The behavior and cytoskeletal system of chick gastrula mesodermal cells on substrata coated with lines of fibronectin

In order to investigate the mechanism of the formation of the mesodermal layer during chick gastrulation, we observed the behavior of fragments of mesodermal cells explanted and cultured on substrata coated with parallel lines of fibronectin (FN). We also examined the distribution of F-actin, alpha-actinin, and vinculin in explanted fragments by immunocytochemical methods noting particularly their distribution with respect to FN lines. Explants of mesodermal cells flattened on FN-coated substrata and then became elliptical with the major axis of the ellipse oriented along the FN lines and migrated along them. The peripheral cells of explants extended filopodia and lamellipodia which attached preferentially to FN lines and then contracted, pulling other mesodermal cells in explants along passively. Vinculin and alpha-actinin in peripheral anchoring filopodia and lamellipodia co-localized with the terminations of F-actin bundles and with FN lines, suggesting that the peripheral cells were the moving force for explant translocation. We propose based on these results that in vivo, peripheral cells of invaginated cell mass are guided by the known FN-rich fibrous extracellular matrix on the basal surface of epiblast to move outwards; the rest linked to the peripheral cells are pulled away from the primitive streak to spread in tandem to form the mesodermal layer.