Evidence for the involvement of receptors for fibronectin in the promotion of chick tail segmentation.

In the chick embryo the paraxial mesoderm forms about 50-53 pairs of somites, the precise number depending on the extent to which segmentation proceeds along the tail. However, the terminal mesoderm of the tail fails to segment despite the fact that it appears to contain a reservoir of potential somites. Why does this mesoderm not segment? Some clues can be obtained by comparing this non-segmenting region with the segmental plate in the trunk. We and others have shown that in the trunk region of the chick, cell adhesion plays a major role in somitogenesis and that this increased cell adhesion is associated with compaction of segments of mesoderm immediately prior to segmentation. This compaction can be brought about prematurely by fibronectin and by the specific adhesion peptide GRGDS. The terminal mesoderm in the tail resembles the segmental plate mesoderm in the trunk in undergoing compaction in response to fibronectin and GRGDS. The tail mesoderm differs from the segmental plate mesoderm in that it can also respond to peptides closely related to GRGDS. The response suggests that, whereas the integrin receptors for fibronectin and GRGDS appear to be specific in the presomitic trunk mesoderm, responding only to the specific adhesion-peptide GRGDS, the tail mesoderm may contain more heterogeneous sets of receptors within the integrin/VLA family that respond to a wider variety of ligands. Coincident with these differences is the phenomenon of regional cell death in the tail bud mesoderm. All of these factors are thought to play a role in the extent of segmentation in the paraxial mesoderm of the embryonic chick.