Arrangement of radial actin bundles in the growth cone of Aplysia bag cell neurons shows the immediate past history of filopodial behavior

Filopodia that protrude forward from the lamellipodium, located at the leading edge of a neuronal growth cone, are needed to guide the extension of a nerve cell. At the core of each filopodium an actin bundle forms and grows into the lamellipodium. By using kymographs of time-lapse polarized light images we examined the relationship between the behavior of the filopodia, the actin bundles immediately proximal to the filopodia, and the shapes and composition of actin bundles in the whole lamellipodium. We find that the shapes of actin bundles, such as tilt, fork, and fused zones, originate at the leading edge and are surprisingly well preserved during retrograde transport of the actin cytoskeleton in the whole lamellipodium. The number of filaments that make up the radial actin bundles, as displayed by their birefringence retardation, also is preserved during retrograde flow over a distance of 4-8 mu m from the leading edge into the lamellipodium. Thus, the disposition of the actin bundles in the lamellipodium frozen at any time point preserves and portrays a history of the past behavior of actin bundles proximal to the filopodia and the behavior of the filopodia themselves. These findings suggest that the arrangement of actin bundles in static image records, such as electron or fluorescence micrographs of fixed and stained specimens, can in fact reveal the sequence of the past history of filopodial behavior and the generation, density, fusion, etc. of the filaments in the actin bundles.