sentences:  Since ephrin ligands (ephrins) and Eph receptors (Ephs) are both membrane-bound proteins, binding and activation of Eph/ephrin intracellular signaling pathways can only occur via direct cell-cell interaction.Eph/ephrin signaling regulates a variety of biological processes during embryonic development including the guidance of axon growth cones, formation of tissue boundaries, cell migration, and segmentation.Additionally, Eph/ephrin signaling has recently been identified to play a critical role in the maintenance of several processes during adulthood including long-term potentiation, angiogenesis, and stem cell differentiation.