B61 (ephrin-A1), a ligand for eph receptor-protein tyrosine kinases, possesses angiogenic and endothelial cell chemoattractant activity. We report the expression profile of ephrin-A1 during mouse embryogenesis (7.25 to 10.5 days post-coitum (dpc)). Ephrin-A1 expression was first detected in the caudal end of the primitive streak and the developing allantois. At mid head fold stage, ephrin-A1 transcripts were observed in the anterior pharyngeal endoderm that underlies the site of the future heart. In 8.0 to 8.5 dpc embryos, ephrin-A1 was expressed in presumptive endocardial cells, the dorsal aorta, and the primary head veins. In 9.5 to 10.5 dpc embryos, ephrin-A1 RNA was detected in intersomitic vessels and the limb bud vasculature. Each site of ephrin-A1 expression corresponded to regions of vasculogenesis and/or angiogenesis.
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