Fig. 5. Brachyury (T) and Nodal expression domains are expanded in Mixl1^–/– embryos. (A-D) T is expressed in a wider domain in 7.5 dpc late-bud stage Mixl1^–/– (–/–) mutant embryos (B) than in the wild-type (+/+) embryo (A). Transverse sections (C,D), at the levels shown in A,B reveal that T expression in the mutant embryo is extended more laterally from the primitive streak (ps) to encompass the adjacent lateral mesodermal (lm) cells. (E) Lateral view of an early somite stage wild-type embryo, and dorsal (F) and lateral (G) views of the mutant embryo showing the expanded T expression domain in the mutant embryo, which may be accentuated by the foreshortening of the anterior part of the embryo. (H,I) Histological sections of the mutant embryo at planes shown in G, showing the T-expressing cells in the notochord-like structure (nc) under the neural folds (nf), and in the dorsal superficial cells in the primitive streak (ps) and the ventral mesendodermal tissue sheet that resembles the notochordal plate (ncp). (J) Nodal ^lacZ expression is expanded in the posterior tissues of the late-streak stage Mixl1^–/– embryos. In the wild-type 8.5 dpc embryo (K), Nodal ^lacZ is expressed in the perinodal tissues of the primitive streak (ps) and the left lateral plate mesoderm (lpm). In the mutant embryo (L), which displays small head folds (hf) and an enlarged allantois (al), Nodal^lacZ is expressed strongly in an expanded domain in the anterior segment of the primitive streak (ps) and in the axial tissues immediately anterior to the primitive streak.