MISEXPRESSION OF CHICK VG1 IN THE MARGINAL ZONE INDUCES PRIMITIVE STREAK FORMATION

In the chick embryo, the primitive streak is the first axial structure to develop. The initiation of primitive streak formation in the poste rior area pellucida is influenced by the adjacent posterior marginal z one (PMZ). We show here that chick Vg1 (cVg1), a member of the TGF bet a family of signalling molecules whose homolog in Xenopus is implicate d in mesoderm induction, is expressed in the PMZ of prestreak embryos. Ectopic expression of cVg1 protein in the marginal zone chick blastod erms directs the formation of a secondary primitive streak, which subs equently develops into an ectopic embryo. We have used cell marking te chniques to show that cells that contribute to the ectopic primitive s treak change fate, acquiring two distinct properties of primitive stre ak cells, defined by gene expression and cell movements. Furthermore, naive epiblast explants exposed to cVg1 protein in vitro acquire axial mesodermal properties. Together, these results show that cVg1 can med iate ectopic axis formation in the chick by inducing new cell fates an d they permit the analysis of distinct events that occur during primit ive streak formation.