Radial glia produce and align the ligand fibronectin during neuronal migration in the developing chick brain

We demonstrated previously that alpha8beta1 integrin regulates the migration and survival of immature neurons during development of the chicken optic tectum; however, the potential extracellular ligand was unknown. We used immunohistochemistry to determine if several potential ligands (fibronectin, tenascin, vitronectin, and osteopontin) were expressed during neuronal migration along radial glia (RG). Fibronectin was localized in a pattern relevant to radial migration and survival of neurons; it was present before and during neuronal migration and appeared oriented along RG fibers by conventional fluorescence microscopy. Confocal microscopy confirmed that fibronectin was localized along RG cells during radial migration. It was more concentrated in some superficial laminae, which might support directional movement. Fibronectin was present after formation of definitive tectal laminae, but was diffuse and not aligned along RG, which persist. Flow cytometry analysis of dissociated optic tectum cells revealed that almost all RG were positive for fibronectin. Short-term cell culture experiments using an exocytosis inhibitor revealed that fibronectin accumulated in most RG cells. Thus, fibronectin is produced by RG and is aligned along their surfaces before and during migration. Fibronectin, therefore, is a potential ligand for general radial neuronal migration in the chick optic tectum. Its predominant source appears to be RG, in contrast with developing mammalian cortex, where fibronectin was not found in a pattern that could guide widespread radial migration and where neurons are the predominant producers of fibronectin during migration.