Analysis of vertical signaling might appear simple, because it is relatively simple to combine parts of the dorsal organizer mesoderm with ectoderm. Unfortunately, it is not very straightforward to evaluate or control the amount of planar signaling that has happened or is continuing along with the vertical signaling. One can explant the prospective neural plate at the early through late gastrula stages, using the fate maps as guides. At progressively later stages, a number of properties of the neural plate become established and can be studied in such an explant. If one makes the explant from late gastrula stages, the dorsal mesoderm can be included with the prospective neural plate. The development of the neural plate with its corresponding mesodermal tissues can be compared with development of the neural plate alone. Although these types of explants have not been used much to date in Xenopus, they were used in the pioneering experiments of Jacobson and his associates on the axolotl and the newt, work that defined many of the morphogenetic issues involved in con-
Fig. 13. Two methods of dissecting a stage 10+ gastrula for the purpose of making Organizer-prospective neural explants are shown. On the left (A), the involuting marginal zone, noninvoluting marginal zone, and animal cap (IMZ-N IMZ-AC) is separated from the postinvolution PM and cut from the VE at the level of the BC (heavy dashed line), such that the PM is left with the embryo. In this case, the vegetal end of the explant (B) consists of prospective notochord (N, coarse stippling) and the overlying superficial endodermal layer (En, cuboidal layer). On the right (C), the IMZ-NIMZ-AC is peeled away from the underlying involuted prechordal mesoderm along the interface (Cleft of Brachet) between the two, but instead of cutting this flap of tissue off the VE at the BC, a cut is made between the PM and the VE (heavy dashed line, C). The involuted, PM is folded vegetally to make the explant (arrow, D).
structing a neural plate and neural tube (86,87). To study regionalization of the neural plate, various parts of the neural plate and mesoderm can be recombined (see ref. 36).
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