Fig. 7. Model of molecular interactions during primitive streak formation. The successive stages in the development of the chick embryo are shown on the left; the sequential molecular signaling steps are shown on the right. (A) Just prior to the gastrulation (stages XI-XIII), WNT8C and TSK are expressed all around the marginal zone, and VG1 in PMZ. TSKA collaborates with chordin to decrease BMP activity in the PMZ. (B) At stages 2-3, the primitive streak starts to form under the influence of VG1+WNT8C and VG1+TSKB. VG1+WNT8C and VG1+TSKB signaling cascade synergize to induce the organizer in the central area where the TSKA+chordin serves to keep BMP expression clear of the center. TSKA also inhibits the activity of BMP7 expressed in the posterior primitive streak.