Fig. 7. Analysis of the sirenomelia phenotype. (A,B) Side view of 9.5 dpc wild-type and mutant embryos hybridized with Bmp4 probe. (A',B') Ventral views of hindlimb bud region of embryos shown in A and B, respectively; the tail has been excised. In the mutant, expression of Bmp4 in the future hindlimb bud mesoderm (hb) forms a transverse horizontal domain instead of two lateral ones on the right (rhb) and left (lhb) hindlimb buds seen in the wild type. (C,D) Side view of 10.5 dpc embryos hybridized with Fgf8 probe, marking the apical ectodermal ridge (aer). (C',D') Ventral views of hindlimb bud region of embryos shown in C and D, respectively. In the mutant, the aer expression domain of Fgf8 adopts a horseshoe-like shape, with the two aer regions meeting posteriorly instead of forming two lateral arches. (E,F) Side view of 10.5 dpc embryos hybridized with sonic hedgehog (Shh) probe. (E',F') Ventral views of hindlimb bud region at higher magnification. Expression of Shh in the zone of polarizing activity (zpa) is not yet detected, and hindgut (hg) staining is reduced in the mutant. (G,H) Side view of 9.5 dpc embryos hybridized with brachyury probe. (G',H') Higher magnification of tail region of embryos stained for brachyury. Staining of tail bud mesoderm (tbm) is decreased ventrally in the mutant; note the abnormal indentation (black arrow) and narrowing of the tail in the mutant. no, node; vm, ventral mesoderm.