Morphometry of cellular protrusions of mesodermal cells and fibrous extracellular matrix in the primitive streak stage chick embryo.

Chick mesodermal cells, having become invaginated and beginning to locomote prior to the formation of the mesodermal cell layer at an early primitive streak stage, extend many filopodia and flatten themselves against the basal surface of the epiblast. Morphometry on scanning electron micrographs of chick mesodermal cells revealed two statistically significant tendencies. Each cell took an extended form and protruded filopodia, preferably along its major axis, suggesting that the force extending the cell body was generated by both ends rich in filopodia. The cells also tended to protrude filopodia most frequently in a direction away from Hensen's node. The orientation of the fibrous extracellular matrix (fECM), running on the basal surface of the epiblast, was assessed quantitatively, and it was proved statistically that the orientation of the fECM was radial around the primitive streak: With an immunogold staining technique, fECM, to which the filopodia of the mesodermal cells attached frequently and closely, was confirmed to be rich in fibronectin (FN). These results lead us to conclude that the mesodermal cells in chick gastrula were guided to locomote towards the periphery of the area pellucida by FN-rich fECM laid on the basal surface of the epiblast, and that this movement was due to an in vivo locomotive mechanism using filopodia.