Trace formation during locomotion of L929 mouse fibroblasts continuously recorded by interference reflection microscopy (IRM)

The recently reported formation of highly ordered traces by migrating cells has been studied on L929 fibroblasts in time lapse experiments by means of interference reflection microscopy (IRM) as well as by conventional microscopy. Formation of pronounced traces on glass substrates correlates to migration after cell division, and the trace arrangement on the substrate depends on migration velocity: slow migration results in a highly branched, broad, and relatively short trace, while fast migration yields a slim and long trace with few branches. IRM-irradiation caused cessation of locomotion and trace formation and accelerated degradation of existing traces. Traces consist of cord-like cytoplasmic strands, which contain F-actin filaments and they seem to be enveloped by a membrane. It is supposed that cell traces are homologous to filopodia. Traces arise mainly from non-retracted filopodia at the rear margin of the migrating cell. The branches within the traces are the result of the repeated stretching out of a backwardly directed lamellipodium. They arise from the formation of new filopodia that emerge at the actin ribs of the lamellipodium.