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K.P. Cracknell, I. Grierson; Can the Modest Increase in Melanin Granule Size Detected in Latanoprost Induced Iris Darkening (LIID) Be Solely Responsible for Bringing About This Change? . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3765.
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Purpose: The morphological change in the iris that has been detected in LIID cases, in our studies is a small increase in the melanin granule size within the melanocytes in the stroma when compared to control irides (green/brown). This raises the question, can such a modest change bring about the dramatic change in eye color that is seen? The aim of this study is to try to answer this question by computational techniques. Methods: In this study we are using mathematical and computational techniques to model the melanocyte, and to also to simulate the physical interaction with light (in the visible spectrum) that strikes the melanocyte. In previous studies we have used electron microscopy to analyze the melanocytes and their melanin granules within the stroma. We have examined 30 irides from both normal (comprised of the range of normally occurring colors that are seen in the Caucasian population) and LIID cases. This data provided us with the ranges of concentrations of melanin, and the size of granules that is found within the iris The first stage is to set up a simulated virtual computer model of a melanocyte. This is filled it with various concentrations of melanin granules (comprised of different numbers of uniformly sized granules), which were be randomly distributed within the cell. The computer codes used have been written by us in FORTRAN90.The next stage involved trying to understand the multiple and complex interactions of each photons path through the cell. We have tackled this using a variety of methods. Initially we used a very simplified mathematical model of the cell, which enabled us to get an analytical solution to a limiting case of the problem. Then we moved on to series of computer simulations, of increasing complexity to enable us to estimate with greater degrees of accuracy the probabilities of transmission and absorption of light that hits the cell. The calculations were performed on a wide range of melanin concentrations and diameters. Results: We found that for our simple model, the fraction of light that passes through the melanocyte is an exponential function of the square of the diameter of the melanin granule. Because of the exponential function a small increase in the granule diameter produces a significant decrease in the amount of the light transmitted. Conclusions: Small changes in melanin particle size can bring about significant changes in the darkness of the iris.
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