Formation of tissue boundaries can be studied in a simple, inexpensive system, theXenopusgastrula. Here, the internalized mesoderm and endoderm are separated from the ectodermal blastocoel roof by Brachet ' s cleft. Non-canonical Wnt signaling mediated by the Wnt receptor, Xfz-7, is essential for this tissue separation event. The function of Wnt pathway components and other factors in tissue separation at Brachet ' s cleft can be tested in a blastocoel roof assay. Small pieces of mesoderm or endoderm are placed on large blastocoel roof explants, and it is observed whether these test explants remain on the surface of their in vivo substratum, or sink into it.
