IDENTIFICATION OF INDUCING, RESPONDING, AND SUPPRESSING REGIONS IN ANEXPERIMENTAL-MODEL OF NOTOCHORD FORMATION IN AVIAN EMBRYOS

The notochord normally arises hem committed cells in the rostral tip o f the primitive streak. After removal of these cells from the avian ga strula, embryos with notochords nevertheless develop in the majority o f cases. A region required for the formation of this reconstituted not ochord Lies lateral to the primitive streak. In the present study we h ave determined that this region acts as an inducer for more lateral ce lls in the epiblast, which actually give rise to the reconstituted not ochord. The strongest inducing region lies between 0-250 mu m lateral to the streak and 500-750 mu m caudal to the rostral end of the streak and chiefly contains cells normally fated to form lateral plate and s emitic mesoderm. The responding region is located 250-500 mu m lateral to the streak and 0-750 mu m caudal to the rostral end of the streak. This area chiefly contains cells normally fated to form neural ectode rm, although cells normally fated to form lateral plate and semitic me soderm are also within this area. The inducing and responding areas in teract to form reconstituted notochord either when the primitive strea k, including its rostral end (Hensen's node), is removed from the cult ured blastoderm or when the inducer and responder are grafted together into an ectopic site. Grafting Hensen's node into isolates containing both inducer and responder blocks formation of reconstituted notochor d, suggesting that Hensen's node suppresses formation of lateral notoc hords during normal development. These findings increase our understan ding of the early interactions between mesoderm and ectoderm and provi de a novel model system that is well defined and accessible for studyi ng inductive events in higher vertebrates. (C) 1995 Academic Press, In c.