Fiber cell membranes have also been implicated in cataract formation through direct scattering of membrane components, alterations of membranes and deposition of proteins on membranes. Recently, Michael et al.,
27 have discussed membrane scattering based on relative refractive index. Using index matching, they found that membranes of the cortex have a higher refractive index than the cytoplasm, thus producing a pattern of alternating refractive index within radial cell columns. They suggested that the expected high scattering from this arrangement is partially canceled by destructive interference from the regular array of cells of the cortex.
24 27 28 In contrast, in the nucleus where fiber cells are less regularly packed, the measured membrane refractive index was lower, matching the refractive index of the cytoplasm, thus minimizing refractive index fluctuations and light scatter.
27 On a finer scale, fiber cell plasma membranes form complex interdigitations, many of which are in the range of 0.2 to 0.5 μm, including finger-like projections, undulating membranes, and complex edge processes.
29 30 31 Because some of these objects appear circular in sections, it has been suggested that they are vesicles that could scatter light as do isolated vesicles in vitro.
32 33 However, careful examination of the ultrastructure of the interdigitations reveals that they are composed of paired membranes in close association, as is typical for fiber cells of the entire nuclear core.
10 30 31 They are not truly circular and are not single-membrane vesicles. In fact the complex pattern is made more complex during aging and cataract formation due to fiber cell compaction.
34 Despite these changes, the membranes are probably not major sources of light scattering based on the observation that the patterns of interdigitation in transparent and nuclear cataractous lenses are indistinguishable.
11 29 An additional factor is that the scattering from the membranes is dependent mainly on the membrane thickness rather than the membrane topology. Minimal light is scattered by sheets that are thin compared with the wavelength of incident light, even if their topology is complex.
35 These properties of membranes, together with the close packing of cytoplasmic crystallins, form the physical basis of lens transparency.
24 28 36 37 38 However, many changes to nuclear fiber cell membranes have been documented that may increase the membrane scattering cross section, such as deposits of cytoplasmic protein,
39 40 protein-like deposits in the extracellular space,
29 and enlarged spaces around the membranes through loss of membranes and adjacent cytoplasmic components.
29 41 Thus far, direct comparisons of nuclear cataracts and age-matched transparent lenses have not revealed any consistent patterns of changes in membrane ultrastructure that could account for increased light-scattering in the early stages of age-related nuclear cataract formation.
29 Nevertheless, it is likely that small changes at or near membranes will contribute to small refractive index fluctuations that increase retinal stray light and high-angle scattering from nuclei of aging and cataractous lenses.