Table 1.

Results of chick misexpression experiments

(A) Experiments on embryos
Marker genes
Node+ (2 hours)+ (1 hour)+ (2 hours)+ (4 hours)+ (9 hours)
Hypoblast+ (2 hours)+ (1 hour)+ (2 hours)
FGF8+ (2 hours)+ (1 hour)+ (4 hours)
BMP4 electroporation*No effect (<6 hours)No effectInhibition
Node (5 hours) + Chordin+ (Maintenance)+
FGF8 (5 hours) + Chordin+ (Maintenance)+
Smad6 + FGF8++
Smad6 + α-Wnt
Smad6 + FGF8 + α-Wnt + Chordin + Noggin++
FGF8 + α-Wnt++
(B) Experiments on explants
Marker genes
No factor+++++
DN-BMPR + SU5402+++
Wnt3A (1×)++
Wnt3A (1×) + FGF2++/–
Wnt3A (1×) + Noggin+++
SU5402 + Noggin++
Wnt3A (3×)++
Wnt3A (3×) + Noggin++
Fz8CRD + SU5402++
Fz8CRD + BMP4++
Fz8CRD + SU5402 + Noggin+++
Fz8CRD + SU5402 (5 μM)++
Fz8CRD + SU5402 (5 μM) + Noggin++
  • (A) Experiments in whole embryos in which a candidate signalling tissue, or COS cells transfected with a secreted factor, are grafted into a test region of the epiblast; or a plasmid encoding a test protein is electroporated into a test region. Data are taken from Streit et al. and Linker and Stern (Streit and Stern, 1999a; Streit and Stern, 1999b; Streit et al., 1998; Streit et al., 2000; Linker and Stern, 2004). In the node, hypoblast and FGF8 experiments, the length of exposure in hours required to obtain marker induction is shown in brackets. Brachyury is a mesoderm marker and SOX2, a `definitive' neural plate marker. ERNI, OTX2, SOX3 and ChCh are expressed in the early epiblast, including the prospective neural territory, but do not indicate commitment to a neural fate. No combination of factors mimics SOX2 induction by the node. +, induction; –, no induction; Inhibition, endogenous expression inhibited; Maintenance, treatment maintains otherwise transient expression; α-Wnt, three Wnt antagonists (crescent, NFz8 and Dkk1) together with Cerberus (a multifunctional Wnt, BMP and Nodal antagonist).

    (B) Experiments in which small epiblast explants from the prospective neural territory (Neur) or rostral, prospective epidermal domain (Epi) from pre-streak embryos are isolated and cultured for 48 hours with or without factors. +/– indicates modest induction. Fz8CRD, a truncated Wnt receptor that inhibits Wnt signalling; SU5402, a synthetic inhibitor of FGF signalling; DN-BMPR, dominant-negative BMP receptor. Blank cells indicate that the experiment was not carried out. Data from Wilson et al. (Wilson et al., 2000; Wilson et al., 2001).

  • * BMP4 misexpressed by electroporation in the neural plate inhibits SOX2 but not the early marker SOX3

  • Chordin induces an ectopic primitive streak when misexpressed inside the embryo but does not induce neural markers in competent extra-embryonic epiblast