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X.–R. Huang, R.W. Knighton, L.N. Cavuoto; Microtubule Contribution to the Reflectance of Retinal Nerve Fiber Layer . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3339.
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© ARVO (1962-2015); The Authors (2016-present)
Clinical assessment of retinal nerve fiber layer (RNFL) by optical methods relies on measurements of the optical properties of RNFL. Different mechanisms may underlie different assessment methods, such as, RNFL birefringence measured by scanning laser polarimetry and reflectivity measured by optical coherence tomography. Cylindrical structures in ganglion cell axons are known to be the anatomic basis for both birefringence and reflectance of the RNFL. Possible structures include axonal membranes, microtubules (MTs), and neurofilaments. MTs have been shown to be the major structure contributing to the RNFL birefringence. This study evaluated the contribution of MTs to RNFL reflectance by using the MT depolymerizing agent colchicine.
Rat retina, isolated free from the pigment epithelium, was measured by means of imaging reflectometry. An experiment consisted of baseline and treatment periods. During baseline, the tissue was perfused with a physiological solution. During the treatment period, the solution was switched either to a control solution identical to the baseline solution or a similar solution containing colchicine. A light source and a CCD camera were set to measure the reflectance of RNFL in the backscattering plane. Images were taken frequently at 460, 580 and 830 nm. A background removal algorithm was used to calculate reflectance of RNFL. The temporal change of the reflectance after solution switch was used to evaluate the contribution of MTs to the RNFL reflectance.
Due to the high reflectance of the RNFL, retinal nerve fiber bundles appeared as bright stripes against a dark background. The reflectance of nerve fiber bundles was stable in control experiments. However, in treatment experiments bundles were bright during the baseline period but the reflectance of bundles decreased rapidly after the colchicine solution was applied. The decrease of bundle reflectance occurred at all measured wavelengths. After about 60 minutes of colchicine treatment, nerve fiber bundles were still visible with low reflectance.
The effect of colchicine suggests that MTs contribute to the RNFL reflectance at all wavelengths. Unlike RNFL birefringence, however, which totally disappeared after colchicine treatment, weak RNFL reflectance remained after colchicine treatment. This result suggests that, in addition to MTs, other mechanisms may contribute to the RNFL reflectance.
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