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Fig. 2. Experiments in Xenopus that support the default model. (A)
Cell-dissociation experiments (results shown in the two left-most columns in
the table) and intact animal cap assays with or without BMP antagonists
(antag.) (results shown in the third and fourth columns). -Ca2+,
Mg2+ indicates that the protein was applied in low
Ca2+/Mg2+ medium. (B) In the most common type of animal
cap experiment, a two- to four-cell stage embryo is injected with RNA encoding
a protein to be tested (here, BMP antagonists: DNBMPR,
dominant-negative BMP receptor; Nog, Noggin; Chd, Chordin;
Smad6, the inhibitory effector Smad6). After incubating the embryo to
the blastula stage, the animal cap is excised and grown in isolation overnight
before assessing its marker gene expression. The table shows the usual results
of these experiments: + indicates expression of markers for the tissue shown;
- indicates no expression; arrows indicate downregulation;? indicates a fate
that has not been tested in published experiments. Modified, with permission,
from Stern (Stern, 2004).
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