Abstract
Purpose: :
To directly observe at a cellular level how lens fiber cells and lens sutures in fresh pig and monkey lenses move and change during accommodation. Prior scanning electron microscopy studies using fixed lenses suggest that lens fiber end segments dynamically overlap at the sutures to allow the lens to change shape during accommodation. Here dynamic confocal imaging in fresh tissues allowed direct observations of the lens fiber cells and lens sutures at high magnification during mechanical stretching simulated disaccommodation-accommodation.
Methods: :
Eyes from ~6 month old pigs and from 7 adult rhesus monkeys were dissected to isolate the intact anterior segment comprising the ciliary body, lens and zonular fibers. 12 arms of an automated mechanical stretching device were glued to the ciliary body. The tissues were subjected to 2-3 mm of repeated mechanical stretch-relax cycles while: 1) measuring lens biometric accommodative changes with ultrasound biomicroscopy, 2) measuring changes in lens optical power using a scanning laser optical system, 3) imaging the anterior subcapsular cortical lens fiber cells and sutures with a Heidelberg Retinal Tomographer (HRT) confocal microscope, and 4) imaging lens fiber cells and sutures with high magnification retroilluminated light microscopy.
Results: :
Repeated 2-3 mm mechanical stretch-relax cycles in pig and monkey lenses reproducibly produced 7-10 D disaccommodative-accommodative changes in lens power and 450-500 µm changes in lens thickness. Confocal microscopy provided ~400x400 µm images and videos of the behavior of lens fiber cells and sutures during stretching. Only subtle changes at the cellular level were observed in the lens fiber cells and sutures. Stretching results in general broadening, expansion and elongation and relaxation results general thinning, compaction and shortening of the lens fibers cells and sutures. Lens fiber end segments at the sutures were observed to separate with stretching in the pig lenses, but no separation, overlapping or other changes in the lens fiber end segments were observed with stretching in monkey lenses.
Conclusions: :
Accommodation of the monkey lens is accompanied by a subtle, uniform elastic molding of the lens fiber cells by the capsule to decrease fiber diameters and fiber lengths. In pig lenses, fluid filled spaces between the fiber ends at the lens sutures appears to increase in size with stretching. This is unlike that observed at the sutures in monkey lenses where no spaces, separation, overlapping or other changes of the lens fiber end terminals were observed at the sutures.
Keywords: accommodation • presbyopia • imaging/image analysis: non-clinical