Migration of chick blastoderm under the vitelline membrane: the role of fibronectin

In the earliest stages of its development the chick blastoderm is a flattened disc at the surface of the yolk. It gradually increases in diameter, partially because the cells are rapidly proliferating, but also because the cells at the periphery (the margin of overgrowth) are migrating in a centrifugal direction. These cells utilize the inner surface of the vitelline membrane as their substratum. In the normal blastoderm, these cells at the edge of the spreading blastoderm are the only cells which are attached to the vitelline membrane. This investigation is concerned with the possible role played by fibronectin in the interaction between these migrating cells and the vitelline membrane. Chick blastoderms, explanted by the New (1955) technique have been treated with synthetic peptides that mimic the adhesive recognition signal of the fibronectin molecule. The pentapeptide GRGDS (containing the specific RGD cell adhesion sequence) caused the edge cells of the blastoderm to detach within minutes, and the expansion of the blastoderm was inhibited for about 4 hr. After this period there was gradual recovery and the cells reattached and spreading resumed. Examination of the margin of the blastoderm by scanning electron microscopy showed that cell processes were lost soon after treatment with GRGDS but concomitant with reattachment and the resumption of spreading, the cell processes reformed. The pentapeptide GRDGS (with the amino acids G and D inverted) produced a brief inhibition of spreading, but after an hour these blastoderms spread at the same rate as controls. Immunocytochemical staining with anti-fibronectin demonstrated that fibronectin was not only present at the interface of the edge cells and the vitelline membrane, but also between the epiblast and the hypoblast. These results indicate that tissue movement during blastoderm spreading is dependent upon fibronectin and that the specific RGD amino acid sequence, and presumably the VLA/integrin family of receptors, is involved in this embryonic morphogenetic movement.