The anteroposterior (AP) axis in mammalian embryos is established by interactions between the embryonic and extra-embryonic tissues; in particular,the extra-embryonic anterior visceral endoderm (AVE) is needed for anterior patterning in mice. However, little is known about what induces AVE formation at the distal tip of the mouse embryo or what directs its migration to the embryo's anterior. Rodriguez and colleagues now report that these processes are regulated by the extra-embryonic ectoderm (ExE; see p. ). By using microsurgery, grafting and video imaging, they show that the ExE restricts AVE induction to the distal tip of the mouse embryo and is required to initiate AVE migration to the prospective anterior of the embryo. The ExE also induces mesoderm markers in the posterior epiblast. Thus, the ExE has a critical role in AP specification in the mouse by patterning both extra-embryonic and embryonic tissues.
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