Figure 4. Fgf8 is not detected in double Sp6;Sp8 mutants.

ISH to transverse sections through the level of the forelimbs at the stage indicated at the top and with the probe indicated on the left. Genotypes are also marked at the top of the figure. In the absence of Sp6 and Sp8, Fgf8 expression in the limb ectoderm is never detected as shown at E9.5 (A–B), E10.5 (I–J) and E11.5 (O–P). However, Bmp4 (C–D), Fgf10 (E–F) and Msx2 (G–H) are normally activated at E9.5 but not maintained at later stages (K–N). Note that the initial budding of the double mutant is similar to normal (A–H) but further growth is impaired (I–N) and complete regression has occurred by E11.5 (O–P). In all panels dorsal is up and distal to the right.