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J.R. Kuszak, R.K. Zoltoski; The Mechanism of Primate Accommodation at the Lens Fiber Level Revealed by Correlative Microscopic and Computer Generated Animations . Invest. Ophthalmol. Vis. Sci. 2005;46(13):731.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The purpose of this study was to examine the mechanism of accommodation at the lens fiber level. Methods: Non–paired human eyes (n = 6) from donors under 45 years of age and paired baboon eyes (n = 9), from animals under 9 years of age, were fixed in either a stretched or un–accommodated state (stretching was accomplished with an ex vivo mechanical device) or in the accommodated state (lenses dissected free of the zonules naturally assume their accommodated configuration). Gross dimensions of lenses (thickness [antero–posterior axis] and width [equatorial axis]) were determined under a stereo surgical dissecting microscope while dimensions and surface structure of fibers along their length (anterior, middle or equatorial and posterior segments) and as a function of age (cortical vs. nuclear fibers) were recorded by scanning electron microscopy. Computer generated animations of accommodative lens shape changes were recreated by placing representative CADs of static SEM images taken in the accommodated and un–accommodated states on a time–line defined by a number of frames and then "tweening" or gradually modifying the lens appearance as it transitions between the un–accommodated and accommodated states. Results: The results of this study demonstrate that there is no variation in fiber morphology or dimensions during dynamic focusing and that accommodative lens shape changes are attributable to the end segments of fibers interfacing at the sutures. This mechanism of accommodation is made possible by opposite end curvature transforming fibers into simple coils or springs. As spring tension is released on the lens capsule fibers within growth shells expand in unison to effect an increase in lens thickness while simultaneously reducing lens width as the diameter of the coils (fibers) decrease as fiber end segments interface at sutures. As spring tension is applied on the lens capsule, fibers within growth shells are compressed in unison to effect a decrease in lens thickness while simultaneously expanding lens width as the diameter of the coils (fibers) increase as fiber end segments abut at sutures. Conclusions: The spring action of fibers as the mechanism of dynamic focusing supports critical aspects of lens physiology necessary for transparency that would be compromised if fiber dimensional and surface structural changes occurred during accommodation. In addition, this novel mechanism explains at least in part, the disparity in accommodative range between primates and non–primates.
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