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A.L. Madurski, A. Ngando, R.K. Zoltoski, J.R. Kuszak; The Limited Range of Non–Primate Accommodation is a Function of Fiber Cell Structure and Sutural Organization . Invest. Ophthalmol. Vis. Sci. 2005;46(13):732.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: We have recently determined that the accommodative shape changes in primates are attributable to fiber end segments interfacing at the sutures. This mechanism is made possible by opposite end curvature transforming fibers into simple coils or springs. In view of the fact that the accommodative range in non–primate lenses is very limited, we have examined the mechanism of non–primate accommodation at the lens fiber level. Methods: Paired rabbit eyes (n = 8) from animals under 1 year 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 in rabbits, like primates there is no change in individual fiber morphology or dimensions during dynamic focusing. However, unlike primate lenses, the end segments of rabbit fibers are both too large and non–uniform to interface at the sutures making the fiber spring action mechanism of accommodation less feasible. In any case, the range of fiber end curvature in rabbits is so broad that it would be more difficult to uniformly effect their expansion or compression in unison to produce the changes in lens thickness and width that produce accommodation. Conclusions: The structure and organization of lens sutures in non–primates precludes the fiber spring action as the mechanism of dynamic focusing. It can be debated whether or not non–primates fail to accommodate due to the structure and organization of their fibers or if they do not develop a structure that supports dynamic focusing because they do not need to accommodate.
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