Visualizing muscle cell migration in situ

Background: Cell migration has been studied extensively by manipulating and observing cells bathed in putative chemotactic or chemokinetic agents on p lanar substrates. This environment differs from that in vivo and, consequen tly, the cells can behave abnormally. Embryo slices provide an optically ac cessible system for studying cellular navigation pathways during developmen t. We extended this system to observe the migration of muscle precursors fr om the somite into the forelimb, their cellular morphology, and the localiz ation of green fluorescent protein (GFP)-tagged adhesion-related molecules under normal and perturbed conditions. Results: Muscle precursors initiated migration synchronously and migrated i n broad, rather than highly defined, regions. Bursts of directed migration were followed by periods of meandering or extension and retraction of cell protrusions. Although paxillin did not localize to discernible intracellula r structures, we found that a-actinin localized to linear, punctate structu res, and the alpha 5 integrin to some focal complexes and/or vesicle-like c oncentrations. Alterations in the expression of adhesion molecules inhibite d migration. The muscle precursors migrating in situ formed unusually large , long-lived protrusions that were polarized in the direction of migration. Unlike wild-type Rac, a constitutively active Rac localized continuously a round the cell surface and promoted random protrusive activity and migratio n. Conclusions: The observation of cellular migration and the dynamics of mole cular organization at high temporal and spatial resolution in situ is feasi ble. Migration from the somite to the wing bud is discontinuous and not hig hly stereotyped. in situ, local activation of Rac appears to produce large protrusions, which in turn, leads to directed migration. Adhesion can also regulate migration.