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M. J. Costello, S. Johnsen, K. O. Gilliland, S. Metlapally, B. Ramamurthy, P. V. Krishna, D. Balasubramanian; Predicted Light Scattering From Membranes in Transparent and Cataractous Human Lenses. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4071.
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© ARVO (1962-2015); The Authors (2016-present)
To characterize the light scattering from fiber cell interfaces in transparent compared to cataractous human lens nuclei from Indian donor lenses and fully opaque cataracts removed by extracapsular surgery.
Fresh Vibratome sections of nuclei were preserved for transmission electron microscopy (TEM). Images of fiber cell interfaces were used to determine the dimensions of the extracellular space (ECS), membranes and junctions. Models of the lamellar interface, such as cytoplasm-membrane-ECS-membrane-cytoplasm, were constructed using literature values of refractive index (RI). The scattering from these models were calculated using a theory for the refractivity of thin layers commonly employed in the optics industry for the manufacture of filter cubes and complex lens assemblies with multiple interfaces. Algorithms were written in MatLab. The theory provides exact solutions of wave equations as a function of wavelength for thin layers having different RI values.
TEM images of interfaces between nuclear fiber cells showed membranes separated by ECS of variable width. The average thickness of the ECS for donor lenses was about 20-30 nm and for some advanced cataracts exceeded 40 nm. Transmittance (percent) was calculated for RI = 1.40 for cytoplasm, RI =1.50 for membranes 7 nm thick and RI = 1.35 for ECS with thickness ranged from 1 nm to 100 nm. Remarkably, the transmittance for 4000 layers with this geometry was nearly 100% for the range of ECS spacings of 22-26 nm. At slightly larger ECS spacing the transmittance decreased (and scattering increased) so that at an ECS of 40 nm the transmittance was only 66%. Surface plots with membrane RI varied from 1.40 to 1.52 were used to explore small variations in the assigned cytoplasmic and ECS RIs and membrane thickness values. A combination of parameters could usually be found to give nearly 100% transmittance for ECS spacing of 15-25 nm.
The optimum ECS thickness for aged transparent lenses may not be zero, probably because the high RI of the membranes needs to be offset by the ECS. The optimum ECS may be about 20 nm given reasonable RI values for the cytoplasm and membranes at the fiber cell interface. An increase of the ECS of only 10-20 nm is sufficient to greatly reduce transmittance and appears to occur in some advanced nuclear cataracts, suggesting that scattering from a membrane interface with an increased ECS can contribute to lens opacity in mature nuclear cataracts.
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