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

Filopodia that protrude forward from the lamellipodium, located at the lead ing 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 i mages we examined the relationship between the behavior of the filopodia, t he actin bundles immediately proximal to the filopodia, and the shapes and composition of actin bundles in the whole lamellipodium. We find that the s hapes of actin bundles, such as tilt, fork, and fused zones, originate at t he leading edge and are surprisingly well preserved during retrograde trans port of the actin cytoskeleton in the whole lamellipodium. The number of fi laments that make up the radial actin bundles, as displayed by their birefr ingence retardation, also is preserved during retrograde flow over a distan ce of 4-8 mu m from the leading edge into the lamellipodium. Thus, the disp osition of the actin bundles in the lamellipodium frozen at any time point preserves and portrays a history of the past behavior of actin bundles prox imal to the filopodia and the behavior of the filopodia themselves. These f indings suggest that the arrangement of actin bundles in static image recor ds, such as electron or fluorescence micrographs of fixed and stained speci mens, can in fact reveal the sequence of the past history of filopodial beh avior and the generation, density, fusion, etc. of the filaments in the act in bundles.