Structural correlates of shape change in the primate crystalline lens
Cytological changes associated with accommodation in macaques and man were investigated using light and scanning electron microscopy to examine lenticular fibre modification and the role of the lens capsule. Accommodation was simulated monocularly in one animal with local administration of 1/4% phospholine iodide and by dislocation in another followed by fixation by perfusion. Three types of junctional structures were observed: angle processes were found at all depths, ball and sockets in outer lens zones, and tongue and grooves in the deeper lens. Interfaces lacking junctional structures were not present and the concept of sliding between fibre layers to permit curvature changes was rejected. The hypothesis of intracellular redistribution of cytoplasm within lens fibres was tested, by comparing fibre cross sectional area throughout the posterior half of accommodated and unaccommodated lenses. In one animal evidence for cytoplasmic flow was found throughout the lens but was greatest in the nucleus. In the other, showing less curvature difference, evidence was restricted the nucleu and superficial cortex. The thin superficial cortex is probably of little significance in effecting shape change. Consequently the results support the notion of greater nuclear than cortical action in accommodative shape changes. The fibres of the intermediate and deep cortex are remarkably thin and indented and are arguably less conducive to cytoplasmic flow. Lens capsule thickness was measured in 23 monkey and 11 human lenses in situ and detached, giving similar results. Profiles were recorded of unfixed monkey lenses with and without capsules. An annular zone of flattening, nearly coincident with maximum capsular thickness, giving the classical 'lenticonus" form, reduced on decapsulation. The young human and monkey capsule thickness variation was consistent with classical rather than more recent data thinnest at the posterior pole and thickest near, not at the equator. The results demonstrate a role for capsular shaping of the accommodated lens (with or without local moulding), effected by cytoplasmic flow most marked in the nucleus.