Microtubule contribution to the reflectance of the retinal nerve fiber layer

PURPOSE. The reflectance of the retinal nerve fiber layer (RNFL) arises from light scattering by cylindrical structures oriented parallel to ganglion cell axons. In amphibian retinas, at 440 nm, microtubules (MTs) contribute about one half of RNFL reflectance. In rodent retinas, MTs are the only structure contributing to RNFL birefringence. To increase understanding of the anatomic basis for clinical RNFL measurements, this study was conducted to evaluate the MT contribution to RNFL reflectance in rodent retinas by using the MT depolymerizing agent colchicine. METHODS. Reflectance of nerve fiber bundles in isolated rat retinas was measured at 460, 580, and 830 nm with a multispectral imaging reflectometer. Images were taken frequently over an extended period. During baseline, the tissue was perfused with a physiological solution. During a treatment period, the solution was switched either to a control solution or to a solution containing colchicine. RESULTS. Because of the high reflectance of the RNFL, nerve fiber bundles appeared as bright stripes against a darker retina. The reflectance of bundles was relatively stable in control experiments. With colchicine treatment, however, bundle reflectance at first decreased rapidly and then became stable. After 70 minutes of colchicine treatment, RNFL reflectance had declined to approximately 50% below baseline at all wavelengths. CONCLUSIONS. MTs contribute to RNFL reflectance at all wavelengths. Unlike RNFL birefringence, however, which totally disappears after colchicine treatment, about one half of RNFL reflectance remained after colchicine treatment. This result suggests that, in addition to MTs, other mechanisms may contribute to RNFL reflectance.