Guidance cues are signals that can attract or repel cells and axons during development.  One of the most well-understood guidance pathways is the Eph/ephrin pathway.  In this pathway, Eph receptors on a cell’s membrane interact with ephrin proteins bound to a different cell.  This interaction typically causes rapid changes in cell adhesion and architecture, and frequently causes the cells to repel each other.  A recent paper describes the role of Eph/ephrin signaling in the motility of muscle stem cells.  Stark and colleagues showed that muscle stem cells, called satellite cells, were repelled by ephrin signals using a well-established “stripe assay.”  In this assay, mouse satellite cells were able to crawl over stripes of ephrin protein coated onto glass coverslips.  As seen in the images above (increasing magnification from left to right), stripes of ephrin ligand (bottom row, blue stripes) repulsed the satellite cells, compared to the more random distribution of cells crawling over control stripes (top row).  In addition, when Stark and colleagues placed mouse satellite cells into developing quail embryos, they responded to the quail embryo’s own ephrin signals and migrated accordingly. 

For a more scientifically detailed description of this image, check out my post on The Node, the community forum for and by developmental biologists.