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Fig. 4 flk-1 is expressed only in cells of the lower trunk and tail regions in clo mutant embryos. flk-1 expression in wild-type and clo mutant embryos. Lateral view of 10-somite stage (A), 20-somite stage (B), 24 hpf (C) and 30 hpf embryos (D-G). (A-C) Wild-type above and clo mutant below. Until the 20-somite stage, the only flk-1 expression observed in mutant embryos is in the ectodermal region of the hindbrain (small arrows); there is also a small area of faint and hazy expression in the tailbud (A, B, arrowheads). At the 20-somite stage (B), staining is observed in the tail region (large arrow) (the tail begins at the level of the anus, which itself is located at the caudal end of the yolk extension). This staining is more noticeable by 24 hpf (C, large arrow) although it is very much weaker than in wild-type. By 30 hpf, the extent of flk-1 staining in mutant embryos (F,G) can be fully appreciated as compared to wild-type (D,E): only cells of the lower trunk and tail regions express flk-1 and this staining is much less intense than in wild-type.