Based on an adhesion gradient, dorsal but not ventral retractions are productive.
HAMMERSCHMIDT/ELSEVIER
The bone morphogenetic proteins (Bmps) are better known as factors that determine cell fate decisions. Bmps appear to affect migration, but this might have been a side-effect of changes in cell fate.
The German group therefore implanted a Bmp-containing bead on the opposite side of a fish embryo from Bmp's normal source. Cells responded by moving away from the bead. Bmp receptors were needed not in the migrating cells but in the surrounding cells on which they migrated. This suggested that the surrounding cells might be creating a gradient of adhesion that was guiding the migrating cells.
Sure enough, the migrating cells showed equal numbers of protrusions at the front and back, but only the protrusions facing away from a Bmp source were able to grab on securely enough to pull the cell body forward.
Other developmental pathways also regulate adhesion. This has been presumed to affect cell survival or cohesion of migrating masses of cells, but the creation of an adhesion gradient is another possibility. 
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