Segregation of arterial and venous markers in subpopulations of blood islands before vessel formation

The neuropilin‐1 (np1) and the neuropilin‐2 (np2) receptors bind vascular endothelial growth factor (VEGF) and class‐3 semaphorins. They form complexes with VEGF tyrosine‐kinase receptors or alternatively with type‐A plexins to transduce respective VEGF or semaphorin signals. We have compared the expression patterns of np1, np2, plexin‐A1, and plexin‐A2 in the emerging vasculature of chick embryos. Double in situ hybridization reveals that six‐somite embryos contain intermingled extraembryonic blood island (BI) subpopulations that express np1 or np2 as well as a BI subpopulation that coexpresses both neuropilins. In 13‐somite embryos, which already contain an extraembryonic vascular plexus, the expression of np1 and np2 is segregated between the arterial and venous parts of the plexus, despite the absence of blood flow. However, the arterial marker ephrin‐B2 was not yet expressed in the plexus at this stage. In 26‐somite embryos, which possess a functional vascular system, np1 and np2 are differentially expressed in arteries and veins as previously reported. At this stage, posterior BIs expressing np2 appear to undergo fusion to form the posterior sinus vein and its tributaries, suggesting that the venous identity of these veins may be established before their formation. The neuropilin coreceptor plexin‐A2 was expressed in extraembryonic veins but not in extraembryonic arteries. In contrast, within the embryo, plexin‐A2 expression was observed in the dorsal aorta as well as in the cardinal vein. Semaphorin‐3F (s3f), an np2 ligand, bound to np2‐expressing cells in 26‐somite embryos regardless of the presence or absence of plexin‐A1 or plexin‐A2. Of interest, even though s3f binds to np1 in vitro, np1‐expressing arteries fail to bind s3f in whole‐mount binding experiments. Developmental Dynamics 232:1047–1055, 2005. © 2005 Wiley‐Liss, Inc.