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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.
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