Birefringence imaging directly reveals architectural dynamics of filamentous actin in living growth cones

We have investigated the dynamic behavior of cytoskeletal fine structure in the lamellipodium of nerve growth cones using a new type of polarized ligh t microscope (the Pol-Scope). Pol-Scope images display with exquisite resol ution and definition birefringent fine structures, such as filaments and me mbranes, without having to treat the cell with exogenous dyes or fluorescen t labels. Furthermore, the measured birefringence of protein fibers In the thin lamellipodial region can be interpreted in terms of the number of fila ments in the bundles. We confirmed that birefringent fibers are actin-based using conventional fluorescence-labeling methods. By recording movies of t ime-lapsed Pol-Scope images, we analyzed the creation and dynamic compositi on of radial fibers, filopodia, and intrapodia in advancing growth cones. T he strictly quantitative information available in time-lapsed Pol-Scope ima ges confirms previously deduced behavior and provides new insight into the architectural dynamics of filamentous actin.