Platelet-derived growth factor-A (PDGF-A) is essential for processes involving cell motility throughout embryonic development. Here we examined the underlying basis of its role as a guidance cue for the directed migration of anterior mesendoderm cells during a period of cell reorganizations called gastrulation. In Xenopus laevis embryos, these cells migrate across a fibronectinrich extracellular matrix (ECM) that lines the blastocoel roof (BCR). We tested the hypothesis that PDGF-A mRNA is in a graded pattern across the BCR that is modulated by mesendoderm cells as they move across it. Quantitative reverse transcriptase polymerase chain reactions were performed on BCR tissue sections. Throughout gastrulation, the overall levels of PDGF-A mRNA increased, and at all developmental stages examined, the same pattern of PDGF-A mRNA was identified. PDGF-A mRNA levels were lowest at the blastopore lip and although higher levels were detected across the rest of the BCR, they were equivalent between tissue sections. Tissues were also dissected from altered embryos in which the position of the mesendoderm remained like that of earlier gastrula staged embryos (stages 10 and 10.5). In these cases, the low levels and distribution of PDGF-A mRNA was similar to that of earlier staged embryos across all developmental stages examined. These data supported our hypothesis and suggested that during gastrulation the mesendoderm moves from a lower to a higher level of PDGF-A expression. They further indicated that the mesendoderm itself plays a role in modulating PDGF-A mRNA during gastrulation. To understand how PDGF-A might be established as a guidance cue for migrating mesendoderm cells, the interaction of PDGF-A with the ECM was investigated. We found a direct interaction between PDGF-A and fibronectin. This interaction is enhanced by heparin, which revealed PDGF-A binding sites by modulating fibronectin structure. We further demonstrated that endogenous heparan sulfate proteoglycans (HSPG's) are required for PDGF-A-guided mesendoderm movement, indicating an in vivo role for HSPGs in mediating the interaction between PDGF-A and fibronectin. Our data further suggested that the expression pattern of PDGF-A in the BCR could lead to a graded distribution of PDGF-A in the ECM by binding to fibronectin at its site of secretion.