The RGD-dependent and the Hep II binding domains of fibronectin govern the adhesive behaviors of amphibian embryonic cells

In early amphibian development, interactions between fibronectin and both ectoderm and mesoderm cells are critical in the progression of gastrulation movements. In the Pleurodeles waltl embryo, it has been established that ectoderm cells of the animal hemisphere organize a fibrillar-extracellular matrix containing fibronectin. Mesoderm cells migrate along the blastocoel roof using these fibronectin fibrils as substratum. Fibronectin is an adhesive glycoprotein which possesses multiple cell-binding domains. From previous studies, it is clear that amphibian ectoderm and mesoderm cells interact with fibronectin in an RGD-dependent manner, whereas the contributions of RGD-independent domains in the adhesive behaviors of gastrula cells has not been defined. To study this question, we have used bacterially expressed Pleurodeles waltl fibronectin-fusion proteins. The approach consisted of in vitro adhesion assays with either isolated cells or tissue fragments of embryos dissected at the onset of gastrulation. Tissues were obtained from regions of the embryo which represent presumptive ectoderm cells or from the dorsal-marginal zone which contains cells of the presumptive cephalic, chordal and semitic mesoderm. The results show that both the RGD-dependent and the Hep II domains of fibronectin mediate attachment and spreading of isolated cells. Both regions cooperate to control the proper expansion of a sheet of dorsal mesoderm cells. The Hep II domain promotes the migration of cells ahead of the mesoderm-cell sheet.