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Fig. 1. Spontaneous initiation of axis formation in isolated embryo fragments.
(A-D) When a pre-primitive streak stage embryo is cut in half (separating
anterior and posterior halves, A), each half forms a primitive streak. The
primitive streak forming in the anterior half expresses brachyury
(B,B') and the organizer marker chordin (C,C'), and a
normal hypoblast expressing crescent develops and moves away from the
new primitive streak (D,D'). (B',C',D') Transverse
sections at the levels shown in the adjacent whole mounts. (E-J) Time-course
of expression of genes associated with primitive streak formation in the
anterior half after cutting. None of the genes is detected 3 hours (E) or 6
hours (F) after cutting; chick Vg1 is the first gene to be expressed,
which appears on one side (arrow) of the posterior edge in the isolated
anterior half 9 hours after cutting (G) and is still detectable at 12 hours
(H, arrow). At this time, Nodal also appears (I, arrow), while the
hypoblast, which expresses FGF8, starts to coalesce into a layer (J).
In this and all subsequent figures, the probe(s) used for in situ
hybridisation are indicated in the lower left-hand corner of each panel.
Yellow. epiblasts; green, extra-embryonic area opaca; red, marginal zone;
blue, hypoblast; white, endoblast; brown, primitive streak. The original
posterior end is shown to the bottom of each panel in all cases.
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