Recent studies in two invertebrate systems, border cells in Drosophila mela
nogaster and distal tip cells in Caenorhabditis elegans, have provided impo
rtant insight into the mechanisms of directed cell migration. These migrati
ng cells are guided by extracellular signals, such as EGF, TGF-P and netrin
. In addition, metalloproteases alter the extracellular matrix of the tissu
e through which these cells migrate. Along the migratory path, migrating ce
lls respond to changes in guidance signals by altering the expression of re
ceptor signaling pathways. Finally, Dock180, CrklI and the GTPase Rac link
the extracellular signals to the cellular machinery that controls cell moti
lity.