Figure 3.

Early deletion of midbrain–hindbrain boundary tissue in Pax5 (−/−) Krd (+/−) embryos. (A–E) β-Galactosidase staining of E9.5 embryos of the indicated genotypes. An arrowhead denotes the expression domain of the Pax5–lacZ allele in the mid–hindbrain boundary region. (F–I) Whole-mount in situ hybridization of E9.5 embryos with Fgf8 and Krox20 probes. A solid arrowhead points to the Fgf8 expression domain in the mid–hindbrain region; open arrowheads denote expression of the Krox20 control gene in rhombomeres 3 and 5 of the hindbrain (33). All embryos were analyzed at the 20- to 21-somite stage except for the embryos in C and F, which contained only 14 or 15 somites. Note that the expression of Krox20 is down-regulated in rhombomere 3 during the transition from the 15- to 20-somite stage (33) and that intensive β-galactosidase staining is already seen at the 7-somite stage in the midbrain region of Pax5 mutant embryos (13).