction in surface followed by an increase in surface, when the stretching was applied. In the human lens model, the surface increased when the lens unaccommodated. The lens volume always decreases as the lens flattens. An explanation for the unexpected surface change is presented and discussed. Furthermore, we determined that the changes in lens volume, as reflected by the speed of the equatorial diameter recovery in in vitro cow and rabbit lenses during simulated accommodation, occurred within a physiologically relevant time frame (200 ms), implying a rapid movement of fluid to and from the lens during accommodation.

Mechanical stretching forces oppose osmotic lens swelling

“…In earlier work we demonstrated, with isolated lenses under in vitro conditions mimicking accommodation, that changes in lens volume occur in accord with the lens shape changes (Gerometta et al ., 2007; Zamudio et al ., 2008). In vivo , human lenses become rounder for closer vision and flatter for distant vision, a process involving a completely reversible change in shape.…”

“…In the present experiments, the lenses were bathed within the stretching apparatus that was described in detail for the rabbit lens (Zamudio et al ., 2008). The stretching device contained eight motors evenly mounted on a circular module.…”

“…The stretching device contained eight motors evenly mounted on a circular module. These motors were synchronized to simulate un-accommodation and accommodation by respectively transmitting radial stretching forces and relaxation to the lens as described in detail before (Zamudio et al ., 2008). The only difference was that the rabbit lens was now photographed perpendicularly to the A-P axis, as done earlier to calculate volume changes in the cow lens during accommodation (Gerometta et al ., 2007).…”

Interaction between mechanical and osmotic forces in the isolated rabbit lens

“…Using large isolated bovine lenses with the accommodation anatomical component intact, we were able to show that the lens volume reversibly changes 3.6 to 6.0% during the process of simulated accommodation. We also predicted a smaller decrease in volume of 2.2 % in the human lens when it flattens out during disaccommodation (Gerometta et al, 2007, Zamudio et al, 2008). This prediction was recently confirmed in young human volunteers demonstrating that the human lens indeed changes volume, albeit small, during accommodation (Sheppard et al, 2011).…”

“…The dissection of the experimental lens was described previously (Gerometta et al, 2007, Zamudio et al, 2008). Briefly, part of the sclera, the iris and the zonulae were left attached to the lens, and this whole complex was glued to a latex rubber diaphragm that could be stretched or relaxed by the pulling pins of a stretcher that was electronically controlled.…”