Computer modeling of secondary fiber development and growth: I. Nonprimate lenses.

PURPOSE: The purpose of this study was to use qualitative and quantitative structural data from nonprimate lenses with branched (Y and line) sutures to generate computer models (animations) of secondary fiber development and suture formation.
METHODS: A minimum of 12-18 adult lenses/species (mice, cows, frogs, and rabbits) were used in this study. Lenses were analyzed by light (LM), transmission (TEM), and scanning electron microscopy (SEM). Fiber width, thickness, and length were ascertained from micrographs and by using formulations to calculate distances between degrees of latitude and longitude on asymmetrical oblate spheroids. This information was then used to create scale computer assisted drawings (CADs) of fibers at different stages of their development. The CADs were then placed on a timeline and animated to produce dynamic representations of secondary fiber development and growth.
RESULTS: Animating secondary fiber development and suture formation with the inclusion of quantifiable differences in fiber dimensions at progressive stages of their differentiation revealed the following: first, there is the presumption that fibers migrate, rotate, and elongate until they reach their sutural destinations is not likely to be correct. When developing fibers reach approximately 60-65% of their eventual total length, their migration and rotation is complete. The remaining fiber elongation (the production of end segments) occurs without either concomitant cellular migration or rotation. Second, it is presumed that suture branches originate peripherally and are then constructed sequentially until all of the branches come to confluence at the poles is also not likely to be correct. While suture branches do originate peripherally, if the rate of elongation is constant in the anterior and posterior directions (intrafiber elongation speed) and between developing fibers within a forming growth shell (interfiber elongation speed), then only a part of their construction proceeds sequentially toward the poles. A second suture branch origin will be established at the poles resulting in a short distal portion of suture branches being formed sequentially in the reverse direction. Suture formation will conclude when a long proximal and a short distal portion of branches come to confluence within unequal anterior and posterior polar cap regions. This segmented suture formation scheme will be more pronounced in line suture lenses than in Y suture lenses. Third, because lenses with branched sutures have growth shells consisting of fibers of unequal length, fiber maturation is likely to be initiated in these lenses before a growth shell as well as suture formation is completed and would proceed in distinct patterns over a period of time. This is in marked contrast to avian lens fiber maturation which does not begin until growth shell and suture (branchless umbilical) formation is completed and then occurs rapidly and essentially simultaneously across the entire growth shell.
CONCLUSIONS: Animations of secondary fiber development and suture formation based on quantitative analysis of electron micrographs reveals important novel aspects of these processes that have not been apparent from the results of previous mechanistic studies. The more complex schemes of fiber differentiation and suture formation presented herein are consistent with the notion that lens function (dynamic focusing) is interdependent on lens structure and physiology. The animations confirm that while all vertebrate lenses have a similar structure, differences in the level of their structural complexity established early in development and maintained throughout life can account for the varying amount of optical quality known to exist between species.