INDUCTION OF PRIMITIVE STREAK AND HENSENS NODE BY THE POSTERIOR MARGINAL ZONE IN THE EARLY CHICK-EMBRYO

In the preprimitive streak chick embryo, the search for a region capab le of inducing the organizer, equivalent to the Nieuwkoop Center of th e amphibian embryo, has focused on Koller's sickle, the hypoblast and the posterior marginal zone. However, no clear evidence for induction of an organizer without contribution from the inducing tissue has been provided for any of these structures. We have used DiI/DiO labeling t o establish the fate of midline cells in and around Koller's sickle in the normal embryo, In the epiblast, the boundary between cells that c ontribute to the streak and those that do not lies at the posterior ed ge of Koller's sickle, except at stage X when it lies slightly more po steriorly in the epiblast, Hypoblast and endoblast (a second lower lay er formed under the streak) have distinct origins in the lower layer, and goosecoid expression distinguishes between them. We then used ante rior halves of chick prestreak embryos as recipients for grafts of qua il posterior marginal zone; quail cells can be identified subsequently with a quail-specific antibody. Anterior halves alone usually formed a streak, most often from the posterior edge. Quail posterior marginal zones without Koller's sickle were grafted to the anterior side of an terior halves. These grafts were able to increase significantly the fr equency of streaks arising from the anterior pole of stage X-XI anteri or halves without contributing to the streak or node. Stage XII anteri or halves no longer responded. A goosecoid-expressing hypoblast did no t form under the induced streak, indicating that it is not required fo r streak formation. We conclude that the marginal zone posterior to Ko ller's sickle can induce a streak and node, without contributing cells to the induced streak.