Cranial neural crest cell migration in the avian embryo and the roles of Eph-A4 and ephrin-A5
The neural crest is a transient population of cells that migrate away from the dorsal neural tube in the vertebrate embryo. As the developing hindbrain constricts into rhombomeres, cranial neural crest cells migrate in three discrete streams adjacent to even-numbered rhombomeres, rhombomere 2 (r2), r4, and r6. To test the role of intrinsic versus extrinsic cues in influencing an individual cell's trajectory, we implanted physical barriers in the chick mesoderm, distal to emerging neural crest cells (NCCs). We analyzed spatio-temporal dynamics as NCCs encountered and responded to the barriers by using time-lapse confocal microscopy and cell tracking analysis. The majority of NCCs were able to overcome physical barriers. Even though the lead cells become temporarily blocked by a barrier, follower cells find a novel pathway around a barrier and become de novo leaders of a new stream. Quantitative analyses of cell trajectories find cells that encounter an r3 barrier migrate significantly faster but less directly than cells that encounter an r4 barrier, which migrate normally. NCCs can also migrate into normally repulsive territory as they reroute. These results suggest that cranial neural crest cell trajectories are not intrinsically determined. NCCs can respond to minor alterations in the environment to retarget a peripheral destination. Both intrinsic and extrinsic cues are important in patterning. We then tested the role of Eph/ephrin signaling on cranial neural crest migration by ectopically expressing full-length ephrin-A5 ligand; a truncated, constitutively active EphA4 receptor; and a truncated, kinase-dead EphA4 receptor within migratory neural crest cells. Ectopic expression of ephrin-A5 specifically causes the r6 subpopulation of neural crest cells to have truncated migration but does not affect directionality, suggesting that the r6 neural crest cells properly follow guidance cues. Our results support a role for ephrin-A5 in regulating the extent of migration. Ectopic expression of constitutively active, truncated EphA4 causes NCCs to migrate aberrantly around the otic vesicle. Pathfinding errors are accompanied by changes in migratory behavior, with the NCCs migrating faster but with less directionality. Expression of a truncated, kinase-dead version of EphA4 also leads to pathfinding errors. Our results suggest Eph activity is involved in guidance and extent of migration.