Eph Receptor Tyrosine Kinases and Their Ligands in Development

A current focus of interest in developmental neurobiology is the identification and characterization of mechanisms and molecules contributing to the formation of neuronal connections. A number of gene families playing a role in this process have recently been identified. They can be classified tentatively according to the basic mechanisms by which axonal pathfinding to, and guidance in, the target area are thought to occur: chemoattraction by Netrins, chemorepulsion by Netrins and secreted Semaphorins; contact-mediated attraction by Ig domain-containing cell adhesion molecules (Ig-CAMs), Cadherins, and extracellular matrix proteins (ECM); and finally, contact-mediated repulsion by transmembrane Semaphorins, ECM proteins, and Ephrins (Tessier-Lavigne and Goodman 1996; Muller 1999).