Distribution of fibronectin in the early phase of avian cephalic neural crest cell migration

Immunofluorescence and immunoperoxidase labeling for fibronectin was used to study the early events of cephalic neural crest cell migration in avian embryos. Prior to crest cell appearance, fibronectin was associated with the basement membranes of all tissues. The loose mesenchymal cells were also surrounded by this glycoprotein. The crest cell individualization phase included a transient rounding up and a rapid increase in cell number in a very limited space. Whereas the neural tube basement membrane was not formed dorsally at the site of emergence of crest cells, it was partially fused laterally with the ectoderm basement membrane apparently preventing immediate crest cell emigration. Further increase in cell number occurred concomitantly with their penetration between the two developing basement membranes of the neural tube and the ectoderm. The localization of migrating crest cells is apparently greatly influenced by local interactions between the ectoderm and the neural tube, whose morphogenesis differs considerably at each axial level: at the mesencephalic-rhombencephalic levels, crest cells rapidly reached a cell-free space that was mostly devoid of fibronectin. Further migration occurred laterally in that space while pioneer crest cells became surrounded by fibronectin in their environment. Crest cells progressed as a confluent multicellular layer with an apparent velocity of 70 μm/hr. At the prosencephalic and median rhombencephalic levels, crest cells accumulated between the fibronectin-rich basement membranes of the ectoderm and the neural tube. Pioneer crest cells were arrested at the site of attachment of the ectoderm and the neural tube basement membranes (i.e., optic vesicles and otic placodes). Crest cells resumed their migration when more space became available during the constriction of the optic vesicles and the invagination of the otic placodes.