Dance of the podosomes

Macrophages crawl across a substrate using podosomes, focal complex-like adhesions that form and disappear rapidly at the cell's leading edge. Beginning on page 697, Evans et al. provide a high-resolution view of the dynamic turnover of these structures, revealing some surprising behavior and suggesting a novel mechanism of cell migration.

Using fluorescently labeled podosome components and quantitative 4-D microscopy, the authors show that the majority of leading edge podosomes either assemble from older podosomes or form through the dramatic fragmentation of a large podosome cluster precursor (PCP). In the first pathway, simple podosomes undergo both fission and fusion events. This often produces a sort of forward stepping movement, when a trailing podosome fuses with one closer to the leading edge. The other pathway begins with a podosome that grows to several times normal size...